System and method for ultrahigh temperature testing of a physical property of a sample

Abstract

A method of testing a physical property' of a sample includes providing the sample, the sample having an axial dimension that is greater than two orthogonal cross dimensions thereof; attaching first and second interconnects at opposing axial ends of the sample; connecting at least one electrical circuit to the first and second interconnects; passing at least one current through at least the first and second interconnects to provide Joule heating of the first and second interconnects; and performing a test of the physical property of the sample. The first and second interconnects are in thermal connection with the sample to thereby heat the sample at least partially by conductive heating with the first and second interconnects.

Description

SYSTEM AND METHOD FOR ULTRAHIGH TEMPERATURE TESTING OF A PHYSICAL PROPERTY OF A SAMPLECROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present patent application claims priority benefit to U.S. Provisional Patent Application No. 63 / 623,005, filed on January 19, 2024, the entire content of which is incorporated herein by reference. All references cited anywhere in this specification, including the Background and Detailed Description sections, are incorporated by reference as if each had been individually incorporated.FEDERAL FUNDING

[0002] This invention was made with government support under grant N000142112462 awarded by the Office of Naval Research (ONR). The government has certain rights in the invention.BACKGROUND1. Technical Field

[0003] The currently claimed embodiments of the present invention relate to systems and methods for testing a physical property of a sample, and more particularly to ultra-high temperature testing of a physical property of a sample.2. Discussion of Related Art

[0004] Ultra-high-temperature (UHT) tensile testing has become more relevant than ever for material development, quality control, and assessing the reliability of in-service components. The advent of going higher and higher on the temperature scale for mechanical testing is driven by the increasing need for materials operating at high service temperatures, like engineering components in the aerospace industry7, land-based turbines, etc., where higher operating temperature meanshigher efficiency. Therefore, the pace of material development and mechanical property evaluation at high temperatures is accelerating. However, a thorough evaluation of high- temperature properties for prospective materials remains challenging due to the limited availability of UHT facilities. The feasibility of performing a full panel of tests on refractory or other exotic high temperature specimens at UHT can be a significant challenge. The available testing facilities for UHT testing are sparse at best and available to a select few industries or research groups. Other limitations to the implementation of UHT tensile testing are the sophistication in instrumentation required for such tests, high capital and running cost per test, and limited availability of test material.

[0005] Evidently, to keep up with the rate of material development, material testing has to accelerate so that more prospective materials can move from feasibility tests to field trials and then to service. One such case is refractory multi-principal alloys (RMPEAs)- a prospective high- temperature material that can potentially be used at high temperatures1. Miracle et al.2compiled available data on tests conducted on RMPEAs, as shown in FIG. 1. Most of these tests are at low temperatures. The limited number of high-temperature tests that have been performed on RMPEAs are in compression, and only a handful are in tension3. There are limited or no studies on long-term creep behavior for such materials. Similarly, in other material systems like superalloys, etc., the cost and limited availability of ultra-high temperature testing presents a need for more robust UHT testing techniques. To bridge this gap, more readily available high- temperature testing facilities must be set up to elucidate the underlying science and probe the feasibility of a new material system for high-temperature applications. The contribution of creep to the overall plasticity at ultrahigh temperatures is another scientific question that needs to be answered by performing batches of UHT tests to understand the time-dependent and timeindependent plasticity and their role in high -temperature deformation. Therefore, there remains a need for improved systems and methods for UHT of materials.SUMMARY

[0006] A method of testing a physical property of a sample according to an embodiment of the current invention includes providing the sample, the sample having an axial dimension that isgreater than two orthogonal cross dimensions thereof; attaching first and second interconnects at opposing axial ends of the sample; connecting at least one electrical circuit to the first and second interconnects; passing at least one current through at least the first and second interconnects to provide Joule heating of the first and second interconnects; and performing a test of the physical property of the sample. The first and second interconnects are in thermal connection with the sample to thereby heat the sample at least partially by conductive heating with the first and second interconnects.

[0007] A system for testing a physical property of a sample according to an embodiment of the current invention includes a sample heater; a power supply electrically connected to the sample heater; a non-contact temperature measurement system arranged to measure temperatures of the sample at a plurality of times during a measurement period; and a feedback control system configured to communicate with the non-contact temperature measurement system to receive temperature measurements therefrom and to provide control signals to the power supply based thereon. The sample heater includes first and second interconnects electrically connected to the power supply and configured to attach to opposing axial ends of a sample to be in thermal connection with the sample to thereby heat the sample at least partially by conductive heating.

[0008] A sample heater for testing a physical property of a sample according to an embodiment of the current invention includes first and second interconnects configured to be electrically connected to a power supply and configured to attach to opposing axial ends of a sample to be in thermal connection with the sample to thereby heat the sample at least partially by conductive heating.BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Embodiments of the present invention, as well as the methods of operation and functions of the related elements of structure and the combination of parts and economies of manufacture, will become more apparent upon consideration of the following description and the appended claims with reference to the accompanying drawings, all of which form a part of this specification, wherein like reference numerals designate corresponding parts in the variousfigures. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the invention.

[0010] FIG. 1 shows compiled data from Miracle et al.2on strength vs. temperature for various RMPEAs (of compositions mentioned in the legend) shows the plateau strength in intermediate temperatures and the prospect application temperature for these materials.

[0011] FIGS. 2A-2C show (a) ASTM E84standard employed for conventional tensile testing, (b) left: complete FEM model of specimen inside loading grips, right: FEM model of the optimized UHT dog-bone geometry that has a lower stress ratio than the ASTM E8 -4standard, and (c) left top: WEDM machining track showing gauge length for triple pass, left bottom: high throughput sample fabrication, right: SEM images of single and triple-pass sections of UHT dog bone samples show significantly reduced recast layers in triple-passed regions.

[0012] FIG. 3A shows ATI C103 grain size analysis from (FIG. 3B) EBSD microstructure.

[0013] FIGS. 4A-4B show photograph (FIG. 4A) and schematic representation (FIG. 4B) of the vacuum UHT testing system showing the vacuum chamber and pumping system, the position of the mechanical load frame, various feed-throughs, and the control system and feedback loop.

[0014] FIG. 5 shows the ADMET load frame (right) within the vacuum chamber (left) and can be seen from above looking in through a quartz viewport (middle) according to an embodiment of the current invention.

[0015] FIG. 6A provides a comparison of thermal profile with (right) and without (left) the use graphite interconnects, and FIG. 6B shows a proof-of-concept experiment showing sample temperatures of 1500°C while a thermocouple just 1cm away reads only 70°C. Further down the ceramic grip to the load cell, the components do not exceed room temperature.

[0016] FIG. 7A shows examples of UHT grip iterations and their corresponding amperages required to melt TZM, used as a standard reference. FIG. 7B shows load frame components for resistive heating according to an embodiment of the current invention. Note the graphite tabs on either edge of the sample and the copper lugs with braided silver wire. The ceramic gripsthermally and electrically insulate the sample from the remainder of the instrument, allowing the system to achieve localized UHT.

[0017] FIG. 8 shows post-test images of TZM showing refractory samples heated to 1500°C and 2650°C (melting temperature) in air and vacuum. The sample heated in the air indicates aggressive and destructive oxidation, whereas the samples heated in a vacuum show no observable signs of oxidation.

[0018] FIG. 9A shows thermal calibration test using a FAR FMP2 and a FLIR thermal camera while recording a melting event of ATI C 103. FIG. 9B shows left top: snapshots of the test are included in the upper left, where the sample can be seen heating, reaching thermal equilibrium, and localizing upon first melting. Middle top: temperature plots from the FLIR and FAR pyrometers. Right top: post-melt test specimen of ATI Cl 03 and bottom: corresponding thermal images at different test stages (D, E. F).

[0019] FIG. 10A provides representative snapshots (optical images) of a complete tension test, initially under load control to account for CTE during heat up and then switching to stroke control upon starting the test after thermal equilibrium. Corresponding temperature and stressstrain curves for a representative test are shown in the graph. The temperature recorded is the average gauge temperature measured by FLIR. Thermal images during uniform deformation and necking are also shown at either side of the stress-strain curve.

[0020] FIG. 10B provides experimentally obtained values for C 103 yield strength (YS) (left) and ultimate tensile strength (UTS) (right), compared to published ATI data41and their predicted strength-temperature curves. Data collected at JHU shows excellent agreement with ATI data41.

[0021] FIG. 11A shows improved speckle pattern clarity, contrast, and depth of field using an IR filter and UV light are visible in proof-of-concept experiments up to 1500°C.

[0022] FIG. 11B show s UV DIC setup according to an embodiment of the current invention, showing a 261nm laser to illuminate the sample, an IR filter to remove higher wavelengths, and a UV camera to capture the detail of the piece as seen from the lower wavelength spectrum.

[0023] FIGS. 12A and 12B show JHU experimental values for C 103 ductility' compared to published ATI data and their predicted ductility-temperature curve. The ductility is more accurately measured at lower temperatures but at high and UHT, localization immediately after necking is more severe, resulting in significant underreporting of ductility at UHT. The plans (middle) for a revised joule heating system and its proof of concept (right) are shown, which can eliminate this undesired phenomenon.

[0024] FIG. 13 provides emissivity' of various alloys (indicated above each plot) measured as a function of temperature in the setup described in accordance with an embodiment of the present invention.

[0025] FIG. 14 shows melting point of various alloys measured in the setup described in accordance with an embodiment of the present invention. Right: melting point measured by described setup compared to values available in literature. Left: melt pool of melted specimens.DETAILED DESCRIPTION

[0026] Some embodiments of the current invention are discussed in detail below. In describing embodiments, specific terminology' is employed for the sake of clarity'. However, the invention is not intended to be limited to the specific terminology so selected. A person skilled in the relevant art will recognize that other equivalent components can be employed, and other methods developed, without departing from the broad concepts of the present invention. All references cited anywhere in this specification are incorporated by reference as if each had been individually incorporated.

[0027] Accordingly, an embodiment of the current invention introduces a novel UHT tester that is shown to be viable for performing UHT tests. Several challenges in implementing UHT tests have been addressed with proof-of-concept tests demonstrating the efficacy of this system.

[0028] Conventionally, tensile test specimen geometry in standard practice is fabricated in accordance with international standards such as ASTM E84. The material required for such specimens is not feasible for testing specialized materials like RMPEAs, composites, specialized alloys, etc., which are usually expensive and in short supply. In such a scenario, sub-scale specimens are more economical to adopt. However, sub-scale specimens of the dimensional orderof millimeters or sub-millimeters are not standardized; therefore, care must be taken to avoid sizeeffect based artifacts, i.e., to ensure that representative volumes are tested to reflect bulk mechanical behavior. The relative scale of the microstructure as compared to the specimen dimensions also becomes a critical consideration in small-scale testing5,6. That being said, modem finite element analysis (FEA) tools and guidelines for selecting a representative volume element7 13are available to design a sub-millimeter-sized geometry that can be adopted for UHT testing.

[0029] High-temperature oxidation is another limiting factor that can involve the formation of an oxide film that changes the dimensions of the specimens and lead to volatilization, cracking or peeling of the film during the test, all of which offset and invalidate the physical measurements made during a test. It is, therefore, preferable to carry out tests in an inert atmosphere7or under a high vacuum to minimize oxidation.

[0030] Achieving UHT between 1200 to 3500 °C poses unique challenges in performing high- temperature tests. Most available furnaces, which are convenient to build at a lab scale and have straightforward instrumentation, are based on Kanthal heating elements that can reach up to ~1200°C while operating at line voltage. For higher temperatures up to 2000 °C. specialized heating elements of molybdenum disilicide, graphite, tungsten, etc., are employed. Such fumaces / heating chambers are reasonably bulkier than the Kanthal counterparts due to their low resistance and hence require step-down transformers and very high operating currents to maintain temperature. Such heating chambers often are supplemented with cooling channels to avoid heat transfer to the body of the furnace, thus further adding to the bulk of the instrument. For the longterm stability of oxide or silicide heating elements, operating them below 1000 °C is discouraged to prevent detrimental phase transformations, thus narrowing the operating temperature range14

[0031] Integrating a UHT furnace into a mechanical load frame requires a compact heating system, especially when the testing is conducted in a vacuum or environmental chamber. Such considerations make joule heating of specimens a viable candidate for reaching UHT. The heating systems that employ j oule heating can be easily integrated into a mechanical tester inside a vacuum or environmental chamber7’15. They can reach over 3000 °C in a fraction of minutes, operate over a range of temperatures, and offer fast heating and cooling rates desired for mechanical testing7. The downside is that one must deal with the effect of electnc current on the mechanical propertiesviz. a viz. electro-migration16, electro-plasticity17, electricity -mediated dislocation motion18 19, etc. Strategies to avoid such effects and apply joule heating to achieve UHT are presented according to some embodiments of the current invention.

[0032] Unlike a conventional furnace where the whole chamber is heated, the localized nature of joule heating confined to the specimen under tension requires temperature measurement at a high response time. The small thermal mass of the millimeter-scale specimen makes the time scale of heat loss to the surrounding grips, and temperature rise in the specimen with respect to the applied current, very fast compared to a conventional furnace. As such, a noncontact temperature sensor like a pyrometer that can provide quick feedback to the control system to compensate for any temperature change is generally used. However, conventional pyrometers rely on emissivity knowledge to determine the surface temperature; therefore, care must be taken to ensure the measured temperature is accurate, especially for newly developed alloys where the emissivity is unavailable in the literature.

[0033] Similarly, strain measurement at UHT is non-trivial. There are no available strain sensors that can withstand temperatures exceeding few hundreds of degrees. Axial extensometers are therefore used in high-temperature tensile tests and creep tests to isolate the strain sensor from the hot section of the furnace20-23. The measurements are still subject to machine compliance because of the additional mechanical components involved in the measurement and have very limited feasibility7for compact mechanical testers due to space constraints. Introducing digital image correlation (DIC) to high-temperature testing has been a step forward in this di recti on24 26

[0034] The following sections elaborate upon each challenge and how they can be overcome according to some embodiments of the current invention. The procedure discussed herein also describes a high-throughput evaluation of high-temperature properties from tensile tests by batch processing of multiple specimens.

[0035] In the following, the term ‘"physical roperty" is intended to have a broad meaning that can include, but is not limited to, mechanical, thermal, optical, and / or chemical properties. The term “ultra-high temperature” as used herein means a temperature of at least 1200 °C, such as, but not limited to, 1400 °C to 3500 °C. The term room temperature as used herein is intended to referto about 25 °C in which “about'’ means to with plus or minus 5 °C. That is, room temperature may typically be 25 °C, but can be as low as 20 °C, or as high as 30 °C.

[0036] The term “milli-scale”, etc. is also used to refer to millimeter-scale.

[0037] Testing small sample volumes at ultra-high temperatures has become more relevant than ever due to the increasing demand for materials that can withstand high service temperatures, such as conventional alloys including TZM, C103, tungsten, tungsten rhenium, superalloys, ceramic matrix composites, and emerging materials, e.g., multi-principal element alloys. The advent of increasing Carnot’s efficiency of an aero-engine and sending space probes to the sun are a few examples of areas where materials with ultra-high temperature strength are in high demand. While a few conventional alloys like TZM, Cl 03, tungsten, tungsten rhenium, etc. superalloys, ceramic matrix composites, and some multi-principal alloys have shown potential for application at or above 1400 °C, their testing, and characterization remain challenging. These materials are often in short supply and oxidize vigorously at high temperatures, which adds to the challenges of performing high-temperature tensile tests. With examples of refractory alloys, we demonstrate that joule heating of milli-sized specimens is a promising approach to address these challenges. Still, it comes with the challenge of electromigration, electro-plasticity, and the requirement of mindful sample gripping. This work describes these challenges and provides a novel high throughput testing methodology for reliable tensile testing from room temperature to 2000 °C according to some embodiments of the current invention.

[0038] Accordingly, a method of testing a physical property of a sample according to an embodiment of the current invention includes providing the sample, the sample having an axial dimension that is greater than two orthogonal cross dimensions thereof; attaching first and second interconnects at opposing axial ends of the sample; connecting at least one electrical circuit to the first and second interconnects; passing at least one current through at least the first and second interconnects to provide Joule heating of the first and second interconnects; and performing a test of the physical property of the sample. The first and second interconnects are in thermal and electrical connection with the sample to thereby heat the sample at least partially by conductive heating with the first and second interconnects.

[0039] In some embodiments, the method further includes monitoring temperatures of the sample over a test period of time and providing feedback control of the passing at least one current through at least the first and second interconnects to thereby provide sample-temperature feedback control. In some embodiments, the monitoring temperatures of the sample is performed by noncontact temperature monitoring. In some embodiments, the passing at least one current through at least the first and second interconnects passes a first current through the first interconnect and passes a second cunent through the second interconnect to provide Joule heating separately to each of the first and second interconnects. In some embodiments, the passing at least one current through at least the first and second interconnects passes a current through the first interconnect, through the sample and through the second interconnect to provide Joule heating to both first and second interconnects and to the sample. In some embodiments, the first and second interconnects have greater electrical resistances than the sample such that the first and second interconnects experience greater Joule heating than the sample. In some embodiments, the first and second interconnects are at least one of carbon in the form of graphite, or carbon fiber felt, zirconium diboride, or tungsten.

[0040] In some embodiments, the method further includes disposing a diffusion barrier between the sample and each of the first and second interconnects. In some embodiments, the diffusion barrier is at least one of tantalum, nickel, nichrome, hafnium, niobium, zirconium, vanadium, or tungsten foil.

[0041] In some embodiments, the performing the test of the physical property of the sample performs a tensile mechanical strength test by displacing the sample along the axial dimension and measuring corresponding strain at a measurement temperature thereof. However, the general concepts of the current invention are not limited to only tensile mechanical tests. For example, other embodiments can perform a creep test in which a mechanical strength is tested by applying a constant load or stress to the sample and measuring the elongation that ensues over time at elevated temperatures. Time dependent plasticity i.e. creep can be measured in the same setup and procedure as described for tensile tests by performing the tests at a constant load or stress over an extended amount of time. Other related properties can include activation energy, stress exponent, etc. Some embodiments of the current invention can be directed to measuring the coefficient of thermal expansion (CTE) or to study microstructural stability and phase transformations, for example.

[0042] We could also do constant load or stress creep experiments to measure the creep strength. In some embodiments, the measurement temperature is at least 1.500 °C. In some embodiments, the measurement temperature is at least 20 °C and less than 3,500 °C.

[0043] In some embodiments, the method further includes providing a vacuum chamber; arranging at least the sample within the vacuum chamber; and forming a vacuum within the vacuum chamber to a preselected pressure level. The performing the test of the physical property of the sample is performed while the sample is in the vacuum chamber at the vacuum.

[0044] In some embodiments, the method further includes providing a chamber; arranging at least the sample within the chamber; and forming a controlled atmosphere within the chamber to a preselected atmosphere. The performing the test of the physical property of the sample is performed while the sample is in the chamber in the controlled atmosphere. For example, the controlled atmosphere could be an atmosphere of a particular volume and type of gas or mixture of gases. For example, the controlled atmosphere can be a highly pure argon atmosphere, a hydrogen atmosphere, a forming gas atmosphere, or an oxygen atmosphere in some embodiments. However, the general concepts of the current invention are not limited to only these examples.

[0045] In some embodiments, the sample has a width of between 0.1 to 2 mm and cross- sectional area of at least 0.01 mm2up to 4 mm2.

[0046] Another embodiment of the current invention is directed to a system for testing a physical property of a sample (see, e.g., FIGS. 4A, 4B, 5, 7A, and 7B for some examples). The system includes a sample heater, a power supply electrically connected to the sample heater, a non-contact temperature measurement system arranged to measure temperatures of the sample at a plurality of times during a measurement period, and a feedback control system configured to communicate with the non-contact temperature measurement system to receive temperature measurements therefrom and to provide control signals to the power supply based thereon. The sample heater includes first and second interconnects electrically connected to the power supply and configured to attach to opposing axial ends of a sample to be in thermal connection with the sample to thereby heat the sample at least partially by conductive heating. In some embodiments, the first and second interconnects include at least one of carbon in the form of graphite, or carbon fiber felt, zirconium diboride, or tungsten. In some embodiments, the first and secondinterconnects further include a diffusion barrier on a surface thereof to make direct contact with the sample. In some embodiments, the diffusion barrier is at least one of tantalum, nickel, nichrome, hafnium, niobium, zirconium, vanadium, or tungsten foil.

[0047] Another embodiment of the current invention is directed to heater for testing a physical property of a sample (see. e.g., FIGS. 4A, 4B, 5, 7A, and 7B for some examples). The heater includes first and second interconnects configured to be electrically connected to a power supply and configured to attach to opposing axial ends of a sample to be in thermal connection with the sample to thereby heat the sample at least partially by conductive heating. In some embodiments, the first and second interconnects include at least one of carbon in the form of graphite, or carbon fiber felt, zirconium diboride, or tungsten. In some embodiments, the first and second interconnects further include a diffusion barrier on a surface thereof to make direct contact with the sample. In some embodiments, the diffusion barrier is at least one of tantalum, nickel, nichrome, hafnium, niobium, zirconium, vanadium, or tungsten foil.

[0048] The following will describe some embodiments in more detail. However, the general concepts of the current invention are not intended to be limited to only these specific embodiments.Material and methodologies

[0049] Two preliminary commercial grade refractory alloys have been studied using the UHT testing methodology: a Mo alloy, TZM, and aNb alloy, ATI Cl 03 from Allegheny Technologies Incorporated (ATI) USA. The materials were received in the form of blocks and machined by a wire EDM (electrical discharge machining) into a scaled-down version akin to ASTM-E84tensile specimens (as shown in FIG. 2A). The milli-tensile specimens (FIG. 2B) were roughly (1- 0.75mm) x (l-0.75mm) in cross-section and about 4mm in gauge length. The milli-sized tensile specimen was designed to ensure that the stress ratio (maximum stress / desired stress) is below 1.3 (as per ASTM4guidelines). Finite element analysis was used to validate the geometry by optimizing the design to minimize stress concentrators.

[0050] Developing a sub-scale method for testing these alloys in a high-throughput fashion without compromising material performance or inducing size scale effects is critical for UHT testing. WEDM (wire electrical discharge machining) recast layer is known to introduce artifactsin mechanical testing27 32. Such effects can be chemical or mechanical in nature and are detrimental to strength and related properties measured in a tensile test27 32. Accordingly, procedures were developed to minimize the recast layer formed on the surface of a machined milli-tensile specimen. The thickness of the recast layer was minimized by reducing the power input during WEDM. The number of surface passes was increased to three times in the gauge section. As shown in FIG. 2C, the difference in the thickness of the recast layer between a single pass and a triple pass is significant. The specimens thus machined were hand polished to remove any ruminant recast surfaces and eliminate the surface roughness that gets inevitably introduced due to WEDM. Additionally, multiple milli-tensile specimens were simultaneously prepared during WEDM and polishing to increase the throughput. The present testing protocol is estimated to offer a 15-fold benefit in terms of material used compared to ASTM E8 standard specimen and a 5 to 10-fold advantage regarding WEDM processing and polishing. This benefit comes from the fact that multiple small specimens can be processed simultaneously in batches (see FIG. 2C).

[0051] Further, the cross-section of the milli-tensile specimen is so chosen that statistically, more than 10 grains end up across each surface3'35 36. As seen in the example of Nb alloy ATI Cl 03 (Error! Reference source not found. 3A, 3B), the grain size is around ~ 40-50 pm, and therefore a 0.75 mm thick by 0.75 wide specimen cross section has around ~ 280 grains which translated to more about 15 grains on each surface. All specimens tested in these examples were between 0.75 to 1 mm thick and had a square cross-section.

[0052] Given the unavailability of low-cost ready -to-buy load frames for material testing in tension at UFIT, a testing rig was developed in-house. As mentioned before, the high vacuum requirements at UFIT can be achieved by designing a testing system that fits inside of a vacuum chamber. The setup consists of a mechanical load frame that fits within a vacuum chamber (see FIGS. 4A, 4B), heats samples via resistive heating, and has viewports for optical strain and temperature monitoring. The specimen is loaded in a custom load frame from the company ADMET (see FIG. 5). The heating system based on joule heating is integrated into the specimen gripping system of ADMET. More details on each component are presented below.Vacuum chamber

[0053] The vacuum enclosure (FIG. 4B) is a ten ft3chamber with numerous feed-throughs, and diffusion and roughing pumps pull a high vacuum. The chamber lid is removable to allow access to the load frame in the chamber, and it includes quartz and germanium viewports that enable line-of-sight DIC and temperature measurements, which in turn provide in-situ measurements of the strain and temperature of the deforming specimen. In the present arrangement, a vacuum of - 9. Ox 10’8Torr is achieved.ADMET load frame

[0054] The load frame required for tensile testing at UHT must be vacuum-compatible and customizable to isolate specimen heat and current from the rest of the instrument. To that end, a custom mechanical tester capable of applying a load of 5 kN load with a resolution of 250 mN, displacement rates between 4 pm / min and 40 mm / min. a symmetrically moving 165mm stroke, and a 100 nm resolution linear scale was acquired from ADMET. The mechanical tester is shown on the left of FIG. 5. The tester has a low profde, small footprint, and is high-vacuum compatible and suitable for experimentation.

[0055] Custom ceramic grips, designed in-house at Johns Hopkins University, grasp the sample's ends at 45° angles, akin to a bowtie, that provide self-alignment of the samples during testing. These ceramic grips are supplemented to the ADMET load frame and ensure electric and thermal isolation of the specimen from the rest of the load frame. The grips are sufficiently large that they do not experience considerable stresses, in addition to thermally and electrically insulating the sample from the rest of the instrument, as elaborated in the following subsection. The ADMET system is shown in FIG. 5, with the supplemental ceramic grips shown in (see FIGS. 7A, 7B), and the sub-scale dog-bone geometry is shown in FIG. 2B.Joule heating apparatus

[0056] The main goal of designing a compact joule heating system is to effectively heat a specimen to UHT without needing a cooling system and is easy to integrate into a benchtop mechanical tester. An iterative design strategy7based on a self-learning approach w as adopted to arrive at the final design. Here, the melting of an alloy w as treated as a physical event of reference to benchmark thermal efficiency. Initially, using jointed metallic grips insulated by a ceramic w asher (see FIG. 7B) a cunent of 100 A was required to melt a sub-scale sample (1mm x 1mmcross section) of TZM (Tm~2623°C). Next, only 50A was required to melt a similar sample of TZM after switching to full ceramic grips and applying current through the sample using copper lugs and braided silver wire. Finally, small graphite interconnects (see FIG. 7B) fastened between the lug and the sample were used to heat the sample by combining heat conduction from the carbon interconnects and direct resistive heating of the specimen (see FIGS. 7A, 7B). This configuration required only 20A to melt a sample of TZM. These proof-of-concept experiments proved that inducing localized UHT over 2600°C was feasible using the newly developed design shown in FIGS. 7A, 7B. The ceramic grips were fabricated from 8 mole % yttria-stabilized zirconia (8YSZ), which has excellent thermal insulation properties due to its low thermal conductivity even at high temperatures37In routine testing, thin foils of Tantalum - 150 pm thick cut in the shape of the specimen grips were placed between the specimen and carbon interconnects, to act as a diffusion barrier against the diffusion of carbon atoms into the specimen. The diffusion of unwanted species into the specimen can be a challenging problem at UHT. Thus, maintaining chemical isolation or providing a diffusion barrier like Ta foils ensures that the specimen's composition is unaltered during high-temperature tests.

[0057] Simulations performed on simulation software by COMSOL using graphite interconnects in conjunction with 8YSZ grips suggest that only 40% of the achieved temperature was a result of direct current through the sample (e.g. traditional joule heating of a tensile specimen), whereas 60% of the acquired temperature was a result of heat conduction from the graphite tabs into the sample. Further, within the time of a tensile test, i.e., typically less than 5- 6 minutes (including heating up and attaining thermal equilibrium), the thermal insulation achieved and isolation of the heat within the inner grips of the system was found to be appreciable. The ceramic grips were measured to be at 70°C just 1 cm in either direction from a sample that was glowing at 1500°C. Further away, the ADMET crossheads and load cell remain at room temperature, as was proven by monitoring the thermocouple reading at these locations (FIGS. 6A, 6B)

[0058] For testing milli scale specimens at higher temperatures (> 1400 °C), additional considerations were taken while designing the control system to account for rapid heat losses to the load trail and response time of system to maintain temperature. The system has been designed to handle high currents that allow reaching UHT in milli-scale samples at a low voltage. Effectively the power input is very low (10-50 watts), incidentally lower than running a hair drier(typically -1000 wats). The electric current injected in the specimen is controlled by a closeloop feedback control using an AC or DC power source, a power controller (solid state relay or a thyristor), a temperature sensor (thermocouple or pyrometer), and a Eurotherm Nanodac PID controller. The low thermal mass of the specimen, the heat lost to the grips by conduction, and to the surrounding via. radiation at high temperatures necessitates using a control system with a response time of micro to milliseconds. Accordingly , the PID. thermocouple or pyrometer, and power controller chosen for this setup are such that they enable micro-second response time. Additionally, owing to the relatively high conductivity of the metallic specimens, the overall resistive load on the power supply is relatively low. Therefore, line voltage cannot be used to operate such setups unlike a Kanthal element-based furnace. Power controllers and pow er sources that operate at the extremely low potential of 1 volt or less and high amperage of 50-80 amps were used in the examples described here. This is relatively easy to achieve in DC power sources compared to AC power sources that need an additional step-down transformer to operate at low voltage and high currents. The ability to rapidly apply and locally control UHT, especially with a dynamic mechanical tester that acts as a thermal sink and evolving sample shape, represents a significant step towards realizing UHT testing. The joule heating apparatus can also be used for static heating and cooling for physical metallurgical, rapid quenching, or thermodynamic studies like phase transformation etc. In the present setup, the specimens can be heated at a rate of -360 °C / s and a cooling rate of -750 °C / s.Results and discussion

[0059] The results presented in this section are focused on demonstrating that UHT testing is realized in the temperature range of 30 to 2000 °C for testing and up to 3400 °C for static heating. In addition, critical issues of oxidation and reliable temperature measurement are addressed in the following subsections.High -Temperature Oxidation

[0060] From a thermodynamic point of view, metal oxides are more stable than metals. Refractory alloys and metals generally have a high affinity for atmospheric oxygen, which increases with increasing temperature38. ATI Cl 03 specimens heated to UHT in the air show aggressive oxidation. In contrast, those heated in a vacuum show no visible signs of oxidation, areassuring observation shown in FIG. 8 for TZM upto 2650 °C. Therefore, achieving a high vacuum (for example > le-6torr) can be important to minimize the oxidation of metallic systems at high temperatures.Temperature measurement and emissivity:

[0061] A dynamic heating arrangement that involves j oule heating a specimen while in tension poses significant challenges in attaching a thermocouple. The high diffusivity of elements at UHT, the detrimental effect of current going through the specimen on the potential difference measured by the thermocouple, and the requirement of a minimal response time for heating suggests that a non-contact mode of temperature measurement is more feasible. Pyrometers are routinely used to measure high temperatures in a non-contact mode. The knowledge of emissivity is used to measure temperature according to Plank's law39(see FIGS. 9A, 9B). Ideally, if the emissivity is known and remains constant throughout the test, a single-color pyrometer can reliably measure temperature. However, the emissivity of all metallic systems changes as a function of temperature, surface conditions, etc.40. Due to variations in emissivity, incorporating a pyrometer measurement into a mechanical tester can be significantly challenging. Ordinary one color pyrometry methods require calibration on each material of interest before testing and operate at a fixed wavelength. However, this is not feasible because emissivity changes during heating up and during deformation due to evolving surface conditions40. Therefore, a pyrometer is required to measure emissivity in a two-color or multi-spectral pyrometer. This is essential for new sample systems that need to be tested, especially in the world of rapidly emerging alloys. We use an FMP2 multi-spectral pyrometer from FAR Associates, designed to read specimen temperatures between 800°C and 2500°C for a material with an emissivity of 1. This range changes with the emissivity’ of the specimen, typically moving to a higher temperature with decrease in emissivity. Its 500-wavelength design can correct for changes in emissivity as a function of time, temperature, and sample material, a feature unique to multi-spectral pyrometers. To this end, a new FMP2 multi -spectral pyrometer with custom optics yielded a spot size of ~ 1 mm or 5 mm (FIGS. 9A, 9B) that is well suited for measuring the temperature of the gauge length of milli-scale samples and was used to measure temperature.

[0062] To validate the efficacy of this technique and instrument, a test was designed to melt ATI C 103 in situ in the testing rig inside the vacuum chamber. Dog-bone specimens of ATI C 103were heated by passing and increasing electric current until they melted, and the corresponding temperatures were recorded using both a FAR pyrometer and FLIR thermal camera (using posttest corrected emissivity). The FLIR camera has a provision to adjust the emissivity post-test, this feature was utilized to segmentize the entire temperature range and use available emissivity data from literature at different temperatures41. This temperature profile measure from FLIR was compared with the measured values from the FAR pyrometer, with a self-adjusting emissivity. Supporting images of this experiment and related material are included, as shown in FIGS. 9A, 9B, showing a good agreement between the expected melting point of ATI Cl 03 and the measurements obtained with the FAR pyrometer and the FLIR thermal camera.

[0063] To test the maximum achievable temperature in this setup, a Tungsten wire of 1 mm diameter and 0.75mm x 0.75mm square cross-section strips of ATI Cl 03 were placed in the testing rig (in place of the tensile specimen). The current was increased by 5A / sec until the material melted, indicating that the temperature was at the melting point of the material. Literature values of the melting point of ATI Cl 03 are 2350 + / -50 °C41(FIGS. 9A, 9B); the temperatures recorded by the FAR pyrometer and by the emissivity-adjusted FLIR thermal camera were found to be in close agreement to this value. The melting temperature of W was bey ond the recording capability7of both the FAR and the FLIR. In this case, the melting of W ire was taken as proof that the sample temperature achieved was > 3400 °C. (The melting point of materials is generally lower in high vacuum. Such minor differences are ignored here and assumed to not have any effect of the broad conclusion drawn here.)

[0064] These proof-of-concept experiments show that, in the present setup, achieving and controlling UHT in milli-scale samples is possible.Testing

[0065] Refractory7alloy ATI Cl 03 was tested in displacement control at various temperatures inside the vacuum chamber. A triple-pass WEDM (FIGS. 2A-2C) and hand polished sample of approximately 0.75mm x 0.75mm in the gauge were placed in the mechanical tester, placed in high vacuum, and heated. After ramping, the sample temperature was allowed to reach a steady thermal equilibrium, after which the tensile test w as commenced. The temperature in the gauge remains uniform until necking, at which point the heat localizes in the narrowest cross-section.This is discussed further in a subsequent section. A detailed image showing representative snapshots of a tensile test, including the stress-strain and temperature data, is shown in FIGS.10A-10B

[0066] The measured yield strength (YS) and ultimate tensile strength (UTS) for ATI C103 were measured for a number of samples, compiled as a function of temperature, and are shown in FIG. 10B in open symbols, where the black data solid symbols represent published ATI data

[0016] , As shown in FIG. 10B, the yield and ultimate tensile strengths of ATI Cl 03, measured up to 2000°C, agree excellently with the published data from ATI

[0016] demonstrating the instrument's capabilities.Strain mapping: UV DIC

[0067] Reliably measuring strain by extensometer-based displacement sensors that are most often used for high-temperature experiments at and above 1000°C can pose a severe challenge due to the small scale of the testing system and increased sensitivity to machine compliance. Noncontact optical strain measurement offers a number of advantages in overcoming or avoiding machine compliance, thermal expansion, thermal gradients and shimmer, and limitations related to vacuum chamber size. Optical strain measurement / mapping via digital image correlation (DIC) is now7commonly applied during conventional tensile testing and has multiple advantages for sub-scale UHT testing. Nevertheless, the acquisition of strain maps on sub-scale specimens, at UHT with intense radiation, through a viewing window, imposes additional challenges that must be addressed.

[0068] Digital cameras and telescopic lenses allow us to monitor the specimen through the quartz window in the lid of the vacuum system (FIG. 5), and the presence of the vacuum eliminates unw anted shimmer. Still, the intense radiation of the sample at UHT may overwhelm the speckle patterns. Therefore, the radiation from the specimen is filtered out by an IR filter bandpass filter that cuts down all radiation except for a narrow wavelength range, ± 5 nm (255- 265 nm) for the current system. The specimen is illuminated by a UV laser (261 nm), and a UV- compatible camera records images of the gauge. Use of a UV camera significantly improves a speckle pattern's clarity, contrast, and depth of field. A proof-of-concept experiment was performed on TZM speckled with alumina and carbon, as shown in FIG. 11A, where a visiblelight filter (300 nm) and a UV light illumination significantly reduce the radiation coming out at high temperatures.Current localization and remedy

[0069] A major drawback in employing joule heating to perform high- temperature testing is temperature dependence on current density and sample shape. The specimen's cross-section changes as a function of deformation in a tensile test. It can be assumed that the tensile specimen deforms uniformly until the ultimate tensile strength is achieved. After this limit, the strain hardening exhausts, and necking, or cavitation, begins. At this point, the current localizes where the cross-section is reduced, at locations of necking or cavitation. At high currents, as in the case of UHT, these current localizations can lead to localized softening and eventually melting and, thus, premature failure. This also leads to artifacts in the measured total strain, as seen in the case of ATI Cl 03 (FIG. 12A). thus posing a challenge to apply joule heating for high- temperature mechanical testing. An alternate strategy is employed to get around this localization phenomenon. Here two current sources are used, one for each grip, to loop current in the specimen grips without the need to pass current through the sample. This allows joule heating of the specimen grip on each side of the gauge and conductive heat transfer to the gauge section, thus bypassing any current through the gauge and hence avoiding localization post UTS.Conclusion

[0070] It was demonstrated that, for sub-scale samples, UHT of the order of 3400 °C can be achieved by joule heating. Tensile tests can be reliably performed until 2000 °C using the present setup without the issue of high-temperature oxidation. Thermal efficiency can be significantly improved by using carbon interconnects, and looping current through the grips allow one to reduce or eliminate current through the same, thus avoiding electromigration and current localization.

[0071] In view of the above, various alternative embodiments may be used, such as, but not limited to the following:(a) Macroscale specimens like ASTM E8 are not feasible due to the sparse availability of specialty materials like RMPEAs.o Use sub-scale / milli-scale tensile specimens for mechanical testing of single and poly crystals.(b) Achieving localized UHT above 1200 °C without using sophisticated environmental chambers and furnaces with temperature capability. o Use resistive heating with a modular grip design that avoids the need for furnaces and heating of the load frame.(c) Avoiding electromigration within the sample o Use low-conductivity interconnects as an additional heat source and supplement the overall heat available to the specimen.(d) Aggressive sample oxidation o Perform experiments under a high vacuum or in a controlled environment.(e) Temperature measurement amidst emissivity variations o Integrate a multi-spectral pyrometer.(f) Strain measurement for milli-scale specimens at UHT o Perform DIC and observe speckle patterns under UV light.Additional data showing melting point and emissivity measurement:

[0072] Emissivity and melting temperature are two important physical properties that can be measured using the described setup. In this setup the same specimen as described in section Material and methodologies is heated at a constant heating rate. The emissivity and temperature are measured by FAR pyrometer inside a vacuum chamber. The mechanism of current injection, interconnect and diffusion barrier is same as described for mechanical testing. The only difference is that while testing the load frame is configured to maintain zero load to avoid thermal expansion or contraction to generate any stress in the specimen. A few examples of Kanthal. carbon, tungsten, ATI Cl 03 and Senkov alloy are shown to demonstrate the measurement of melting point and emissivity.

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[0074] While various embodiments of the present invention have been described above, they have been presented by way of example only, and not limitation. Thus, the breadth and scope of the present invention should not be limited by any of the above-described illustrative embodiments but should instead be defined only in accordance with the following claims and their equivalents.

[0075] The embodiments illustrated and discussed in this specification are intended only to teach those skilled in the art how to make and use the invention. In describing embodiments of the disclosure, specific terminology is employed for the sake of clarity. However, the disclosure is not intended to be limited to the specific terminology so selected. The above-described embodiments of the disclosure may be modified or varied, without departing from the invention, as appreciated by those skilled in the art considering the above insights. It is therefore to be understood that, within the scope of the claims and their equivalents, the invention may be practiced otherwise than as specifically described. For example, it is to be understood that the present disclosure contemplates that, to the extent possible, one or more features of any embodiment can be combined with one or more features of any other embodiment.

Claims

WE CLAIM:

1. A method of testing a physical property of a sample, comprising: providing said sample, said sample having an axial dimension that is greater than two orthogonal cross dimensions thereof; attaching first and second interconnects at opposing axial ends of said sample; connecting at least one electrical circuit to said first and second interconnects; passing at least one current through at least said first and second interconnects to provide Joule heating of said first and second interconnects; and performing a test of said physical property' of said sample, wherein said first and second interconnects are in thermal connection with said sample to thereby heat said sample at least partially by conductive heating with said first and second interconnects.

2. The method according to claim 1 , further comprising monitoring temperatures of said sample over a test period of time and providing feedback control of said sample passing at least one current through at least said first and second interconnects to thereby provide sampletemperature feedback control.

3. The method according to claim 2, wherein said monitoring temperatures of said sample is performed by non-contact temperature monitoring.

4. The method according to any one of claims 1-3, wherein said passing at least one current through at least said first and second interconnects passes a first current through said firstinterconnect and passes a second current through said second interconnect to provide Joule heating separately to each of said first and second interconnects.

5. The method according to any one of claims 1-3, wherein said passing at least one current through at least said first and second interconnects passes a current through said first interconnect, through said sample and through said second interconnect to provide Joule heating to both of said first and second interconnects and to said sample.

6. The method according to claim 5, wherein said first and second interconnects have greater electrical resistances than said sample such that said first and second interconnects experience greater Joule heating than said sample.

7. The method according to any one of claims 1-6. wherein said first and second interconnects are at least one of carbon in the form of graphite or carbon fiber felt, or of zirconium diboride, or of tungsten.

8. The method according to any one of claims 1-7, further comprising disposing a diffusion barrier between said sample and each of said first and second interconnects.

9. The method according to claim 8, wherein said diffusion barrier is at least one of tantalum, nickel, nichrome, hafnium, niobium, zirconium, vanadium, or tungsten foil.

10. The method according to any one of claims 1-9. wherein performing said test of said physical property of said sample performs a tensile mechanical strength test by applying strainto said sample along said axial dimension and measuring corresponding strain at a measurement temperature thereof.

11. The method according to claim 10, wherein said measurement temperature is at least 1,500 °C.

12. The method according to claim 10, wherein said measurement temperature is at least 20 °C and less than 3,500 °C.

13. The method according to any one of claims 1-12, further comprising: providing a vacuum chamber; arranging at least said sample within said vacuum chamber; forming a vacuum within said vacuum chamber to a preselected pressure level, wherein said performing said test of said physical property of said sample is performed while said sample is in said vacuum chamber in said vacuum.

14. The method according to any one of claims 1-12, further comprising: providing a chamber; arranging at least said sample within said chamber; forming a controlled atmosphere within said chamber to a preselected atmosphere, wherein said performing said test of said physical property of said sample is performed while said sample is in said chamber in said controlled atmosphere.

15. The method according to any one of claims 1-14, wherein said sample has a width of between 0. 1 to 2 mm and cross-sectional area of at least 0.01 mm2up to 4 mm2.

16. A system for testing a physical property of a sample, comprising: a sample heater; a power supply electrically connected to the sample heater; a non-contact temperature measurement system arranged to measure temperatures of said sample at a plurality of times during a measurement period; and a feedback control system configured to communicate with said non-contact temperature measurement system to receive temperature measurements therefrom and to provide control signals to said power supply based thereon, wherein said sample heater comprises first and second interconnects electrically connected to said power supply and configured to attach to opposing axial ends of a sample to be in thermal connection with said sample to thereby heat said sample at least partially by conductive heating.

17. The system according to claim 1 , wherein said first and second interconnects comprise at least one of carbon in the form of graphite or carbon fiber felt, or of zirconium diboride, or of tungsten.

18. The system according to claim 17, wherein said first and second interconnects further comprise a diffusion barrier on a surface thereof to make direct contact with said sample.

19. The system according to claim 18, wherein said diffusion barrier is at least one of tantalum, nickel, nichrome, hafnium, niobium, zirconium, vanadium, or tungsten foil.

20. A sample heater for testing a physical property of a sample, comprising: first and second interconnects configured to be electrically connected to a power supply and configured to attach to opposing axial ends of a sample to be in thermal connection with said sample to thereby heat said sample at least partially by conductive heating.

21. The sample heater according to claim 20, wherein said first and second interconnects comprise at least one of carbon in the form of graphite, carbon fiber felt, zirconium diboride, or tungsten.

22. The sample heater according to claim 21, wherein said first and second interconnects further comprise a diffusion barrier on a surface thereof to make direct contact with said sample.

23. The sample heater according to claim 22, wherein said diffusion barrier is at least one of tantalum, nickel, nichrome, hafnium, niobium, zirconium, vanadium, or tungsten foil.

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