A test method for the second type of residual stress of 4J33 Kovar alloy

Abstract

The present invention discloses a method for testing the second type of residual stress of 4J33 Kovar alloy, which relates to the field of analytical instruments and material testing technology. The method comprises the following steps: loading an annealed stress-free specimen and a specimen pre-stressed with equibiaxial known stress into a nanoindenter; performing a nanoindentation experiment on the grains entering a marked circle with the test point as the center and twice the average grain size as the radius, obtaining all indentation data pressed into each grain in the marked circle; averaging all indentation data pressed into each grain in the marked circle to obtain an average value of indentation data corresponding to each grain in the marked circle; summing and averaging the average values ​​of indentation data corresponding to all grains in the marked circle to obtain an average value of indentation data corresponding to all grains in the marked circle, and substituting the average value into a residual stress calculation model to obtain the second type of residual stress. The present invention can effectively reduce the influence of grain orientation differences on the accuracy of residual stress testing using the nanoindentation method.

Description

Technical Field

[0001] The present invention relates to the technical field of analytical instruments and material testing, and in particular to a method for testing the second type residual stress of a 4J33 Kovar alloy. Background Art

[0002] Residual stresses are generally classified into three categories based on their distribution range: Type I residual stresses exist between regions of a deformed body; Type II residual stresses exist between grains; and Type III residual stresses exist within grains. Currently, Type I residual stresses are the most widely studied, while Type II and Type III residual stresses are relatively understudied. Nanoindentation is an effective method for measuring mechanical properties such as hardness and elastic modulus. Due to its applicability to micro- and nanoscale measurements and its nearly non-destructive nature, nanoindentation offers significant technical advantages in measuring Type II residual stresses in metals.

[0003] In nanoindentation experiments on polycrystalline materials, although their grain orientation may appear isotropic overall, adjacent grains are still anisotropic at the local microscopic level. Research has shown that differences in grain orientation can lead to significant differences in mechanical properties and residual stress test results obtained within different grains. Therefore, when the grain size is small, many nanoindentation experiments will use a larger indentation depth or indentation load, with an indentation diameter of tens to dozens of microns, so that the indentation spans several or even dozens of grains to avoid errors caused by grain orientation.

[0004] However, when the grain size of the sample to be tested is large, reaching tens of microns or more, the indentation size can only be of the order of magnitude of the grain size or even much smaller than the grain size. Obviously, the above test method cannot avoid the influence of inter-grain variations on the experiment by making the indentation span multiple grains. On the other hand, for thin film materials or coating materials, in order to avoid the substrate effect, the indentation depth is generally stipulated not to exceed one tenth of the material thickness when conducting nanoindentation experiments. In this case, the indentation depth or the size of the indentation load is limited. In addition, when using nanoindentation to measure the residual stress of the workpiece, most calculation models such as the Suresh model and the Lee model require a stress-free specimen as a reference and use the difference in indentation data such as load-displacement curve and contact area under stress and non-stress to calculate the residual stress. For polycrystalline materials, if the indentation can only fall within one grain, then the possibility that the grains where the indentations are located in the two stress states are not the same grain orientation is very high, and the difference in the indentation data is likely to be affected by the different grain orientations.

[0005] Annealed 4J33 Kovar alloy is a polycrystalline material with large grain sizes, characterized by random and highly variable grain orientations. Its average grain size is approximately 50 μm. When nanoindenting 4J33 Kovar alloy using a low-load tip, the indentation radius is only about 1 μm, significantly smaller than the average grain size. Therefore, when nanoindentation is performed on stress- and unstressed specimens within a single grain, the residual stress calculations are significantly affected by grain orientation differences.

[0006] Therefore, for materials like annealed 4J33 Kovar alloy, grain orientation differences significantly affect the accuracy of residual stress measurements using conventional nanoindentation methods, potentially introducing significant errors in nanoindentation measurements of mechanical properties and residual stress. Therefore, new testing methods are needed for nanoindentation measurements of mechanical properties and residual stress in polycrystalline 4J33 Kovar alloy. Summary of the Invention

[0007] The purpose of the present invention is to provide a method for testing the second type residual stress of 4J33 Kovar alloy, which can effectively reduce the influence of grain orientation difference on the residual stress testing accuracy of nanoindentation method.

[0008] To achieve the above object, the present invention provides the following solutions:

[0009] A method for testing type II residual stress of 4J33 Kovar alloy, the method comprising:

[0010] The samples of 4J33 Kovar alloy to be tested were processed to obtain annealed stress-free specimens and specimens pre-stressed with equibiaxial known stress;

[0011] The annealed stress-free sample and the sample pre-applied with equibiaxial known stress are placed in a nanoindenter, and a nanoindentation experiment is performed on the grains entering the marked circle with the test point as the center and twice the average grain size as the radius, to obtain all the indentation data for each grain in the marked circle; the marked circle is the circle marked for grain selection;

[0012] averaging all indentation data pressed into each grain in the marked circle to obtain an average value of the indentation data corresponding to each grain in the marked circle;

[0013] The average values ​​of the indentation data corresponding to all the grains in the marked circle are summed and then averaged to obtain the average value of the indentation data corresponding to all the grains in the marked circle;

[0014] The average value of the indentation data corresponding to all the grains in the marked circle is substituted into the calculation model of the residual stress to obtain the second type of residual stress.

[0015] Optionally, the sample of the 4J33 Kovar alloy to be tested is processed to obtain an annealed stress-free sample and a sample pre-stressed with an equibiaxial known stress, specifically comprising:

[0016] The 4J33 Kovar alloy sample to be tested was ground and polished, and then electrolytically polished to obtain an annealed stress-free sample and a sample pre-loaded with equibiaxial known stress.

[0017] Optionally, when performing a nanoindentation experiment on the grains entering the marked circle, the experimental parameters adopted at each indentation point are exactly the same; the indentation point is the point where nanoindentation is performed on the grains in the marked circle.

[0018] Optionally, the experimental parameters include loading speed, holding time and unloading speed.

[0019] Optionally, when performing a nanoindentation experiment on the grains entering the marked circle, n indentations are pressed into each grain in the marked circle; the value of n satisfies that the deviation between the average value of the first n-1 indentation data pressed into the grain and the average value of the n indentation data is less than or equal to 5%.

[0020] Optionally, when more than 70% of the area of ​​the grain is located in the marking circle, it is determined that the grain enters the marking circle.

[0021] Optionally, the calculation model of the residual stress is a Suresh model.

[0022] Optionally, the sample pre-applied with equibiaxial known stress is obtained by a stress pre-processing device.

[0023] Optionally, the indentation data includes hardness, elastic modulus, contact area and contact depth.

[0024] According to the specific embodiments provided by the present invention, the present invention discloses the following technical effects:

[0025] The present invention discloses a method for testing the second type of residual stress of 4J33 Kovar alloy. The method uses a radius twice the average grain size, performs the same nanoindentation experiment on each grain entering the marked circle, obtains all the indentation data of each grain, calculates the average value of all the indentation data of each grain, and then adds up all the calculated average values ​​and calculates the average value again to reduce the influence of the orientation difference between the grains on the experiment, thereby effectively reducing the influence of the grain orientation difference on the accuracy of the residual stress test by the nanoindentation method. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0027] Figure 1 Flow chart of an embodiment of a method for testing the second type of residual stress of 4J33 Kovar alloy according to the present invention;

[0028] Figure 2 This is a flow chart of a method for testing the second type residual stress of 4J33 Kovar alloy according to the present invention;

[0029] Figure 3 This is a schematic diagram of the marked circle range, the indentation position within the grain, and the number of example indentations during the nanoindentation experiment conducted by the present invention;

[0030] Figure 4 Schematic diagram of residual stress calculation results of the present invention;

[0031] Figure 5 Schematic diagram of the standard deviation of the residual stress calculation results of the present invention. DETAILED DESCRIPTION

[0032] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0033] The purpose of the present invention is to provide a method for testing the second type residual stress of 4J33 Kovar alloy, which can effectively reduce the influence of grain orientation difference on the residual stress testing accuracy of nanoindentation method.

[0034] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0035] Figure 1 Flow chart of an embodiment of the method for testing the second type of residual stress of 4J33 Kovar alloy according to the present invention. Figure 1 As shown, this embodiment provides a method for testing the second type residual stress of 4J33 Kovar alloy, comprising the following steps:

[0036] Step 101: Process the sample of 4J33 Kovar alloy to be tested to obtain an annealed stress-free sample and a sample pre-loaded with equibiaxial known stress.

[0037] The step 101 specifically includes:

[0038] The 4J33 Kovar alloy sample to be tested was ground and polished, and then electrolytically polished to obtain an annealed stress-free sample and a sample pre-loaded with equibiaxial known stress.

[0039] The specimens pre-applied with known equibiaxial stress are obtained through a stress pre-processing device.

[0040] Step 102: Place the annealed stress-free sample and the sample pre-applied with equibiaxial known stress into the nanoindenter, and perform nanoindentation experiments on the grains entering the marked circle with the test point as the center and twice the average grain size as the radius to obtain all the indentation data for each grain in the marked circle; the marked circle is the circle marked for grain selection.

[0041] In step 102, when performing nanoindentation experiments on the grains within the marked circle, the experimental parameters adopted at each indentation point are exactly the same. The indentation point is the point where nanoindentation is performed on the grains within the marked circle. The experimental parameters include loading speed, holding time, and unloading speed.

[0042] When more than 70% of the area of ​​the grain is located in the marked circle, it is determined that the grain has entered the marked circle.

[0043] When performing a nanoindentation experiment on the grains entering the marked circle in step 102, n indentations are pressed into each grain in the marked circle; the value of n satisfies that the deviation between the average value of the first n-1 indentation data pressed into the grain and the average value of the n indentation data is less than or equal to 5%.

[0044] Among them, the indentation data includes hardness, elastic modulus, contact area and contact depth.

[0045] Step 103: averaging all indentation data pressed into each grain in the marked circle to obtain an average value of the indentation data corresponding to each grain in the marked circle.

[0046] Step 104: The average values ​​of the indentation data corresponding to all the grains in the marked circle are summed up and then averaged to obtain the average value of the indentation data corresponding to all the grains in the marked circle.

[0047] Step 105: Substitute the average value of the indentation data corresponding to all the grains in the marked circle into the residual stress calculation model to obtain the second type of residual stress.

[0048] In step 105, the residual stress calculation model is the Suresh model. The basic principle of the Suresh model is as follows:

[0049] The Suresh model determines the residual stress based on the difference in contact area between stress-exposed and stress-free specimens at the same indentation depth, or the difference in indentation depth between stress-exposed and stress-free specimens at the same load. When residual tensile stress exists in the specimen, the residual stress can be obtained using formulas (1) and (2):

[0050]

[0051]

[0052] When residual compressive stress exists in the specimen, the calculation is performed using formula (3) and formula (4) respectively:

[0053]

[0054]

[0055] Among them, σ R represents residual stress, H represents material hardness; α represents the angle between the surface of the conical indenter and the material surface; h and h0 represent the indentation depth of materials with and without residual stress, respectively; A and A0 represent the surface indentation area of ​​materials with and without residual stress, respectively.

[0056] The technical solution of the present invention is described below with a specific embodiment:

[0057] The present invention provides a method for testing the second type residual stress of 4J33 Kovar alloy, such as Figure 2 As shown, the method mainly includes the following steps:

[0058] Step S1: Nanoindentation experiments are performed on annealed and pre-stressed 4J33 Kovar alloys, respectively. With the test point as the center and a radius of twice the average grain size (ensuring that the number of grains taken is at least 10), nanoindentation is performed on the grains that enter the marked circle (i.e., the marked circle), and n indentations are made in each grain.

[0059] Step S2: Take the average value of all indentation data (indentation data includes hardness, contact area, contact depth, etc.) and substitute it into the residual stress calculation model (i.e., Suresh model) to obtain the calculation result of material residual stress (i.e., second-type residual stress).

[0060] Among them, the material is the material to be tested; the material to be tested is annealed 4J33 Kovar alloy, whose grain size is relatively large, reaching more than tens of microns. Under the low-load indentation mode, the indentation radius is much smaller than the grain size.

[0061] The nanoindentation experiment in step S1 specifically includes the following steps:

[0062] Step S1.1: The test sample is pre-treated by grinding and polishing, and then electrolytically polished to reduce roughness and remove surface stress.

[0063] Step S1.2: The experimental parameters adopted at each indentation point should be exactly the same, including loading speed, holding time, unloading speed, etc., and a nanoindentation experiment should be performed.

[0064] Among them, the method for obtaining the material nanohardness value (i.e. hardness) H is calculated as follows:

[0065]

[0066] In formula (5), A is the indentation contact area. The true contact area should take into account the indentation accumulation (indentation protrusion), etc. The three-dimensional morphology of the indentation can be obtained by SPM and the like, and its true contact area can be obtained; F is the applied force, i.e., the load; F can be directly obtained by direct nanoindentation testing.

[0067] In step S1, the circle marked for grain selection (marked circle for grain selection) has a radius of twice the average grain size (average grain size) to ensure that the selected area contains at least 10 grains. At this time, the residual stress calculation results tend to be stable, indicating that the influence of grain orientation is reduced to within the required range, that is, the influence of grain orientation is significantly reduced. In addition, when more than 70% of the area of ​​the grain is within the marked circle, the grain is considered to be within the marked circle, and the indentation should be as close to the center of the grain as possible.

[0068] In step S1, the sample pre-applied with known stress is obtained by a stress pre-processing device.

[0069] In step S1, n indentations are pressed into each grain, and the value of n should satisfy that the average value of the first n-1 indentation data (including hardness, elastic modulus, contact area) pressed into the grain deviates from the average value of the n data by no more than 5%.

[0070] The present invention targets 4J33 Kovar alloy and conducts nanoindentation experiments across multiple grains in a test area to reduce the influence of orientation differences between grains on the experiment.

[0071] The present invention provides a method for testing the second type residual stress of 4J33 Kovar alloy. For 4J33 Kovar alloy, which is a polycrystalline material with a large grain size, the method adopts a method of pressing into multiple grains and taking an average value. Specifically, with a radius of twice the average grain size, the same nanoindentation experiment is performed on each grain entering the marked circle to obtain indentation data for each grain, and then the average value is calculated to reduce the influence of orientation differences between grains on the experiment. When the material to be tested is a polycrystalline material such as 4J33 Kovar alloy with a large grain size, the method for testing the second type residual stress of 4J33 Kovar alloy can effectively reduce the influence of grain orientation differences on the accuracy of residual stress testing by nanoindentation.

[0072] To further illustrate the technical effect of the method for testing the second type residual stress of 4J33 Kovar alloy provided by the present invention, the method is applied as follows:

[0073] (1) The 4J33 Kovar alloy sample was ground and polished and then electropolished to obtain a sample with good surface quality and clear grains. The average grain size was about 50 μm.

[0074] (2) The annealed stress-free specimen and the equibiaxial known stress specimen pre-applied with 235 MPa tensile stress were loaded into the nanoindenter. With the test point as the center and a radius of 2.5 times the average grain size (including 18 grains), the nanoindentation experiment was performed on the grains entering the marked point (marked circle). A fixed depth of 300 nm was used for quasi-static loading. The distance between adjacent test points was 10 times the indentation radius. Two indentations were made in one grain, and the indentation position was as close to the center of the grain as possible.

[0075] The specific steps of the nanoindentation experiment are as follows: (a) The sample to be tested is pre-treated by grinding and polishing, and electrolytic polishing is used to reduce the roughness and remove surface stress. (b) The experimental parameters adopted at each indentation point should be exactly the same, including loading speed, holding time, unloading speed, etc. The schematic diagram of the marking circle range, indentation position within the grain, and the number of example indentations during the nanoindentation experiment is shown in the figure below. Figure 3 shown.

[0076] (3) All the indentation data in step (2) including hardness, contact area, contact depth, etc. are obtained (the contact area is obtained by the nanoindentation in-situ imaging system), and the indentation data in 2, 3, 4, ..., 17 grains are randomly selected and averaged (50 sets of data are randomly selected in each case), and then substituted into the equibiaxial calculation model Suresh model. The residual stress calculation results and their standard deviations are as follows: Figure 4 and Figure 5 As shown, Figure 4The upper dashed line in the middle represents the known stress pre-applied by the tooling, and the lower dashed line represents the residual stress value calculated from all 18 intra-grain indentation data.

[0077] In each case, 50 sets of data are randomly selected. For example, data of two grains out of 18 grains are randomly selected. There are more than 50 ways to select them according to the permutations and combinations, but considering the actual situation, only 50 are randomly selected. The 50 sets of data are used to calculate the average value of the data in each grain of the two grains and then the average value of the data of the two grains is calculated again.

[0078] (4) The calculation results show that when more than 10 grain data are taken, the calculation results of the residual stress are in a relatively stable range, and the standard deviation is less than 15% of the residual stress calculated from the average value of all 18 grains (e.g. Figure 5 At this point, the influence of grain orientation differences on residual stress calculation has been greatly reduced. Considering the experimental quantity, it is considered reasonable to take more than 10 grains. For the convenience of marking, for most test areas, a marking circle of twice the average grain size can meet this requirement.

[0079] The various embodiments in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the various embodiments can be referenced to each other.

[0080] This document uses specific examples to illustrate the principles and implementation methods of the present invention. The above examples are only intended to help understand the method and core concept of the present invention. At the same time, those skilled in the art will find that the specific implementation methods and application scopes may vary based on the concept of the present invention. In summary, the contents of this specification should not be construed as limiting the present invention.

Claims

1. A method for testing the second type residual stress of 4J33 Kovar alloy, characterized in that: The test method includes: The samples of 4J33 Kovar alloy to be tested were processed to obtain annealed stress-free specimens and specimens pre-stressed with equibiaxial known stress; The annealed stress-free sample and the sample pre-applied with equibiaxial known stress are placed in a nanoindenter, and a nanoindentation experiment is performed on the grains entering the marked circle with the test point as the center and twice the average grain size as the radius, to obtain all the indentation data for each grain in the marked circle; the marked circle is the circle marked for grain selection; averaging all indentation data pressed into each grain in the marked circle to obtain an average value of the indentation data corresponding to each grain in the marked circle; The average values ​​of the indentation data corresponding to all the grains in the marked circle are summed and then averaged to obtain the average value of the indentation data corresponding to all the grains in the marked circle; The average value of the indentation data corresponding to all the grains in the marked circle is substituted into the calculation model of the residual stress to obtain the second type of residual stress.

2. The method for testing the second type residual stress of 4J33 Kovar alloy according to claim 1, characterized in that: The 4J33 Kovar alloy sample to be tested is processed to obtain an annealed stress-free sample and a sample pre-stressed with an equibiaxial known stress, specifically comprising: The 4J33 Kovar alloy sample to be tested was ground and polished, and then electrolytically polished to obtain an annealed stress-free sample and a sample pre-loaded with equibiaxial known stress.

3. The method for testing the second type residual stress of 4J33 Kovar alloy according to claim 1, characterized in that: When the nanoindentation experiment is performed on the grains entering the marked circle, the experimental parameters adopted at each indentation point are exactly the same; the indentation point is the point where the nanoindentation is performed on the grains in the marked circle.

4. The method for testing the second type residual stress of 4J33 Kovar alloy according to claim 3, characterized in that: The experimental parameters include loading speed, holding time and unloading speed.

5. The method for testing the second type residual stress of 4J33 Kovar alloy according to claim 1, characterized in that: When performing a nanoindentation experiment on the grains entering the marked circle, n indentations are pressed into each grain in the marked circle; the value of n satisfies that the deviation between the average value of the first n-1 indentation data pressed into the grain and the average value of the n indentation data is less than or equal to 5%.

6. The method for testing the second type residual stress of 4J33 Kovar alloy according to claim 1, characterized in that: When more than 70% of the area of ​​a grain is located within the marked circle, it is determined that the grain enters the marked circle.

7. The method for testing the second type residual stress of 4J33 Kovar alloy according to claim 1, characterized in that: The calculation model of the residual stress is the Suresh model.

8. The method for testing the second type residual stress of 4J33 Kovar alloy according to claim 1, characterized in that: The sample pre-applied with equibiaxial known stress is obtained through a stress pre-processing device.

9. The method for testing the second type residual stress of 4J33 Kovar alloy according to claim 1, characterized in that: The indentation data includes hardness, elastic modulus, contact area and contact depth.

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