A method for measuring static elastic constants of single crystal high temperature alloy

Abstract

The present invention provides a method for measuring the static elastic constants of a single crystal high-temperature alloy, which is applicable in a range from room temperature to high temperatures of 1200°C. First, a flat single crystal high-temperature alloy blank is prepared by directional solidification technology and a spiral crystal selection method, and is processed into a flat tensile specimen by mechanical processing, and then the crystal orientation of the blank is measured by a method such as the Laue method. A tensile test is performed on the tensile specimen on a tensile testing machine, and its in-plane deformation is measured by a full-field deformation measurement method. According to the stress-strain relationship and the first orientation and second orientation angles, the three elastic constants S of the single crystal high-temperature alloy at different temperatures can be calculated through a single specimen. 11 , S 12 and S 44 The present invention avoids the shortcomings of the traditional testing method, and can obtain all three elastic constants of the single crystal high temperature alloy at the temperature by only using a sample with a random crystal orientation to carry out a uniaxial tensile test.

Description

Technical Field

[0001] The invention belongs to the field of measurement technology, and in particular relates to a method for measuring static elastic constants of a nickel-based single crystal high-temperature alloy at room temperature and high temperature. Background Art

[0002] Nickel-based single crystal high-temperature alloys have good high-temperature fatigue strength and creep resistance. They are the core materials for aircraft engine and gas turbine blades at present and for a long time to come. The elastic constants of aviation structural materials are essential parameters for structural design and strength verification. Their accuracy directly affects the simulation accuracy of stress and strain in the structure, and also has an important impact on the accuracy of damage evolution and life prediction.

[0003] Single crystal turbine blades are generally prepared by directional solidification, usually using spiral crystal selection or seed crystal method to allow grains to grow in a specific direction. The angle between the blade axis and the preferred orientation is called the first orientation. Due to the randomness of casting, the first orientation is generally controlled within 10°; the angle of rotation of the blade plane around the axis is called the second orientation, which is generally difficult to control. Nickel-based single crystal alloys are coherent structures of γ phase and γ' phase, both of which are FCC structures, so there are only three independent elastic constants: S 11 , S 12 and S 44 (or C 11 , C 12 and C 44 ). After obtaining these three elastic constants, the elastic constant matrix in any coordinate system can be calculated by coordinate transformation according to the actual crystal orientation angle of the turbine blade, so as to perform finite element calculation on the turbine blade in the corresponding coordinate system.

[0004] The existing single crystal elastic constant measurements are mainly divided into static methods and dynamic methods, which are represented by the dense moiré interferometry method and the resonance method respectively. However, the existing methods have certain shortcomings: first, most of these methods require the preparation of an ideal single crystal plate with the first orientation and the second orientation being one of

[001] ,

[010] and

[100] , which is impossible to achieve perfectly with the current casting technology, and these methods do not consider the influence of small deviation angles on the measurement results. Furthermore, these methods generally require more than two tests on samples with more than two orientations to measure Young's modulus, shear modulus and Poisson's ratio, and then perform inversion to measure the three elastic constants. The test cycle is long, and multiple independent tests are prone to introduce errors into the derivation results. Summary of the invention

[0005] In view of the above-mentioned situation of the prior art, the purpose of the present invention is to provide a method for measuring the static elastic constants of a nickel-based single crystal high-temperature alloy at different temperatures. All three elastic constants of the single crystal high-temperature alloy at this temperature can be obtained by conducting a uniaxial tensile test on a sample with a random crystal orientation.

[0006] The technical solution of the present invention is as follows:

[0007] A method for measuring static elastic constants of a single crystal high temperature alloy comprises the following steps:

[0008] (1) Preparing a single crystal high-temperature alloy blank by a directional solidification method;

[0009] (2) Cut the plate-type tensile specimen and the oriented measurement specimen with the same sampling direction next to the sampling position;

[0010] (3) measuring the crystal orientation angle of the orientation measurement sample by Laue method, four-circle diffractometer method or electron backscatter diffraction (EBSD) method;

[0011] (4) Install a plate-type tensile specimen on a tensile testing machine, and arrange deformation tracking carriers such as speckle and grating on the surface of the specimen. For high-temperature tests, use a high-temperature furnace with an optical observation window for heating, and use a heat-resistant deformation tracking carrier;

[0012] (5) Measure the transverse and longitudinal strains and shear strains of single crystal high-temperature alloys at different stress levels by non-contact deformation measurement method;

[0013] (6) Static elastic constant S 11 , S 12 and S 44 The calculation formula is as follows:

[0014]

[0015] Where: ψ, θ are the Euler angles representing the orientation of the single crystal high temperature alloy, σ is the normal stress in the Z direction, ε z is the positive strain in the Z direction, ε y is the positive strain in the Y direction, γ yz is the shear strain in the YZ plane.

[0016] In step (2), the sample is cut in a random direction. However, if all three static elastic constants are obtained through one test, it is necessary to ensure that the orientation of the cut plate tensile sample satisfies the Euler angles ψ and θ of not less than 15°, and cos 2 θ(sin 2 ψsin 2 θ+cos 2 θ)-cos 2 ψsin 2θ(sin 2 ψsin 2 θ+cos 2 ψsin 2 θ)≠0.

[0017] In step (5), the non-contact deformation measurement method needs to eliminate the influence of off-plane displacement on the measurement result, including three-dimensional digital image correlation (3D-DIC) method, two-dimensional digital image correlation (2D-DIC) method based on telecentric lens, moiré interferometry (MI) method, electronic speckle pattern interferometry (ESPI) method, etc.

[0018] The method for measuring the static elastic constants of the nickel-based single crystal high-temperature alloy at different temperatures provided by the present invention only uses a sample with a random crystal orientation, and a single uniaxial tensile test can be performed to obtain all three elastic constants of the single crystal high-temperature alloy at the temperature. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a flow chart of the method for measuring elastic constants of single crystal high temperature alloys of the present invention;

[0020] Figure 2 It is a schematic diagram of sample coordinates of the method for measuring elastic constants of single crystal high temperature alloys of the present invention;

[0021] Figure 3 It is a schematic diagram of the orientation represented by the Euler rotation adopted in the present invention. DETAILED DESCRIPTION

[0022] In order to more clearly understand the purpose, technical solutions and advantages of the present invention, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments.

[0023] The present invention provides a method for measuring the static elastic constants of a single crystal high temperature alloy. Figure 1 Flow chart of the method for measuring elastic constants of single crystal high temperature alloy of the present invention. Figure 1 As shown, the method of the present invention comprises the following steps:

[0024] S1: Single crystal high temperature alloy blank production

[0025] The spiral crystal selection method or seed crystal method is used to select suitable grains, and the single crystal high-temperature alloy plate blank is prepared by directional solidification technology. The total length of the blank exceeds 100mm, the width is greater than 20mm, and the thickness is arbitrary. There is no secondary orientation and residual stress in the blank.

[0026] S2: Single crystal sample preparation

[0027] Cut out a long blank from the blank, and try to avoid the first and second orientations of

[001] ,

[010] and

[100] . Process the long blank into a flat tensile specimen, and cut an orientation measurement specimen with the same sampling direction next to the tensile specimen for orientation measurement.

[0028] S3: Specimen Orientation Measurement

[0029] Assume that the axial direction of the specimen is the Z direction, and the direction perpendicular to the specimen plane is the X direction, such as Figure 2 The crystal orientation of the sample is measured by Laue method, four-circle diffractometer method or electron backscatter diffraction (EBSD) method, and the three Euler angles Φ, θ, and ψ of the sample coordinate system and the crystal orientation are converted by software, as shown in Figure 3 shown.

[0030] S4: Preparation for elastic constant measurement experiment

[0031] S41: Selecting an appropriate non-contact deformation measurement method requires the ability to measure the in-plane transverse and longitudinal strains and shear strains and to eliminate the influence of out-of-plane displacement on the measurement results, including the three-dimensional digital image correlation (3D-DIC) method and the two-dimensional digital image correlation (2D-DIC) method based on a telecentric lens, the dense moiré interferometry (MI) method, the electronic speckle pattern interferometry (ESPI) method, etc.

[0032] S42: Prepare a speckle tracking carrier on the flat tensile test surface, such as speckle in the DIC method, grating in the MI method, and developer in the ESPI method.

[0033] S43: The load in the uniaxial tensile test is applied by a tensile testing machine. Before the test, the plate-type tensile specimen is installed on the tensile testing machine and connected by pins or clamps.

[0034] S44: If the test condition is high temperature, the corresponding high temperature test method must be adopted: heating uses a high temperature furnace with an optical observation window, the deformation tracking carrier has sufficient heat resistance, cold light is used for illumination, and a warm light filter is installed on the lens surface. The tracking carrier on the YZ plane and surface of the sample can be clearly collected through the observation window.

[0035] S5: Elastic constant measurement test

[0036] The test was loaded by a tensile testing machine with a nominal stress of σ, and the strain in the YZ plane was measured using a non-contact deformation measurement device, including the longitudinal strain ε z , transverse strain ε y and shear strain γ yz .

[0037] S6: Calculation of elastic constants

[0038] Three Euler angles are used to describe the orientation of single crystal high temperature alloy. The first rotation is a rotation angle Φ around the Z axis, and the corresponding transformation matrix is ​​[A z ]; the second rotation is to rotate around the X axis by angle θ, and the corresponding transformation matrix is ​​[A x ]; The first rotation is to rotate around the Z axis by an angle of ψ, and the corresponding transformation matrix is ​​[A z’ ]. [A] is the transformation matrix after three rotations.

[0039]

[0040] When an axial stress σ of magnitude is applied to the specimen, a 1×6 stress vector [σ] can be obtained through coordinate transformation.

[0041]

[0042] The flexibility matrix of a single crystal superalloy is [S], and the stiffness matrix is ​​[C], each with three independent variables. The two are inverse matrices of each other.

[0043]

[0044] If the above three equations are independent, the three flexibility constants S can be obtained by solving the equations: 11 , S 12 , S 44 .

[0045]

[0046] The stiffness constant of the material can also be obtained through the flexibility constant. The calculation formula is as follows: C 11 =1 / S 11 , C 12 =-S 12 / S 11 , C 44 =1 / S 44 .

[0047] It can be seen from the calculation formula that if the equation is to be independent, it is necessary to ensure that the orientation of the cut sample satisfies the Euler angles ψ and θ of not less than 15°, and cos 2 θ(sin 2 ψsin 2 θ+cos 2 θ)-cos 2 ψsin 2 θ(sin 2 ψsin 2 θ+cos 2 ψsin 2 θ)≠0. Only when the above conditions are met can all three elastic constants be obtained from one specimen.

[0048] Example

[0049] The second generation nickel-based single crystal alloy DD6 was used as the test material. The single crystal alloy test plate was prepared by directional solidification technology and spiral crystal selection method, with a length of 130 mm, a width of 40 mm, and a thickness of 12 mm.

[0050] A long strip of blank was cut out from the blank and processed into a flat tensile specimen and an orientation measurement specimen of 5mm×5mm×2mm in size. The orientation of the single crystal alloy was measured by Laue method, and the Euler angles Φ=26.7°, θ=35.8°, ψ=35.6° were obtained by conversion, which met the requirements for measuring all elastic constants of a single specimen.

[0051] A high-temperature speckle paint composed of aluminum oxide powder and alcohol is randomly sprayed on the surface of the tensile specimen with a spray gun, and after curing, high-temperature speckles with appropriate size and uniform density are formed.

[0052] The tensile specimen was clamped on a universal electronic testing machine by pins, and the specimen was heated in a high-temperature furnace with an observation window, raised to a set temperature and kept warm for 20 minutes. The loading was carried out with a beam displacement speed of 0.5 mm / min, and the two-dimensional DIC system with a double telecentric lens was used to record the image of the specimen after deformation during the stretching process. The acquisition rate was 20 frames / s, and the loading was carried out in the elastic state of the material. The loading was stopped and unloaded when it was close to 2 / 3 of the yield strength of the material.

[0053] The strain in the sample surface during the stretching process is calculated by DIC software, and the load in each image state is obtained by using the signal synchronization device to form an array matrix {σ i ; ε zi ; ε yi ; γ yzi}, the static elastic constants of the material at this temperature are obtained by multidimensional linear fitting using the above formula (1).

Claims

1. A method for measuring the static elastic constants of a single crystal high temperature alloy, comprising the following steps: (1) Preparing a single crystal high-temperature alloy blank by a directional solidification method; (2) Cut the plate-type tensile specimen and the oriented measurement specimen with the same sampling direction next to the sampling position; (3) measuring the crystal orientation angle of the orientation measurement sample; (4) Installing a plate-type tensile specimen on a tensile testing machine and arranging a deformation tracking carrier on the surface of the specimen; (5) Measure the transverse and longitudinal strains and shear strains of single crystal high-temperature alloys at different stress levels by non-contact deformation measurement method; (6) Static elastic constant S 11 , S 12 and S 44 The calculation formula is as follows: Where: ψ, θ are the Euler angles representing the orientation of the single crystal high temperature alloy, σ is the normal stress in the Z direction, ε z is the positive strain in the Z direction, ε y is the positive strain in the Y direction, γ yz is the YZ plane shear strain, The orientation of the cut plate tensile specimens satisfies the Euler angles ψ and θ of not less than 15°, and cos 2 θ(sin 2 ψsin 2 θ+cos 2 θ)-cos 2 ψsin 2 θ(sin 2 ψsin 2 θ+cos 2 ψsin 2 θ)≠0.

2. The method according to claim 1, wherein the crystal orientation angle is measured by a Laue method, a four-circle diffractometer method or an electron backscatter diffraction method.

3. The method according to claim 1, wherein the deformation tracking carrier is a speckle or a grating.

4. The method according to claim 1, wherein the non-contact deformation measurement method comprises a three-dimensional digital image correlation method, a two-dimensional digital image correlation method based on a telecentric lens, a moiré interferometry method or an electronic speckle pattern interferometry method.

5. The method according to claim 1, wherein the directional solidification method is a directional solidification method using a spiral crystal selection method or a seed crystal method.

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