A method for determining the critical point of dynamic recrystallization of mixed crystal materials

Abstract

This invention relates to the fields of materials science and metal processing technology and discloses a method for determining the critical point of dynamic recrystallization of mixed-crystal materials. This method, based on the assumption that peak stress represents the saturated stress value of dynamic recovery, uses precise fitting of true stress-true strain curves to reduce noise, plots the relationship between work hardening rate and true stress, and utilizes the Tangent plug-in in Origin software to precisely locate the critical point of dynamic recrystallization. The critical stress and strain values ​​corresponding to the critical point are recorded, providing key process parameters for material processing and significantly improving the systematicity and practicality of the entire process.

Description

Technical Field

[0001] The invention belongs to the technical field of material science and metal processing, and particularly relates to a method for determining a dynamic recrystallization critical point of a mixed crystal structure material. Background Art

[0002] Mixed-grain materials exhibit unique deformation behavior during hot working due to the diversity of their grain size and shape. This structural heterogeneity leads to the complexity of dynamic recrystallization behavior, making it challenging to determine the critical strain for dynamic recrystallization. Since the critical strain is a key parameter affecting the material's processing properties and the quality of the finished product, accurately determining the critical strain is crucial for controlling the material's microstructure and ultimate properties.

[0003] At present, the main method for determining the dynamic recrystallization critical point of mixed crystal structure materials is proposed by POLIAK et al. This method determines the dynamic recrystallization critical point by analyzing the inflection point of the relationship curve between the strain hardening rate (θ = dσ / dε) and the flow stress (σ). However, in practical applications, this method has some limitations. First, in order to accurately identify the inflection point, it is often necessary to fit the data multiple times, which not only increases the complexity of the calculation, but also may introduce fitting errors. Secondly, due to data noise or the characteristics of the material itself, the inflection point may not be obvious in the curve, which makes the determination of the critical point difficult and inaccurate. Summary of the Invention

[0004] In view of the above shortcomings of the prior art, the present invention provides a method for determining the dynamic recrystallization critical point of mixed crystal structure materials.

[0005] To achieve the above object, the technical solution adopted by the present invention is:

[0006] A method for determining the dynamic recrystallization critical point of a mixed crystal structure material comprises the following steps:

[0007] (1) Through thermal simulation experiments, uniaxial hot compression or hot tensile tests are performed on mixed crystal structure materials, and the true stress and true strain data under a specific deformation temperature and strain rate are recorded to draw the true stress-true strain curve;

[0008] (2) Using polynomial functions to fit the true stress and true strain data during the plastic deformation stage, a fitted true stress-true strain curve is obtained;

[0009] The polynomial function is

[0010]

[0011] σ is true stress, ε is true strain, A n , A n-1, ..., A1, A0 are the coefficients obtained through the fitting process, and n is the order of the polynomial;

[0012] (3) By taking the derivative of the polynomial function formula (1), a continuous work hardening rate θ curve is obtained:

[0013]

[0014] (4) According to formula (1) and formula (2), draw the relationship curve between work hardening rate θ and true stress σ;

[0015] (5) Obtain the measured peak stress σ from the true stress-true strain curve obtained in step (1) p , assuming that the measured peak stress σ p is the dynamic recovery saturation stress value σ when the dynamic recovery process reaches the saturation state drvs , the measured peak stress σ in the true stress-true strain curve p The corresponding strain is the peak strain ε p ;

[0016] (6) In the relationship curve between work hardening rate θ and true stress σ, find the corresponding peak stress σ on the horizontal axis p The location point (σ p , 0);

[0017] (7) Using the Tangent plug-in of Origin software, the position of the tangent point of the relationship curve between the work hardening rate θ and the true stress σ is adjusted. By adjusting the tangent position, the intersection of the tangent and the x-axis is located at point (σ p , 0), the tangent point of the relationship curve between the tangent line and the work hardening rate θ and the true stress σ is the critical point (σ c ,θ c );

[0018] (8) In the fitted true stress-true strain curve obtained in step (2), find the critical point (σ c ,θ c ) corresponds to the critical stress value σ c , and record the critical stress value σ c The corresponding critical strain value ε c , the dynamic recrystallization critical point (σ c , ε c );

[0019] (9) Calculate the peak strain ε p and critical strain value ε c to test the peak strain ε p and critical strain value ε c Whether the ratio is consistent with the prediction range of the Sellars model;

[0020] If ε c =(0.3~0.9)ε p , then the peak strain ε p and critical strain value ε c The ratio of is consistent with the prediction range of the Sellars model, then the point (σ c , ε c ) is the critical point of dynamic recrystallization.

[0021] The present invention is based on a key assumption: the peak stress σ p It represents the saturation stress value during the dynamic recovery process. At this time, the dislocation density inside the material reaches a dynamic equilibrium state and no longer increases significantly. Dynamic recovery is a prelude to dynamic recrystallization. The end of this stage is marked by the appearance of peak stress. Subsequently, the formation of new grains inside the material marks the beginning of dynamic recrystallization, thus completing the microstructural evolution from dynamic recovery to dynamic recrystallization.

[0022] As a preferred embodiment of the present invention, the average grain size of the mixed crystal structure material is less than 1 μm, and the proportion of grains within 1 μm exceeds 50%.

[0023] As a preferred embodiment of the present invention, the mixed crystal structure material is obtained by deforming the alloy material at high temperature; the deformation treatment is one or more of equal channel angular extrusion, high pressure torsion, cumulative rolling, multi-directional forging, thermomechanical treatment, powder metallurgy and laser melting deposition.

[0024] As a preferred embodiment of the present invention, the microstructure of the sample is tested by metallographic microscope, scanning electron microscope, transmission electron microscope and electron backscatter diffraction technology to obtain the grain size of the mixed crystal structure material.

[0025] As a preferred embodiment of the present invention, the present invention identifies the node where elastic deformation transitions to plastic deformation by changing the slope of the true stress-true strain curve in step (1). Specifically, the node where elastic deformation transitions to plastic deformation exhibits a significant decrease in slope on the true stress-true strain curve, indicating that the material transitions from the linear elastic deformation stage following Hooke's law to the nonlinear plastic deformation stage. In the elastic stage, the true stress σ and the true strain ε maintain a linear relationship, following the formula σ=E*ε, where E is the Young's modulus of the material. However, once the material enters the plastic deformation region, this linear relationship no longer holds, and the true stress-true strain curve begins to exhibit nonlinear characteristics. Therefore, the data after the node where elastic deformation transitions to plastic deformation is the data segment of plastic deformation.

[0026] As a preferred embodiment of the present invention, the polynomial function automatically generates the true stress-true strain curve of the plastic deformation stage by Origin software, and finds the correlation coefficient R by adjusting the polynomial order. 2 The best fitting polynomial function with a value greater than 0.995 was selected. The specific steps include: importing the entire data segment of plastic deformation into Origin, plotting the data points using the scatter plot function, and selecting the correlation coefficient R for the polynomial function with a polynomial order of ≤9 using the Simple Fit plug-in. 2 Fit polynomial functions with a polynomial order greater than 0.995, or manually enter higher-order correlation coefficients R using Nonlinear Curve Fit for polynomial functions with a polynomial order greater than 9. 2 >0.995. After fitting, Origin displays the adjusted polynomial function and the corresponding fitted curve on the chart along with the original data points.

[0027] Compared with the prior art, the beneficial effects of the present invention are as follows: the present invention uses a polynomial function of appropriate order to accurately fit the true stress-true strain curve, effectively reducing the noise in the measurement data and improving the signal-to-noise ratio. At the same time, the general form of the polynomial function is used to flexibly adapt to the deformation complexity of different materials, ensuring the accuracy of the fitting. In terms of critical point positioning, the present invention uses reasonable assumptions and clearly defined dynamic recovery saturation stress values, and uses the Tangent plug-in of the Origin software to adjust the tangent point to accurately determine and identify the dynamic recrystallization critical point, and records the critical stress value and critical strain value corresponding to the critical point, providing key process parameters for material processing, thereby significantly improving the systematicity and practicality of the entire process.

[0028] Finally, the method described in the present invention also includes steps such as sample preparation, microstructure characterization, thermal simulation experiments and data recording, aiming to simplify the process of determining the critical strain value, improve accuracy and efficiency, and provide a reliable basis for optimizing thermal processing process parameters. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 EBSD map and grain size distribution map of the Ti-6Al-4V titanium alloy with mixed crystal structure in Example 1; (a) is the EBSD map, and (b) is the grain size distribution map.

[0030] Figure 2 In Example 1, the deformation temperature is 550℃ and the strain rate is 0.1s -1 True stress-true strain curve obtained under deformation conditions.

[0031] Figure 3This is a specific process diagram for identifying the entire data segment after entering plastic deformation based on the unique characteristics of elastic deformation and plastic deformation in the true stress-true strain curve in Example 1.

[0032] Figure 4 This is a graphical representation of the results of fitting the true stress-true strain curve of the entire data segment after entering plastic deformation using a polynomial function of appropriate order in Example 1.

[0033] Figure 5 In Example 1, the deformation temperature is 550℃ and the strain rate is 0.1s -1 The relationship curve between work hardening rate and true stress under specific deformation conditions.

[0034] Figure 6 This is a diagram illustrating the result of determining the tangent point position after adjusting the tangent point position in the curve using the magnification function of the origin software and the Tangent plug-in in Example 1.

[0035] Figure 7 In Example 2, the deformation temperature is 700℃ and the strain rate is 0.005s -1 True stress-true strain curve obtained under deformation conditions.

[0036] Figure 8 This is a specific process diagram for identifying the entire data segment after entering plastic deformation based on the unique characteristics of elastic deformation and plastic deformation in the true stress-true strain curve in Example 2.

[0037] Figure 9 This is a diagram showing the results of fitting the true stress-true strain curve of the entire data segment after entering plastic deformation using a polynomial function of appropriate order in Example 2.

[0038] Figure 10 In Example 2, the deformation temperature is 700℃ and the strain rate is 0.005s -1 The relationship curve between work hardening rate and true stress under specific deformation conditions.

[0039] Figure 11 This is a diagram illustrating the result of determining the tangent point position after adjusting the tangent point position in the curve using the magnification function of the origin software and the Tangent plug-in in Example 2.

[0040] Figure 12 The results of determining the critical point position using the conventional method in Comparative Example 1 are shown in Figure 1; (a) is the deformation temperature of 700 ° C and the strain rate of 0.005s -1 The relationship curve between work hardening rate and true stress under the conditions of (b) deformation temperature of 700℃ and strain rate of 0.005s-1 Under the conditions curve chart. DETAILED DESCRIPTION

[0041] In order to better illustrate the purpose, technical solutions and advantages of the present invention, the present invention will be further described below in conjunction with specific embodiments.

[0042] Example 1

[0043] A method for determining the dynamic recrystallization critical point of a mixed crystal structure material comprises the following steps:

[0044] (1) The mixed crystal structure material of this embodiment is a Ti-6Al-4V titanium alloy with a mixed crystal structure. The Ti-6Al-4V titanium alloy with a mixed crystal structure is obtained by subjecting the Ti-6Al-4V titanium alloy to three multi-directional isothermal closed die forging deformation passes at a temperature of 550°C. Subsequently, the microstructural characteristics of the Ti-6Al-4V titanium alloy with a mixed crystal structure were characterized using EBSD technology. The results are shown in FIG. Figure 1 As shown in the figure, the structure shows a distribution of large and small grains. Grains smaller than 1 μm account for approximately 53.6%, the largest grain size is approximately 9.64 μm, and the average grain size is approximately 0.85 μm. Therefore, the Ti-6Al-4V titanium alloy with a mixed crystal structure meets the requirements for a mixed crystal structure.

[0045] (2) The Ti-6Al-4V titanium alloy sample with mixed crystal structure described in step (1) was finely processed into a φ8mm×12mm cylindrical specimen. Subsequently, a hot compression test was performed on the cylindrical specimen using a Gleeble-3500 thermal simulation tester. During the experiment, the temperature was set to 550°C and the strain rate was set to 0.1s. -1 During the experiment, the temperature was raised to a preset temperature of 550°C at a heating rate of 5°C / s, then kept at that temperature for 300 seconds, and then a hot compression test was performed until the sample reached a deformation of 50% (equivalent to a strain of about 0.69). Finally, a Ti-6Al-4V titanium alloy sample with a mixed crystal structure was obtained under this specific condition (deformation temperature of 550°C, strain rate of 0.1s -1 ) under the true stress and true strain data, plotted as true stress-true strain curve, such as Figure 2 As shown in Figure 2, the true stress-true strain curve is a curve with true strain as the horizontal axis and true stress as the vertical axis.

[0046] (3) According to the characteristics of elastic deformation and plastic deformation in the true stress-true strain curve, the entire data segment after entering plastic deformation in the true stress-true strain curve in step (2) is defined. The result is as follows Figure 3 As shown, in Figure 3In the process of deformation analysis, the node where elastic deformation transitions to plastic deformation is identified by the change in the slope of the true stress-true strain curve. Specifically, this node is manifested as a significant decrease in the slope of the curve, marking the transition of the material from the linear elastic stage following Hooke's law to the nonlinear plastic deformation stage. In the elastic stage, the relationship between true stress σ and true strain ε remains linear, following the formula σ = E * ε, where E is the Young's modulus of the material. However, once the material enters the plastic deformation region, this linear relationship no longer holds, and the true stress-true strain curve begins to exhibit nonlinear characteristics. The data after this transition point is then determined to be the entire data segment after entering plastic deformation. Figure 3 The point pointed by the middle arrow is the transformation node, and the data after the transformation node is the entire data segment after entering plastic deformation.

[0047] (4) The true stress-true strain curve of the entire data segment after entering plastic deformation defined in step (3) is fitted using a 9th-order polynomial function (Formula (1) and Table 1). The fitting results are shown in Figure 4 , the order of the polynomial function under this condition is 9, and the specific implementation method is as follows: First, import the entire data segment after entering plastic deformation into the Origin software. Use the scatter plot function of the Origin software to draw the data points of true stress and true strain. Then, start the fitting process by selecting the "Simple Fit" plug-in in the "Analysis" menu. In the Simple Fit plug-in, according to the characteristics of the data and the needs of fitting, manually select the order of the polynomial function so that the fitting correlation coefficient R 2 >0.995. If the order of the polynomial is ≤9, you can directly select the corresponding order in the Simple Fit dialog box. If you need to use a polynomial with an order greater than 9, select "Nonlinear Curve Fit" in the "Analysis" menu and manually enter the formula of the polynomial function in the "NLFit" dialog box. After performing the fitting operation, the Origin software automatically adjusts the polynomial coefficients to minimize the difference between the data points and the fitted curve to obtain the polynomial function. Finally, the software automatically generates the true stress-true strain curve and displays it in a chart together with the original data points.

[0048] The 9th-order polynomial function of this embodiment is shown in Formula (1) and Table 1:

[0049] The polynomial function of order 9 is:

[0050] Table 1

[0051] i <![CDATA[A i ]]> 0 -91.09228 1 26127.79869 2 -360352.10595 3 2891604.87506 4 <![CDATA[-1.42636×10 7 ]]> 5 <![CDATA[4.44145×10 7 ]]> 6 <![CDATA[-8.74552×10 7 ]]> 7 <![CDATA[1.05493×10 8 ]]> 8 <![CDATA[-7.1095×10 7 ]]> 9 <![CDATA[2.04918×10 7 ]]>

[0052] (5) Taking the first-order derivative of the data obtained by fitting the 9th-order polynomial function, the work hardening rate θ is obtained:

[0053]

[0054] (6) The true stress σ data fitted in step (4) and the work hardening rate θ data derived in step (5) are used to draw a curve of the relationship between the work hardening rate and the true stress (θ-σ), as shown in the following example: Figure 5 shown.

[0055] (7) In the original true stress-true strain curve of step (2), it is identified that the deformation temperature is 550℃ and the strain rate is 0.1s -1 Peak stress σ under conditions p is 842.22MPa, and the corresponding peak strain value ε p It is 0.28377.

[0056] (8) In step (6) Figure 5 First, remove the negative portion of the work hardening rate, then set the end value of the horizontal axis to 842.22. Then, use the zoom function of the origin software and the Tangent plug-in to adjust the position of the tangent point in the curve so that the tangent line coincides with the end point of the horizontal axis (842.22, 0). At this point, the tangent point position (832.88, 251.8) is the critical point. The result after the plug-in realizes the adjustment of the tangent point position is as follows Figure 6 shown.

[0057] (9) Find the corresponding stress value 832.88 in the true stress-true strain curve data after fitting the 9th-order polynomial function in step (4), and record the strain value corresponding to this stress value as 0.2216. Therefore, the critical point for dynamic recrystallization under this deformation condition is (0.2216, 832.88).

[0058] (10) The critical strain value determined in step (9) is 0.2216, which is the ratio of the peak strain of 0.28377 identified in step (7) This ratio is within the prediction range of the Sellars model, namely ε c =(0.3~0.9)ε p This result verifies the reliability and accuracy of the proposed method and confirms its effectiveness in identifying and predicting the critical conditions of dynamic recrystallization.

[0059] Example 2

[0060] A method for determining the dynamic recrystallization critical point of a mixed crystal structure material comprises the following steps:

[0061] (1) The mixed crystal structure material of this embodiment is a Ti-6Al-4V titanium alloy with a mixed crystal structure. The Ti-6Al-4V titanium alloy with a mixed crystal structure is obtained by subjecting the Ti-6Al-4V titanium alloy to three multi-directional isothermal closed die forging deformation passes at a temperature of 550°C. Subsequently, the microstructural characteristics of the Ti-6Al-4V titanium alloy with a mixed crystal structure were characterized using EBSD technology. The results are shown in FIG. Figure 1 As shown in the figure, the structure shows a distribution of large and small grains. Grains smaller than 1 μm account for approximately 53.6%, the largest grain size is approximately 9.64 μm, and the average grain size is approximately 0.85 μm. Therefore, the Ti-6Al-4V titanium alloy with a mixed crystal structure meets the requirements for a mixed crystal structure.

[0062] (2) The Ti-6Al-4V titanium alloy sample with mixed crystal structure described in step (1) was finely processed into a φ8mm×12mm cylindrical specimen. Subsequently, a hot compression test was performed on the cylindrical specimen using a Gleeble-3500 thermal simulation tester. During the experiment, the temperature was set to 700°C and the strain rate was set to 0.005s -1 During the experiment, the temperature was raised to a preset temperature of 700°C at a heating rate of 5°C / s, and then kept at that temperature for 300 seconds. Then, a hot compression test was performed until the sample reached a deformation of 50% (equivalent to a strain of about 0.69). Finally, a Ti-6Al-4V titanium alloy sample with a mixed crystal structure was obtained under this specific condition (deformation temperature of 700°C, strain rate set to 0.005s -1 ) under the true stress and true strain data, plotted as true stress-true strain curve, such as Figure 7 As shown in Figure 2, the true stress-true strain curve is a curve with true strain as the horizontal axis and true stress as the vertical axis.

[0063] (3) According to the characteristics of elastic deformation and plastic deformation in the true stress-true strain curve, the entire data segment after entering plastic deformation in the true stress-true strain curve in step (2) is defined. The result is as follows Figure 8 As shown in the figure, the node where elastic deformation transitions to plastic deformation is identified by the change in the slope of the true stress-true strain curve. Specifically, this node is manifested as a significant decrease in the slope on the curve, marking the transition of the material from the linear elastic stage following Hooke's law to the nonlinear plastic deformation stage. In the elastic stage, the true stress σ and the true strain ε maintain a linear relationship, following the formula σ = E * ε, where E is the Young's modulus of the material. However, once the material enters the plastic deformation region, this linear relationship no longer holds, and the true stress-true strain curve begins to exhibit nonlinear characteristics. The data after this transition point is then determined to be the entire data segment after entering plastic deformation. The arrow points to the transition node, and the data after the transition node is the entire data segment after entering plastic deformation.

[0064] (4) A 12th-order polynomial function (Formula (1) and Table 2) is used to fit the true stress-true strain curve of the entire data segment after entering plastic deformation defined in step (3). The fitting results are shown in Figure 9 , the order of the polynomial function under this condition is 12. The specific implementation method is as follows: First, import the entire data segment after entering plastic deformation into the Origin software. Use the scatter plot function of the Origin software to plot the data points of true stress and true strain. Then, by selecting the "Simple Fit" plug-in in the "Analysis" menu, start the fitting process so that the correlation coefficient R after fitting is 2 >0.995. In the Simple Fit plug-in, manually select the order of the polynomial function according to the characteristics of the data and the needs of fitting. If the order of the polynomial is ≤9, you can directly select the corresponding order in the Simple Fit dialog box. If you need to use a polynomial with an order greater than 9, select "Nonlinear Curve Fit" in the "Analysis" menu and manually enter the formula of the polynomial function in the "NLFit" dialog box. After performing the fitting operation, the Origin software automatically adjusts the polynomial coefficients to minimize the difference between the data points and the fitted curve to obtain the polynomial function. Finally, the software automatically generates the true stress-true strain curve and displays it in a chart together with the original data points.

[0065] The 12th-order polynomial function of this embodiment is shown in Formula (1) and Table 2:

[0066] The polynomial function of order n is:

[0067] Table 2

[0068] i <![CDATA[A i ]]> 0 -33.85444 1 18793.37641 2 -483663.79685 3 6971114.75134 4 <![CDATA[-6.28251×10 7 ]]> 5 <![CDATA[3.74566×10 8 ]]> 6 <![CDATA[-1.52561×10 9 ]]> 7 <![CDATA[4.30773×10 9 <!-- 6 -->]]> 8 <![CDATA[-8.42266×10 9 ]]> 9 <![CDATA[1.11839×10 10 ]]> 10 <![CDATA[-9.61933×10 9 ]]> 11 <![CDATA[4.83209×10 9 ]]> 12 <![CDATA[-1.07596×10 9 ]]>

[0069] (5) Take the first-order derivative of the data obtained by fitting the 12th-order polynomial function to obtain the work hardening rate θ:

[0070]

[0071] (6) The true stress data fitted in step (4) and the work hardening rate θ data obtained by derivation in step (5) are used to draw a curve of the relationship between the work hardening rate and the true stress (θ-σ), as shown in the following example: Figure 10 shown.

[0072] (7) In the original true stress-true strain curve of step (2), it is identified that the deformation temperature is 700℃ and the strain rate is 0.005s -1Peak stress σ under conditions p is 289.94 MPa, and the corresponding peak strain value ε p It is 0.08038.

[0073] (8) In step (6) Figure 10 First, remove the negative portion of the work hardening rate, then set the end value of the horizontal axis to 289.94. Then, use the zoom function of the origin software and the Tangent plug-in to adjust the position of the tangent point in the curve so that the tangent line coincides with the end point of the horizontal axis (289.94, 0). At this point, the tangent point position (248.73, 2031.51) is the critical point. The result after the plug-in realizes the adjustment of the tangent point position is as follows Figure 11 shown.

[0074] (9) Find the corresponding stress value of 248.73 in the data after fitting the 12th-order polynomial function in step (4), and record the strain value corresponding to this stress value as 0.0332. Therefore, the critical point for dynamic recrystallization under this deformation condition is (0.0332, 248.73).

[0075] (10) The critical strain value determined in step (9) is 0.0332, which is the ratio of the peak strain of 0.08038 identified in step (7) This ratio is within the prediction range of the Sellars model, namely ε c =(0.3~0.9)ε p This result confirms the ability of this method to identify the critical point of dynamic recrystallization when processing curves without obvious inflection points, ensuring its reliability and accuracy. At the same time, this also verifies the effectiveness of this method in identifying and predicting the critical conditions of dynamic recrystallization.

[0076] Comparative Example 1

[0077] A method for determining the dynamic recrystallization critical point of a mixed crystal structure material comprises the following steps:

[0078] The steps (1) to (6) are the same as those in Example 2.

[0079] (7) In step (6) Figure 10 First, remove the part with negative work hardening rate, and then use the correlation coefficient R 2 The 5th order polynomial function with a value greater than 0.995 was fitted. The fitting results are shown in Figure 12 (a) and Table 3.

[0080] The polynomial function of order 5 is:

[0081] Table 3

[0082] i <![CDATA[A i ]]> 0 1539024.73706 1 -34335.7734 2 307.66815 3 -1.37538 4 0.00306 5 <![CDATA[-2.71888×10 -6 ]]>

[0083] (8) Taking the derivative of the obtained fifth-order polynomial, we get Make The curve graph is drawn and the peak point is found in the curve graph. The abscissa of the peak point represents the critical stress, and the corresponding strain is defined as the critical strain value under the deformation condition. Figure 12 As shown in (b), under this deformation condition, after the derivation of the 5th-order polynomial function fitting, Two peak points, (199.25, 59.5) and (238.15, 70.63), were observed in the curve.

[0084] (9) In step (4), the corresponding stress values ​​of 199.25 MPa and 238.15 MPa were found from the data after fitting the 12th-order polynomial function, and the corresponding strain values ​​of these stress values ​​were recorded as 0.01895 and 0.03049. Therefore, the critical points for dynamic recrystallization under this deformation condition may be (0.01895, 199.25) and (0.03049, 238.15).

[0085] (10) The critical strain values ​​determined in step (9) are 0.01895 and 0.03049, respectively, and their ratios to the peak strain of 0.08038 identified in step 7 of [Example 2] are 0.236 and 0.379, respectively. The ratio of 0.379 is within the prediction range of the Sellars model, i.e., ε c =(0.3~0.9)ε p The ratio of 0.236 is not within the prediction range of the Sellars model, so the critical strain value determined by this method is 0.03049.

[0086] Because the work hardening rate was fitted quadratically in Comparative Example 1, fitting errors inevitably occur. Therefore, the resulting data points are not fixed but rather fluctuate. While quadratic fitting of the work hardening rate-true stress curve is a common practice, choosing different polynomial functions can lead to different fitting errors. Due to these fitting errors, the data points determined by different fitting methods (such as the critical point of dynamic recrystallization) may not be precise single values, but are more likely to fluctuate within a specific range.

[0087] Compared to existing technologies, this method significantly reduces the number of fitting operations required to determine the dynamic recrystallization critical point and the potential fitting errors by using peak stress as a direct reference point. Furthermore, peak stress is a specific value obtained directly from experimental data, bypassing the reliance on mathematical models and eliminating complex mathematical models and multiple fitting steps. This simplifies the data analysis process and makes the determination of the dynamic recrystallization critical point more intuitive and faster.

[0088] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A method for determining the dynamic recrystallization critical point of a mixed crystal structure material, characterized in that: The steps include: (1) Perform unidirectional hot compression or hot tensile tests on mixed crystal structure materials through thermal simulation experiments, record the true stress and true strain data under a specific deformation temperature and strain rate, and draw the true stress-true strain curve; (2) Using polynomial functions to fit the true stress and true strain data during the plastic deformation stage, a fitted true stress-true strain curve is obtained; The polynomial function is σ is true stress, ε is true strain, A n , A n-1 ,…,A1,A0 are the coefficients obtained by fitting, and n is the order of the polynomial; (3) Derivative the polynomial function formula (1) to obtain a continuous work hardening rate θ curve; The work hardening rate θ is: (4) According to formula (1) and formula (2), draw the relationship curve between work hardening rate θ and true stress σ; (5) Obtain the measured peak stress σ from the true stress-true strain curve obtained in step (1) p , assuming that the measured peak stress σ p is the dynamic recovery saturation stress value σ when the dynamic recovery process reaches the saturation state drvs , the measured peak stress σ in the true stress-true strain curve p The corresponding strain is the peak strain ε p ; (6) In the relationship curve between the work hardening rate θ and the true stress σ obtained in step (4), find the corresponding measured peak stress σ on the horizontal axis. p The location point (σ p , 0); (7) Using the Tangent plug-in of Origin software, the tangent point position of the relationship curve between the work hardening rate θ and the true stress σ is adjusted. By adjusting the tangent position, the intersection point of the tangent and the x-axis is located at point (σ p , 0), the tangent point of the relationship curve between the tangent line and the work hardening rate θ and the true stress σ is the critical point (σ c ,θ c ); (8) In the fitted true stress-true strain curve obtained in step (2), find the critical point (σ c ,θ c ) corresponds to the critical stress value σ c , and record the critical stress value σ c The corresponding critical strain value ε c , the dynamic recrystallization critical point (σ c , ε c ); (9) Calculate the peak strain ε p and critical strain value ε c to test the peak strain ε p and critical strain value ε c Whether the ratio is consistent with the prediction range of the Sellars model; If ε c =(0.3~0.9)ε p , then the peak strain ε p and critical strain value ε c The ratio of is consistent with the prediction range of the Sellars model, then the point (σ c , ε c ) is the critical point of dynamic recrystallization.

2. The method for determining the dynamic recrystallization critical point of a mixed crystal structure material according to claim 1, wherein: The average grain size of the mixed crystal structure material is less than 1 μm, and the proportion of grains within 1 μm exceeds 50%.

3. The method for determining the dynamic recrystallization critical point of a mixed crystal structure material according to claim 1, wherein: In the true stress-true strain curve obtained in step (1), the mixed crystal structure material transitions from the elastic deformation stage to the plastic deformation stage as the true strain increases during the loading process; In the elastic deformation stage, in the true stress-true strain curve, the true stress σ and the true strain ε are in a linear relationship, following the formula σ=E*ε, where E is Young's modulus; in the plastic deformation stage, in the true stress-true strain curve, the true stress σ and the true strain ε are in a nonlinear relationship.

4. The method for determining the dynamic recrystallization critical point of a mixed crystal structure material according to claim 1, wherein: In step (2), the polynomial function is obtained by importing the true stress and true strain data of the plastic deformation stage into the Origin software, and then the polynomial function with a polynomial order of ≤9 is selected through the Simple Fit plug-in with a correlation coefficient R 2 Fitting polynomial functions with a polynomial order greater than 0.995, or manually entering the correlation coefficient R using Nonlinear Curve Fit for polynomial functions with a polynomial order greater than 9. 2 The polynomial formula with a polynomial order of >0.995 is used. After fitting is completed, the adjusted polynomial function and the corresponding fitted true stress-true strain curve are obtained.

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