Method for evaluating mechanical properties of lamellar structure TC18 titanium alloy

Abstract

The invention belongs to the technical field of titanium alloy performance evaluation, and particularly relates to a method for evaluating the mechanical property of a lamellar structure TC18 titanium alloy, which comprises the following steps: acquiring crystal orientation information of alpha phase and beta phase of an undeformed TC18 titanium alloy, and analyzing the types of alpha variants and the proportion of each alpha variant; applying external force to the undeformed TC18 titanium alloy to enable the undeformed TC18 titanium alloy to generate 5% plastic deformation; obtaining crystal orientation information after deformation; a slippage trace generated by deformation is calibrated, and a slippage system and a Schmidt factor of each alpha variant are calculated according to the slippage trace; analyzing slippage transmission from the alpha phase to the beta matrix, and determining a slippage system started by the beta phase and a Schmidt factor of the slippage system; calculating a Berger vector and a slip transfer geometric compatibility factor remaining on an alpha / beta interface; and evaluating the yield strength of the TC18 titanium alloy according to the slip mode of the alpha variant in combination with the Schmidt factor of the alpha variant, the Schmidt factor of the beta phase in combination with the Berger vector of the alpha / beta interface and the geometric compatibility factor of slip transmission.

Description

Technical Field

[0001] The present invention belongs to the technical field of TC18 titanium alloy property evaluation, and particularly relates to a method for evaluating the mechanical properties of lamellar TC18 titanium alloy. Background Art

[0002] TC18 titanium alloy has high strength, corrosion resistance and good welding performance, and is widely used as a structural material in the aerospace field. An appropriate ratio of α-phase and β-phase can improve toughness while maintaining high strength. At present, most of them adjust the mechanical properties of TC18 titanium alloy by adjusting the heat treatment process.

[0003] Evaluating the mechanical properties of TC18 titanium alloy includes experimental testing and theoretical analysis methods; the mechanical property experimental testing can directly measure the mechanical properties of TC18 titanium alloy through standardized experimental testing methods. This method has accurate results, but a long time period. The theoretical analysis method is to understand the distribution, ratio and morphology of α-phase and β-phase in TC18 titanium alloy through microstructure observation, so as to indirectly evaluate its mechanical properties. For lamellar TC18 titanium alloy, the Hall-Petch formula is used to establish the corresponding relationship between hot working process - microstructure - mechanical properties, so as to evaluate the mechanical properties of lamellar TC18 titanium alloy.

[0004] The Hall-Petch formula is an empirical formula in materials science used to describe the relationship between the yield strength and grain size of polycrystalline materials (such as metals and alloys); the formula is σ y =σ0 + kd -1 / 2 , σ y is the yield strength of the material, σ0 is the friction stress inside the grains (independent of grain size), k is the Hall-Petch constant (related to material properties), and d is the average grain size. The Hall-Petch formula shows that the yield strength of the material increases with the decrease of grain size, and it is an important theoretical basis in material design and processing. However, for two samples with similar microstructures, it is often found that their mechanical properties may also have significant differences, and it is no longer applicable to judge mechanical properties based on grain size. Therefore, the Hall-Petch formula has great limitations in the actual production process.

[0005] The Chinese invention patent with the patent number 202311485453.3 discloses a method for rapidly and accurately predicting the microstructure and mechanical properties of titanium alloy forging deformation. By extracting the image features of the microstructure, different forging deformation parameters are used as input data in the training data, and the low-dimensional tissue data and mechanical properties extracted under the corresponding forging deformation parameters are used as output data in the training data to construct a machine learning model, realizing the rapid and accurate prediction of the evolution of forging parameters - microstructure - mechanical properties. However, the establishment of a machine learning model requires a large amount of data, and it is only limited to predicting some products with good research foundations. For new products, there is often a lack of data, and accumulating a large amount of experimental data is time-consuming, laborious, and costly. At the same time, when extracting the microstructure features, only a small number of image features are often extracted, and it is an empirical prediction without theoretical support, and the prediction results are often not accurate enough. Summary of the Invention

[0006] The object of the present invention is to provide a method for evaluating the mechanical properties of lamellar microstructure TC18 titanium alloy. This method evaluates the yield strength of the alloy by obtaining the crystal orientation of the α phase and then combining the slip system and Schmid factor, thereby realizing the evaluation of the mechanical properties of lamellar microstructure TC18 titanium alloy.

[0007] To achieve the above object, the present invention adopts the following technical solutions:

[0008] A method for evaluating the mechanical properties of lamellar microstructure TC18 titanium alloy, comprising:

[0009] Obtain the crystal orientation information of the α phase and β phase of the undeformed TC18 titanium alloy, and analyze the types of α variants and the proportion of each α variant based on the Burgers orientation relationship between the α phase and β phase;

[0010] Apply an external force to the undeformed TC18 titanium alloy to cause 5% plastic deformation; obtain the crystal orientation information after deformation; and calibrate the slip traces generated by the deformation, calculate the slip system and its Schmid factor of each α variant according to the slip traces; analyze the slip transfer from the α phase to the β matrix, determine the slip system and its Schmid factor that initiate the β phase; calculate the Burgers vector and the geometric compatibility factor of slip transfer remaining at the α / β interface;

[0011] Evaluate the yield strength of the TC18 titanium alloy according to the slip mode of the α variant, combining the Schmid factor of the α variant, the Schmid factor of the β phase, the Burgers vector at the α / β interface, and the geometric compatibility factor of slip transfer.

[0012] Furthermore, the crystal orientation information is obtained by electron backscatter diffraction technology. The measurement area of the electron backscatter diffraction technology should contain at least 20 original β grains, and based on the width of the lamellar α, the step size is selected as 1 / 5 of the width of the lamellar α.

[0013] Furthermore, the types of the α variants are determined according to the Euler angles of the α phase in the α-phase crystal orientation information, and the types of the α variants include 12 kinds.

[0014] Furthermore, the slip systems and their Schmid factors of each α variant are calculated according to the slip traces, including:

[0015] First, all possible theoretical slip systems (slip plane + slip direction) are preset according to the crystallographic characteristics of the α phase (hexagonal structure) and the β phase (body-centered cubic structure);

[0016] Then, according to the crystal orientation information of the α phase and the β phase of the deformed TC18 titanium alloy, the theoretical slip trace directions of each slip system on the sample surface are calculated;

[0017] Finally, the actual slip traces generated on the surface of the deformed sample are observed by SEM or optical microscope, the actual trace direction is compared with the theoretical trace direction, and the included angle deviation between the two is calculated; if the trace directions of multiple slip systems coincide, the slip system with the largest Schmid factor (SF) is preferentially selected as the activated slip system;

[0018] Among them, the slip system includes a slip plane and a slip direction, and the normal angle of the slip plane refers to the angle between the external force direction and the normal of the slip plane, and the slip direction angle λ refers to the angle between the external force direction and the slip direction; the Schmid factor its value range is [0, 0.5].

[0019] When the resolved shear stress acting on the slip plane along the slip direction reaches a certain critical value, the crystal begins to slip.

[0020] Furthermore, the Burgers vector remaining at the α / β interface is calculated, including:

[0021] Based on the dislocation conservation principle, the Burgers vector of the input slip system is equal to the sum of the Burgers vector of the output slip system and the residual Burgers vector; that is, b1 = b2 + b r ; b1 is the Burgers vector based on the input slip system (α phase); b2 is the Burgers vector based on the input slip system (β phase); b r represents the dislocation component that remains uncoordinated at the α / β interface after slip transfer, and the smaller its value, the more efficient the slip transfer.

[0022] The slip transfer geometric compatibility factor m' = cosθ1cosθ2; θ1 is the included angle between the input / output slip plane normals, and θ2 is the slip direction angle. The larger m' (close to 1), the easier the slip transfer and the smaller the residual Burgers vector.

[0023] Slip transfer between two phases is considered to be related to low residual Burgers vectors, high resolved shear stress on slip systems, and good alignment between slip systems.

[0024] Furthermore, the yield strength of the TC18 titanium alloy σ x1 is the contribution of the Schmidt factor SF1 of the α variant to the yield strength; σ x2 is the contribution of the Schmidt factor SF2 of the β phase to the yield strength; σ x3 is the contribution of the slip transfer geometric compatibility factor m′ to the yield strength; x is a certain type of α variant, and V x is the texture strength of a certain type of α variant.

[0025] σ x1 = A × SF1, where A is the weight coefficient of the α variant. The weight coefficient A is related to the slip mode of the slip α variant. When the slip mode is basal plane and prismatic plane slip, A = 2; when the slip mode is conical plane slip, A = 2 / 3;

[0026] σ x2 = B × SF2, where B = 2 is the weight coefficient of the β phase;

[0027] m′ is the slip transfer geometric compatibility factor;

[0028] V x = number of pixels of variant x / total number of α pixels; V x represents the contribution weight of a certain variant to the overall mechanical properties of the alloy. The larger the value, the more significant the influence of this variant on the yield strength.

[0029] In view of the limitations of the traditional method for evaluating the mechanical properties of titanium alloys by the grain size of the microstructure and the Hall-Petch relationship, the present invention proposes an innovative method based on crystal orientation, which significantly improves the efficiency and accuracy of evaluating the mechanical properties of the lamellar microstructure TC18 alloy. The specific advantages are as follows: (1) Low-cost and efficient evaluation: Only by extracting the crystal orientation information of the α phase through the EBSD technique can the mechanical properties be predicted, greatly simplifying the experimental process and reducing costs, and avoiding the dependence on a large amount of experimental data by the traditional method.

[0030] (2) Dynamic determination of the hardness and softness of α variants: A multi-parameter comprehensive criterion is proposed; when a certain α variant preferentially initiates basal plane or prismatic plane slip (the Schmidt factor SF1 is the largest) and transfers slip to the β phase, when the Schmidt factor SF2 of the β phase slip system is the largest, the interfacial residual Burgers vector (b r ) is the smallest, and the slip transfer factor (m′) is the largest, it can be determined that this variant is the "softest variant" with the lowest yield strength among the 12 α variants, thus accurately locking the easily deformable area.

[0031] (3) Texture strength statistical correlation performance: By statistically analyzing the texture strength (V x ) of the "softest α variant", combined with the contribution value of the slip transfer parameter (σ x1 +σ x2 +σ x3 ), the overall yield strength of the alloy is directly calculated, and a quantitative relationship between crystal orientation - slip behavior - mechanical properties is established.

[0032] (4) Breaking through the limitations of traditional theories: Abandoning the traditional Hall - Petch empirical formula that depends on grain size, starting from the crystal orientation and dislocation transfer mechanism, it solves the engineering problem of significant performance differences in microstructures with similar microstructures, providing a more general and accurate theoretical framework for the mechanical property evaluation of lamellar microstructure TC18 alloy.

[0033] The present invention deeply combines crystallography theory with engineering applications, not only filling the technical gap of quickly predicting the properties of titanium alloys through orientation, but also guiding process optimization and assisting in the research and development and quality control of high - performance titanium alloys in the aerospace field. Brief Description of the Drawings

[0034] Figure 1 It is a schematic diagram of the Schmid factor principle.

[0035] Figure 2 It is a schematic diagram of the residual Burgers vector and the slip transfer factor principle.

[0036] Figure 3 They are SEM and IPF diagrams of sample 1 at different deformation stages.

[0037] Figure 4 They are SEM and IPF diagrams of sample 2 at different deformation stages.

[0038] Figure 5 They are the corresponding α variants of sample 1 and sample 2. Detailed Implementation Manner

[0039] As Figure 1 shown, a method for evaluating the mechanical properties of lamellar microstructure TC18 titanium alloy provided in this embodiment includes the following steps:

[0040] Step 1: Obtain the crystal orientation information of the α - phase and β - phase of the undeformed TC18 titanium alloy, and analyze the types of α variants and the proportion of each α variant based on the Burgers orientation relationship between the α - phase and β - phase.

[0041] The crystal orientation information of the undeformed lamellar TC18 alloy was measured by electron backscatter diffraction (EBSD) technique. Based on the size of the actual β grains, at least 20 original β grains should be included in the test area; based on the width of the actual lamellar α, the step size was selected to be about 1 / 5 of the width of the lamellar α.

[0042] The Euler angles of the α phase were extracted based on the EBSD test results. Using the Burgers orientation relationship satisfied by the α and β phases, the MTEX toolbox of the open-source software Matlab was used to separate these α variants and calculate the proportion of each variant.

[0043] Step 2: Apply an external force to the undeformed TC18 titanium alloy to produce 5% plastic deformation; obtain the crystal orientation information after deformation; and calibrate the slip traces generated by the deformation. According to the slip traces, calculate the slip system and its Schmid factor of each α variant; analyze the slip transfer from the α phase to the β matrix, determine the slip system and its Schmid factor initiated by the β phase; calculate the Burgers vector and the geometric compatibility factor of slip transfer remaining at the α / β interface.

[0044] Calculating the slip system and its Schmid factor of each α variant according to the slip traces includes:

[0045] According to the crystallographic characteristics of the α phase (hexagonal structure) and the β phase (body-centered cubic structure), all possible theoretical slip systems (slip plane + slip direction) were predefined. For example, common slip systems of the α phase include basal slip ((0001)<11-20>), prismatic slip ({10-10}<12-10>), etc.

[0046] Using the crystal orientation (Euler angles), calculate the theoretical slip trace direction of each slip system on the sample surface. The trace direction is determined by the intersection line of the slip plane normal and the sample surface (geometric projection).

[0047] Observe the actual slip traces (usually in the form of parallel lines) generated on the surface of the deformed sample by SEM or optical microscope. Compare the actual trace direction with the theoretical trace direction and calculate the angular deviation between the two. Judgment criterion: The theoretical slip system with the smallest deviation is identified as the actually initiated slip system. If the trace directions of multiple slip systems coincide, the slip system with the largest Schmid factor (SF) is preferentially selected as the initiated slip system (the larger the SF, the higher the resolved shear stress and the easier it is to initiate).

[0048] The calculation of the Schmid factor is as Figure 1As shown in the figure, assuming the angle between the tensile force F and the normal line n of the slip plane is φ, and the angle between F and the slip direction b is λ, the resolved shear stress acting along the slip direction on the slip plane can be expressed by the following formula: τ = F / Acosφcosλ = γSF, where τ is the shear stress, φ is the angle between the normal line of the slip plane and F, λ is the angle between the slip direction and F, SF = cosφcosλ is the orientation factor or Schmid factor, and γ = F / A is the tensile stress. When the resolved shear stress acting along the slip direction on the slip plane reaches a certain critical value, the crystal begins to slip.

[0049] The calculation of the residual Burgers vector and the slip transfer factor is as Figure 2 shown Figure 2 in a schematic diagram of slip transfer; Crystal 1 is the input slip system, b1 is the Burgers vector of the input slip system, L1 is the slip direction of the input slip system, n1 is the normal line of the slip plane of the input slip system. Similarly, Crystal 2 is the output slip system, and b2, L2, and n2 are the corresponding Burgers vector, slip direction, and normal line of the slip plane respectively. The residual Burgers vector br can be calculated by the following formula: b1 = b2 + b r ; the geometric compatibility factor can be calculated by the following formula: m′ = cosθ1·cosθ2.

[0050] Step 3: Evaluate the yield strength of TC18 titanium alloy according to the slip mode of the α variant, combining the Schmid factor of the α variant, the Schmid factor of the β phase, the Burgers vector of the α / β interface, and the geometric compatibility factor of slip transfer.

[0051] If the slip system initially activated in one of the α variants is prismatic or basal slip, and the Schmid factor SF1 is the largest, followed by slip transfer to the β matrix, the Schmid factor SF2 of the activated slip system of the β phase is also the largest and the Burgers vector remaining at the α / β interface is the smallest, and finally the slip transfer factor m' is also the largest, then it can be determined that this variant is the one with the lowest yield strength and the easiest to deform among the 12 variants. A decrease in any one of the parameters SF1, SF2, and m' will result in an increase in the yield strength, and the contributions of the three parameters to the yield strength are equally important.

[0052] Therefore, the contributions of the three parameters to the yield strength can be set to be between 0 and 1. The closer to 1, the lower the yield strength, and the cumulative contribution to the yield strength is between 0 and 3.

[0053] For any type of variant x, the contribution of SF1 to the yield strength can be set as σ x1 , since the approximate ratio of the critical shear stresses for initiating basal, prismatic, and pyramidal slips is 1:1:3, the contribution of basal and prismatic slips to the yield strength is σ x1 = SF1 / 0.5, and the contribution of pyramidal slip to the yield strength is σx1 = SF1 / 1.5; For any type of variant x, the contribution of SF2 to the yield strength is σ x2 , then σ x2 = SF2 / 0.5; For any type of variant x, the contribution of m' to the yield strength can be set as σ x3 , then when m' > 0.72, σ x3 = m', when m' < 0.72, σ x3 = 0. For any type of variant, its texture strength is V x , then the contribution to the yield strength is σ x =(σ x1 + σ x2+ σ x3 )V x , so the total contribution of the 12 variants to the yield strength is The larger σ is, the lower the yield strength. The strength of the lamellar TC18 alloy can be judged based on this.

[0054] The following uses Sample 1 and Sample 2 to prove the effectiveness of the method described in this embodiment.

[0055] Sample 1 and Sample 2 are taken from different positions of the same TC18 forging. The lamellar α sizes are similar, being 1.14 microns and 1.16 microns respectively. The SEM and IPF at their different deformation stages are as shown in Figure 3 and Figure 4 . According to the Hall - petch relationship, the mechanical properties of the two samples cannot be judged from the perspective of grain size.

[0056] The EBSD characterization diagram is as shown in Figure 5 . It can be seen from the EBSD results that the crystal orientations of the two samples are quite different, and there must be differences in mechanical properties. Therefore, the mechanical properties of the two samples can be evaluated according to the method proposed in this embodiment.

[0057] Table 1 Euler angles and contents of each variant of Sample 1 and Sample 2

[0058]

[0059] Table 1 shows the α Euler angles of Sample 1 and Sample 2 measured, and their proportions. The deformed microstructure is tested by EBSD. According to the slip traces, the slip systems and Schmid factors activated by each variant during deformation are characterized, as well as the slip systems, Schmid factors activated by the β phase and the Burgers vectors remaining at the α / β interface when slip transfer occurs to the β matrix. Finally, the geometric compatibility factor of slip transfer is calculated based on the included angles between the slip plane normal and the slip direction of the input and output slip systems. The results are shown in Figure 2.

[0060]

[0061] Evaluate the mechanical properties of the two samples by calculating the contribution of each parameter to the yield strength from the data in Table 2; the yield strength of Sample 1, σ1 = 1.608; the yield strength of Sample 2, σ2 = 0.639; therefore, the yield strength of Sample 2 is higher.

[0062] The above are only the preferred embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Any modifications and substitutions based on the technical solutions and inventive concepts provided by the present invention should be covered within the protection scope of the present invention.

Claims

1. A method for evaluating the mechanical properties of lamellar TC18 titanium alloy, characterized in that: include: Obtain the crystal orientation information of the α phase and β phase of the undeformed TC18 titanium alloy, and analyze the types of α variants and the proportion of each α variant based on the Burgers phase relationship between the α phase and the β phase; An external force was applied to the undeformed TC18 titanium alloy to cause 5% plastic deformation; Obtaining crystal orientation information after deformation; and calibrating the slip traces generated by the deformation, calculating the slip system and the Schmidt factor of each α variant according to the slip traces; analyzing the slip transfer from the α phase to the β matrix, and determining the slip system initiated by the β phase and its Schmidt factor; The Burgers vector remaining at the α / β interface and the slip transfer geometric compatibility factor are calculated; The yield strength of TC18 titanium alloy was evaluated based on the slip mode of α variant combined with the Schmidt factor of α variant, the Schmidt factor of β phase combined with the Burgers vector of α / β interface, and the geometric compatibility factor of slip transfer.

2. A method for evaluating the mechanical properties of lamellar TC18 titanium alloy according to claim 1, characterized in that: The crystal orientation information is obtained by electron backscatter diffraction technology. The measurement area of ​​the electron backscatter diffraction technology must contain at least 20 original β grains. Based on the α width of the lamellar structure, the step size is selected to be 1 / 5 of the α width of the lamellar structure.

3. A method for evaluating the mechanical properties of lamellar TC18 titanium alloy according to claim 1, characterized in that: The types of the α variants are determined according to the Euler angle of the α phase in the α phase crystal orientation information, and the types of the α variants include 12 types.

4. A method for evaluating the mechanical properties of lamellar TC18 titanium alloy according to claim 3, characterized in that: The slip system and Schmidt factor of each α variant are calculated according to the slip trajectory, including: First, all theoretical slip systems are pre-set according to the crystallographic characteristics of the α-phase and the β-phase; Then, the theoretical slip track direction of each slip system on the surface of TC18 titanium alloy is calculated according to the crystal orientation information of the α phase and β phase of the deformed TC18 titanium alloy; Next, observe the actual slip traces generated on the sample surface after deformation, compare the actual trace direction with the theoretical trace direction, and calculate the angle deviation between the two; if the trace directions of multiple slip systems coincide, select the slip system with the largest Schmidt factor as the starting slip system; Finally, the slip system includes a slip plane and a slip direction, and the normal angle of the slip plane is refers to the angle between the external force direction and the normal of the slip surface, the slip direction angle λ refers to the angle between the external force direction and the slip direction; the Schmidt factor Its value range is [0,0.5]; When the shear stress acting on the slip plane along the slip direction reaches a certain critical value, the crystal begins to slip.

5. A method for evaluating the mechanical properties of lamellar TC18 titanium alloy according to claim 4, characterized in that: The Burgers vector remaining at the α / β interface is calculated, including: Based on the dislocation conservation principle, the Burgers vector of the input slip system is equal to the sum of the Burgers vector of the output slip system and the residual Burgers vector; that is, b1 = b2 + b r ; b1 is the Burgers vector based on the input slip system (α phase); b2 is the Burgers vector based on the input slip system (β phase); b r It represents the dislocation component that remains at the α / β interface and is not fully coordinated after slip transfer; The slip transfer geometric compatibility factor m′=cosθ1cosθ2; θ1 is the input / output slip surface normal angle, and θ2 is the slip direction angle.

6. A method for evaluating the mechanical properties of lamellar TC18 titanium alloy according to claim 5, characterized in that: The yield strength of the TC18 titanium alloy σ x1 is the contribution of the Schmidt factor SF1 of the α variant to the yield strength; σ x2 is the contribution of Schmidt factor SF2 of β phase to yield strength; σ x3 is the contribution of the slip transfer geometric compatibility factor m′ to the yield strength; x is a certain type of α variant, V x is the texture strength of a certain α variant; Among them, σ x1 =A×SF1, A is the weight coefficient of the α variant, the weight coefficient A is related to the slip mode of the α variant, when the slip mode is base and cylindrical slip, A=2; when the slip mode is cone slip, A=2 / 3; σ x2 =B×SF2, B=2 is the weight coefficient of β phase; m′ is the geometric compatibility factor of slip transfer; V x = number of pixels of variant x / total number of α pixels.

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