Method for producing plastic forming metal

Abstract

To provide a method for producing a plastic forming metal capable of simultaneously exhibiting work hardening and ductility.SOLUTION: A method includes: forming plate deposits 10 along the (0001) crystal plane parallel to a basal plane in a metal having an hcp structure, for suppressing non-basal slip and performing plastic processing; thereby producing a plastically formed metal in which the volume fraction of processed grains 11 which exhibit a fibrous texture with <10-10> orientation corresponding to a prismatic plane normal direction in the hcp structure aligned with a plastic forming direction, is between 25% and 80%; and producing the plastically formable metal in which, during plastic forming, the critical resolved shear stress of a slip system active in the processed grains 11 is 40 MPa or less for the basal slip, and the critical resolved shear stresses for the prismatic slip and the pyramidal <c+a> slip are each at least 20 times or greater than that of the basal slip.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for producing a plastically worked metal having an hcp structure. [Background technology]

[0002] As lightweight structural materials that can sustainably support an expanding consumer society, light metal materials are attractive because they have high heat resistance and thermal conductivity, and a good balance of strength and ductility compared to inorganic and polymeric materials. In particular, magnesium (Mg) alloys, which are the lightest structural metal materials, are attracting increasing attention (see, for example, Patent Document 1).

[0003] However, magnesium metal has few slip systems that allow freedom of deformation, and its poor ductility and workability remain major issues that need to be resolved. In order to aim for expanded use of magnesium alloys as structural components for transportation equipment, it is urgent for material developers to provide transportation equipment manufacturers, i.e., material users, with wrought materials that are high in strength and ductility. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-075183 Summary of the Invention [Problem to be solved by the invention]

[0005] Under these circumstances, the development of new alloy compositions and plastic processing processes suited to these compositions has been promoted without being bound by existing commercial alloy compositions. As a result, it has been discovered that the contradictory properties of strength and ductility are simultaneously expressed by the bimodal transformation of the αMg matrix into fine recrystallized grains with random crystal orientation and coarse processed grains with a strong texture (<10-10> fibrous texture) in which the <10-10> orientation, which is the normal direction to the prism plane of the hcp structure, is aligned parallel to the extrusion direction. 97It has been revealed that in LPSO-type magnesium alloys, which have a long-period stacking ordered structure (LPSO) phase as the second phase, such as the Zn1Y2 (at%) composition, a multimodal microstructure consisting of two phases and three regions is formed, which in particular improves strength. Currently, we are exploring methods for designing high-strength, high-ductility magnesium alloys that do not rely on the LPSO phase.

[0006] An object of various aspects of the present invention is to provide a method for producing a plastically worked metal that can simultaneously exhibit work hardening and ductility. [Means for solving the problem]

[0007] Various aspects of the present invention are described below. [1] In a metal having an hcp structure, plate precipitates are formed along the (0001) crystal plane parallel to the basal plane to suppress non-basal sliding, and by plastic processing, a plastically processed metal is produced in which the volume fraction of processed grains having a fibrous texture in which the <10-10> orientation, which is the normal direction of the prism plane of the hcp structure, is aligned with the direction of plastic processing is 25% to 80%. The critical resolved shear stress of the slip system active in the processed grain during the plastic processing is The slip is less than 40 MPa, and the columnar surface glides and cones<c+a> The critical resolved shear stress for each slip is A method for producing plastically processed metals, characterized in that the slip is 20 times or more greater than that of the normal slip.

[0008] [2] In [1] above, A method for producing a plastically processed metal, characterized in that the processed grains in the plastically processed metal are dispersed in a recrystallized grain region in which crystal orientations are randomly oriented, with an orientation relationship in which the <10-10> orientation is aligned in the direction of plastic processing.

[0009] [3] In [1] or [2] above, In the metal with the hcp structure, the work hardening rate resulting from the dislocation motion mechanism defined by shear stress is Δh τ The Schmidt factor is SF, and the work hardening rate of the processed grain is Δh σ Then, the following formula 1 is established, and the work hardening rate Δh σ The method for producing a plastically worked metal is characterized in that the plastically worked metal is made to exhibit ductility accompanied by work hardening by utilizing the fact that the Schmidt factor SF increases inversely proportional to the square of the Schmidt factor SF. Δh σ =Δh τ / SCIENCE FICTION 2 ...Formula 1

[0010] [4] In [1] or [2] above, 1. A method for producing a plastically worked metal, wherein the metal having an hcp structure is any one of an Mg alloy, a Ti alloy, and a Zn alloy. [Effects of the Invention]

[0011] According to various aspects of the present invention, it is possible to provide a method for producing a plastically worked metal that can simultaneously exhibit work hardening and ductility. [Brief explanation of the drawings]

[0012] [Figure 1] 1A to 1C are diagrams illustrating a method for producing a plastically worked metal according to one embodiment of the present invention. [Figure 2] 10(A) to 10(D) are diagrams illustrating a comparative example. [Figure 3] FIG. 10 is a diagram showing the evaluation results of the AMID effect by crystal plasticity FEM (FEM: finite element method) when polycrystals are assumed. [Figure 4] FIG. 10 is a diagram showing the evaluation results of the AMID effect by crystal plasticity FEM when a single crystal is assumed. [Figure 5](A) is a SEM (Scanning Electron Microscope) image of Sample 1 on which bottom surface precipitates according to Example 1 are formed, (B) is an SEM image of Sample 2 on which bottom surface precipitates according to Example 1 are formed, and (C) is an SEM image of Sample 3 on which bottom surface precipitates according to Example 1 are formed. [Figure 6] FIG. 1 is a diagram showing a tensile stress-strain curve of an extruded material of a Mg-3.5 atomic % Y-1.0 atomic % Al alloy after a continuous two-stage heat treatment. [Figure 7] FIG. 1 is a diagram showing a tensile stress-strain curve of an extruded material of a Mg-3.5 at. % Y-1.0 at. % Al alloy after discontinuous two-stage heat treatment. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, the present invention is not limited to the following description, and it will be readily understood by those skilled in the art that various changes in form and details can be made without departing from the spirit and scope of the present invention. Therefore, the present invention should not be interpreted as being limited to the description of the embodiments shown below.

[0014] (Comparative Example) 2(A) to 2(D) are diagrams illustrating comparative examples. Fig. 2(A) is a perspective view showing a crystal structure expressing plate precipitates 21 along the {10-10} crystal plane parallel to the prismatic plane, and Fig. 2(B) is a top view of the crystal structure shown in Fig. 2(A). Fig. 2(C) is a perspective view showing a crystal structure expressing plate precipitates 22 along the {11-20} crystal plane, and Fig. 2(D) is a top view of the crystal structure shown in Fig. 2(C).

[0015] In order to suppress the activity of basal slip, which is the most easily activated slip system, it has been considered to form plate precipitates 21 and 22 shown in Figures 2(A) to 2(D) to make the mechanical properties isotropic and obtain strength and ductility.

[0016] (Embodiment) Fig. 1 is a diagram illustrating a method for producing a plastically worked metal according to one embodiment of the present invention. One embodiment of the present invention utilizes an "Anisotropic Mechanical Property-Induced Ductilization (AMID)" mechanism, which is completely opposite to the comparative example shown in Fig. 2, and which does not aim for isotropy of mechanical properties, but rather maximizes the plastic anisotropy of the reinforcing phase to produce ductility as an alloy.

[0017] As shown in Figure 1, plate precipitates 10 are formed in a metal (e.g., an Mg alloy) having an hcp structure along the (0001) crystal plane parallel to the basal plane to suppress non-basal glide. Methods for forming these plate precipitates 10 include the addition of trace amounts of alloying elements, such as the addition of a trace amount of a combination of Zn and a rare earth element, or the addition of a trace amount of a combination of Al and a rare earth element.

[0018] The metal having the hcp structure on which the plate precipitates 10 are formed is subjected to plastic working, thereby producing a plastically worked metal in which the volume fraction of worked grains 11 having a fibrous texture in which the <10-10> orientation, which is the normal direction to the cylinder plane of the hcp structure, is aligned with the direction of plastic working is 25% or more and 80% or less. In other words, the metal having the above-mentioned hcp structure is plastically processed so as to produce a plastically processed metal in which the volume fraction of processed grains 11 having a fibrous texture in which the <10-10> orientation is aligned in the direction of plastic processing is 25% or more and 80% or less.

[0019] The critical resolved shear stress (CRSS) of the slip system active in the processed grain 11 during the above plastic processing is The slip is less than 40 MPa, and the columnar surface glides and cones<c+a> The critical resolved shear stress for slip is This is more than 20 times that of a slip.

[0020] The processed grains 11 in the above-mentioned plastically processed metal are dispersed in a recrystallized grain region 12, where the crystal orientation is randomly oriented, with the <10-10> orientation, which is the normal direction to the cylindrical surface of the hcp structure, aligned in the direction of the plastic processing (see Figure 1).

[0021] FIG. 3 is a diagram showing the evaluation results of the AMID effect by crystal plasticity FEM (FEM: finite element method) when polycrystals are assumed.

[0022] In the Voce hardening law equation shown in Figure 3, τ0 represents the critical shear stress, τ1 represents a constant of 100 MPa, θ0 represents a constant of 5000 MPa, and θ1 represents a constant of 1. In this equation, the extent to which the AMID effect is manifested is calculated by changing only τ0. In other words, Figure 3 shows the calculation results using the crystal plasticity orientation plane finite element method.

[0023] The DRX 100% material shown in Figure 3 is a material composed of only randomly oriented recrystallized grains. The critical shear stress τ0 for slip is 20 MPa, and the Critical shear stress τ0 for slip and cone surface<c+a> The critical shear stress τ0 for slip was calculated as 100 MPa.

[0024] On the other hand, the DRX75% / TEX25% material is a bimodal material containing 25% processed grains with a fibrous texture. Slip, pillar surface Slip and cone surfaces<c+a> The critical shear stress τ0 for each slip is the same as above. For TEX, the basal The critical shear stress τ0 for slip is 20 MPa, and the glides and cones<c+a> The calculations were performed assuming that the critical shear stress τ0 for slippage was 400 MPa.

[0025] The TEX (deformed grains with fibrous texture) shown in Figure 3 is a deformed grain 11 with a fibrous texture in which the <10-10> orientation, which is the normal direction to the cylindrical surface of the hcp structure shown in Figure 1, is aligned with the direction of plastic processing. The DRX (randomly oriented recrystallized grains) is a recrystallized grain region 12 shown in Figure 1 in which the crystal orientation is randomly oriented.

[0026] From the above, it can be seen that the critical resolved shear stress (CRSS) of the slip system active in the processed grain 11 during the above-mentioned plastic processing is The slip is less than 40 MPa, and the columnar surface glides and cones<c+a> The critical resolved shear stress for each slip is This is more than 20 times the rate of slippage.

[0027] The graph showing the relationship between nominal stress and nominal strain in Figure 3 reveals that the 75% DRX / 25% TEX material, a bimodal material containing 25% deformed grains, can achieve higher ductility than the 100% DRX material composed only of recrystallized grains. In other words, by forming a volume fraction of 25% or more of deformed grains (TEX) with a fibrous texture in which the <10-10> orientation, the normal to the prism plane of the hcp structure, is aligned with the direction of plastic deformation, the work-hardening rate increases along with the strength. Note that % here refers to the volume fraction.

[0028] Furthermore, the graph showing the relationship between the hardening rate and nominal strain in Figure 3 reveals that the 75% DRX / 25% TEX material, which is a bimodal material containing 25% deformed grains, can achieve a higher work-hardening rate than the 100% DRX material composed only of recrystallized grains. In other words, it can be seen that the work-hardening rate rises sharply when the volume fraction of deformed grains (TEX) is 25% or more, and the TEX has a fibrous texture in which the <10-10> orientation, which is the normal direction to the prism plane of the hcp structure, is aligned with the direction of plastic deformation.

[0029] As mentioned above, the reason why the upper limit of the volume fraction of TEX (deformed grains) is set to 80% is that sufficient ductility cannot be obtained unless the volume fraction of DRX (recrystallized grains) with random orientation is 20% or more.

[0030] Figure 4 shows the evaluation results of the AMID effect using crystal plasticity FEM assuming a single crystal. Figure 4 shows the simulation results using the crystal plasticity orientation plane finite element method.

[0031] The diagram showing the orientation dependence of the Schmid factor (SF) reveals that only the basal slip system is active, and that the work hardening is linear.

[0032] In metals with hcp structure, the work hardening rate due to the dislocation motion mechanism determined by shear stress is Δh τ The Schmidt factor is SF, and the work hardening rate of the processed grain 11 shown in Fig. 1 is Δh σ Then, the following formula 1 is established. Therefore, the work hardening rate Δh σ As mentioned above, this is done by utilizing the fact that increases in inverse proportion to the square of the Schmid factor SF, to make the plastically processed metal exhibit ductility accompanied by work hardening. Δh σ =Δh τ / SCIENCE FICTION 2 ...Formula 1

[0033] The table shown in Figure 4 shows the Δh τ When the load is 100 MPa, the angle θ with respect to the loading axis is 45°, and SF is 0.5, Δh σ is 400, the angle θ is 60°, and SF is 0.433, then Δh σ is 533, and the angle θ is 75° and SF is 0.25, then Δh σ becomes 1600.

[0034] Furthermore, according to the graph of Fig. 4, which shows the relationship between nominal stress and nominal strain assuming a single crystal, it can be seen that as SF decreases, both the strength and work hardening rate increase. Note that the value of the work hardening rate h shown on the graph showing the relationship between nominal stress and nominal strain includes the increase due to geometric hardening caused by lattice rotation, so Δh in the table σ values ​​greater than the value of

[0035] In this embodiment, Mg alloy is used as an example of a metal having an hcp structure, but metals having an hcp structure other than Mg alloy include Ti alloy and Zn alloy. In addition, Mg alloy means that more than 50% of Mg is contained, and the same applies to Ti alloy and Zn alloy.

[0036] The reason why metals having an hcp structure can exhibit ductility accompanied by work hardening in plastically processed metals as in this embodiment is that plastic processing forms a bimodal structure in which processed grains having a fibrous texture in which the <10-10> orientation, which is the normal direction to the cylinder plane of the hcp structure, is aligned with the direction of plastic processing in the recrystallized grain region in which the crystal orientation is randomly oriented, and the AMID effect is exhibited in the processed grains, resulting in significant work hardening. [Example]

[0037] FIG. 5(A) is a SEM (Scanning Electron Microscope) image of Sample 1 in which bottom surface precipitates were formed according to Example 1. FIG. 5(B) is an SEM image of Sample 2 in which basal precipitates were formed according to Example 1. FIG. 5(C) is an SEM image of Sample 3 in which bottom surface precipitates were formed according to Example 1.

[0038] Samples 1 to 3 shown in FIGS. 5(A) to 5(C) were fabricated as follows. A casting material with an alloy composition of Mg-3.5 atomic % Y-1.0 atomic % Al alloy is produced by high-frequency induction melting in an Ar gas atmosphere.

[0039] Next, the cast material was subjected to a two-hour heat treatment at 500°C, followed by a two-hour, four-hour, and six-hour consecutive heat treatment at 450°C. This resulted in the formation of basal precipitates in the α-Mg matrix, producing Samples 1 to 3. The pre-extrusion heat treatment conditions are shown in Table 1. Specifically, Sample 1 was subjected to a two-hour second heat treatment, Sample 2 to a four-hour second heat treatment, and Sample 3 to a six-hour second heat treatment. The other differences were the same production method. The basal precipitates refer to plate precipitates 10 along the (0001) crystal plane parallel to the basal plane, as shown in Figure 1.

[0040] SEM images of the heat-treated materials of Samples 1 to 3 produced by the above-described method are shown in FIGS. 5(A) to 5(C).

[0041] Next, extrusion processing was performed on each of the heat-treated materials of Samples 1 to 3. The extrusion processing was performed at an extrusion temperature of 300°C, an extrusion ratio of R10, and an extrusion ram speed of 2.5 mm / sec.

[0042] Next, the extruded material was subjected to a tensile test at room temperature. The results are shown in Table 1 and Figure 6. Figure 6 shows the tensile stress-strain curve of the extruded material of the Mg-3.5 at.% Y-1.0 at.% Al alloy after continuous two-stage heat treatment. As a comparative example, Figure 6 also shows the tensile test result of a heat-treated material that was only subjected to heat treatment at 500°C for 2 hours before extrusion.

[0043] The 0.2% tensile yield strength, ultimate tensile strength and elongation of each of the extruded materials of Samples 1 to 3 were as shown in Table 1.

[0044] [Table 1]

[0045] According to Table 1, it was confirmed that by performing a continuous two-stage heat treatment (2 hours at 500°C, then 4 hours at 450°C) before extrusion, high strength was achieved, with a 0.2% yield strength of 390 MPa. [Example]

[0046] The method for producing the cast materials of Samples 4 to 6 in which bottom precipitates were formed according to Example 2 was the same as the method for producing the cast materials of Samples 1 to 3 in Example 1, and the alloy composition was also the same, Mg-3.5 atomic % Y-1.0 atomic % Al alloy.

[0047] The above cast material was heat-treated at 500°C for 2 hours, water-cooled, and then subjected to discontinuous two-stage heat treatment at 450°C for 2, 4, and 6 hours, to produce heat-treated samples 4 to 6 in which basal precipitates were formed in the α-Mg matrix. That is, the pre-extrusion heat treatment conditions are shown in Table 2. Specifically, the second stage of heat treatment for sample 4 was 2 hours, for sample 5 it was 4 hours, and for sample 6 it was 6 hours; otherwise, the production method was the same.

[0048] Next, each of the heat-treated materials of Samples 1 to 3 was subjected to the same extrusion processing as in Example 1. That is, the extrusion processing was performed at an extrusion temperature of 350°C, an extrusion ratio of R10, and an extrusion ram speed of 2.5 mm / sec.

[0049] Next, the extruded material was subjected to a tensile test at room temperature. The results are shown in Table 2 and Figure 7. Figure 7 shows the tensile stress-strain curve of an extruded material after discontinuous two-stage heat treatment of an Mg-3.5 at.% Y-1.0 at.% Al alloy. As a comparative example, Figure 7 also shows a tensile test result of an extruded material that was extruded after heat treatment at 500°C for 2 hours, followed by water cooling to room temperature and then heat treatment at 450°C.

[0050] The 0.2% tensile yield strength, ultimate tensile strength and elongation of each of the extruded materials of Samples 4 to 6 were as shown in Table 2.

[0051] [Table 2]

[0052] According to Table 2, it was confirmed that discontinuous two-stage heat treatment (2 hours at 500°C + 4 hours at 450°C) before extrusion resulted in high strength with a 0.2% yield strength of 367 MPa, but it was also confirmed that the extruded material that underwent continuous two-stage heat treatment was superior. [Explanation of symbols]

[0053] 10 Plate precipitates along the (0001) crystal plane parallel to the basal plane 11 Deformed grains with a fibrous texture in which the <10-10> orientation, which is the normal direction to the cylinder plane of the hcp structure, is aligned in the direction of plastic deformation. 12 Recrystallized grain region with randomly oriented crystal orientation

Claims

1. In a metal having an hcp structure, plate precipitates are formed along the (0001) crystal plane parallel to the basal plane to suppress non-basal slip, and the metal is subjected to plastic processing, thereby producing a plastically processed metal in which the volume fraction of processed grains having a fibrous texture in which the <10-10> orientation, which is the normal direction of the cylindrical surface of the hcp structure, is aligned with the direction of plastic processing, is 25% or more and 80% or less; A method for producing plastically processed metal, characterized in that the critical resolved shear stress of the slip systems active in the processed grains during the plastic processing is 40 MPa or less for basal slip, and the critical resolved shear stresses of each of prismatic slip and conical <c+a> slip are 20 times or more that of basal slip.

2. In claim 1, A method for producing a plastically processed metal, characterized in that the processed grains in the plastically processed metal are dispersed in a recrystallized grain region in which crystal orientations are randomly oriented, with an orientation relationship in which the <10-10> orientation is aligned in the direction of plastic processing.

3. In claim 1 or 2, In the metal having the hcp structure, the work hardening rate derived from the dislocation motion mechanism defined by shear stress is Δh τ The Schmidt factor is SF, and the work hardening rate of the processed grain is Δh σ Then, the following formula 1 is established, and the work hardening rate Δh σ ductility accompanied by work hardening is expressed in the plastically worked metal by utilizing the fact that the Schmidt factor SF increases inversely proportional to the square of the Schmidt factor SF. Δh σ = Δh τ / SF ２ ・・・ Equation 1

4. In claim 1 or 2, A method for producing a plastically worked metal, characterized in that the metal having the hcp structure is any one of an Mg alloy, a Ti alloy, and a Zn alloy.

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