A heat treatment method for eliminating or inhibiting the continuous grain boundary α layer of high-strength titanium alloy

Abstract

The present invention discloses a heat treatment method for eliminating or suppressing continuous grain boundary α layer in high-strength titanium alloy, and the method comprises: First, keeping the high-strength titanium alloy at a temperature 30°C to 50°C above the phase transformation point T β for 30 min to 60 min; Second, water-cooling to room temperature; Third, slowly heating in the furnace to 350°C to 400°C and keeping for 90 min to 180 min; Fourth, water-cooling to room temperature; Fifth, quickly heating in the furnace to 510°C, keeping for 120 min to 240 min, and air-cooling to room temperature. By performing low-temperature aging treatment with slow heating and high-temperature aging treatment with rapid heating on the high-strength titanium alloy after solution treatment, the present invention effectively suppresses or eliminates the continuous grain boundary α layer in the high-strength titanium alloy, reduces strain localization or accumulation during plastic deformation, coordinates the deformation between each constituent phase, suppresses intergranular brittle fracture, improves the plasticity and fracture toughness of the high-strength titanium alloy, and is applicable to the field of aerospace weaponry and equipment.

Description

Technical Field

[0001] The invention relates to the technical field of titanium alloys, and in particular to a heat treatment method for eliminating or inhibiting a continuous grain boundary alpha layer of a high-strength titanium alloy. Background Art

[0002] High-strength titanium alloy is the metal structural material with the highest specific strength to date. It is an important and indispensable advanced material urgently needed for lightweight aerospace weapons and equipment and national economic construction. This type of alloy is expected to replace some traditional high-strength steels and achieve a significant weight reduction in aircraft. However, in actual use, it is found that high-strength titanium alloys, especially ultra-high-strength titanium alloys with a tensile strength of more than 1300MPa, have insufficient plasticity and poor toughness, which greatly limits their promotion and application.

[0003] As we all know, the microstructure of a material determines its macroscopic mechanical properties. Studies have found that one of the key reasons for the lack of plasticity and toughness of high-strength titanium alloys is that the precipitation of the straight and continuous α layer at the grain boundary weakens the β / β grain boundary. Specifically, during the plastic deformation process, strain is easily accumulated at the straight and continuous grain boundary α, which in turn triggers the initiation and rapid expansion of cracks at the grain boundary α; at the same time, the precipitation of the continuous grain boundary α layer will discharge a large amount of β-stabilizing elements into the β matrix, resulting in the formation of a precipitation-free zone (PFZ zone) around it. The strength difference between the precipitation-free zone and the aged matrix is ​​huge, which greatly weakens the alloy's ability to resist crack growth, resulting in reduced fracture toughness and fatigue performance.

[0004] In response to the above problems, the latest research has found that adding a small amount of carbon to titanium alloys using grain boundary engineering can effectively eliminate the continuous grain boundary α layer and its influence on the macroscopic mechanical properties. The reason is that the carbides formed by carbon are distributed on the β / β grain boundaries, destroying the continuity of the grain boundary α layer and inhibiting the initiation and rapid expansion of cracks. Although this method can eliminate the adverse effects of the continuous grain boundary α layer to a certain extent, for high-strength titanium alloys of specified grades, the addition of carbon elements is almost unacceptable due to the strict requirements on alloy composition in national standards (especially aviation standards). Therefore, for now, the suppression or elimination of the continuous grain boundary α layer with the help of more practical heat treatment control methods is still the key direction of high-strength titanium alloy research today and in the future. Summary of the invention

[0005] The technical problem to be solved by the present invention is to provide a heat treatment method for eliminating or inhibiting the continuous grain boundary α layer of high-strength titanium alloy in view of the deficiencies of the above-mentioned prior art. The method effectively inhibits or eliminates the continuous grain boundary α layer in the high-strength titanium alloy by performing a low-temperature aging treatment with slow heating and a high-temperature aging treatment with rapid heating in sequence after the high-strength titanium alloy is solid-solution treated, reduces the strain localization or accumulation during the plastic deformation process, coordinates the deformation between the constituent phases, inhibits the intergranular brittle fracture, improves the plasticity and fracture toughness of the high-strength titanium alloy, and effectively solves the problem that the continuous grain boundary α layer of the existing high-strength titanium alloy is difficult to inhibit or eliminate.

[0006] In order to solve the above technical problems, the technical solution adopted by the present invention is: a heat treatment method for eliminating or inhibiting the continuous grain boundary α layer of high-strength titanium alloy, characterized in that the method comprises the following steps:

[0007] Step 1: Place the high-strength titanium alloy at a temperature equal to the phase transition point T β Keep in the high temperature heat treatment furnace at 30℃~50℃ for 30min~60min;

[0008] Step 2, cooling the high-strength titanium alloy after heat preservation in step 1 to room temperature, so as to retain the structure morphology at room temperature;

[0009] Step 3: placing the high-strength titanium alloy cooled to room temperature in step 2 in a low-temperature heat treatment furnace, slowly heating the furnace to 350° C. to 400° C. and keeping the temperature for 90 min to 180 min;

[0010] Step 4, cooling the high-strength titanium alloy after heat preservation in step 3 to room temperature, so as to retain the structure morphology at room temperature;

[0011] Step 5: Place the high-strength titanium alloy cooled to room temperature in step 4 in a low-temperature heat treatment furnace, quickly heat it to 510° C. and keep it for 120 min to 240 min, then air-cool it to room temperature to obtain a high-strength titanium alloy after heat treatment.

[0012] High-strength titanium alloys are usually metastable β-type titanium alloys, with high β-stabilizing element content and long and straight β / β grain boundaries. After traditional solution aging heat treatment, this type of alloy is prone to form a straight and continuous grain boundary α layer on the β / β grain boundary, accompanied by a wide precipitation-free zone, which seriously affects the tensile plasticity, fracture toughness and fatigue properties of the alloy. To address this problem, the present invention designs a novel heat treatment method through an ingenious conception, which is different from the traditional solution aging heat treatment. Specifically, the high-strength titanium alloy is first subjected to a solution treatment, and the metastable phase is retained to room temperature by a rapid cooling method using water cooling; then the temperature is slowly raised to a low-temperature aging zone for a heat preservation treatment, so that the α phase nucleates synchronously in the grain and at the grain boundary, and the microstructure is retained to room temperature by a rapid cooling method using water cooling; then the temperature is rapidly raised to a high-temperature aging zone for a heat preservation treatment, so as to reduce thermal stress and stabilize the internal microstructure of the alloy, so that the α phase precipitated in the low-temperature zone is quickly dissolved back. After the dissolution, the remaining area serves as the nucleation site of the α phase during high-temperature aging, avoiding the defect that a straight continuous grain boundary α layer is easily formed and the grain boundary is weakened in the conventional heat treatment process, effectively solving the problem that the continuous grain boundary α layer of the existing high-strength titanium alloy is difficult to suppress or eliminate, and providing technical support for promoting the application of such alloys.

[0013] The above-mentioned heat treatment method for eliminating or inhibiting the continuous grain boundary α layer of high-strength titanium alloy is characterized in that the high-strength titanium alloy in step 1 is Ti-1500 alloy, and the phase transition point T β ≈820℃.

[0014] The above-mentioned heat treatment method for eliminating or inhibiting the continuous grain boundary α layer of high-strength titanium alloy is characterized in that the rate of slowly heating with the furnace in step three is 2°C / min to 5°C / min.

[0015] The above-mentioned heat treatment method for eliminating or inhibiting the continuous grain boundary α layer of high-strength titanium alloy is characterized in that the rate of rapid furnace heating in step five is 1000°C / min.

[0016] The above-mentioned heat treatment method for eliminating or suppressing the continuous grain boundary α layer of high-strength titanium alloy is characterized in that the microstructure of the high-strength titanium alloy after the heat treatment in step five is that the continuous grain boundary α layer is eliminated or suppressed.

[0017] Compared with the prior art, the present invention has the following advantages:

[0018] 1. The present invention performs a solid solution treatment and rapid cooling on the high-strength titanium alloy to retain the metastable phase structure, then slowly heats up to a low-temperature aging zone for heat preservation treatment and rapid cooling, so that the α phase is synchronously nucleated in the grain and at the grain boundary, and then rapidly heats up to a high-temperature aging zone for heat preservation treatment and slow cooling, so that the α phase is quickly dissolved back, and the remaining area serves as the nucleation site of the α phase during high-temperature aging, thereby avoiding the defect of the conventional heat treatment process that a straight continuous grain boundary α layer is easily formed and the grain boundary is weakened, and effectively eliminating or inhibiting the continuous grain boundary α layer of the high-strength titanium alloy.

[0019] 2. The heat treatment method of the present invention can not only effectively inhibit or eliminate the continuous grain boundary α layer in the high-strength titanium alloy, reduce the strain localization or accumulation in the grain boundary α layer or the non-precipitation zone during plastic deformation, but also coordinate the deformation between the constituent phases and inhibit the occurrence of intergranular brittle fracture, thereby further improving the plasticity and fracture toughness of the high-strength titanium alloy, and is suitable for the field of aerospace weapons and equipment.

[0020] 3. The heat treatment method of the present invention has a simple process and is easy to operate and adjust. At the same time, the structure of the grain boundary-free α layer and the PFZ-free zone obtained by heat treatment provides the strength of the high-strength titanium alloy while minimizing the loss of plasticity, toughness and fatigue properties. This structural design strategy can be applied and promoted to other precipitation-strengthened high-strength alloys to optimize the matching between comprehensive mechanical properties.

[0021] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 The figure is a schematic diagram of the heat treatment method for eliminating or inhibiting the continuous grain boundary α layer of high-strength titanium alloy according to the present invention.

[0023] Figure 2 This is the microstructure diagram of the Ti-1500 high-strength titanium alloy after heat treatment in Example 1 of the present invention.

[0024] Figure 3 This is the microstructure diagram of the Ti-1500 high-strength titanium alloy after heat treatment in Example 2 of the present invention.

[0025] Figure 4 This is the microstructure diagram of the Ti-1500 high-strength titanium alloy after heat treatment in Example 3 of the present invention.

[0026] Figure 5 This is the microstructure diagram of the Ti-1500 high-strength titanium alloy after heat treatment in Example 4 of the present invention.

[0027] Figure 6 This is the microstructure diagram of the Ti-1500 high-strength titanium alloy after heat treatment in Example 5 of the present invention.

[0028] Figure 7This is the organizational diagram of the Ti-1500 high-strength titanium alloy after heat treatment in Comparative Example 1 of the present invention.

[0029] Figure 8 This is the organizational diagram of the Ti-1500 high-strength titanium alloy after heat treatment in Comparative Example 2 of the present invention.

[0030] Fig. 9 This is the organizational diagram of the Ti-1500 high-strength titanium alloy after heat treatment in Comparative Example 3 of the present invention. DETAILED DESCRIPTION

[0031] Example 1

[0032] like Figure 1 As shown, this embodiment includes the following steps:

[0033] Step 1: Place the Ti-1500 high-strength titanium alloy in a high-temperature heat treatment furnace at 860°C for 60 minutes;

[0034] Step 2, cooling the Ti-1500 high-strength titanium alloy after heat preservation in step 1 to room temperature, so as to retain the structure morphology at room temperature;

[0035] Step 3: Place the Ti-1500 high-strength titanium alloy cooled to room temperature in step 2 in a low-temperature heat treatment furnace, slowly heat it to 400°C at a heating rate of 2°C / min and keep it at that temperature for 90 minutes;

[0036] Step 4, cooling the Ti-1500 high-strength titanium alloy after heat preservation in step 3 to room temperature, so as to retain the microstructure at room temperature;

[0037] Step 5: Place the Ti-1500 high-strength titanium alloy cooled to room temperature in step 4 in a low-temperature heat treatment furnace, rapidly heat it to 510°C at a heating rate of 1000°C / min, keep it warm for 120 minutes, and then air-cool it to room temperature to obtain the heat-treated Ti-1500 high-strength titanium alloy.

[0038] Figure 2 The microstructure of the Ti-1500 high-strength titanium alloy after heat treatment in this embodiment is shown in FIG. Figure 2 It can be seen that there is no continuous grain boundary α layer in the structure of the Ti-1500 high-strength titanium alloy after the heat treatment, that is, the continuous grain boundary α layer is eliminated.

[0039] Example 2

[0040] like Figure 1 As shown, this embodiment includes the following steps:

[0041] Step 1: Place the Ti-1500 high-strength titanium alloy in a high-temperature heat treatment furnace at 870°C for 30 minutes;

[0042] Step 2, cooling the Ti-1500 high-strength titanium alloy after heat preservation in step 1 to room temperature, so as to retain the structure morphology at room temperature;

[0043] Step 3: Place the Ti-1500 high-strength titanium alloy cooled to room temperature in step 2 in a low-temperature heat treatment furnace, slowly heat it to 400°C at a heating rate of 3°C / min, and keep it warm for 10 minutes;

[0044] Step 4, cooling the Ti-1500 high-strength titanium alloy after heat preservation in step 3 to room temperature, so as to retain the microstructure at room temperature;

[0045] Step 5: Place the Ti-1500 high-strength titanium alloy cooled to room temperature in step 4 in a low-temperature heat treatment furnace, rapidly heat it to 510°C at a heating rate of 1000°C / min, keep it warm for 150 minutes, and then air-cool it to room temperature to obtain the heat-treated Ti-1500 high-strength titanium alloy.

[0046] Figure 3 The microstructure of the Ti-1500 high-strength titanium alloy after heat treatment in this embodiment is shown in FIG. Figure 2 It can be seen that there is no continuous grain boundary α layer in the structure of the Ti-1500 high-strength titanium alloy after the heat treatment, that is, the continuous grain boundary α layer is eliminated.

[0047] Example 3

[0048] like Figure 1 As shown, this embodiment includes the following steps:

[0049] Step 1: Place the Ti-1500 high-strength titanium alloy in a high-temperature heat treatment furnace at 870°C for 50 minutes;

[0050] Step 2, cooling the Ti-1500 high-strength titanium alloy after heat preservation in step 1 to room temperature, so as to retain the structure morphology at room temperature;

[0051] Step 3: Place the Ti-1500 high-strength titanium alloy cooled to room temperature in step 2 in a low-temperature heat treatment furnace, slowly heat it to 350°C at a heating rate of 3°C / min, and keep it warm for 180 minutes;

[0052] Step 4, cooling the Ti-1500 high-strength titanium alloy after heat preservation in step 3 to room temperature, so as to retain the microstructure at room temperature;

[0053] Step 5: Place the Ti-1500 high-strength titanium alloy cooled to room temperature in step 4 in a low-temperature heat treatment furnace, rapidly heat it to 510°C at a heating rate of 1000°C / min, keep it warm for 240 minutes, and then air-cool it to room temperature to obtain the heat-treated Ti-1500 high-strength titanium alloy.

[0054] Figure 4 The microstructure of the Ti-1500 high-strength titanium alloy after heat treatment in this embodiment is shown in FIG. Figure 4 It can be seen that the continuous grain boundary α layer in the Ti-1500 high-strength titanium alloy structure after the heat treatment is very small and thin, that is, the continuous grain boundary α layer is suppressed.

[0055] Example 4

[0056] like Figure 1 As shown, this embodiment includes the following steps:

[0057] Step 1: Place the Ti-1500 high-strength titanium alloy in a high-temperature heat treatment furnace at 860°C for 60 minutes;

[0058] Step 2, cooling the Ti-1500 high-strength titanium alloy after heat preservation in step 1 to room temperature, so as to retain the structure morphology at room temperature;

[0059] Step 3: Place the Ti-1500 high-strength titanium alloy cooled to room temperature in step 2 in a low-temperature heat treatment furnace, slowly heat it to 350°C at a heating rate of 5°C / min, and keep it warm for 150 minutes;

[0060] Step 4, cooling the Ti-1500 high-strength titanium alloy after heat preservation in step 3 to room temperature, so as to retain the microstructure at room temperature;

[0061] Step 5: Place the Ti-1500 high-strength titanium alloy cooled to room temperature in step 4 in a low-temperature heat treatment furnace, rapidly heat it to 510°C at a heating rate of 1000°C / min, keep it warm for 120 minutes, and then air-cool it to room temperature to obtain the heat-treated Ti-1500 high-strength titanium alloy.

[0062] Figure 5 The microstructure of the Ti-1500 high-strength titanium alloy after heat treatment in this embodiment is shown in FIG. Figure 5 It can be seen that there is no continuous grain boundary α layer in the structure of the Ti-1500 high-strength titanium alloy after the heat treatment, that is, the continuous grain boundary α layer is eliminated.

[0063] Example 5

[0064] like Figure 1 As shown, this embodiment includes the following steps:

[0065] Step 1: Place the Ti-1500 high-strength titanium alloy in a high-temperature heat treatment furnace at 850°C for 60 minutes;

[0066] Step 2, cooling the Ti-1500 high-strength titanium alloy after heat preservation in step 1 to room temperature, so as to retain the structure morphology at room temperature;

[0067] Step 3: Place the Ti-1500 high-strength titanium alloy cooled to room temperature in step 2 in a low-temperature heat treatment furnace, slowly heat it to 370°C at a heating rate of 4°C / min and keep it warm for 100 minutes;

[0068] Step 4, cooling the Ti-1500 high-strength titanium alloy after heat preservation in step 3 to room temperature, so as to retain the microstructure at room temperature;

[0069] Step 5: Place the Ti-1500 high-strength titanium alloy cooled to room temperature in step 4 in a low-temperature heat treatment furnace, rapidly heat it to 510°C at a heating rate of 1000°C / min, keep it warm for 220 minutes, and then air-cool it to room temperature to obtain the heat-treated Ti-1500 high-strength titanium alloy.

[0070] Figure 6 The microstructure of the Ti-1500 high-strength titanium alloy after heat treatment in this embodiment is shown in FIG. Figure 6 It can be seen that the continuous grain boundary α layer in the Ti-1500 high-strength titanium alloy structure after the heat treatment is very small and thin, that is, the continuous grain boundary α layer is suppressed.

[0071] Comparative Example 1

[0072] This comparative example comprises the following steps:

[0073] Step 1: Place the Ti-1500 high-strength titanium alloy in a high-temperature heat treatment furnace at 860°C for 60 minutes;

[0074] Step 2, cooling the Ti-1500 high-strength titanium alloy after heat preservation in step 1 to room temperature, so as to retain the structure morphology at room temperature;

[0075] Step 3: Place the Ti-1500 high-strength titanium alloy cooled to room temperature in step 2 in a low-temperature heat treatment furnace, heat it to 510°C at a heating rate of 10°C / min, keep it warm for 120 minutes, and then air-cool it to room temperature to obtain the heat-treated Ti-1500 high-strength titanium alloy.

[0076] Figure 7 The microstructure of the Ti-1500 high-strength titanium alloy after heat treatment in this embodiment is shown in FIG. Figure 7 It can be seen that the continuous grain boundary α layer in the Ti-1500 high-strength titanium alloy structure after heat treatment is relatively large and thick.

[0077] By comparing Example 1 of the present invention with Comparative Example 1, it can be seen that compared with the conventional heat treatment in Comparative Example 1, the present invention avoids the defect of the conventional heat treatment process that a straight continuous grain boundary α layer is easily formed and the grain boundary is weakened by adopting the method of slowly heating the temperature to the low-temperature aging zone for heat preservation treatment and rapid cooling, and then rapidly heating the temperature to the high-temperature aging zone for heat preservation treatment and slow cooling, thereby effectively eliminating or inhibiting the continuous grain boundary α layer of the high-strength titanium alloy.

[0078] Comparative Example 2

[0079] This comparative example comprises the following steps:

[0080] Step 1: Place the Ti-1500 high-strength titanium alloy in a high-temperature heat treatment furnace at 870°C for 30 minutes;

[0081] Step 2, cooling the Ti-1500 high-strength titanium alloy after heat preservation in step 1 to room temperature, so as to retain the structure morphology at room temperature;

[0082] Step 3: Place the Ti-1500 high-strength titanium alloy cooled to room temperature in step 2 in a low-temperature heat treatment furnace, heat it to 510°C at a heating rate of 3°C / min, keep it warm for 240 minutes, and then air-cool it to room temperature to obtain the heat-treated Ti-1500 high-strength titanium alloy.

[0083] Figure 8 The microstructure of the Ti-1500 high-strength titanium alloy after heat treatment in this embodiment is shown in FIG. Figure 8 It can be seen that the continuous grain boundary α layer in the Ti-1500 high-strength titanium alloy structure after heat treatment is relatively large and thick.

[0084] By comparing Example 2 of the present invention with Comparative Example 2, it can be seen that compared with the conventional heat treatment in Comparative Example 2, the present invention avoids the defect of the conventional heat treatment process that a straight continuous grain boundary α layer is easily formed and the grain boundary is weakened by adopting the method of slowly heating the temperature to the low-temperature aging zone for heat preservation treatment and rapid cooling, and then rapidly heating the temperature to the high-temperature aging zone for heat preservation treatment and slow cooling, thereby effectively eliminating or inhibiting the continuous grain boundary α layer of the high-strength titanium alloy.

[0085] Comparative Example 3

[0086] This comparative example comprises the following steps:

[0087] Step 1: Place the Ti-1500 high-strength titanium alloy in a high-temperature heat treatment furnace at 850°C for 60 minutes;

[0088] Step 2, cooling the Ti-1500 high-strength titanium alloy after heat preservation in step 1 to room temperature, so as to retain the structure morphology at room temperature;

[0089] Step 3: Place the Ti-1500 high-strength titanium alloy cooled to room temperature in step 2 in a low-temperature heat treatment furnace, heat it to 510°C at a heating rate of 4°C / min, keep it warm for 180 minutes, and then air-cool it to room temperature to obtain the heat-treated Ti-1500 high-strength titanium alloy.

[0090] Fig. 9 The microstructure of the Ti-1500 high-strength titanium alloy after heat treatment in this embodiment is shown in FIG. Fig. 9 It can be seen that the continuous grain boundary α layer in the Ti-1500 high-strength titanium alloy structure after heat treatment is relatively large and thick.

[0091] By comparing Example 5 of the present invention with Comparative Example 3, it can be seen that compared with the conventional heat treatment in Comparative Example 3, the present invention avoids the defect of the conventional heat treatment process that a straight continuous grain boundary α layer is easily formed and the grain boundary is weakened by adopting the method of slowly heating the temperature to the low-temperature aging zone for heat preservation treatment and rapid cooling, and then rapidly heating the temperature to the high-temperature aging zone for heat preservation treatment and slow cooling, thereby effectively eliminating or inhibiting the continuous grain boundary α layer of the high-strength titanium alloy.

[0092] The above is only a preferred embodiment of the present invention and does not limit the present invention in any way. Any simple modification, change and equivalent change made to the above embodiment according to the technical essence of the invention still falls within the protection scope of the technical solution of the present invention.

Claims

1. A heat treatment method for eliminating or inhibiting the α layer of the continuous grain boundary of a high-strength titanium alloy, characterized in that: The method comprises the following steps: Step 1: Place the high-strength titanium alloy at a temperature equal to the phase transition point T β Keep in a high temperature heat treatment furnace at 30℃~50℃ for 30min~60min; Step 2, cooling the high-strength titanium alloy after heat preservation in step 1 to room temperature, so as to retain the structure morphology at room temperature; Step 3: placing the high-strength titanium alloy cooled to room temperature in step 2 in a low-temperature heat treatment furnace, slowly heating the furnace to 350°C-400°C and keeping the temperature for 90min-180min; Step 4, cooling the high-strength titanium alloy after heat preservation in step 3 to room temperature, so as to retain the structure morphology at room temperature; Step 5: Place the high-strength titanium alloy cooled to room temperature in step 4 in a low-temperature heat treatment furnace, quickly heat it to 510°C and keep it for 120min to 240min, then air-cool it to room temperature to obtain a high-strength titanium alloy after heat treatment.

2. A heat treatment method for eliminating or inhibiting the continuous grain boundary α layer of high-strength titanium alloy according to claim 1, characterized in that: The high-strength titanium alloy in step 1 is Ti-1500 alloy, and the phase transition point T β =820℃.

3. A heat treatment method for eliminating or inhibiting the α layer of the continuous grain boundary of high-strength titanium alloy according to claim 1, characterized in that: The rate of slowly heating up with the furnace in step 3 is 2°C / min to 5°C / min.

4. A heat treatment method for eliminating or inhibiting the continuous grain boundary α layer of high-strength titanium alloy according to claim 1, characterized in that: The rate of rapid heating in the furnace in step 5 is 1000°C / min.

5. The heat treatment method for eliminating or inhibiting the continuous grain boundary α layer of high-strength titanium alloy according to claim 1, characterized in that: The microstructure of the high-strength titanium alloy after the heat treatment in step 5 is characterized in that the continuous grain boundary α layer is eliminated or suppressed.

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