A method for high temperature solution treatment and thermo-mechanical treatment of a new high magnesium content aluminum alloy

Abstract

This invention discloses a novel high-magnesium content aluminum alloy high-temperature solution treatment and deformation heat treatment method, comprising the following steps: (1) preparing the novel high-magnesium content aluminum alloy hot-working material by any of the methods of rolling, extrusion and forging, and performing high-temperature solution treatment, with a holding temperature of 495~545℃ and a holding time of 30~150min; (2) rapidly cooling the hot-working material after high-temperature solution treatment to room temperature using a quenching medium, and performing pre-deformation treatment, with a pre-deformation amount of 1~6%; (3) after pre-deformation treatment, performing low-temperature long-term aging treatment at 30~75℃ within 72h, with a holding time of 96~720h; (4) after low-temperature aging, performing deformation treatment, with a deformation amount of 1~6%; (5) after deformation treatment, performing three-stage aging treatment. The novel high-magnesium content aluminum alloy hot-working material obtained by the method of this invention has a tensile strength ≥430MPa, a spalling corrosion resistance not lower than EA grade, and a strengthening precipitate system consisting of a T phase and a β' phase dual-phase co-precipitation strengthening system.

Description

Technical Field

[0001] This invention relates to heat treatment methods for aluminum alloy materials, specifically to a novel high-temperature solution treatment and deformation heat treatment method for high-magnesium-content aluminum alloys. Background Technology

[0002] Aluminum alloys are indispensable lightweight structural materials in the manufacture of aerospace vehicles and transportation equipment. Reducing the weight of aluminum alloy structural components is not only of great significance for improving the performance of aerospace vehicles and transportation equipment, reducing energy consumption and emissions of harmful substances, but also can enhance the application competitiveness of aluminum alloys.

[0003] Mg is one of the common main alloying elements in aluminum alloys, with a density of only 1.74 g / cm³. 3 For every 1 wt.% increase in Mg content in aluminum alloys, the density decreases by 0.38%, effectively reducing the alloy's density. By adding Mg and employing a suitable solution treatment process, Mg atoms dissolve in the Al matrix, causing lattice distortion and achieving solid solution strengthening. However, when the alloy temperature falls below a certain level, excess Mg segregates, forming coarse grain boundary precipitates that severely deteriorate the overall properties of the aluminum alloy. Previous studies have shown that adding an appropriate amount of Zn to conventional Al-Mg aluminum alloys can combine with Mg elements exceeding the matrix's solubility to form age-strengthening precipitates of Mg. 32 (Al,Zn) 49 The addition of a certain amount of Si (T phase) effectively enhances the alloy's strength. Furthermore, the addition of appropriate amounts of Si can also combine with Mg to form the Mg2Si phase, which is the main strengthening phase in Al-Mg-Si alloys. In addition, appropriate heat treatment processes (such as solution treatment and aging) can effectively improve the overall performance of the alloy.

[0004] Therefore, it is necessary to conduct further research on the heat treatment process of novel high-magnesium-content Al-Mg-Zn-Si alloys in order to obtain excellent matching of key properties. Summary of the Invention

[0005] The purpose of this invention is to provide a novel high-temperature solution treatment and deformation heat treatment method for high-magnesium content aluminum alloys, so that the strengthening precipitates of the alloy hot-working material are a dual-phase co-precipitation strengthening system of T phase and β' phase, thereby obtaining excellent matching of key properties.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: A novel high-magnesium content aluminum alloy is disclosed using a high-temperature solution treatment and deformation heat treatment method. The aluminum alloy comprises: 6.0~9.9 wt% Mg, 1.1~3.01 wt% Zn, 0.1~1.15 wt% Si, and at least one of Mn, Cu, Zr, Sc, and Ti elements with a total content not exceeding 0.8 wt%, the remainder being Al and unavoidable impurities. The method includes the following steps: (1) The novel high magnesium content aluminum alloy hot-working material is prepared by any of the rolling, extrusion and forging methods, and the high temperature solution treatment is carried out by heating from room temperature to solution holding temperature, with a heating rate of 10~20℃ / h, a holding temperature of 495~545℃ and a holding time of 30~150min. (2) The hot-worked material after high-temperature solution treatment is rapidly cooled to room temperature using a quenching medium. The temperature of the quenching medium is 20~30℃ and the cooling rate is 40~75℃ / s. Pre-deformation treatment is then performed, with a pre-deformation amount of 1~6%. (3) After pre-deformation treatment, perform low-temperature long-term aging treatment at 30~75℃ within 72h, and keep warm for 96~720h; (4) After low-temperature aging, deformation treatment is performed, with a deformation of 1-6%; (5) After deformation treatment, a three-stage aging treatment is carried out. The first stage aging regime is 90~135℃ for 10~24h, the second stage aging regime is 240~350℃ for 10~30min, and the third stage aging regime is a two-stage aging treatment of 90~135℃ for 10~24h and 140~190℃ for 8~24h.

[0007] Preferably, the aluminum alloy is composed of the following components: Mg 6.3~9.9wt%, Zn 1.1~2.9wt%, Si 0.15~1.0wt%, and at least one of Mn, Cu, Zr, Sc and Ti elements with a total content not exceeding 0.6wt%, with the remainder being Al and unavoidable impurities.

[0008] Preferably, in step (1), the heat preservation temperature of the high-temperature solution treatment is 510~535℃, and the heat preservation time is 30~60min.

[0009] Preferably, in step (2), the quenching medium is a water-based quenching liquid, comprising, by mass: 10% polyethylene glycol, 10% butyl acrylate, 2% styrene, 3% acrylic acid, 5% isododecyl alcohol, 0.3% preservative, 0.1% defoamer, 0.5% antioxidant, with the remainder being water; the preservative is nitrite, the defoamer is polyether-modified silicone, and the antioxidant is tert-butylhydroquinone.

[0010] Preferably, in step (2), the pre-deformation amount of the hot-working material is 2~5.5%, the deformation rate is 0.5~1.2mm / min, the pressure holding time is 50~100s, and the time interval between the completion of quenching and the pre-deformation does not exceed 5h.

[0011] Preferably, in step (3), the temperature of the low-temperature aging is 40~65℃ and the heat preservation time is 168~720h.

[0012] Preferably, in step (4), the deformation amount of the deformation treatment is 1~2%, the deformation rate is 0.5~1.2mm / min, and the pressure holding time is 50~100s.

[0013] Preferably, in step (5), the second stage of aging is carried out by heating to room temperature, and after aging is completed, it is cooled to room temperature, and the cooling medium is room temperature cooling water.

[0014] Preferably, in step (5), after the first and third stage aging treatments, the room temperature is cooled to room temperature using any one of the following methods: air cooling, water cooling, or wind cooling.

[0015] Preferably, in step (5), after the first stage of aging treatment and cooling to room temperature, the second stage of aging treatment is carried out within 336 hours of being left at room temperature; after the second stage of aging treatment and cooling to room temperature, the third stage of aging treatment is carried out immediately.

[0016] A novel high-magnesium-content aluminum alloy hot-working material is prepared by the method described above. The hot-working material has a tensile strength ≥430MPa, a spalling corrosion resistance not lower than EA grade, and the strengthening precipitate is a dual-phase co-precipitation strengthening system of T phase and β' phase.

[0017] The beneficial effects of this invention are: This invention provides a novel high-temperature solution treatment and deformation heat treatment method for high-magnesium content aluminum alloys. Specifically targeting novel high-magnesium content aluminum alloys for hot working, it innovatively employs a low solution heating rate and a high solution holding temperature for high-temperature solution heating. This allows the AlMgZn phase in the alloy to fully dissolve during the slow heating process, and the MgSi phase to partially dissolve during the high-temperature solution holding process, without over-burning the alloy matrix. After high-temperature solution pre-deformation, a long-term low-temperature aging treatment is used to promote the formation of strengthening phase precursor clusters. A three-stage aging process is employed: the first stage is pre-aging for extensive nucleation; the second stage is high-temperature aging, where the MgSi precipitate (β' phase) grows rapidly, and the natural aging clusters dissolve; the third stage is a two-stage aging process, promoting the formation of TMg. 32 (Al,Zn) 49 A large amount of phase precipitates.

[0018] According to the high-temperature solution treatment and deformation heat treatment method of the novel high-magnesium content aluminum alloy of the present invention, the obtained novel high-magnesium content aluminum alloy hot-working material has a tensile strength ≥430MPa, a spalling corrosion resistance not lower than EA grade, and the strengthening precipitate phase is a dual-phase co-precipitation strengthening system of T phase and β' phase. Detailed Implementation

[0019] This invention employs a multi-element alloying composition ratio, simultaneously adding Zn and Si elements to Al-Mg aluminum alloys to develop a novel high-magnesium-content aluminum alloy, making the alloy contain TMg. 32 (Al,Zn) 49 Age-hardening aluminum alloys of the Mg and Si phases. The novel high-magnesium aluminum alloy is composed of the following components: Mg 6.0~9.9wt%, Zn 1.1~3.01wt%, Si 0.1~1.15wt%, and at least one of Mn, Cu, Zr, Sc and Ti elements with a total content not exceeding 0.8wt%, the remainder being Al and unavoidable impurities.

[0020] More preferably, the novel high-magnesium aluminum alloy is composed of the following components: Mg 6.3~9.9wt%, Zn 1.1~2.9wt%, Si 0.15~1.0wt%, Mn≤0.1wt%, Cu≤0.1wt%, Zr≤0.15wt%, Sc≤0.1wt%, Ti≤0.1wt%, with the remainder being Al and unavoidable impurities.

[0021] The novel high-temperature solution treatment and deformation heat treatment method for high-magnesium content aluminum alloys provided by this invention includes the following steps: (1) High-temperature solution insulation Any hot-worked material prepared from the new high-magnesium-content aluminum alloy by rolling, extrusion, and forging is subjected to high-temperature solution treatment at 495~545℃ for 30~150min.

[0022] Further preferred, the heat-working material prepared by the novel high-magnesium content aluminum alloy in step (1) has a high-temperature solution treatment temperature of 495~545℃, a holding time of 30~150min, and a heating rate from room temperature to the solution treatment temperature of 10~20℃ / h.

[0023] (2) Quenching and pre-deformation treatment The high-temperature solution-insulated heat-worked material is rapidly cooled to room temperature using a quenching medium and then pre-deformed, with a pre-deformation amount of 1~6%.

[0024] Further optimization is made by using a water-based quenching medium in the hot-working material prepared from the novel high-magnesium aluminum alloy in step (2). The quenching medium comprises, by mass: 10% polyethylene glycol, 10% butyl acrylate, 2% styrene, 3% acrylic acid, 5% isododecyl alcohol, 0.3% corrosion inhibitor, 0.1% defoamer, 0.5% antioxidant, with the remainder being water. The corrosion inhibitor is nitrite, the defoamer is polyether-modified silicone, and the antioxidant is tert-butylhydroquinone. The quenching medium temperature is 20-30°C, and the cooling rate is 40-75°C / s. The deformation after quenching and cooling is 2-5.5%, the deformation rate is 0.5-1.2 mm / min, the holding pressure is 50-100 s, and the time interval between quenching completion and pre-deformation does not exceed 5 hours.

[0025] (3) Low-temperature long-term aging treatment The pre-deformed high-magnesium aluminum alloy was used to prepare a hot-working material, which was then subjected to a low-temperature long-term aging treatment at 30~75℃ for 72 hours, and the holding time was 96~720 hours.

[0026] Further optimization is that the heat-working material prepared by the novel high-magnesium content aluminum alloy in step (3) adopts a low-temperature aging temperature of 30~60℃ and an aging holding time of 168~720h.

[0027] (4) Deformation treatment After low-temperature aging, deformation treatment is performed, with a deformation of 1-6%.

[0028] Further preferred, the hot-working material prepared by the novel high-magnesium content aluminum alloy in step (4) has a deformation of 1~2%, a deformation rate of 0.5~1.2 mm / min, and a holding pressure of 50~100 s.

[0029] (5) Three-stage timeliness processing The deformed heat-worked material undergoes a three-stage aging treatment. The first stage aging regime is 90~135℃ for 10~24h, the second stage aging regime is 240~350℃ for 10~30min, and the third stage aging regime is a two-stage aging treatment consisting of 90~135℃ for 10~24h and 140~190℃ for 8~24h. The second stage aging uses heating to the set temperature, followed by cooling to room temperature using room temperature water as the cooling medium. After the first and third stage aging treatments, the material is cooled to room temperature using any one of the following methods: air cooling, water cooling, or air cooling. After the first stage aging treatment and cooling to room temperature, the material is left at room temperature for 336h before undergoing the second stage aging treatment; after the second stage aging treatment and cooling to room temperature, the third stage aging treatment is immediately carried out.

[0030] The following examples illustrate the implementation of the present invention in detail, thereby enabling a full understanding and implementation of how the present invention uses technical means to solve technical problems and achieve technical effects.

[0031] Example 1 This embodiment provides a novel high-temperature solution treatment and deformation heat treatment method for high-magnesium content aluminum alloys, which is implemented according to the following steps: (1) The new high magnesium content aluminum alloy hot working material is prepared with the following alloy mass ratios: Mg 7.5wt%, Zn 2.9wt%, Si 0.15wt%, Mn≤0.1wt%, Cu≤0.1wt%, Zr≤0.15wt%, Sc≤0.1wt%, Ti≤0.1wt%, and the remainder is Al and unavoidable impurities.

[0032] (2) The new high magnesium content aluminum alloy hot working material in step (1) is subjected to high temperature solution heat preservation treatment in the range of 530℃ / 60~90min, and the heating rate of the material from room temperature to solution temperature is in the range of 20~25℃ / h.

[0033] (3) A water-based quenching liquid (by mass, comprising: 10% polyethylene glycol, 10% butyl acrylate, 2% styrene, 3% acrylic acid, 5% isododecyl alcohol, 0.3% preservative, 0.1% defoamer, 0.5% antioxidant, with the remainder being water; the preservative is nitrite, the defoamer is polyether-modified silicone, and the antioxidant is tert-butylhydroquinone) is used for spray quenching at a cooling rate of 70~75℃ / s. After quenching and cooling, a pre-stretching treatment is performed with a deformation rate of 0.5mm / min and a deformation amount in the range of 1.8~2.5%, with a holding pressure range of 50~100s. The time interval between the completion of quenching and pre-deformation does not exceed 5h.

[0034] (4) Within 24 hours of being left at room temperature, perform a low-temperature long-term aging treatment at 60℃ / 600~650h.

[0035] (5) The heat-treated material subjected to low temperature and long-term aging treatment is subjected to deformation treatment with a deformation rate of 0.5 mm / min, a holding pressure of 50~100s, and a deformation of 1.0~1.3%.

[0036] (6) A three-stage aging treatment is carried out. The first stage aging regime is 120℃ / 15h, the second stage aging regime is 335℃ / 30min, and the third stage aging regime is a two-stage aging treatment of 120℃ / 15h + 175℃ / 8h. The second stage aging uses heating to reach the set temperature, and after aging, it is water-cooled to room temperature. After the first and third stage aging treatments, it is air-cooled to room temperature. After the first stage aging treatment and cooling to room temperature, the second stage aging treatment is carried out within 336h at room temperature; after the second stage aging treatment and cooling to room temperature, the third stage aging treatment is carried out immediately.

[0037] Tests and analyses were conducted on the aged hot-worked material, which showed a tensile strength of 441 MPa, an exfoliation corrosion resistance of EA grade, and a dual-phase co-precipitation strengthening system consisting of T phase and β' phase.

[0038] Comparative Example 1 The difference between this comparative example and Example 1 is that the novel high-magnesium content aluminum alloy hot-working material prepared in step (1) has the following alloy mass ratios: Mg 7.7wt%, Zn 3.9wt%, Si 0.3wt%, Mn 0.15wt%, Cu≤0.1wt%, Zr≤0.15wt%, Sc≤0.1wt%, Ti≤0.1wt%, with the remainder being Al and unavoidable impurities. All other steps are the same as in Example 1.

[0039] Tests and analyses were conducted on the hot-worked materials that underwent high-temperature solution treatment and deformation heat treatment. It was found that the alloy exhibited overheating, with a tensile strength of 417 MPa, an exfoliation corrosion resistance of EB grade, and a dual-phase co-precipitation strengthening system consisting of T phase and β' phase, with a significant decrease in the number density of precipitates.

[0040] Comparative Example 2 The difference between this comparative example and Example 1 is that the novel high-magnesium content aluminum alloy hot-working material prepared in step (1) is subjected to high-temperature solution treatment by the "temperature-to-sampling" method, that is, the temperature is first raised to 530°C, and then the hot-working material is directly placed into the heat treatment equipment for high-temperature solution treatment. The other steps are the same as those in Example 1.

[0041] Tests and analyses were conducted on the hot-worked materials that had undergone high-temperature solution treatment and deformation heat treatment. It was found that the alloy exhibited overheating, with a tensile strength of 408 MPa, an exfoliation corrosion resistance of EB grade, and a dual-phase co-precipitation strengthening system consisting of T phase and β' phase, with a significant decrease in the number density of precipitates.

[0042] Comparative Example 3 The difference between this comparative example and Example 1 is that the solution temperature of the novel high-magnesium aluminum alloy hot-working material prepared in step (1) is reduced to 480°C, while the other steps are the same as in Example 1.

[0043] The heat-worked material after solution treatment and deformation heat treatment was tested and analyzed. The tensile strength was 433 MPa, the exfoliation corrosion performance was EA grade, and the strengthening precipitate phase was only T phase.

[0044] Comparative Example 4 The difference between this comparative example and Example 1 is that, in step (3) of the low-temperature long-term aging process, the sample was left at room temperature for 24 hours and then subjected to a low-temperature aging process of 50°C for 12 hours. The other steps are the same as those in Example 1.

[0045] The heat-worked material after solution treatment and deformation heat treatment was tested and analyzed. The tensile strength was 396 MPa, the exfoliation corrosion performance was EA grade, the strengthening precipitate was a two-phase co-precipitation strengthening system of T phase and β' phase, the number density of precipitates decreased and the size decreased.

[0046] Comparative Example 5 The difference between this comparative example and Example 1 is that the deformation treatment in step (4) is not performed, but the other steps are the same as in Example 1.

[0047] The heat-worked material after solution treatment and deformation heat treatment was tested and analyzed. The tensile strength was 414 MPa, the exfoliation corrosion performance was EA grade, the strengthening precipitate was a two-phase co-precipitation strengthening system of T phase and β' phase, and the number density of precipitates decreased.

[0048] Comparative Example 6 The difference between this comparative example and Example 1 is that the second stage of aging in the three-stage aging process in step (5) is not performed. After the first stage of aging at 120℃ / 15h, the third stage of aging at 120℃ / 15h + 175℃ / 8h is performed. The other steps are the same as those in Example 1.

[0049] The heat-worked material after solution treatment and deformation heat treatment was tested and analyzed. The tensile strength was 427 MPa, the exfoliation corrosion performance was EA grade, and the strengthening precipitate was a two-phase co-precipitation strengthening system of T phase and β' phase. The number density and size of the β' phase decreased.

[0050] Comparative Example 7 The difference between this comparative example and Example 1 is that in the three-stage aging process in step (5), after the second stage of aging at 335℃ / 30min, the air cooling method is used to cool it to room temperature. The other steps are the same as those in Example 1.

[0051] The heat-worked material after solution treatment and deformation heat treatment was tested and analyzed. The tensile strength was 425 MPa, the exfoliation corrosion performance was EA grade, and the strengthening precipitate was a two-phase co-precipitation strengthening system of T phase and β' phase. The number density of T phase decreased and the size increased sharply.

[0052] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention in any other way. Any person skilled in the art may make changes or modifications to the above-disclosed technical content to create equivalent embodiments. However, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A novel high-temperature solution treatment and deformation heat treatment method for high-magnesium-content aluminum alloys, characterized in that, The aluminum alloy is composed of the following components: Mg 6.0~9.9wt%, Zn 1.1~3.01wt%, Si 0.1~1.15wt%, and at least one of Mn, Cu, Zr, Sc, and Ti with a total content not exceeding 0.8wt%, the remainder being Al and unavoidable impurities; the method includes the following steps: (1) The novel high magnesium content aluminum alloy hot-working material is prepared by any of the rolling, extrusion and forging methods, and the high temperature solution treatment is carried out by heating from room temperature to solution holding temperature, with a heating rate of 10~20℃ / h, a holding temperature of 495~545℃ and a holding time of 30~150min. (2) The hot-worked material after high-temperature solution treatment is rapidly cooled to room temperature using a quenching medium. The temperature of the quenching medium is 20~30℃ and the cooling rate is 40~75℃ / s. Pre-deformation treatment is then performed, with a pre-deformation amount of 1~6%. (3) After pre-deformation treatment, perform low-temperature long-term aging treatment at 30~75℃ within 72h, and keep warm for 96~720h; (4) After low-temperature aging, deformation treatment is performed, with a deformation of 1-6%; (5) After deformation treatment, a three-stage aging treatment is carried out. The first stage aging regime is 90~135℃ for 10~24h, the second stage aging regime is 240~350℃ for 10~30min, and the third stage aging regime is a two-stage aging treatment of 90~135℃ for 10~24h and 140~190℃ for 8~24h.

2. The high-temperature solution treatment and deformation heat treatment method for the novel high-magnesium content aluminum alloy according to claim 1, characterized in that, The aluminum alloy is composed of the following components: Mg 6.3~9.9wt%, Zn 1.1~2.9wt%, Si 0.15~1.0wt%, and at least one of Mn, Cu, Zr, Sc and Ti elements with a total content not exceeding 0.6wt%, with the remainder being Al and unavoidable impurities.

3. The high-temperature solution treatment and deformation heat treatment method for the novel high-magnesium content aluminum alloy according to claim 1 or 2, characterized in that, In step (1), the heat preservation temperature of the high-temperature solution treatment is 510~535℃, and the heat preservation time is 30~60min.

4. The high-temperature solution treatment and deformation heat treatment method for the novel high-magnesium content aluminum alloy according to claim 1 or 2, characterized in that, In step (2), the quenching medium is a water-based quenching liquid, which, by mass, includes: 10% polyethylene glycol, 10% butyl acrylate, 2% styrene, 3% acrylic acid, 5% isododecyl alcohol, 0.3% preservative, 0.1% defoamer, 0.5% antioxidant, and the balance is water; the preservative is nitrite, the defoamer is polyether-modified silicone, and the antioxidant is tert-butylhydroquinone.

5. The high-temperature solution treatment and deformation heat treatment method for the novel high-magnesium content aluminum alloy according to claim 1 or 2, characterized in that, In step (2), the pre-deformation amount of the hot-working material is 2~5.5%, the deformation rate is 0.5~1.2mm / min, the pressure holding time is 50~100s, and the time interval between the completion of quenching and the pre-deformation does not exceed 5h.

6. The high-temperature solution treatment and deformation heat treatment method for the novel high-magnesium content aluminum alloy according to claim 1 or 2, characterized in that, In step (3), the temperature of the low-temperature aging is 30~60℃ and the heat preservation time is 168~720h.

7. The high-temperature solution treatment and deformation heat treatment method for the novel high-magnesium content aluminum alloy according to claim 1 or 2, characterized in that, In step (4), the deformation amount of the deformation treatment is 1~2%, the deformation rate is 0.5~1.2mm / min, and the pressure holding time is 50~100s.

8. The high-temperature solution treatment and deformation heat treatment method for the novel high-magnesium content aluminum alloy according to claim 1 or 2, characterized in that, In step (5), the second stage of aging is carried out by heating to room temperature, and after aging is completed, it is cooled to room temperature. The cooling medium is room temperature cooling water.

9. The high-temperature solution treatment and deformation heat treatment method for the novel high-magnesium content aluminum alloy according to claim 1 or 2, characterized in that, In step (5), after the first and third stage aging treatments, the room temperature is cooled using any one of the following methods: air cooling, water cooling, or wind cooling.

10. The high-temperature solution treatment and deformation heat treatment method for the novel high-magnesium content aluminum alloy according to claim 1 or 2, characterized in that, In step (5), after the first stage of aging treatment and cooling to room temperature, the second stage of aging treatment is carried out within 336 hours of being left at room temperature; after the second stage of aging treatment and cooling to room temperature, the third stage of aging treatment is carried out immediately.

11. A novel high-magnesium-content aluminum alloy hot-working material, characterized in that, The hot-working material prepared by any one of claims 1 to 10 has a tensile strength ≥ 430 MPa, a spalling corrosion resistance not lower than EA grade, and the strengthening precipitate is a dual-phase co-precipitation strengthening system of T phase and β' phase.