热处理对轧制态GWZ932镁合金微观组织和力学性能的影响

Effect of Heat Treatment on Microstructure and Mechanical Properties of Rolled GWZ932 Alloy

摘要:

通过光学显微镜、X射线衍射仪、维氏硬度计、扫描电子显微镜、透射电子显微镜、高角度环形暗场扫描透射电子显微镜等研究了固溶与时效处理对轧制态GWZ932镁合金组织演变和力学性能的影响。结果表明，轧制态合金组织主要由 α-Mg、块状18R-LPSO相、片层状14H-LPSO相、富稀土相和Zn-Zr相构成。经500 ℃固溶处理2 h片层14H-LPSO几乎完全溶入基体，固溶4 h后从α-Mg基体中析出细长针状14H-LPSO，且随着时间的延长其体积分数逐渐升高，固溶6 h时针状14H-LPSO体积分数达到16%，固溶8 h后针状14H-LPSO相溶解并重新析出片层14H-LPSO（长约14.9 μm，宽约8.2 μm），少量未溶解的针状14H-LPSO相长大形成棒状14H-LPSO（长23.4 μm，宽1.98 μm）。提高固溶温度至520 ℃，溶质原子扩散速率加快，导致片层14H-LPSO完全溶解和再析出时间分别提前至1和2 h，再析出的片层14H-LPSO相尺寸（长12.6 μm，宽5.1 μm）小于500 ℃固溶8 h再析出的尺寸。520 ℃固溶过程中未出现针状14H-LPSO的析出与转变，说明针状14H-LPSO相的析出与溶解及其含量受固溶温度和时间的影响。在此基础上，研究了合金经520 ℃固溶处理4 h后的时效硬化行为，结果表明合金在225 ℃时效处理64 h时达到峰值硬度。峰值时效条件下合金的室温抗拉强度（UTS）、屈服强度（YS）和伸长率（EL）分别为：396.3 MPa、274 MPa和12.7%。相比轧制态分别提高了23.8%，7.4%和69.3%。合金优异强度和塑性来源于 α-Mg基体中析出的长约28.9 nm，宽约8.9 nm，体积分数约为11.7%的柱面β''相和基面18R-LPSO/γ''相。

Abstract:

The effects of solid solution and aging treatments on the microstructural evolution and hardness of the GWZ932 alloy in the rolled state were investigated by means of optical microscope, X-ray diffractometer, Vickers hardness tester, scanning electron microscope, transmission electron microscope, and high-angle annular dark-field scanning transmission electron microscope. The results show that the alloy microstructure in the rolled state mainly consists of α-Mg, massive 18R-LPSO phase, lamellar 14H-LPSO phase, rare-earth-rich phase and Zn-Zr phase. The lamellar 14H-LPSO is almost completely dissolved into the matrix by solid solution treatment at 500 ℃ for 2 h; the elongated acicular 14H-LPSO is precipitated from the α-Mg matrix after 4 h of solid solution, and its volume fraction gradually increases with the extension of time; the volume fraction of acicular 14H-LPSO reaches 16% after 6 h of solid solution; the acicular 14H-LPSO phase dissolves and re-precipitates the lamellar 14H-LPSO (about 14.9 μm in length and 8.2 μm in width), and a small amount of undissolved acicular 14H-LPSO phase grows to form rod-like 14H-LPSO (23.4 μm in length and 1.98 μm in width). Increasing the solid solution temperature to 520 ℃, the solute atom diffusion rate is accelerated, thus the complete dissolution and re-precipitation time of lamellar 14H-LPSO is advanced to 1 and 2 h, respectively, and the size of the re-precipitated lamellar 14H-LPSO phase (12.6 μm in length, 5.1 μm in width) is smaller than that of the re-precipitated one after solid solution at 500 ℃ for 8 h. Precipitation and transformation do not occur during solid solution treatment at 520 ℃, indicating that the precipitation and dissolution of the acicular 14H-LPSO phase and its content are affected by the solid solution temperature and time. The age-hardening curves reach peak hardness under conditions of 520 ℃, 4 h+225 ℃, 64 h. On this basis, the age-hardening behaviour of the alloy after solution treatment at 520 ℃ for 4 h was investigated and the results show that the alloy reaches peak hardness after age treatment at 225 ℃ for 64 h. The room-temperature tensile strength (UTS), yield strength (YS) and elongation (EL) of the alloy under peak aging conditions are 396.3 MPa, 274 MPa, and 12.7% increase by 23.8%, 7.4%, and 69.3%, compared to those of the rolled state, respectively. The excellent strength and plasticity of the alloys arise from the precipitation of a columnar β'' phase (about 28.9 nm in length and 8.9 nm in width with an average area fraction of about 11.7%) and a basal 18R-LPSO/γ'' phase in the α-Mg matrix.