热处理对超细晶Ti-55511近β钛合金显微组织和力学性能的影响Effect of heat treatment on microstructure and mechanical properties of ultra-fine grained Ti-55511 near β titanium alloy

采用热轧制备晶粒尺寸为0.1~0.5 μm的超细晶Ti-55511近β钛合金，利用SEM和TEM等研究热处理工艺对超细晶合金显微组织和力学性能的影响。结果表明，在350~650 ℃的温度条件下，合金强度和硬度随温度的升高呈现先增高后降低的趋势，在450 ℃时达到峰值强度1486.09 MPa；在450 ℃退火时，随着退火时间的延长，合金强度首先急剧上升至1536 MPa后趋于稳定，伸长率呈现先增加后下降的趋势；合金在退火过程中主要经历动态回复过程，未发生明显的粗化长大现象，晶粒尺寸均小于1 μm。动态回复过程在消除加工硬化的同时促进了晶界/相界的稳定化，增强细晶强化作用；退火过程中发生α→α₂和β→ω→α相变过程，第二相粒子弥散强化效应增强。但是当第二相粒子尺寸增大至一定程度时，会显著降低合金的塑性。退火过程中合金力学性能的变化与强化机制的作用有关。

The ultra-fine grained (UFG) Ti-55511 near β titanium alloy with grain size of 0.1-0.5 μm was prepared by hot rolling. The effects of heat treatment on the microstructure and mechanical properties were investigated by SEM and TEM. The results indicate that both the strength and hardness of the alloy increase firstly and then reduce with increasing the heat treatment temperature from 350 ℃ to 650 ℃. The peak strength of 1486.09 MPa appears at 450℃. The strength dramatically reaches 1536 MPa and then becomes stable with prolonging the holding time when heat-treated at 450 ℃. While the maximum elongation increases firstly and then decreases. During the annealing process the dynamic recovery occurs in the alloy and the grain size remains smaller than 1 μm. The dynamic recovery stimulates the grain refinement effect by eliminating the hardening process and improves the stability of the grain boundary/phase boundary. The phase transitions of α→α₂ and β→ω→α enhance the second phase particle dispersion effect during the annealing process. However, the ductility of alloy significantly decreases when the second phase particles grow up to a certain size. The mechanical property evolution during annealing process is mainly related to the effect of strengthening mechanisms.

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