TiAl基合金超塑性变形的力学行为研究

Mechanical Properties of TiAl Based Alloy during Superplastic Deformation

摘要:

采用恒应变速率和应变速率递增实验研究了变形态Ti-48Al-2.3Cr-0.2Mo (at%)合金的超塑性变形力学行为，并根据计算得到的变形激活能，结合超塑性变形的流变曲线形态，对TiAl基合金的超塑性变形机理进行了分析。超塑性拉伸试验分别在800~900 ℃区间和950~1100 ℃区间和应变速率?=1′10-3~5′10-5 s-1的条件下进行。结果表明，变形态TiAl基合金超塑性变形的应变-应力曲线上几乎没有稳态塑性流变阶段。在950~1100 ℃区间，加工硬化现象显著。当T>1025 ℃或≤5′10-4 s-1时，应力-应变曲线呈典型的加工硬化形态，并且随着变形温度升高和应变速率降低，加工硬化阶段增长。原始组织中的高密度位错是引起加工硬化的原因。在800~900 ℃区间，应变速率敏感性因子m的最佳值在0.52~0.67之间，超塑性变形的表观激活能为Qapp=178 kJ/mol，晶界扩散是超塑性的速率控制机制。在950~1100 ℃区间，m的最佳值在0.63~0.77之间，超塑性变形的表观激活能值Qapp=290 kJ/mol，晶格扩散是超塑性变形的速率控制机制。

Abstract:

Superplastic mechanical behavior of Ti-48Al-2.3Cr-0.2Mo (at%) alloy with deformed microstructure was investigated at temperatures ranging from 800 to 1100 oC with strain rates between 5′10-5 and 1′10-3 s-1 by constant strain rate tests and incremental strain rate tests. Depending on the plots of true stress-true strain and the data of activation energy calculated in the tests, the mechanism of superplasticity was discussed. Results show that for the Ti-48Al-2.3Cr-0.2Mo alloy, there is almost no constant stress stage but hardening is observed in the plots of true stress-true strain. Particularly, at the temperatures above 1025 oC or strain rates below 5′10-4 s-1, a continuous strain hardening stage is observed, and with the increasing of the temperature and the decreasing of the strain rate, the stage increases. High-density dislocation caused by pre-deformation is suggested the reason of hardening. At temperatures from 800 to 900oC, the maximum strain rate sensitivity (m) values are 0.52~0.67, and the activation energy of the alloy is calculated as Qapp =178 kJ/mol, indicating that the dominant mechanism of superplastic deformation is grain boundary siding controlled by grain boundary diffusion. At temperatures from 950 to 1100 oC, the maximum strain rate sensitivity (m) values are 0.63~0.77, and the activation energy of the alloy is calculated as Qapp =290 kJ/mol, suggesting that the dominant mechanism of superplastic deformation is grain boundary siding controlled by lattice diffusion.