(1. 中南大学 粉末冶金国家重点实验室， 长沙 410083;
2. 郑州轻金属研究院， 郑州 450041;
3. 中南大学 材料科学与工程学院， 长沙 410083)
摘 要: 采用恒应变速率和应变速率递增实验研究了Ti-48Al-2Cr-0.2Mo(摩尔分数，%)合金在常压空气中的低温超塑性变形力学行为， 并且探讨了TiAl基合金的低温超塑性变形机理。 研究结果表明， TiAl基合金的变形组织具有良好的空气中低温超塑性变形性能。 在t=900℃， ε·=5×10-4s-1时， 伸长量δ达到最大值为413%， 即使在较低的温度（t=800℃）和较高的应变速率（ε·=1×10-3s-1）下变形， 伸长量δ值仍然超过300%。 在整个变形区间m值均大于0.3， mmax为0.78。 当t＞900℃或ε·＜5×10-4s-1时， 剧烈氧化导致超塑性变形性能的恶化和脆性断裂。 在900～950℃之间， TiAl基合金超塑性变形的热激活机制发生转变。 实验测得TiAl基合金在800～900℃ 时超塑性变形的热激活能为Qav=178kJ/mol， 这个数值介于γ-TiAl的蠕变体积激活能和TiAl基合金的空位迁移能之间， 而接近于后者， 因此， TiAl基合金低温超塑性变形的速率控制机制是晶界扩散。
关键字: TiAl基合金； 超塑性； 速率控制机制；
（1. State Key Laboratory for Powder Metallurgy,
Central South University , Changsha 410083, China.
2. Zhengzhou Light-Metal Institution, Zhengzhou 450041,China;
3. School of Materials Science and Engineering,
Central South University,Changsha 410083, China）
Abstract:Superplastic behaviors of a TiAl based alloy with deformed and duplex microstructure were investigated in air at temperatures ranging from 800℃ to 950℃ and at strain rates ranging from 5×10-5s-1 to 1×10-3s-1. The results show that the TiAl alloy with deformed microstructure exhibits excellent low temperature superplasticity. A maximal elongation of 413% is obtained at 900℃ (at a strain rate of 5×10-4s-1), and the elongation values over 300% are obtained at 800℃(at a strain rate of 5×10-4s-1) or at a strain rate of 1×10-3s-1(at 900℃). The lower superplasticity in duplex microstructure was observed, owing to the larger grain size and the effect of the grain boundary structure and state on the flow. With increased temperatures or decreased strain rates, fracture occurs because of oxidation. The flow softening and the strain hardening are observed in the deformation, a lower rate of strain hardening is corresponding to a larger elongation. The strain rate sensitivity values were examined by incremental strain rate tests. The alloy shows relatively high m value which keeps above 0.3 in the deforming range, the maximal value of m is 0.78. At temperature range from 900℃ to 950℃, the activation mechanism of the superplastic deformation changes, and the activation energy is calculated to be 178kJ/mol at temperature range from 800℃ to 900℃, which is greatly lower than the values of activation energy of volume diffusion in γ-TiAl and in α2-Ti3Al, otherwise closely to the value of activation energy of grain boundary diffusion in γ-TiAl. Therefore, the dominant mechanism during superplatic deformation at temperature range from 800℃ to 900℃ is grain boundary sliding controlled by grain boundary diffusion.
Key words: TiAl based alloy; superplastic deformation; mechanism of rate controlling
