Effect of alloying additions on work hardening, dynamic recrystallization, and mechanical properties of Ti-44Al-5Nb-1Mo alloys during direct hot-pack rolling

摘要

There is significant gap in the understanding of the effect of alloying elements, particularly V and B, on the dynamic recrystallization (DRX) behavior, and the corresponding microstructure evolution in Ti-44Al-5Nb-lMo-(V, B) alloys during uniaxial hot compression. In this regard, we have studied here the relationship of work hardening rate (0) and critical strain of dynamic recrystallization (ε_c) on strain, deformation parameters. The 8 values showed typical dynamic recrystallization characteristics, and were sensitive to temperature and strain rate. Deformation mechanism involving twinning-dislocation interactions and phase transformation contributed to characteristics of θ. DRX occurred in Ti-44Al-5Nb-1Mo-2V-0.2B alloys even at higher rolling speed and at ambient temperatures over a wide range, which implied a wider processing window during the direct hot-pack rolling process. Crack-free sheets were directly obtained from the as-HIPed alloys at 1250 ℃ with rolling speeds of 0.3 m/s, and 10%-20% reduction per pass. Their microstructure, high-temperature tensile properties and fracture mechanisms are discussed. As rolling progressed, Ti-44Al-5Nb-1Mo alloy was characterized by residual lamellae with locally dissolved α2 laths, broadening y laths, and fine sub-grains. Dislocation-induced DRX and phase transformation of α2/γ lamellae were the primary deformation modes. In contrast, on the addition of V and B, deformation twinning was activated. Multi-phase microstructure was characterized by equiaxed grains of ～ 5-20 μm. The as-rolled sheets of both the alloys exhibited superior performance at 800 ℃. Ti—44Al-5Nb-1Mo alloy showed a higher ultimate tensile stress of —689 MPa, while superior ductility of greater than 75% was obtained for Ti-44Al-5Nb-1Mo-2V-0.2B alloy. The microstructure with different degree of dynamic recrystallization contributed to various aspects of high-temperature tensile properties.