Introducing mechanism of nano planar defect structure in Ti-48Al-2Nb-2Cr alloy through muti-step heat treatment and its effect on mechanical properties

Abstract

Improving the comprehensive properties of γ-TiAl alloys is of important engineering significance, and nano planar defect structure (NPDS), including stacking faults (SFs) and nano twins, etc. have shown a great strengthening effect in many alloys. In this paper, a relatively simple and feasible multi-step heat treatment method was adopted to introduce NPDS into an as-forged Ti-48Al-2Nb-2Cr alloy. The repeated phase transformation in cyclic heat treatment (CHT) resulted in the formation of NPDS in equiaxed grains, which was closely related to the phase transformation of α(α₂) → FCC → γ. The morphology of NPDS was associated with the formation rate of FCC regions. With the increase of cycle times, complex twin boundaries and SF regions were formed. After short heat treatment (SHT) and aging, the NPDS reinforced alloy with duplex phase (DP) structure was finally obtained, which exhibited similar grain size with the as-forged alloy. The tensile strength of the heat-treated alloy at room temperature and 700 °C reached 839 MPa and 714 MPa. The significant advantage in tensile strength can be attributed to the obvious inhibition of NPDS on dislocation slip. The formation of the interface dislocation loops as well as the wide distributions of pseudo twin (PT) and rotational boundary (RB) played a beneficial role in improving the plasticity and fracture toughness of the alloy.