Microstructural evolution and mechanical properties of Ti55531–0.5Nb fabricated by selective laser melting under different heat treatments

Abstract

The metastable Ti55531–0.5Nb alloy was deposited by selective laser melting (SLM) technology. The microstructure and phase orientation along different deposition directions were analyzed, and the microstructural evolution and mechanical properties of the alloy with different heat treatments have been investigated. It is indicated that the microstructures of the as-build samples were β phase without α phase, and the β phase orientation was dominated by 〈100〉 texture. The volume fraction of primary α decreases with increasing of solution temperature, while the secondary α exhibits an opposite trend, which is mainly related to the growth rate and dissolution rate α phase. The relatively higher ultimate tensile strength of 1300 ± 20 MPa and lower elongation of 2.4 ± 0.1% was observed when solution temperature is 790 °C, and the lower ultimate tensile strength of 1190 ± 8 MPa with higher elongation of 8.40 ± 0.6% was obtained at solution temperature of 700 °C. The tensile fracture exhibits a mixture fracture mode dominated by ductile fracture when there is primary α phase in the microstructure, while the brittle fracture was obtained owing to the absence of primary α phase. The crack nucleation mode is mainly microvoid coalescence, which mainly nucleate grain boundaries α phase and propagates along the grain boundary observed from the tensile fracture and cross-sectional microstructure near the fracture of tensile specimens. There results can provide some theoretical support for metastable β titanium alloys fabricated by additive manufacturing technology and subsequent heat treatment in the future.