Mechanisms of secondary phases formation and resultant mechanical properties of a beta-type Ti-13.3Mo-7.2Al–4Zr-0.25Ni alloy fabricated by laser aided additive manufacturing

Abstract

The microstructures of Ti alloys are sensitive to the thermal history during the laser aided additive manufacturing (LAAM) process, which inevitably affects the mechanical properties. The dimension of the deposited part is one of the important factors influencing the heat accumulation and dissipation. In this work, the effect of sample size on the LAAM-built β-titanium Ti-13.3Mo-7.2Al–4Zr-0.25Ni (wt.%) was investigated. The results demonstrate that the vertical sample with smaller cross-section area has larger grain size than the horizontal sample with larger cross-section area. In addition, three types of secondary phases, i.e., Ti3Al, Ti2Ni and TiNi, are formed in the vertical sample due to thermal accumulation caused by the smaller cross-section area. The formation of the secondary phases leads to higher Vickers hardness and brittle failure of the vertical sample. However, the horizontal sample with no secondary phase demonstrates excellent combination of high yield strength (923 MPa) and low elastic modulus (68.7 GPa), which is beneficial for structural and biomedical application. Furthermore, the low modulus is mainly ascribed to the crystallographic texture along <001> direction.