Constructing a bimodal microstructure via cyclic thermal treatment in selective laser melted Ti6Al4V-5vol%TiB composite

Abstract

Titanium matrix composites are considered promising materials in engineering fields that demand high strength and wear-resistant materials in combination with lightweight. Selective laser melting (SLM) recently has been used to fabricate TMCs, but their plasticity is generally poor as reported. Bimodal microstructures consisting of globular α are known to possess excellent comprehensive mechanical properties in Ti-based alloys. Heat treatment for the transformation of lamellar to globular microstructures is usually preceded by thermomechanical processing. This work reports a cyclic thermal treatment (CTT) for constructing a bimodal microstructure in SLM-processed Ti6Al4V-5vol%TiB composite without the need for predeformation. The CTT process involves repeated thermal cycling between 875 and 975 °C to induce notable changes in the fractions of α and β. Additionally, a β-region annealing process (1050 °C, 1 h) was designed and implemented before CTT to accelerate the globularization process. After a CTT of 10 cycles, a typical bimodal microstructure with highly spherical αp was successfully prepared. Meanwhile, the segregated TiB whisker clusters were significantly replaced by uniformly distributed individual TiB whiskers. Boundary splitting and termination migration are two competing mechanisms in the globularization process. TiB whiskers promote the globularization process through repeated heterogeneous nucleation in the cooling part. Compressive testing suggested that the sample with a bimodal microstructure (54.5% αp) exhibited improved plasticity (12.3%–27.3%) at the cost of acceptable yield strength reduction (1743 MPa–1392 MPa), which is desirable for structural applications.