Effect of Hot Rolling on the Microstructure and Mechanical Properties of a Ti-15Mo/TiB Metal-Matrix Composite

Abstract

A Ti-15Mo/TiB metal matrix composite was produced by the spark plasma sintering process at 1400 °C using a Ti-14.25 wt.% Mo-5 wt.% TiB2 powder mixture. The microstructure and mechanical properties of the composite were studied after non-isothermal rolling of specimens heated to 1000 °C to a thickness strain of ~0.7. Transmission and scanning electron microscopy, as well as X-ray analysis were used for microstructure examination; mechanical properties were evaluated using tensile testing and microhardness measurement. In the initial condition, the Ti-15Mo/TiB composite consisted of 8.5 vol.% of TiB needle-like particles heterogeneously distributed within the β matrix. A small volume of fractions of the α″ and ω phases was also found in the microstructure. Microstructure evolution of the composite during hot rolling was associated with dynamic recrystallization of the bcc titanium matrix and shortening of the TiB whiskers by a factor of ~2. The Ti-15Mo/TiB composite after hot rolling showed considerable improvement in ductility without substantial loss of strength and hardness. The hot rolled specimen was not fractured during the compression test even after 45% thickness reduction, while in the initial condition, the compression ductility was 22%. The yield strength for both conditions was quite similar (~1350 MPa). The hot rolled composite also showed some improvement in ductility to ~12% elongation at elevated temperature (500 °C) compared to the initial condition, the tensile elongation of which did not exceed 2%. The observed difference in the mechanical behavior was associated with the presence of the metastable α″ and isothermal ω phases in the initial condition and the more stable α phase in the hot rolled condition.