Effect of heat treatment within alpha/beta dual-phase field on the structure and tensile properties of binary TiMo alloys

Abstract

The present study investigated the effect of heat treatment within the alpha (¡)/beta (¢) dual-phase field on the structure and tensile properties of Ti(1.59.5) mass% Mo alloys. The alloys were prepared using an arc-melting vacuum-pressure type casting system. The cast alloys were heat-treated at 700, 750 and 800°C in vacuum for 30 minutes followed by quenching in ice water. The X-ray diffraction (XRD) results indicated that beta (¢) phase intensities increased while ¡/alpha prime (¡A) intensities decreased with increased heat treatment temperature (HTT) and Mo concentration. The ¢ phase was observed to dominate the 800°C-treated Ti9.5Mo alloy, while the highest alpha double prime (¡AA) phase content was observed in the 800°C-treated Ti7.5Mo alloy. Both optical and scanning electron microscopy indicated that a relatively coarse ¡ platelet was always observed in Ti1.5Mo. A fine, uniformly-distributed acicular microstructure was observed in Ti7.5Mo, while an equi-axed ¢ granular microstructure was clearly seen in Ti9.5Mo. The tensile properties were found sensitive to the HTT and Mo concentration. When heat-treated at 700°C, the yield strength (YS) and ultimate tensile strength (UTS) increased while the elongation generally decreased with Mo concentration. The highest YS and UTS were found in Ti7.5Mo and Ti9.5Mo. When heat-treated at 750°C, the strength of Ti5.5Mo was improved without reducing elongation. With Mo concentration increased to 7.5% or higher, the elongation further increased while the strength maintained a similar level. When treated at 800°C, the YS of Ti3.5Mo, Ti5.5Mo and Ti7.5Mo maintained a lower level than Ti1.5Mo and Ti9.5Mo. A fully satisfactory interpretation for the tensile properties and their relationships to the complicated microstructures might not be a simple task due to several different factors simultaneously involved, yet practically it is interesting to note that selected alloys heat-treated within the dual-phase field demonstrated quite promising overall mechanical properties.