High temperature softening mechanism of powder metallurgy TA15 alloy

Abstract

The understanding of softening mechanisms for high-temperature titanium alloys at service temperature is essential to guarantee their service stability and safety. High-temperature softening is mainly manifested as grain boundary softening for metallic materials. To clarify the softening mechanism and failure mode of high-temperature titanium alloy TA15 (Ti-6.5Al–2Zr–1Mo–1V, wt.%) at temperature around service temperatures, coarse prior β grains were obtained by spark plasma sintering at 1300 °C, other than 1000 °C which is the traditional power metallurgy (P/M) consolidation temperature for TA15. Meanwhile, the effect of grain boundary softening was amplified. The high-temperature tensile test was carried out at 500–650 °C at intervals of 50 °C. The experimental results showed the maximum prior β grain size of P/M TA15 alloy is 2.9 μm, and its tensile strength decreases from 579 to 389 MPa with the increase of tensile temperature from 500 °C to 650 °C. Combined with calculation according to the Read-Shockley formula and analysis by macro-fracture, which show that the grain boundary strength is higher than the grain strength at 500 and 550 °C, and there are obvious grain and phase boundary torsion. The fracture mode demonstrated as a trans-granular fracture. The specimens soften rapidly, grain boundary strength decreases to less than grain strength, which led to fracture mode changed to inter-granular fracture at tensile temperatures up to 600 °C. If the crack extends to the critical length, the stress at the crack tip reaches easily the fracture strength of P/M TA15 alloy, and crack propagates rapidly at the temperature above 600 °C. Moreover, which corresponds to the elongation of true stress-strain decreasing with the increase of test temperature. OM and EBSD result demonstrated that the equi-cohesive temperature of TA15 alloy between 550 °C and 600 °C. Our findings provide a promising route to improve service temperature of high temperature TA15 alloy by raising the equi-cohesive temperature area in zone Ⅱ to a higher temperature.