High temperature softening mechanism of powder metallurgy TA15 alloy

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 hightemperature titanium alloy TA15 (Ti-6.5Al-2Zr-1Mo-1V, wt.%) at temperature around service temperatures, coarse prior & beta; grains were obtained by spark plasma sintering at 1300 & DEG;C, other than 1000 & DEG;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 & DEG;C at intervals of 50 & DEG;C. The experimental results showed the maximum prior & beta; grain size of P/M TA15 alloy is 2.9 & mu;m, and its tensile strength decreases from 579 to 389 MPa with the increase of tensile temperature from 500 & DEG;C to 650 & DEG;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 & DEG;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 & DEG;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 & DEG;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 & DEG;C and 600 & DEG;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.