High-temperature "Inverse" Hall-Petch relationship and fracture behavior of TA15 alloy

The Hall-Petch relationship is important for material design at room temperature. However, it is not well studied at high temperatures. In this work, the influence of different microstructures on the high-temperature mechanical behavior and corresponding softening mechanisms of hightemperature titanium alloy TA15 (Ti-6.5Al-2Zr-1Mo-1 V, wt.%) were studied. Specimens with duplex, Widmansta<spacing diaeresis>tten, and coarse Widmansta<spacing diaeresis>tten microstructures were obtained through powder metallurgy. High-temperature tensile tests were carried out between 500 and 650 degrees C. It was found that the fracture mode of the Widmansta<spacing diaeresis>tten microstructure was transgranular at 500 degrees C and 550 degrees C, but intergranular at 600 degrees C and 650 degrees C. EBSD analysis revealed that the hightemperature deformation was facilitated by several mechanisms including GB softening, GB migration, grains rotation, and activation of multiple slip systems. High temperature nanoindentation indicated sofenting of individual grains with increasing temperature. In-situ tensile testing under SEM revealed deformation was primarily in grain interior at room temperature, and GBs played a significant role at 650degree celsius. The GB behavior at high temperatures is believed responsible for the inverse Hall-Petch relationship. These findings provide a new perspective for improving the high-temperature mechanical properties and microstructure control of titanium and its alloys.