Mechanisms behind the superplastic behavior of as-extruded TiBw/Ti6Al4V composites with a network architecture

The as-extruded TiB whisker reinforced Ti6Al4V (TiBw/Ti6Al4V) composites with tailored architecture were fabricated by powder metallurgy followed by hot extrusion. High temperature tensile tests were conducted to study the superplastic deformation behavior at 900 °C, 925 °C, 950 °C and 975 °C with strain rates of 0.000316/s, 0.001/s and 0.00316/s, respectively. Microstructural observation showed that the fraction of β transformed microstructure increased with increasing strain, which indicated that the phase transformation was drastically affected by hot deformation. Voids next to the TiBw reinforcements were observed due to the incoordination deformation at small strain, but this kind of voids was refilled at large strain, which is attributed to the combined processes of recrystallization and coordination deformation. Furthermore, new voids were formed at the α/β interface at large strain, considered to be caused by the incoordination of phase boundary sliding. EBSD results implied that the texture intensity decreased during the superplastic tensile process due to recrystallization and grain rotation. TEM analysis revealed that the grains next to the TiBw reinforcements presented a higher dislocation density forming new boundaries by recrystallization. This effect led to grain refinement which was beneficial to grain boundary sliding, which resulted in the composites' high elongation.