Influence of TiBw volume fraction on microstructure and high-temperature properties of in situ TiBw/Ti6Al4V composites with TiBw columnar reinforced structure fabricated by pre-sintering and canned extrusion

摘要

Tailoring TiBw volume fraction was utilized in TiBw/Ti6Al4V composites to pursue performance optimization for potential high-temperature applications. Detailed investigations focused on the influence of TiBw volume fraction on microstructure and its correlations with mechanical properties mainly at high temperatures ranging from 500 degrees C to 700 degrees C. Highly aligned TiB whiskers along extrusion direction caused the formation of TiBw columnar reinforced structure, which was composed of ductile TiBw-poor interior regions and hardened TiBw-rich boundaries. Refined prior beta phases arising from dynamic recrystallization led to the size reduction in alpha colonies. This tendency commonly accompanied with the formation of equiaxed alpha phase was evidently enhanced by the restriction of increasing TiB whiskers at the boundaries. Much better strengthening of TiB whiskers was achieved on the premise of good interfacial bonding with Ti matrix at high temperatures, and the strengthening depended linearly on their volume fractions, with strength increments of about 44.3 MPa/vol.% at 500 degrees C, and 27.5 MPa/vol.% at 600 degrees C, yet merely 6.3 MPa/vol.% at 700 degrees C. Attributed to the softening of Ti matrix and the crack retardation of ductile TiBw-poor regions by raising temperature, enhanced ductility was negatively correlated with TiBw volume fraction below 600 degrees C, but positively at 700 degrees C under the possible grain boundary sliding inspired by ample equiaxed alpha phase. (C) 2017 The Society of Powder Technology Japan. Published by Elsevier B.V.