Effects of multiaxial forging on microstructure and high temperature mechanical properties of powder metallurgy Ti-45Al-7Nb-0.3W alloy

Abstract

Uniaxial forging (UAF) and multiaxial forging (MAF) were conducted on a Ti-45Al-7Nb-0.3W (at. %) alloy produced by powder metallurgy (PM) technique, and the deformed microstructures and deformation mechanism were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Results show that the near-gamma microstructure (NG) transformed into a fine and uniform duplex microstructure (DP) after MAF, and the mean grain size of the multiaxial forged alloy was 5 μm. The microstructure of the uniaxial forged alloy was inhomogeneous and anisotropy, while the microstructure of multiaxial forged alloy was very uniform and a higher amount of the β phase was distributed at the grain boundaries. During forging, deformation induced microstructure changes include dislocation movements, mechanical twinning, layer decomposition and dynamic recrystallization (DRX). The DRX occurred repeatedly and more severely during MAF than UAF process. Correspondingly, the multiaxial forged TiAl alloy exhibited excellent high temperature mechanical properties compared to the uniaxial forged alloy. At a temperature of 750 °C, the yield strength, ultimate tensile strength and elongation of the multiaxial forged alloy were 623 MPa, 697 MPa and 4.5%, respectively. It was noteworthy that the multiaxial forged alloy achieved highly plastic deformation and the elongation reached 173% at 900 °C with a strain rate of 1 × 10−3 s−1.