Synthesis of TiAl-Based Intermetallics with Nanocrystalline Structure By Mechanical LAloying and Hot Isostatic Pressing: Microstructure Stability and properties

摘要

Two fully dense nanocrystalline TiAl-based compacts, Ti-47.5 Al-3Cr and Ti-48 Al-2Nb-2Cr, were synthesized by hot isostatic pressing(HIP'ing) from mechanically alloyed(MA'd) powders. Microstructure evolution and phase transformations after MA'ing, HIP'ing, and following annealing were studied. Formation of amorphous phases was detercted in both powders after 15 hour MA'ing. During HIP'ing, crystallization of the amorphous phases occurred and very fine equiazed gamma-TiAl grains were formed. The rain size decreased when the temperature of HIP'ing was decreased, and it was 42 nm after HIP'ing at 725 deg C. Grain growth occurred during annealing of the HIP'd alloys, which could be descibed by a single thermally activated porcess limited by a permanent pinning force. The activation energy of the grain growth was close to the activation energy for lattice diffusion of Ti in TiAl. Microhardness decreased with the annealing time and followed the Hall-Petch dependence on the grain size.