Synthesis of TiAl-based intermetallics with nanocrystalline structure by mechanical alloying and hot isostatic pressing: Microstructure stability and properties

Two fully dense nanocrystalline TiAl-based compacts, Ti-47.5Al-3Cr and Ti-48Al-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 detected in both powders after 15 hour MA'ing. During HIP'ing, crystallization of the amorphous phases occurred and very fine equiaxed gamma-TiAl grains were formed. The grain size decreased when the temperature of HIP'ing was decreased, and it was 42 nm after HIP'ing at 725 °C. Grain growth occurred during annealing of the HIP'd alloys, which could be described by a single thermally activated process 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-Perch dependence on the grain size.