Stability of a nanocrystalline structure in TiAl-based alloys

Two fully dense nanocrystalline TiAl-based compacts, i.e. Ti-47Al-3Cr and Ti-48Al-2Nb-2Cr, were synthesized by hot isostatic pressing (HIP'ing) from mechanically alloyed powders. Microstructure evolution and phase transformations during mechanical alloying, HIP'ing, and heating were studied with the use of TEM, SEM, XRD and DTA. Formation of amorphous phases was detected in both powders after 15-hour mechanical alloying. During HIP'ing, crystallization of the amorphous phases occurred and a very fine equiaxed grain structure was 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 in both alloys. An analysis showed that the grain growth in the TiAl-based nanocrystalline alloys can be satisfactorily described by a single thermally activated process following equation Dⁿ-D₀ⁿ = K₀·exp(-Q/RT), with the parameters n = 4, Q = 335 kJ/mol and K₀ = (2-4)·10²² nm⁴/h. The most probable reason for the high value of n is the presence of impurities which retard the grain boundary mobility by pinning and dragging effects.