Atomic diffusion and microstructural evolution at the Nb/TiAl heterogenous interfaces during annealing at elevated temperatures

Abstract

The relevant phase transformations induced by atomic diffusion at the vicinity of Nb/TiAl heterogenous interface were systematically investigated by establishing a novel system of TiAl-Nb couple via metallurgical bonding of Nb block and γ-TiAl powders by hot pressing sintering and post-annealing treatments. As a result, the resultant diffusion layer, acting as adhesive region linking the Nb and TiAl substrates, thickened with the increase of annealing temperature and the prolongation of holding time, which could be well described by the developed power function. A series of intermediate phases initially nucleated at the Nb/TiAl interface and grew toward both substrates, which was the result of the exchange, partitioning and recombination of solute atoms in the light of vacancy-mediated substitutional diffusion pattern. The phase transformation path on the Nb side was β→η→η + σ→σ→β0→β0 + γn, while that on the TiAl side was γ→γss→γss+ α→(γ + α)+ (α + β0)→α + β0→β0 + γn. Then the disorder-order transition of α→α2 and β0→B2 occurred during cooling. The progress of phase transformation slowed down as the diffusion proceeded, which was attributed to the decrease in atomic substitution efficiency due to the reduction of atomic concentration gradient and the extension of migration distance. In such a case, the banded σba split into spike σsp along the direction of maximum concentration gradient, and then shrunk to isolated σis until completely dissolved in β0 regime, principally dominated by Ti occupying Nb sublattices. The lenticular γn existed mainly in the form of twins, resulting from the lattice reconstruction in accordance with the orientation relationship of 111γ//110β0 and 1̅10γ//11̅1β0 in the Al partitioning region during the spontaneous fluctuation of atomic concentration of β0 regime. These findings filled some fundamental gaps with respect to TiAl-Nb diffusion reaction system, and afforded profound insights into the development of high-performance TiAl/Nb composites with tailorable microstructure.