Microstructural Evolution and Mechanical Properties of a Multicomponent Nb-16Si-22Ti-2Al-2Hf-2Cr Alloy Prepared by Reactive Hot Press Sintering

In this paper, the effect of reactive hot press sintering (RHPS) parameters, such as milling time, hot pressing temperature, and heat treatment, on phase constitution, microstructure, and room temperature mechanical properties of a multicomponent Nb-16Si-22Ti-2Hf-2Al-2Cr alloy prepared from ball milled powder mixture is investigated. Evaluation of the microstructure revealed that all as-sintered and heat-treated samples mainly consisted of particles of Nb and Ti solid solutions (Nb_SS and Ti_SS), as well as a niobium silicide β-Nb₅Si₃ matrix, with a small quantity of Hf solid solution particles (Hf_SS). By sintering at 1773 K and 1873 K (1500 °C and 1600 °C), most of the Nb_SS phase in the samples prepared from 5 and 10 ball milled powder mixtures showed a narrow strip morphology. Upon increasing the milling time to 20 hours, the morphology changed to a near-equiaxed shape, which became finer with increased milling time. In contrast, the Ti_SS phase in all as-sintered samples more or less had a near-equiaxed shape. Spheroidizing tendency took place in both Nb_SS and Ti_SS phases during an annealing heat treatment at 1773 K (1500 °C) for 50 hours. Interestingly, when the milling time was extended from 5 to 20 hours, the volume fractions of the β-Nb₅Si₃ and Ti_SS phases increased, whereas that of the Nb_SS phase decreased. This resulted in the reduction of the fracture toughness K _Q and an enhancement of the Vickers hardness Hv of the bulk as-sintered and heat-treated samples. A fractography analysis was also carried out to elucidate the fracture behavior of phases, with particular emphasis on the interaction between phases and cracks.