Microstructural Evolution and Mechanical Properties of a Three-Phase Alloy in the Cr–Mo–Nb System

A new three-phase alloy of 50Cr-30Mo-20Nb (at%) was studied based on the Cr-Mo-Nb ternary phase diagram, and was composed of a Cr-rich BCC1 phase, a Mo-rich BCC2 phase and a NbCr2 Laves phase after heat treatment at around 1523K or lower. A supersaturated BCC single-phase solid solution alloy obtained by homogenization at 1973K for 1 h underwent microstructural evolution during heat treatment at 1473 K. Intragranular precipitation of the Cr-rich BCC1 phase occurred, which led to the formation of an alternating BCC1/BCC2 two-phase microstructure through a discontinuous precipitation process, followed by precipitation of the Laves phase at the BCC1/BCC2 interphase boundaries. At a higher temperature of 1523 K, a similar microstructure was observed, with increased BCC decomposition and Laves precipitation rates, while the alloy consisted of BCC and Laves phases at 1773 K. The mechanical properties of alloys heat treated at 1473K for various periods after the solid-solution treatment were also investigated. A maximum fracture strength of 1493 MPa and a minimum hardness of 773 +/- 7 HV were obtained for an alloy aged for 24 h, where the BCC1/BCC2 two-phase microstructure dominated. The Vickers hardness of an alloy aged for 72 h, which had a fine-grained microstructure that included the Laves phase, was 839 +/- 8 HV under a load of 0.5 kgf, and no obvious microcracks were observed.