Experimental investigation of phase equilibria in the Co-Cr-Re, Cr-Ni-Re and Co-Ni-Re systems

Phase relations in the solidification interval of the Co-Cr-Re, Cr-Ni-Re and Co-Ni-Re systems were studied for the first time by a combination of scanning electron microscopy (SEM), electron probe microanalysis (EPMA), X-ray diffraction (XRD), and differential thermal analysis (DTA). The results are presented as liquidus and solidus projections, melting diagrams, Scheil reaction schemes and isopleths. The phase diagram of the binary Co-Re system was revised. It is evident that remarkable solubility of both Ni and Co occurs in the Cr 2 Re 3 (σ) phase at solidus temperature. For instance, solubility of Co in the σ phase is up to 40.2 at.% at the solidus temperature, while the solubility of Ni in it is ∼20 at.%. No ternary compounds were found in either system. The liquidus projection of the Co-Cr-Re system is characterized by the presence of primary solidification fields of the binary-based σ phase and component-based solid solutions (αCo), (Cr) and (Re). The solidus projection is characterized by the presence of two three-phase regions (αCo) + σ + (Re) and (αCo) + σ + (Cr), which result from U-type four-phase invariant equilibria L U1 + (Re) ⇄ (αCo) + σ and L U2 + σ ⇄ (αCo) + (Cr) taking place at 1529 and 1495°C, respectively. The liquidus projection of the Cr-Ni-Re system is characterized by the primary regions of (Ni), (Cr), (Re) and σ phases, and the solidus projection is characterized by the coexistence of the σ phase with all phases of the system, forming two three-phase regions (Re) + σ + (Ni) and (Cr) + σ + (Ni). These result from four-phase U-type reactions L U1 + (Re) ⇄ (Ni) + σ and L U2 + σ ⇄ (Ni) + (Cr) at 1493 and 1390°C, respectively. The Co-Ni-Re ternary system is very simple; there are only two single-phase regions of (αCo,γNi) and (Re) and one two-phase region (αCo,γNi) + (Re) at solidus temperatures. In addition, the effect of Re addition on the microstructure and mechanical properties of the CoCrNi alloy was investigated. It was shown that the addition of Re to CoCrNi increases the yield strength.