Phase equilibria in the T–Al–C (T: Co, Ni, Rh, Ir) and T–Al–B (T: Rh, Ir) systems for the design of E21–Co3AlC based heat resistant alloys

Phase equilibria in Co–Al–C ternary and Co–Ni–Al–C quaternary systems were investigated to establish the basis for designing new class of α(Co) based heat resistant alloys strengthened by the E21 type intermetallic compound Co3AlC. Phase stability of E21 (Co, Ni)3AlC was examined from the viewpoint of magnetic properties such as Curie temperature and saturation magnetization. The possibility of two-phase separation is indicated between E21(E21′) (Co,Ni)3AlC and E21(L12) (Ni,Co)3Al(C) in the Co–Ni–Al–C quaternary system, where we denote E21′ as standing for the ordered crystal structure of (Co,Ni)3AlC0.5 formed by the extra ordering of carbon atoms. Phase diagrams information was determined by means of electron probe microanalysis and microstructural observation for the T–Al–C (T: Co, Ni, Rh, Ir) and T–Al–B (T: Rh, Ir) systems to examine the phase equilibria in each alloy system focusing on the existence of E21 T3AlC and T3AlB. The existence of E21 Ir3AlB (E21′ Ir3AlB0.5) phase has been revealed in the Ir–Al–B system by diffraction analysis of transmission electron microscopy and electron probe microanalysis.