Effects of liquid quenching and subsequent heating on the structure of Co-Al Alloys

Abstract

The phase composition and structure of Co-Al alloys after liquid-quenching (LQ) and subsequent heating were determined by x-ray diffraction, differential thermal analysis, and differential scanning calorimetry. After LQ, the Co-19.5 at % Al alloy consisted of two phases: CoAl (B2 structure, a=0.2852 nm) and an fcc Co1 − xAlx solid solution (a=0.3573 nm). The phase composition of the LQ alloy and the lattice parameters of the intermetallic phase CoAl and solid solution corresponded to a solidification temperature of 1100°, rather than to the eutectic temperature (1400°C). Heating to 1000°C brought the alloy to the equilibrium state. In the range 25–28 at % Al, the LQ alloys were single-phase and consisted of Co-enriched CoAl (B2). Decomposition of this intermetallic phase during heating in the calorimeter gave rise to an exothermic peak at 680°C and led to precipitation of hcp Co, which converted to an fcc Co1 − xAlx solid solution on heating to 1000°C. The activation energy of the decomposition process, evaluated by the Kissinger method, was Ea=125 ± 10 kJ/mol. The LQ Co-71.4 at % Al alloy consisted of three phases: metastable quasicrystalline phase (similar in structure to Al14Co3Ni3), Co2Al5, and CoAl. Heating brought the alloy to the equilibrium state (Co2Al5), with an exotherm near 690°C. The activation energy of this phase transformation was Ea=195 ± 15 kJ/mol. The LQ alloys containing 76.5 and 82 at % Al were single-phase and consisted of monoclinic Co4Al13 or Co2Al9 with reduced lattice parameters.