Phase transformations in the B2 phase of Co-rich Co–Al binary alloys

Phase transformations in the β (B2) phase of Co–21 and –23 at.% Al alloys were examined using transmission electron microscopy, energy dispersive X-ray spectroscopy and differential scanning calorimetry. The microstructures obtained from as-quenched specimens were found to be strongly affected by the quenching condition. While relatively thick sheet-specimens with a lower quenching rate showed bainitic plate precipitates with a fcc structure, a martensite-like structure was observed by optical microscopy in relatively thin specimens with a higher quenching rate. Regardless of the quenching condition, a spinodal-like microstructure composed of A2 and B2 phases was also detected and the A2 phase changed to a metastable hcp phase during further aging.

Research highlights▶ Bainitic transformation and a martensite-like structure from B2-CoAl were observed depending on quenching rate. ▶ The phase separation into the metastable A2 + B2 structure was found in the as-quenched B2-CoAl. ▶ The two-phase structure of A2 and B2 was found to show some coercive force after aging under a magnetic field.