Structural Transformations during the Mechanochemical Synthesis and Heating of Co–Al Alloys

Abstract

X-ray diffraction analysis and differential scanning calorimetry are used to follow the structural changes induced in cobalt by high-energy ball milling and the processes involved in the mechanochemical synthesis (MCS) of Co–Al alloys containing 3, 10, 20, and 50 at % Al. The results demonstrate that short-term (2–3 min) high-energy milling of cobalt, consisting, as a rule, of two phases, hcp and fcc, causes the high-temperature (fcc) phase to disappear. Longer term milling leads to the reverse, hcp-to-fcc phase transformation. This milling-induced transformation is attributable to the increase in the probability of stacking faults, which attains 20%. Similar structural changes occur in the initial stages of MCS at Al contents from 3 to 20 at %. The formation of fcc Co1 – xAlx solid solutions (x < 0.5) is accompanied by an increase in the probability of stacking faults. The presence of Al stabilizes stacking faults in hexagonal cobalt and accelerates the transition to the cubic phase, which dissolves the aluminum. The milling of a mixture containing 20 at % Al leads to the formation of a solid solution and, at intermediate milling times, CoAl. The latter phase disappears at sufficiently long milling times. Mechanical alloying of an equiatomic mixture (Co + 50 at % Al) yields phase-pure CoAl. Heating of the MCS alloys to 720°C stabilizes the Co1 – xAlx solid solution and the intermetallic phase CoAl. Heating to 720°C after medium-term milling leads to the formation of the metastable phase Co3Al (L12 type), which disappears at higher temperatures.