PHASE DECOMPOSITION OF THE y PHASE IN A Mn-30 at. Cu ALLOY DURING AGING

摘要

The phase decomposition process of γ phase in a Mn-30 at. Cu alloy, when aged at 723 K from 2 to 50 h, is investigated with electrical resistivity and magnetic susceptibility measurement. In con- junction with the antiferro-magnetic transition of the Mn-rich regions during cooling to room temperature from the aging temperature, the temperature coefficient of electrical resistivity shows a continuous increase in a certain temperature range. The temperature where the coefficient has the maximum increasing rate is defined as the T_N temperature of the Mn-rich regions. It was found that the T_N temperature was 20-30 K higher than the concomitant f.c.c.-f.c.t. transformation temperature T_t determined with the minima of Young's modulus in the aged samples. The increment of temperature coefficient of electrical resistivity involved in the magnetic transition is used to estimate the changes of volume fraction for Mn-rich regions vs aging time. At the same time, the paramagnetic feature above the spin-freezing transition temperature for quenched Cu-rich alloys is summarized, and the Mn concentration in Cu-rich regions of aged samples is calculated. It should be noted that Mn and Cu-rich regions had already formed in the 2 h-aged Mn-30 at. Cu sample, and longer aging further enriched Mn or Cu, while the volume fraction of Mn-rich regions decreased slightly with aging time. Electrical resistivity measurement sensitive to Mn-rich regions and the magnetic susceptibility measurement for Cu-rich regions have shown the compositional heterogen- eity in decomposed phases. TEM observation