Effect of Ambient Aging on Heat-Treated Mechanically Alloyed Mn-Al-C Powders

The effect of ambient aging upon the magnetic and structural properties of heat-treated mechanically alloyed Mn_0.53Al_0.47-xC_x powders was systematically investigated. Their properties were examined after 6, 15, 21 and 52 weeks of aging. XRD data revealed no detectable structural changes in the carbon-doped alloys. With x = 0.01 and 0.015, the magnetic properties were stable up to 52 weeks with only a slight variation of ~7%-10%. Conversely, with x = 0, magnetic property variations greater than 30% were demonstrated. This confirms that carbon doping of the MnAl alloys not only improves the stability of the τ phase during heat-treatment but also over a prolonged period of time. Curie temperature (T_C) of the alloys was determined using the first derivative of the differential scanning calorimetry (DSC) data. Only 1% decrease in T_C was observed for the aged heat-treated alloy with x =0.01 compared to that for the pre-aged alloy.