| The specimens have compositions of 70wt% Mn, 28-29wt% Al and three different carbon contents (0.48wt%, 0.92wt%, 1.24wt%). After solutionized at 1100℃ for 3 hr, the specimens were quenched into solicon oil maintained at zoom temperature and then annealed in a salt bath for the various period of time at the predetermined temperatures ranging from 400℃ to 700℃. Their phase transformations and magnetic properties were investigated. The results obtained from the present experiments can be summarized as follows; 1) The specimen with 0.48wt% C showed the best magnetic properties when annealed for 60 to 120 min. at 550℃. It was believed that the transformation of εto τphase was overshadowed by the decomposition of τinto β-Mn and γphase. 2) The specimen with 1.24wt% C which is more than two times of carbon solubility of τphase (0.48wt% C), showed the highest values of (BH)_(max) when compared with the others. The reason for it is believed to be resulted from a particular morphology of carbide precipitates, i.e. a repeated lamellar structures of Mn₃AlC in the τphase. This type of structure seems to inhibits the decomposition process of τphase to β-Mn phase in addition to the negative effect on the domain wall motion in that phase by pinning it. |
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