Effect of Mn Content on Microstructure and Superelastic Property of Fe-Mn-Al-Ni Alloy Fabricated by Direct Energy Deposition

This study investigated the effects of the Mn content on the microstructure and superelastic properties of the Fe-Mn-Al-Ni alloys fabricated using direct energy deposition. As the Mn content increased from 36 at.% (36Mn) to 42 at.% (42Mn) with fixed Al and Ni contents, the fraction of the FCC-γ phase significantly increased in the as-built state. After six hours of homogenization at 1,200ºC, both the 36Mn and 42Mn alloys exhibited the BCC-α phase. In the results of cyclic compression test, both the 36Mn and 42Mn alloys exhibited superelastic properties. 42Mn, however, showed a superelastic displacement of approximately 3.6% at a maximum compressive displacement of 5%, which was lower than 4.0% of the 36Mn alloy. This appears to be due to the stabilization of the FCT-γ' phase caused by the high Mn content and the increase in permanent deformation caused by low critical slip stress. The results of this study indicate that alloy design that can promote reverse transformation into BCC-α upon the removal of stress by lowering the stability of the FCT-γ' phase while maintaining high critical slip stress is required to improve or maintain superelastic properties.