Effect of nitrogen on shape memory effect of a Fe-Mn-based alloy

In this work we have investigated the effect of nitrogen on the shape memory effect (SME) of a Fe-Mn-based shape memory alloy (SMA). Addition of 0.26 wt% nitrogen in solid solution to the alloy system increased the strength of the alloy and suppressed the SME by approximate to 80%. A reduction of hcp phase (hexagonal close packed) formation found to be the dominant factor in suppression of the SME. This was related to: (i) increase of stacking fault energy in the alloy of interest in this work, as predicted by our thermodynamic model; and (ii) the strengthening and pinning effect of nitrogen on Shockley partial dislocations. It was shown by a thermodynamic model that at low concentration of nitrogen increasing the stacking fault energy of a fee (face centred cubic) Fe-Mn-based alloy due to increase of stacking fault energy arising from the bulk of the material is the dominating effect in lowering the probability of hcp phase formation. However, at high concentration of nitrogen the stacking fault energy arising from segregation of alloying elements to stacking fault is large. This suggests the strong occurrence of a strengthening and pinning effect on dislocations which can be a dominating factor in lessening the probability of hcp formation in a fee Fe-Mn-based alloys. (C) 1999 Elsevier Science S.A. All rights reserved.