Effect of pre-deformation at room temperature on shape memory properties of stainless type Fe–15Mn–5Si–9Cr–5Ni–(0.5–1.5)NbC alloys

Substantial improvement of shape memory properties of the stainless type Fe–15Mn–5Si–9Cr–5Ni–(0.5–1.5)NbC (mass%) alloys has been achieved by pre-deformation at room temperature. It shows that a suitable pre-extension can result in excellent shape memory effect (SME). A 90% shape recovery of an initial 4% strain and 200 MPa shape recovery stress are attained when the Fe–15Mn–5Si–9Cr–5Ni–0.53Nb–0.06C alloy is 12% pre-extended at room temperature, being followed by aging at 1070 K for 10 min. If the pre-extension is more than 12%, the shape recovery is reduced with an increasing amount of the pre- extension. It is found by X-ray diffraction and transmission electron microscopy (TEM) that such a decrease in shape recovery is due to the formation of α′ phase (body-centered cubic). The mechanism to obtain a nearly perfect shape memory recovery at 12% pre-extension is studied by atomic force microscopy and TEM. It is concluded that the uniform distribution of stacking faults associated with fine NbC precipitates produces the single variant martensite plates, which is the key factor in obtaining an excellent SME.