TEM observation of the internal structures in NbC containing Fe-Mn-Si-based shape memory alloys subjected to pre-deformation above room temperature

Shape recoveries comparable to those of the trained conventional alloys are obtained in Fe-Mn-Si-based alloys containing Nb and C by pre-rolling at 870 K and subsequent short ageing at 1070 K to produce NbC precipitates. In addition, when the amount of thickness reduction by pre-rolling is between 14 and 30%, these newly developed alloys attain recovery stresses high enough to make them useful for practical applications such as pipe joints. In this work, the microstructures resulting from the pre-rolling treatment at 870K and subsequent ageing are investigated by means of transmission electron microscopy (TEM) in Fe-28Mn-6Si-5Cr-0.53Nb-0.06C and Fe-15Mn-5Si-9Cr-5Ni-0.53Nb-0.06C (mass%) alloys. Observations of the as-rolled microstructure indicate that the pre-rolling treatment produces a high density of stacking faults in the fcc matrix. A fine distribution of semi-coherent, nanometric scale NbC precipitates is obtained after ageing the pre-rolled samples. The amount of pre-rolling affects the size and distribution of precipitates and most of the precipitates are seen to be in close relation with stacking faults in the aged condition. The stress-induced hcp martensite is produced as lamella structure, probably due to the abundance of sites for its nucleation i.e., stacking faults and precipitates, and the width of martensite plates is extremely small. These thin martensite plates are highly recoverable, which is the indispensable condition for obtaining a large shape memory effect. (C) 2003 Elsevier B.V. All rights reserved.