Effect of silicon on atomic distribution and shape memory in Fe-Mn base alloys

Electron spin resonance and transmission electron microscopy are used for studying the ordering-clustering-precipitation phenomenon in Fe-Mn base austenitic solid solutions. The effect of 5% of Si on the short-range atomic order in the Fe alloys containing (mass%) 17Mn, 9Cr and 4Ni was derived from the analysis of change in the temperature dependence of magnetic susceptibility caused by single atoms of d-elements and their clusters. It is shown that Si increases the size of superparamagnetic clusters consisting mainly of Mn atoms. The size of the clusters was estimated to be of about 1.6 nm in the Si-free and 2.1 run in the Si-added alloy. Based on the Si-increased concentration of conduction electrons and previously observed correlation between the electronic structure and short-range atomic order, it is concluded that, due to prevailing Fe-Si atomic bonds, the Mn atoms are pushed out of the solid solution thereby forming the clusters. This conclusion is supported by TEM observation of precipitates after ageing at 873 K for 36 h. The increase in the Mn content up to 20 mass% in the alloy Fe-20Mn-9Cr-6Si and to 31 mass% in the alloy Fe-31Mn-6Si enhances the precipitation of particles having the crystal structure and lattice parameters of beta-Mn and the size of 5-25 nm. TEM data are consistent with ThermoCalc calculations of the phase equilibrium, according to which the addition of 6 mass% of Si to the Fe-Mn system shifts the concentration border of beta-Mn phase stability at 873 K from 55 down to 25% of Mn. Clusters and precipitates with structure of beta-Mn are considered to be responsible for a positive effect of Si on shape memory in Fe-Mn base alloys. (c) 2005 Elsevier B.V. All rights reserved.