Positron annihilation and transmission electron microscopy study of the evolution of microstructure in cold-rolled and nitrided FeNiTi foils

Abstract

Positron beam analysis (PBA) and transmission electron microscopy (TEM) were applied to study structural transformations in cold-rolled Fe0.94Ni0.04Ti0.02 foils, which were subjected to different thermal treatments in an atmosphere of a gas mixture of NH3 + H2 (nitriding). Positrons proved to be sensitive probes for the microstructure evolution and formation of nitride precipitates. The nitriding of the samples in the α-region (αN) of the Lehrer diagram for the Fe-N system produced a large decrease of the central part of the Doppler broadened annihilation γ-peak (S-parameter) and an increase of the contribution to the wings of the peak (W-parameter). The effect, ascribed to replacing of vacancy type positron traps by nitride-related traps, was much more pronounced for the α-nitrided samples than for samples annealed in vacuum at the same temperature. A reduction of the αN samples by annealing in H2 atmosphere brings the S-parameter back to a higher value. Further nitriding of αN samples in the γ'-region (αN + γ'N) of the Lehrer diagram increases S and lowers the W-parameter compared with the αN samples. The changes in S- and W-parameters are interpreted on the basis of the evolution of microstructure of the films during the processing.