Examination of microstructures and microanalysis of an Fe-9% Ni alloy

Abstract

Phase transformations in a continuously cooled Fe-9.14 mass%Ni alloy have been studied. Samples furnace cooled at 0.13 K·s-1 gave a thermal arrest of 555±5°C, while dilatometry indicated that this corresponded to 25±5% with initial transformation occurring at 575±5°C. Optical examination revealed a typical massive ferrite microstructure, while examination of pre-polished specimens showed this to consist of grain boundary ferrite and Widmanstatten ferrite within the grains. Transmission electron microscopy confirmed the presence of both approximately equi-axed grains and Widmanstätten ferrite within the prior austenite grains. Some areas of martensite/austenite (MA) constituent were observed indicating that partition of carbon had occurred during transformation. Quantitative analysis of the dilatation curve showed it to consist of two portions. One between 575±5°C and 558±3°C corresponding to the formation of grain boundary ferrite while the portion between 558 to 500°C was thought to correspond to Widmanstätten ferrite formation. In the latter temperature interval -ln (1-y) varied linearly with temperature, where y=fraction transformed. Microanalysis using a FEG-STEM system with a windowless LINK X-ray detector was carried out. This showed that the Ni content across a ferrite grain was constant at 8.8±0.2 mass%Ni with statistically significant values at grain boundaries e.g. 11.05±0.18 mass%Ni and 12.98±0.43 mass%Ni. The results are discussed in terms of paraequilibrium. A short description of microstuctures and transformation temperatures obtained on water quenching & oil quenching (bainitic ferrite) and ice brine quenching, (lath/massive martensite) is also given in the paper.