Quantitative high spatial resolution microchemical analysis of Fe-Co-Ni transformational-toughened steels

Abstract

Superior strength-toughness combinations can be obtained in Aermet 100 by an austenite→martensite phase transformation of small intralath austenite precipitates. The enhanced toughness in these alloys has been attributed to the presence of “metastable” austenite precipitates, the stability of which is intimately related to the size and composition. The austenite precipitates require high stability to resist transformation under all stress conditions except when in the triaxial stress field of a crack tip. The two primary factors determining austenite stability are size and composition. Thus the microstructureproperty correlations and further alloy development require quantitative microchemical analysis of the 50-150Å austenite precipitates.Of the two obvious analytical electron microscopy techniques, energy dispersive spectrometry (EDS) is superior for quantitative results. Several artifacts must be considered when performing quantitative EDS, for example; the continuum background, absorption/fluorescence effects, k-factor measurement and peak overlap. The continuum background fitting is elementary due to the high peak/background ratio and extensive research in this area.