Ni-Sn interaction in temper embrittled steel detected by Mössbauer spectroscopy

Abstract

PREREQUISITES for reversible temper embrittlement of steel¹ are the presence of a trace impurity² (Sb, Sn, As or P) and a major alloying element^3,4 (Ni, Cr or Mn). Auger electron spectroscopy (AES) demonstrated that in the embrittled state these elements are segregated to within the first few monolayers of the high angle boundaries^5,6. The segregation is reversible and can be removed by heating at temperatures above the embrittlement range, whilst still within the α field (that is > 600 °C). It has been postulated⁷ that the segregation behaviour can be explained using equilibrium thermodynamics. It is proposed that the trace impurities interact more strongly with the major alloying elements than the Fe matrix and hence the chemical potential of the system is reduced by segregation to the high angle boundaries with the formation of bi-dimensional complexes (such as Ni₃Sn₂) Mn₃P and Cr₃P). No experimental evidence is available for the existence of such complexes. AES gives no information regarding the chemical bonding of the segregants but it has been shown⁸, using X-ray photoelectron spectroscopy (XPS), that the segregated Mn on embrittled fracture surfaces of a 2% Mn steel is in a different chemical state to that on unembrittled surfaces. We report here preliminary results from a specially prepared 3% Ni steel, doped with ¹¹⁹Sn, which were obtained using Mössbauer spectroscopy to monitor changes in the nearest-neighbour atoms of the Sn atoms as a function of heat treatment in the temper embrittlement temperature range.