Absence of atomic ordering in Fe-31 at.% Ni alloy

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Abstract
Atomic ordering in bulk austenitic single‐crystal samples of composition Fe‐31 at.% Ni was investigated using x‐ray and neutron diffraction techniques. Samples were annealed between 450–550 °C for time periods between 1.5 h and 55 days. No superlattice peak was observed in any case providing no evidence of long‐range order in these samples. X‐ray studies on martensitic samples of the same composition revealed a tetragonality of about 0.4% which was increased insignificantly by the ’’ordering’’ heat treatment given to the austenite prior to the martensitic transformation. If martensite tetragonality is linearly related to the difference in the first and second short‐range order parameters of the austenite (as seems likely in this alloy) this observation supports and extends the results of the studies on the austenitic single crystals.