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Effect of thermal history on the short-range order in Fe-Cr alloys

Effect of a thermal history of Fe(100-x)Cr(x) (x < 20) samples on a Cr atoms distribution within the first (1NN) and the second (2NN) neighbor-shells was studied with the Mössbauer spectroscopy. The distribution was expressed in terms of the Cowley-Warren short-range order (SRO) parameters: <alpha1> for 1NN, <alpha2> for 2NN and <alpha12> for 1NN-2NN. It was shown to be characteristic of the thermal treatment and of the neigbor shell. For quenched samples, <alpha1> is positive for all x-values, while <alpha2> shows inversion at x about 8 from positive to weakly negative. Similar character has <alpha12>, but the degree of ordering in 1NN-2NN is lower than that in 2NN. Isochronally annealed samples exhibit similar behvior for x > ~8, but significantly different for x < ~8 where the inversion both in <a1> and <a2> occurs at x about 3, yet in the opposite direction. The <alpha12> follows the trend predicted by Erhart et. al. [PRB 77, 134206 (2008)]. A clear-cut inversion induced by an isothermal annealing at 415 C was found for the Fe85Cr15 sample.