Theory of mechanical relaxation due to changes in short-range order in alloys produced by stress (Zener relaxation)

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Abstract
A general expression for the relaxation strength Δ M for Zener relaxation is obtained in terms of the rates of change with strain of the short-range order parameters and of the assumed pair-wise interaction energies between atoms. A Bethe-Peierls description of the degree of order yielded values of Δ M in agreement with experiment (Le Claire and Lomer 1954) but is unable to explain more recent observations in a AgZn and LiMg alloys of (1) a marked anisotropy of Δ M (Seraphim and Nowick 1961) and (2) a positive critical temperature, increasing with solute concentration, in the Curie-Weiss type temperature dependence of Δ M (Li and Nowick 1961). Both these defects are fully remedied when the more complete Cowley (1950, 1960) equations are used to describe the degree of order. First and second nearest-neighbour interaction energies only need to be taken into account and order parameters out to eighth neighbours. Good agreement with experimental measurements on a AgZn alloys is obtained with a choice of parameters that find support from other evidence.