Elastic and Anelastic Behavior during the Martensitic Transformation in Indium-Thallium Alloys

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Abstract
Measurement of internal friction at strain amplitudes −5 in In‐rich In–Tl alloys has shown an amplitude‐dependent peak associated with the fcc→fct martensitic transformation. The peak is thermally activated when the transformation occurs near the melting point Tm, but not at lower temperatures. It is suggested that the peaks originate from the stress‐assisted motion of twin boundaries that are known to spontaneously coalesce immediately below the transformation temperature. The activation energy associated with the transformation peak near Tm is 10–13 kcal/mole higher than for the diffusion of Tl in In. A mechanism is proposed for this relaxation, invoking an interaction between twin boundaries and solute, whereby reordering of solute is effected in differently orientated tetragonal regions by twin‐boundary motion. Shear modulus measurements indicated abrupt changes at the transformation temperature and positive temperature coefficients of modulus in the high‐temperature fcc phase, implying structural instability and an ``entropy‐elastic'' state. At 0.9 Tm, about 60% of the elasticity in a 19.5% Tl–In alloy arises from a change in vibrational entropy with extension. The results are consistent with the known crystallography of the transformation.