The Martensitic Transformation in In-Tl Alloys Revisited

Seminar/Forum

 The Martensitic Transformation in In-Tl Alloys Revisited

Hercus Theatre
Physics South
Spencer Road

Map

Indium-Thallium alloys (In-xat%Tl alloys, for 15.5  x  30.5) exhibit a structural transformation from cubic to tetragonal, on being cooled below a transition temperature, Ms, where 425K  Ms  0K for the above composition range [1]. This transformation has been classified as martensitic and traditionally has been described as proceeding via a double shear such as: (101)[1 ̅01]; (011)[01 ̅1], on the basis of optical microscopy and x-ray observations [2] and measurements of the (c11 - c12)/2 elastic constant [3]. These early results, together with a calculation of the phonon dispersion relations based on a model pseudopotential and the measured elastic constants as input parameters [4], suggested that the transformation was driven by the softening of low-, [0][ ̅0] phonons, which provided the motivation for a measurement of the phonon dispersion relations using neutron, inelastic scattering [5]. One model for this dynamical behaviour was based on topological solitons, as advanced by Barsch and Krumhansl [6], In this talk, I shall give a brief introduction to martensitic transformations and touch on one of the most important applications for some materials exhibiting such a transformation, namely shape-memory behaviour. Specifically concerning In-Tl alloys, I shall outline some of their historical background, for those unfamiliar with the topic. However, the suggested low-, [0][ ̅0] phonon softening has never been observed experimentally, despite phonon measurements to as low as  = 0.02 rlu, which have been made possible with the advent of the cold-neutron, triple-axis spectrometer, SIKA, now available at our Australian Research Reactor, OPAL. An alternative model for the formation of coherent nuclei and growth along conjugate {111} planes was once proposed by Geisler [7]. This model is consistent with some electron diffuse scattering data as well as yielding identical x-ray pole figure results as those for the double-shear mechanism. Appropriate nuclei could be generated by 〈111〉〈112 ̅ 〉 atomic displacements. To test such an idea, we have measured the []T phonon branch for a good quality In-Tl crystal in a recent experiment at SIKA. The initial results have shown that the zone-boundary, []T phonon softens with decreasing temperature, which may provide the dynamical behaviour consistent with the Geisler model for the transformation.

[1] J.T.A. Pollack and H.W. King, J. Mater. Sci. 3, 372 (1968). [2] J.S. Bowles, C.S. Barrett and L. Guttman, Trans. AIME 188, 1478 (1950). [3] D.B. Nvotny and J.F. Smith, Acta Metall. 13, 881 (965). [4] D.J. Gunton and G.A. Saunders, Solid State Commun. 12, 569 (1973). [5] T.R. Finlayson, M. Mostoller, W. Reichardt and H.G. Smith, Solid State Commun. 53, 461 (1985). [6] G.R. Barsch and J.A. Krumhansl, Proc. Int. Conf. on Martensitic Transformations (1992), eds. C.M. Wayman and J. Perkins (Monterey Inst. of Adv. Studies, 1993) p 53. [7] A.M. Geisler, Acta Metall. 1, 260 (1953).

Presenter

  •  Trevor Finlayson
    Trevor Finlayson, School of Physics, University of Melbourne