Thursday 2:00 PM

July 13, 2017

Room: Wrigley

Location: Hyatt Regency Chicago

Micrographs of indium-thallium alloys appear in text books to illustrate the microstructure typical of some martensitic transformations. The In-Tl transformation is from cubic to tetragonal, with c/a typically 1.028. The transformation was suggested to arise from [0][ ̅0] phonon softening, based on reports of the temperature dependence of the (c11 c12)/2 elastic modulus. However, on account of the large attenuation of [110][11 ̅0] ultrasonic waves, this modulus could only be estimated from the measurements of other elastic moduli. Previous measurements of the low-q [0][ ̅0] phonon frequencies, with decreasing temperature, showed an increase in frequency, not a decrease, as expected for phonon softening. In our current research on In-27.5at%Tl, both the neutron Laue technique and cold, triple-axis spectrometry are being used to study this martensitic transformation. The results from these experiments using the KOALA and SIKA instruments on the OPAL Research Reactor at Lucas Heights, NSW, will be presented.

Effects of driving rate on non-isothermal martensitic transition modes and spatiotemporal patterns are studied based on Ginzburg-Landau theory and heat equation. We find that the traditional nucleation-growth paradigm breaks down due to fast self-heating under high driving rate as far as the adiabatic elastic modulus becomes positive. With the increase of driving rate, the mode of phase transition gradually changes from the traditional nucleation-growth of scattered domains to emergence of periodic domain patterns and eventually to stable and homogenous deformation. Such changes is essentially governed by the ratio of external time scale of driving (heat release) and internal time scale of heat conduction. Moreover, the scaling law of the spatiotemporal patterns and the experiment verification using Digital Image Correlation method on nano-gained polycrystalline NiTi thin strips are presented.

Molecular dynamics simulations on bent multi-twinned α-Fe nanowires, with the orientation of x-[100], y-[011], z-[0-10], show a novel interface driven pseudoelasticity upon loading/unloading cycles. Further studies show that the underlying mechanisms are orientation dependent. When the wire is bent in x-y plane, the pseudoelasticity is mediated by the accumulation and disappearance of a/6<111> partial dislocations in the conventional {112}/<111> twin boundaries. This kind of pseudo-elasticity shows almost no size independence, and can be extend to bulk materials. In contrast, when the wire is bent in x-z plane, the pseudo-elasticity is stemmed from formation of non-conventional {110} interfaces, which provide a large part of the driving force for shape recovery upon unloading. Such bending pseudo-elasticity is related to twin boundary density, and can be extended to a wide range of wire diameters by seeding enough conventional twin boundaries in the sample.

Space group consideration of martensitic transformation in ferrous alloy is discussed. Crystal structure of austenite is face centered cubic, whose space group is Fm3m (225) and that of martensite is BCT structure, whose space group is P42/n (86). Number of symmetry operations of Fm3m is 192 and that of P42/n is 8. Therefore, 24 variants appear. Group-subgroup consideration is applied; that is, (1) removing translational symmetry and two-fold axis at the first step, (2) removing three-fold axis, and (3) removing two-fold axis again.