Non-equilibrium processes in martensitic phase transformations by X-ray photon correlation spectroscopy
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Non-equilibrium processes in martensitic phase transformations by X-ray photon correlation spectroscopy Michael Widera1 and Uwe Klemradt1, 1 nd
2 Institute of Physics and JARA-FIT, RWTH Aachen University, D-52056 Aachen, Germany.
ABSTRACT Through undulator sources at 3rd generation synchrotrons, highly coherent X-rays with sufficient flux are nowadays routinely available, which allow carrying over photon correlation spectroscopy (PCS) from visible light to the X-ray regime. X-ray photon correlation spectroscopy (XPCS) is based on the auto-correlation of X-ray speckle patterns during the temporal evolution of a material and provides access both to equilibrium and non-equilibrium properties of materials at the Angstrom scale. Owing to technical limitations (detector readout), XPCS has typically been used for the detection of slow dynamics on the scale of seconds. The variety of scattering geometries employed in conventional X-ray analysis can be combined with XPCS. In this work, we report on bulk diffraction (XRD) used to study the prototypical shape memory alloy Ni63Al37 undergoing a structural, diffusionless (martensitic) transformation. Twotime correlation functions reveal non-equilibrium dynamics superimposed with microstructural avalanches. INTRODUCTION The shape memory effect is based on a diffusionless, structural (martensitic) phase transition. Although shape memory alloys are key functional materials with considerable use in technical applications, the underlying transformation mechanism is still little understood. Being of first order, the martensitic transition (MT) proceeds by nucleation and growth. However, typically the transition is only weakly of first order, e.g. with a small latent heat and accompanied by a hysteresis width on the order of about 10 K. Owing to the reduced symmetry of the low temperature phase, typically several variants (domains) of the martensitic phase are nucleated on cooling. Although these possess the same structure, the variants are nucleated with different orientations (unless prevented by some biasing field). This leads to the buildup of twinrelated variants and self-accommodation in order to minimize strain. Since the variants are related to each other by orientation relations, which can be considered compatibility relations of the self-accommodation process, the MT necessarily involves the temperature-dependent upfolding of a surface relief [1, 2]. In contrast to steel, the MT in shape memory alloys appears to be close to a second order phase transition. Precursor effects that herald the new phase well above the actual transition temperature are frequently observed, for example pronounced phonon softening [3-5] in neutron scattering, anomalous elastic constants in ultrasound experiments [6] and tweed patterns observed in transmission electron microscopy [1]. By definition, a prototypical MT proceeds without diffusion [7]. This explains why largely athermal transformations are observed, where the amount of material transformed depends solely on the degree of undercooling,
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