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TT4.01 - Non-Ergodic Processes in Martensitic Phase Transformations by X-Ray Photon Correlation Spectroscopy: Bulk vs. Surface Behavior 
December 2, 2014   1:45pm - 2:15pm
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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 current technical limitations (detector readout), XPCS is typically 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, as done in this work, where bulk diffraction (XRD) and surface-sensitive scattering (GISAXS) were used to study shape memory alloys undergoing a structural, diffusionless (martensitic) transformation.

The vast majority of martensitic transformations (MT) is viewed as being athermal, although time-dependent phenomena such as incubation time or ageing effects have been known for a long time. XPCS is able to reveal non-equilibrium dynamics using two-time correlation functions [1]. This technique has only recently been applied to MT [2,3]. In particular, it can reveal - on a purely experimental basis - non-ergodic processes in the vicinity of the MT, elucidating the actual transformation path. Microstructural avalanches that accompany MT can be observed and quantified [2] as well as ageing processes [3].

Our experiments on Au50.5Cd49.5 and Ni63Al37 single crystals indicate the following:
a) Bulk — Non-equilibrium features can be observed for both alloys in the direct vicinity of the MT, which are partially superimposed with signatures of microstructural avalanches. The revealed slow dynamics can be quantified in terms of stretched exponential functions and is consistent with the “symmetry conforming short-range-order” model based on short-range diffusion [4].

b) Surface — Grazing incidence small angle X-ray scattering (GISAXS) was employed to study the temperature-dependent evolution of surface roughness from the precursor regime (several 10 K above the MT) to the fully developed martensitic surface relief. The measurements indicate pronounced non-equilibrium dynamics in the precursor regime and a peculiar type of precursor not observed in the bulk (non-monotonous correlation function). We conclude that significant premonitory surface fluctuations must be present in the austenitic state. This precursor effect is more pronounced in Ni-Al than in Au-Cd.

[1] A. Malik et al., Phys. Rev. Lett. 81, 5832 (1998).
[2] C. Sanborn et al., Phys. Rev. Lett. 107, 015702 (2011).
[3] L. Müller et al., Phys. Rev. Lett. 107, 105701 (2011).
[4] X. Ren et al., Phys. Rev. Lett. 85, 5 (2000).

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