Martensitic relief formation on an electropolished Ni-37 at.% Al (001) surface by diffuse X-ray scattering under grazing angles

The development of a martensitic relief on an electropolished (001) surface of a Ni-37 at.% Al crystal was studied in a high-resolution, in-situ experiment by diffuse X-ray reflectivity using synchrotron radiation. A one-dimensional detector was used to monitor the distribution of the reflected intensity in the scattering plane as a function of temperature between 300 and 270 K. The presence of surface precursor effects with second-order (continuous) character was found far above the phase transition, whereas the discontinuous surface morphology change from nanoscopic roughness to macroscopic relief occurred within a temperature interval of less than 1 K. We observed three stages in the relief formation, which were followed with a temperature resolution of 0.1 K: (1) enhancement of the Fresnel transmission (Yoneda) peak by more than a factor of three on cooling from 272.5 to 272.0 K, reflecting a changing surface height-height correlation function that heralds the build-up of the martensitic relief; (2) reduction of the Yoneda peak intensity by more than two orders of magnitude with simultaneous appearance of a second peak, indicative of an intermediate faceting transition; and (3) suppression of the Yoneda peaks and development of a diffuse, featureless signal due to extensive surface roughening. The observed scattering patterns were reversible on heating. Resistivity measurements from the same sample suggest that surface relief represents only the final stage of the bulk transformation, preceded by non-equilibrium, intermediate stages without surface tilts.