Markus Aspelmeyer

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

Abstract 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.

An experimental method to investigate the structure and kinetics of patterned surfaces using laser light diffraction

We describe a novel experimental method using the diffraction of a He–Ne laser beam to study surfaces patterned with structures on mesoscopic to macroscopic length scales. The technique provides high spatial and temporal resolution; it is not limited to periodic, artificial structures, but is also well suited to study the development of self-organized surface relief. Measurements can be performed under in situ conditions in a diffraction mode or an imaging mode, providing (1) qualitative and quantitative information on the surface structures, (2) information on time-dependent surface changes with a resolution of 10 μs or better, (3) observation of incubation processes (including determination of incubation time) in first-order, displacive phase transformations, and (4) observation of the surface in real space, in particular, the pattern evolution as a function of temperature or other parameters. As an example we show results of the application of our method to a Ni0.63Al0.37 single crystal undergoing a ma...

Premonitory Martensitic Surface Relief Via Novel X-Ray Diffuse and Laser Light Reflectivity from The (001)-Surface of A Ni 63 Al 37 Single Crystal

Both x-ray diffuse reflectivity and laser light scattering have been used to investigate the temperature-dependent surface behavior of a Ni{sub 63}Al{sub 37} single crystal on different length scales. In-situ experiments were performed above the conventional martensitic start temperature M{sub s}. to search for premartensitic phenomena. X-ray experiments showed the presence of a surface precursor with second-order (continuous) character several 10 K above M{sub s}. This premonitory effect corresponds to a height-height-correlation function which changes on the nanometer scale as the martensitic transformation is approached. At the martensitic transformation, the surface morphology changed from nanoscopic roughness to macroscopic relief within a temperature interval of less than 1 K via intermediate stages. Laser light scattering was employed to study time-dependent aspects of the athermal martensitic transformation above M{sub s}. The occurrence of a martensitic transformation on isothermal holding after a certain incubation period was observed in Ni-Al for the first time. The measured incubation times increased by four orders of magnitude within a temperature interval of 0.5 K.