Evolution of laser-induced strain in a Ge crystal for the [111] and [100] directions probed by time-resolved X-ray diffraction

Ultra-short laser-pulse-induced strain propagation in a Ge crystal is studied in the [111] and [100] directions using time-resolved X-ray diffraction (TXRD). The strain propagation velocity is derived by analysis of the TXRD signal from the strained crystal planes. Numerical integration of the Takagi–Taupin equations is performed using open source code, which provides a very simple approach to estimate the strain propagation velocity. The present method will be particularly useful for relatively broad spectral bandwidths and weak X-ray sources, where temporal oscillations in the diffracted X-ray intensity at the relevant phonon frequencies would not be visible. The two Bragg reflections of the Ge sample, viz. 111 and 400, give information on the propagation of strain for two different depths, as the X-ray extinction depths are different for these two reflections. The strain induced by femtosecond laser excitation has a propagation velocity comparable to the longitudinal acoustic velocity. The strain propagation velocity increases with increasing laser excitation fluence. This fluence dependence of the strain propagation velocity can be attributed to crystal heating by ambipolar carrier diffusion. Ge is a promising candidate for silicon-based optoelectronics, and this study will enhance the understanding of heat transport by carrier diffusion in Ge induced by ultra-fast laser pulses, which will assist in the design of optoelectronic devices.

Elastic Parameters of Various Rafted Structures of Model Ni-Base Alloys

Two-phase microstructure of ordered cube-shaped precipitates in the disordered matrix is characteristic of Ni-base superalloys. This microstructure degrades under the applied stress: depending on the stress direction, lattice misfit and elastic parameters of both constituent phases, the precipitates coalesce and change their overall shape. Various atomic configurations were modeled in this work representing various morphologies of precipitates developed under applied stress. A model Ni-base alloy containing six alloying elements typical of advanced Ni-based superalloys was used. Generated configurations were further subject to study of elastic parameters by means of computer straining experiments. Relaxation of atomic positions in the strained crystal blocks was implemented using molecular dynamics calculations with phenomenological Lennard-Jones pair potentials and interactions involving three coordination spheres. Changes of elastic parameters due to varying precipitates morphology are discussed.