Determination of thermoelastic material properties by differential heterodyne detection of impulsive stimulated thermal scattering

Guided acoustic waves in thin epitaxial films: Experiment and inverse problem solution for NiTi

Despite the fundamental and technological importance of the elastic constants, a suitable method for their full characterization in epitaxial films is missing. Here we show that transient grating spectroscopy (TGS) with highly k-vector-selective generation and detection of acoustic waves is capable of determination of all independent elastic coefficients of an epitaxial thin film grown on a single-crystalline substrate. This experimental setup enables detection of various types of guided acoustic waves and evaluation of the directional dependence of their speeds of propagation. For the studied model system, which is a 3μm thin epitaxial film of the NiTi shape memory alloy on an MgO substrate, the TGS angular maps include Rayleigh-type surface acoustic waves as well as Sezawa-type and Love-type modes, delivering rich information on the elastic response of the film under different straining modes. The resulting inverse problem, which means the calculation of the elastic constants from the TGS maps, is subsequently solved using the Ritz-Rayleigh numerical method. Using this approach, tetragonal elastic constants of the NiTi film and their changes with the austenite→martensite phase transition are analyzed.

Revisiting impulsive stimulated thermal scattering in supercooled liquids: Relaxation of specific heat and thermal expansion

Impulsive stimulated thermal scattering (ISTS) allows one to access the structural relaxation dynamics in supercooled molecular liquids on a time scale ranging from nanoseconds to milliseconds. Till now, a heuristic semi-empirical model has been commonly adopted to account for the ISTS signals. This model implicitly assumes that the relaxation of specific heat, C, and thermal expansion coefficient, γ, occur on the same time scale and accounts for them via a single stretched exponential. This work proposes two models that assume disentangled relaxations, respectively, based on the Debye and Havriliak-Negami assumptions for the relaxation spectrum and explicitly accounting for the relaxation of C and γ separately in the ISTS response. A theoretical analysis was conducted to test and compare the disentangled relaxation models against the stretched exponential. The former models were applied to rationalize the experimental ISTS signals acquired on supercooled glycerol. This allows us to simultaneously retrieve the frequency-dependent specific heat and thermal expansion up to the sub-100 MHz frequency range and further to compare the fragility and time scale probed by thermal, mechanical, and dielectric susceptibilities.

Remote Thermoelastic Characterization of Candidate Structural and Protective Coatings for Lead-Bismuth Eutectic Cooled Nuclear Reactors

A laser ultrasonics approach to remotely characterize the elastic and thermal properties of a coating-substrate structure, used to protect fuel claddings in lead-bismuth eutectic cooled nuclear reactors, is presented and validated with experimental results. A transient grating geometry is used for multiple wavelength excitation of thermoelastic displacements, and laser beam deflection for detection. The value of Young’s modulus of the coating layer as calculated from the coating’s Rayleigh velocity of 190 ± 50 GPa is in accordance with a nano-indentation measurement. Using a priori knowledge concerning the density and elastic parameters of the coating and substrate, scanning the sample allows the obtaining of a coating thickness map in a fully remote, all-optical way, with an accuracy of about 4 microns for a coating of around 20-micron average thickness. Analyzing the transient thermal grating decay yields a thermal diffusivity value of ( 5.0 ± 1.6 ) × 10 − 6 m 2 /s, in the range of low-carbon steels. The consistency of the results infers that the all-optical laser ultrasonics approach should be feasible for remote inspection of the quality of optically rough coated claddings in the harsh environment of a nuclear reactor.

Characterization of Texture Evolution during Recrystallization by Laser-Induced Transient Thermal Grating Method

The analyses of texture evolution of cold rolled interstitial free (IF) steel sheets during annealing and recrystallization are presented, in which the dispersion curves of surface acoustic waves (SAW) excited by laser-induced transient thermal grating method are measured. The results show that the angular anisotropy of the SAW velocity changes due to the texture changes at different stages of recrystallization. The theoretically simulated angular dispersion of SAW velocity within individual crystal revealed that the change of SAW velocity is closely related to recrystallization texture evolution. A model for the angular dependence of the SAW velocity in textured polycrystalline IF steel with different oriented crystals is presented and the simulations are yielded, which show that the results agree with those of experiments.