Superficial amorphization and re-crystallization of silicon in and orientation after irradiation by femtosecond laser pulses (790 nm, 30 fs) are studied using optical imaging and transmission electron microscopy. Spectroscopic imaging ellipsometry (SIE) allows fast data acquisition at multiple wavelengths and provides experimental data for calculating nanometric amorphous layer thickness profiles with micrometric lateral resolution based on a thin-film layer model. For a radially Gaussian laser beam and at moderate peak fluences above the melting and below the ablation thresholds, laterally parabolic amorphous layer profiles with maximum thicknesses of several tens of nanometers were quantitatively attained. The accuracy of the calculations is verified experimentally by high-resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray spectroscopy (STEM-EDX). Along with topographic information obtained by atomic force microscopy (AFM), a comprehensive picture of the superficial re-solidification of silicon after local melting by femtosecond laser pulses is drawn.

Thermodynamic driving forces and growth rates in rapid solidification are analysed. Taking into account the relaxation time of the solute diffusion flux in the model equations, the present theory uses, in a first case, the deviation from local chemical equilibrium, and ergodicity breaking. The second case of ergodicity breaking may exist in crystal growth kinetics of rapidly solidifying glass-forming metals and alloys. In this case, a theoretical analysis of dendritic solidification is given for congruently melting alloys in which chemical segregation does not occur. Within this theory, a deviation from thermodynamic equilibrium is introduced for high undercoolings via gradient flow relaxation of the phase field. A comparison of the present derivations with previously verified theoretical predictions and experimental data is given. This article is part of the theme issue ‘Heterogeneous materials: metastable and non- ergodic internal structures’.

Results of a study on microstructural evolution of eutectic Sn-57 wt.% Bi processed with cooling rates of 10 ⁻² , 1 K s ⁻¹ and approximately 10 ⁵ K s ⁻¹ are presented. In order to distinguish different mechanisms of microstructure formation, a comparison with microstructures of different hypoeutectic alloys with compositions down to below the maximum solubility of Bi in Sn–Bi is undertaken. It is found that at the cooling rates of 10 ⁻² and 1 K s ⁻¹ , coupled eutectic growth occurs, leading to lamellar structures with different length scales. At the rapid quenching rates of approximately 10 ⁵ K s ⁻¹ , structure formation in the eutectic alloy is qualitatively different. Partitionless solidification resulting in a supersaturated solid solution with the initial composition is observed in both eutectic and hypoeutectic alloys. It is shown that the observed microstructure of the rapidly solidified alloys forms by the decomposition of the supersaturated solid solution. This article is part of the theme issue ‘Heterogeneous materials: metastable and non-ergodic internal structures’.

The effect of substitution of Nb by Mo in Nb₄₀Ti₃₀Ni₃₀was investigated with respect to microstructural features and hydrogen dissolution, diffusion and permeation. As-cast Nb_40−xMo_xTi₃₀Ni₃₀(x = 0, 5, 10) alloys consist of primary bcc-Nb phase and binary eutectic (bcc-Nb + B2-TiNi). The substitution of Nb by Mo reduces the hydrogen solubility in alloys, but may increase (x = 5) or decrease (x = 10) the apparent hydrogen diffusivity and permeability. As-cast Nb₃₅Mo₅Ti₃₀Ni₃₀exhibits a combined enhancement of hydrogen permeability and embrittlement resistance as compared to Nb₄₀Ti₃₀Ni₃₀. This work confirms that Mo is a desirable alloying element in Nb that can contribute to a reduction in hydrogen absorption and an increase in intrinsic hydrogen diffusion, thus improving embrittlement resistance with minimal permeability penalty.

Layered (Bi_1−xIn_x)₂Te₃-In₂Te₃ (x = 0.075) composites of pronounced anisotropy in structure and thermoelectric properties were produced by zone melting and subsequent coherent precipitation of In₂Te₃ from a (Bi_1−xIn_x)₂Te₃ (x > 0.075) matrix. Employing solid state phase transformation, the Bi₂Te₃/In₂Te₃ interface density was tuned by modifying the driving force for In₂Te₃ precipitation. The structure-property relationship in this strongly anisotropic material is characterized thoroughly and systematically for the first time. Unexpectedly, with increasing Bi₂Te₃/In₂Te₃ interface density, an increase in electrical conductivity and a decrease in the absolute Seebeck coefficient were found. This is likely to be due to electron accumulation layers at the Bi₂Te₃/In₂Te₃ interfaces and the interplay of bipolar transport in Bi₂Te₃. Significantly improved thermoelectric properties of Bi₂Te₃-In₂Te₃ composites as compared to the single phase (Bi_1−xIn_x)₂Te₃ solid solution are obtained.