卷 120, 编号 8 (2019)

In this study, the effect of grain size on the mechanical properties and hydrogen embrittlement of a NiTi shape memory alloy (SMA) was investigated. Samples with different grain sizes were prepared through different annealing processes. For creating and evaluating hydrogen embrittlement, samples were immersed in an aqueous solution of 2% acidulated phosphate fluoride (APF) for 8, 16, and 72 h. Optical microscopy (OM), scanning electron microscopy (SEM), tensile test, and three-point bending test were conducted to characterize the samples. Results demonstrated samples with large grain sizes to be more susceptible to hydrogen embrittlement. Improvement in shape memory properties was achieved at smaller grain sizes. Besides, with increasing the immersion time, fracture stress reduced, while detwinning stress increased. Overall, the results of this study have demonstrated that grain refinement can bring about improved mechanical properties and abated hydrogen embrittlement in NiTi alloys, which can be of vital importance in biomedical applications.

An experimental depth-resolved method for analyzing the local atomic structure of low-contrast multilayered thin films is presented in this work. A combination of X-ray reflectometry and angle-resolved EXAFS spectroscopy is considered. The following methods for solving ill-posed inverse problems are used to determine structural characteristics: the Tikhonov regularization method (for linear integral equations) and the Levenberg–Marquardt algorithm (for nonlinear equations). The proposed algorithms do not require information on the studied system such as the interface width and shape, the thickness of layers of specific elements and the depth at which they are located, and the atomic structure. This allows one to retrieve data on the local atomic structure of individual interface layers and the surface. Model numerical experiments for a Cr/Fe/Cr/Fe/Cr sample were conducted to assess the potential of this method.

Martensitic transformation B2–L1₀ in the ordered alloy Ni₅₀Mn₅₀, which occurs at comparatively high temperatures (980–920 K), is discussed with the use of dynamic concepts of the wave control of the threshold deformation. The proximity of the observed orientations of martensite-crystal habits (and of twin boundaries) to the planes of the {110} family makes it possible to use the longitudinal waves along the axes 〈001〉 (in the basis of the initial phase) as the driving factors. It is shown that at temperatures of the onset of the transformation there is a satisfactory correspondence between the calculated and experimental data on the tetragonality of martensite and on the volume effect. The opportunity of different dynamic scenarios of the formation of the final phase is noted, namely, of separate crystals; layered structures, in which the crystals of martensite with the identical orientation relationships alternate with the untransformed regions of austenite; and packets of pairwise-twinned crystals. Examples are given of morpho-types corresponding to these scenarios.

The influence of T6 heat treatment on the microstructure, hardness and tensile properties of cast AA6063 aluminum alloy was investigated as a function of squeeze casting pressure. The specimens elaborated were solution treated at 500°C for 8 hours and then were subjected to aging at 140°C. Various test techniques, including optical microscopy, scanning electron microscope, X-ray diffraction, tensile tests, and hardness were used to examine the effect of this alloy produced by the gravity and squeeze casting processes. The experimental results showed that Mg₂Si and Al₁₅Si₂(FeMn)₃ were the primary particles observed in the α-Al matrix. The effect of precipitation on the mechanical properties was investigated. The changes in mechanical properties were correlated with changes in microstructure evolution and Mg–Si precipitation. Furthermore, hardness and tensile properties increased with both heat treatment and squeeze pressure.

The nonstationary distribution function characterizing the energy distribution of a cascade of moving atoms (with their multiplication factored in) is determined by solving the kinetic Boltzmann equation. The development of such cascades in materials consisting of identical atoms is examined without regard to the lattice-site binding energy. It is assumed that the interaction cross section is inversely proportional to velocity and the scattering of moving atoms is elastic and spherically symmetric in the center-of-mass system. These simplifying assumptions provide an opportunity to derive simple analytical formulas for the nonstationary distribution function of a cascade of decelerating atoms and analyze its features. The obtained results may also be used to estimate the accuracy of various approximate solutions.