Internal stresses in pre-stressed micron-scale aluminum core-shell particles and their improved reactivity

Dilatation of aluminum (Al) core for micron-scale particles covered by alumina (Al2O3) shell was measured utilizing x-ray diffraction with synchrotron radiation for untreated particles and particles after annealing at 573 K and fast quenching at 0.46 K/s. Such a treatment led to the increase in flame rate for Al + CuO composite by 32% and is consistent with theoretical predictions based on the melt-dispersion mechanism of reaction for Al particles. Experimental results confirmed theoretical estimates and proved that the improvement of Al reactivity is due to internal stresses. This opens new ways of controlling particle reactivity through creating and monitoring internal stresses

Influence of Aluminum Passivation on the Reaction Mechanism: Flame Propagation Studies

Currently, two main known mechanisms of aluminum (Al) nanoparticle reaction are discussed in the literature, namely those based on diffusion through an oxide shell and melt-dispersion. The two mechanisms lead to opposite predictions in nanoparticle design. The diffusion mechanism suggests that the reduction or complete elimination of the oxide shell will increase Al reactivity, whereas the meltdispersion mechanism suggests an increase in initial oxide thickness up to an optimal value. The goal of this study is to perform critical experiments in a confined flame tube apparatus to compare these two predictions. Specifically, the flame propagation rates of perfluoroalkyl carboxylic acid (C 13F27COOH)-treated Al nanoparticles with and without an alumina shell were measured. Results show that when there is no alumina passivation shell encasing the Al core, the flame rate decreases by a factor of 22-95 and peak pressure deceases by 3 orders of magnitude, in comparison with the Al particles with an oxide shell. These results imply that the melt-dispersion reaction mechanism is responsible for high flame propagation rates observed in these confined tube experiments

Phase field approach to interaction of phase transformations and plasticity at large strains

Thermodynamically consistent phase field approach (PFA) for multivariant martensitic phase transformations (PTs) and twinning for large strains is developed [1, 2]. Thermodynamic potential in hyperspherical order parameters is introduced, which allowed us to describe each martensite‑martensite (i.e., twin) interface with a single order parameter [3]. These theories are utilized for finite element simulation of various important problems [1‑4]. Phase field approach to dislocation evolution was developed during the last decade and it is widely used for the simulation of plasticity at the nanoscale. Despite significant success, there are still a number of points for essential improvement. In our study [5], a new PFA to dislocation evolution is developed. It leads to a well-posed formulation and mesh-independent solutions and is based on fully large-strain formulation. Our local potential is designed to eliminate stress-dependence of the Burgers vector and to reproduce desired local stress–strain curve, as well as to obtain the mesh-independent dislocation height H for any dislocation orientation. The gradient energy contains an additional term, which excludes localization of dislocation within height smaller than H but disappears at the boundary of dislocation and the rest of the crystal; thus, it does not produce interface energy and does not lead to a dislocation widening. Problems for nucleation and evolution of multiple dislocations along the multiple slip systems are studied. The interaction between PT and dislocations is the most basic problem in the study of martensite nucleation and growth. Here, a PFA is developed to a coupled evolution of martensitic PTs and dislocations [6], including inheritance of dislocation during direct and reverse PTs. A complete system of equations, including Ginzburg–Landau equations is presented. It is applied to studying the hysteretic behavior and propagation of an austenite‑martensite interface with incoherency dislocations, the growth and arrest of martensitic plate for temperature-induced PTs, the evolution of phase and dislocation structures for stress-induced PTs, and the evolution of dislocations and high pressure phase in a nanograined material under pressure and shear [6, 7]. REFERENCES [1] Levitas, V.I., Levin, V.A., Zingerman, K.M., Freiman, E.I. Phys. Rev. Lett. 2009, 103, 025702. [2] Levitas, V.I. Int. J. Plasticity. 2013, 49, 85‑118. [3] Levitas, V.I., Roy, A.M., Preston, D.L. Phys. Rev. B. 2013, 88, 054113. [4] Levin, V.A., Levitas, V.I., Zingerman, K.M., Freiman, E.I. Int. J. Solids & Struct. 2013, 50, 2914‑2928. [5] Levitas, V.I., Javanbakht, M. Phys. Rev. B., Rapid Commun. 2012, 86, 140101. [6] Levitas, V.I., Javanbakht, M. Appl. Phys. Lett. 2013, 102, 251904. [7] Levitas, V.I., Javanbakht, M. Nanoscale. 2014, 6, 162‑166