Plastic straining and concomitant microstructure recrystallization of Ni-Cu alloy in the undercooled condition

Microstructure and microtexture of rapidly solidified undercooled Ni-Cu alloys were investigated. The characteristic undercooling of Ni80Cu20 alloy was determined as 45K, 90K and 160K. Dendrite deformation due to rapid solidification led to strong deformation microtexture. Due to recrystallization upon annealing after recalescence, many subgrains were formed in the microstructure. Further, annealing the quenched alloy at 900°, new microtextures and subgrains were formed, which was due to recrystallization and dislocation network rearrangement. The results of comparative experiment proved the recrystallization mechanism of the microstructure refinement in the non-equilibrium solidification structure of the undercooled binary alloy

Deformed and stressed states of materials at the rolling of three layer stacks, dislocation structure of inner layer – nickel foil

The deformed and stressed states during rolling of a three-layer stack from various materials with a nickel foil inner layer are considered. The technique of determining the density of dislocations is described. The data about the influence of deformation conditions on the distribution and density of dislocations during rolling of nickel foil in various stacks are presented, including the registration or determination of the dislocation structure of nickel foil before deformation and at various degrees of deformation. It is shown that the mechanical scheme of deformation of the inner layer of the stack, namely, the deformation of the nickel foil by non-uniform compression with shear, has a decisive influence on the development of the dislocation structure and properties. It is established that the dislocation density is determined not only by the degree of deformation, but also by a scheme of the deformed and stressed state of matter, and for the case of shear deformation with increasing degree of deformation the dislocation density increases more rapidly than in the case of tensile strain or compression without shear; the result of shear deformation is a significant refinement of the structure of materials: with increasing degree of plastic deformation of the material a three-dimensional cellular network of dislocation is formed, wherein the borders of cells are formed by tangles of dislocations. With increasing degree of deformation, the density of dislocations at the cell boundaries increases, and the size of the cells decreases; in this case, the areas inside the cells of the dislocation network are always free of dislocations. The obtained results allow recommending the schemes with shear deformation for new promising processes of production of materials with unique properties.

Structural complexity and the metal-to-semiconductor transition in lead telluride

Lead chalcogenides are known for their thermoelectric properties since the first work of Thomas Seebeck on the discovery of this phenomenon. Yet, the electronic properties of lead telluride are still of interest due to the incomplete understanding of the metal-to-semiconductor transition at temperatures around 230 °C. Here, a temperature-dependent atomic-resolution transmission electron microscopy study performed on a single crystal of lead telluride reveals structural reasons for this electronic transition. Below the transition temperature, the formation of a dislocation network due to shifts of the NaCl-like atomic slabs perpendicular to {100} was observed. The local structure modification leads to the appearance of in-gap electronic states and causes metal-like electronic transport behavior. The dislocation network disappears with increasing temperature, yielding semiconductor-like electrical conductivity, and re-appears after cooling to room temperature restoring the metal-like behavior. The structural defects coupled to the ordering of stereochemically active lone pairs of lead atoms are discussed in the context of dislocations' formation.

Dissociated prismatic loop punching by bubble growth in FCC metals

Materials performance can be significantly degraded due to bubble generation. In this work, the bubble growth process is elaborated in Cu by atomistic modeling to bridge the gap of experimental observations. Upon continuous He implantation, bubble growth is accommodated first by nucleation of dislocation network from bubble surface, then formation of dissociated prismatic dislocation loop (DPDL), and final DPDL emission in $$\langle 110\rangle$$ ⟨ 110 ⟩ directions. As the DPDL is found capable of collecting He atoms, this process is likely to assist the formation of self-organized bubble superlattice, which has been reported from experiments. Moreover, the pressurized bubble in solid state manifests the shape of an imperfect octahedron, built by Cu $$\{111\}$$ { 111 } surfaces, consistent with experiments. These atomistic details integrating experimental work fill the gap of mechanistic understanding of athermal bubble growth in Cu. Importantly, by associating with nanoindentation testings, DPDL punching by bubble growth arguably applies to various FCC (face-centered cubic) metals such as Au, Ag, Ni, and Al.

Influence of Temperature on Void Collapse in Single Crystal Nickel under Hydrostatic Compression

Employing atomistic simulations, we investigated the void collapse mechanisms in single crystal Ni during hydrostatic compression and explored how the atomistic mechanisms of void collapse are influenced by temperature. Our results suggest that the emission and associated mutual interactions of dislocation loops around the void is the primary mechanism of void collapse, irrespective of the temperature. The rate of void collapse is almost insensitive to the temperature, and the process is not thermally activated until a high temperature (∼1200–1500 K) is reached. Our simulations reveal that, at elevated temperatures, dislocation motion is assisted by vacancy diffusion and consequently the void is observed to collapse continuously without showing appreciable strain hardening around it. In contrast, at low and ambient temperatures (1 and 300 K), void collapse is delayed after an initial stage of closure due to significant strain hardening around the void. Furthermore, we observe that the dislocation network produced during void collapse remains the sample even after complete void collapse, as was observed in a recent experiment of nickel-base superalloy after hot isostatic pressing.

A New Atomistic Mechanism for Heterogeneous Nucleation in the Systems with Negative Lattice Misfit: Creating a 2DTemplate for Crystal Growth

Heterogeneous nucleation is a widespread phenomenon in both nature and technology. However, our current understanding is largely confined to the classical nucleation theory (CNT) postulated over a century ago, in which heterogeneous nucleation occurs stochastically to form a spherical cap facilitated by a substrate. In this paper, we show that heterogeneous nucleation in systems with negative lattice misfit completes deterministically within three atomic layers by structural templating to form a two-dimentional template from which the new phase can grow. Using molecular dynamics (MD) simulations of a generic system containing metallic liquid (Al) and a substrate of variable lattice misfit (fcc lattice with fixed Al atoms), we found that heterogeneous nucleation proceeds layer-by-layer: the first layer accommodates misfit through a partial edge dislocation network; the second layer twists an angle through a partial screw dislocation network to reduce lattice distortion; and the third layer creates a crystal plane of the solid (the 2D nucleus) that templates further growth. The twist angle of the solid relative to the substrate as a signature of heterogeneous nucleation in the systems with negative lattice misfit has been validated by high resolution transmission electron microscopic (HRTEM) examination of TiB2/Al and TiB2/α-Al15(Fe, Mn)3Si2 interfaces in two different Al-alloys.