Unraveling the origin of extra strengthening in gradient nanotwinned metals

Materials containing heterogeneous nanostructures hold great promise for achieving superior mechanical properties. However, the strengthening effect due to plastically inhomogeneous deformation in heterogeneous nanostructures has not been clearly understood. Here, we investigate a prototypical heterogeneous nanostructured material of gradient nanotwinned (GNT) Cu to unravel the origin of its extra strength arising from gradient nanotwin structures relative to uniform nanotwin counterparts. We measure the back and effective stresses of GNT Cu with different nanotwin thickness gradients and compare them with those of homogeneous nanotwinned Cu with different uniform nanotwin thicknesses. We find that the extra strength of GNT Cu is caused predominantly by the extra back stress resulting from nanotwin thickness gradient, while the effective stress is almost independent of the gradient structures. The combined experiment and strain gradient plasticity modeling show that an increasing structural gradient in GNT Cu produces an increasing plastic strain gradient, thereby raising the extra back stress. The plastic strain gradient is accommodated by the accumulation of geometrically necessary dislocations inside an unusual type of heterogeneous dislocation structure in the form of bundles of concentrated dislocations. Such a heterogeneous dislocation structure produces microscale internal stresses leading to the extra back stress in GNT Cu. Altogether, this work establishes a fundamental connection between the gradient structure and extra strength in GNT Cu through the mechanistic linkages of plastic strain gradient, heterogeneous dislocation structure, microscale internal stress, and extra back stress. Broadly, this work exemplifies a general approach to unraveling the strengthening mechanisms in heterogeneous nanostructured materials.

The research development of the structure in a-Fe Microcrystals with the Orientation deformed by compression with different values of the friction coefficient

The paper analyzes the change in the dislocation structure in a-Fe microcrystals with orientations <100> and <110> deformed by compression under conditions of limited flow with different values of the friction coefficient. The orientation stability of the microcrystal and texture is investigated when the compression axis deviates from the normal to the compression plane. Shooting and analysis of limited pole figures made it possible to establish that the operation of the OSS in the MC with the [100](001) orientation at different values of the friction coefficient at the sample-punch interface leads to a rotation of the latticework relatively to the MC axis. The specific conditions of plastic deformation under compression determined by the MC morphology prohibit displacement deyz., as the deformation is observed under conditions of limited flow. The operation of the OSS, consequently, leads to the MC partition with the [100] (001) orientation deformed by compression with the friction coefficient Kmax into parts and the rotation of the latticework around the growth axis. It is established the connection between the friction coefficient at the crystal-punch interface and the sliding geometry in a-Fe MCs deformed by compression under conditions of limited flow. The authors carried out the dimension of the friction ratio has a significant effect on the slip geometry and the dislocation structure formed in the MC of the [100] orientation deformed by compression along the (001) plane. Changes in the friction coefficient and the angle deviation of the compression axis from the normal to the compression plane in the MC with the [100] (001) orientation do not affect the slip geometry and the dislocation structure formed during plastic deformation.

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.

On Incipient Plasticity of InP Crystal: A Molecular Dynamics Study

With classical molecular dynamics simulations, we demonstrated that doping of the InP crystal with Zn and S atoms reduces the pressure of the B3→B1 phase transformation as well as inhibits the development of a dislocation structure. On this basis, we propose a method for determining the phenomenon that initiates nanoscale plasticity in semiconductors. When applied to the outcomes of nanoindentation experiments, it predicts the dislocation origin of the elastic-plastic transition in InP crystal and the phase transformation origin of GaAs incipient plasticity.