Effect of Ti addition on the structural, thermodynamic, and elastic properties of Tix(HfNbTaZr)(1−x)/4Ti_{x}(HfNbTaZr)_{(1-x)/4} alloys

The structure and thermodynamic properties of $Ti_{x}(HfNbTaZr)_{(1-x)/4}$ refractory highly entropy multicomponent alloys have been studied using a comprehensive Monte-Carlo Special Quasi-random Structure (MCSQS) realization of the disordered atomic structure and DFT calculations. We have shown that to model the random structure in a small supercell, it is necessary to study a large space of random configurations with respect to the nearest shells. Mimicking the randomness with the many-body terms does not lead to significant improvements in the formation energy but modeling the random structure with the few nearest neighbor pairs leads to improvements in the formation energy. We have also demonstrated the existence of weak to intermediate SRO for equimolar compositions. Chemical ordering is studied by linking a large number of MCSQS realizations to DFT calculations, and the SRO results are rationalized in terms of the crystallographic structure of the element pairs and binary phase diagrams. The formation energy of $Ti_{x}(HfNbTaZr)_{(1-x)/4}$ alloys remains slightly positive for all $x$ when Ti is added. For $x$ > 0.5, a phase transition in favor of an hcp structure is observed in agreement with the Bo-Md diagram. A dual phase is predicted at $x$ = 0.5. The Ti content in this class of alloys appears to be a practical way to select the phase structure and tailor the structure and elastic properties to specific applications

Assessment of interatomic potentials for atomistic analysis of static and dynamic properties of screw dislocations in W

Screw dislocations in bcc metals display non-planar cores at zero temperature which result in high lattice friction and thermally activated strain rate behavior. In bcc W, electronic structure molecular statics calculations reveal a compact, non-degenerate core with an associated Peierls stress between 1.7 and 2.8 GPa. However, a full picture of the dynamic behavior of dislocations can only be gained by using more efficient atomistic simulations based on semiempirical interatomic potentials. In this paper we assess the suitability of five different potentials in terms of static properties relevant to screw dislocations in pure W. As well, we perform molecular dynamics simulations of stress-assisted glide using all five potentials to study the dynamic behavior of screw dislocations under shear stress. Dislocations are seen to display thermally-activated motion in most of the applied stress range, with a gradual transition to a viscous damping regime at high stresses. We find that one potential predicts a core transformation from compact to dissociated at finite temperature that affects the energetics of kink-pair production and impacts the mechanism of motion. We conclude that a modified embedded-atom potential achieves the best compromise in terms of static and dynamic screw dislocation properties, although at an expense of about ten-fold compared to central potentials

Unraveling the temperature dependence of the yield strength in single-crystal tungsten using atomistically-informed crystal plasticity calculations

We use a physically-based crystal plasticity model to predict the yield strength of body-centered cubic (bcc) tungsten single crystals subjected to uniaxial loading. Our model captures the thermally-activated character of screw dislocation motion and full non-Schmid effects, both of which are known to play a critical role in bcc plasticity. The model uses atomistic calculations as the sole source of constitutive information, with no parameter fitting of any kind to experimental data. Our results are in excellent agreement with experimental measurements of the yield stress as a function of temperature for a number of loading orientations. The validated methodology is then employed to calculate the temperature and strain-rate dependence of the yield strength for 231 crystallographic orientations within the standard stereographic triangle. We extract the strain-rate sensitivity of W crystals at different temperatures, and finish with the calculation of yield surfaces under biaxial loading conditions that can be used to define effective yield criteria for engineering design models

On the evaluation of the Bauschinger effect in an austenitic stainless steel—The role of multiscale residual stresses

In this work, a physically based self-consistent model is developed and employed to examine the microscopic lattice response of pre-strained Type 316H polycrystalline austenitic stainless steel subjected to uniaxial tensile and compressive loading. The model is also used to explain the Bauschinger effect observed at the macroscopic length-scale. Formulated in a crystal based plasticity framework, the model incorporates detailed strengthening effects associated with different microstructural elements such as forest dislocation junctions, solute atoms and precipitates on individual crystallographic slip planes of each individual grain within the polycrystal. The elastoplastic response of the bulk polycrystal is obtained by homogenizing the response of all the constituent grains using a self-consistent approach. Micro-plasticity mechanisms and how these influence the Bauschinger effect are illustrated in terms of the role of residual stresses at different length-scales. Overall, predictions are in good agreement with experimental data of the Bauschinger effect and the corresponding meso-scale lattice response of the material, with the latter measured by neutron diffraction. The results demonstrate that transient softening of the material is related to residual stresses at different length scales. In addition, the (Type III) residual stress at the micro-scale slip system level extends the strain range over which the tensile and compressive reloading curves of the pre-strained material merge

Analytical integration of the tractions induced by non-singular dislocations on an arbitrary shaped triangular quadratic element

We present fully analytical expressions to evaluate nodal forces induced by the stress field of non-singular dislocations at quadratic surface elements

On the saturation stress of deformed metals

Crystalline materials exhibit an hysteresis behaviour when deformed cyclically. The origins of this tension-compression asymmetry have been fully understood only recently as being caused by an asymmetry in the junction strength and a reduced mean free path of dislocations inherited from previous deformation stage. Here, we investigate the saturation stress in fcc single- and poly-crystals using a Crystal Plasticity framework derived from dislocation dynamics simulations. In the absence of plastic localization and damage mechanism, the single-crystal mechanical response eventually saturates. We show that the cyclic saturation stress converges asymptotically to the monotonic saturation stress as the cycle plastic increment increases, and this convergence can be observed for some experimental conditions. The analysis of the experimental literature suggests that the mechanisms controlling the saturation in single crystals are the same controlling the cyclic response of polycrystals with large grains. We propose also analytical and approximated models to predict the saturation stress over the considered loading conditions. The saturation stress appears as a fundamental property of dislocations, explaining the consistency observed in the experimental literature. This work provides a unified view on the monotonous and cyclic responses of fcc single and poly-crystals, which may help in interpreting experimental data