Combined theoretical and experimental efforts are put forward to study the critical factors influencing deformation mode transitions in face-centered cubic materials. We revisit the empirical relationship between the stacking fault energy (SFE) and the prevalent deformation mechanism. With ab initio calculated SFE, we establish the critical boundaries between various deformation modes in the model Cr-CoNi solid solution alloys. Satisfying agreement between theoretical predictions and experimental observations are reached. Our findings shield light on applying quantum mechanical calculations in designing transformation-induced plasticity and twinning-induced plasticity mechanisms for achieving advanced mechanical properties.

The effect of magnetism on the solid solution strengthening (SSS) of CrxCoyNi1?x?y alloys formulated by Varvenne?s and Toda-Caraballo?s (TC?s) models has been investigated comparatively by first-principles. It is found that for a reliable estimation of the model parameters for the paramagnetic (PM) and ferromagnetic (FM) alloys one should adopt the atomic radii and elastic moduli of the constituent metals corresponding to the same magnetic state. Furthermore, we show that Varvenne?s model underestimates the SSS when compared to the experimental data, while TC?s model correctly predicts the SSS of PM CrxCoyNi1?x?y alloys when adopting the PM parameters as inputs. Therefore, the magnetic states should be correctly accounted when estimating the atomic radii and elastic moduli of the constituent elements as input parameters for both models to capture the SSS in complex concentrated alloys properly.

In order to efficiently explore the nearly infinite composition space in multicomponent solid solution alloys for reaching higher mechanical performance, it is important to establish predictive design strategies using computation-aided methods. Here, using ab initio calculations we systematically study the effects of magnetism and chemical composition on the generalized stacking fault energy surface (gamma-surface) of Cr-Co-Ni medium entropy alloys and show that both chemistry and the coupled magnetic state strongly affect the gamma-surface, consequently, the primary deformation modes. The relations among various stable and unstable stacking fault energies are revealed and discussed. The present findings are useful for studying the deformation behaviors of Cr-Co-Ni alloys and facilitate a density functional theory based design of transformation-induced plasticity and twinning-induced plasticity mechanisms in Cr-Co-Ni alloys.

We used density-functional theory to assess the electronic structure, elastic properties and planar fault energies of the B2 Ti(AlxNb1-x) (0.2 <= x <= 0.8) phase in relation to the composition and chemical ordering. We found that the covalent bonding becomes stronger for B2 Ti(AlxNb1-x) with higher Al concentration and long range order (LRO) parameter. Based on a universal ductile-to-brittle criterion by integrating Pettifor's Cauchy pressure with Pugh's modulus ratio, the deformability becomes less for Ti(AlxNb1-x) with higher Al concentration and LRO parameter, which is well correlated with the bonding character. Rice's ratio has an anti-correlation with Pugh's modulus ratio for Ti(AlxNb1-x). According to Rice's criterion, Ti(AlxNb1-x) with various Al concentration and LRO parameter are brittle in pure mode I loading, however, Nb-enriched disordered and low-ordered Ti(AlxNb1-x) may satisfy Rice's criterion for nucleation of dislocation and thus, are ductile in mode II or III loading. The hardness increases but the fracture toughness decreases obviously with increasing the degree of covalent bonding in Ti(AlxNb1-x).

Twin boundaries (TBs) in Ni-based superalloys are vulnerable sites for failure in demanding environments, and a current lack of mechanistic understanding hampers the reliable lifetime prediction and performance optimisation of these alloys. Here we report the discovery of an unexpected gamma '' precipitation mechanism at TBs that takes the responsibility for alloy failure in demanding environments. Using multiscale microstructural and mechanical characterisations (from millimetre down to atomic level) and DFT calculations, we demonstrate that abnormal gamma '' precipitation along TBs accounts for the premature dislocation activities and pronounced strain localisation associated with TBs during mechanical loading, which serves as a precursor for crack initiation. We clarify the physical origin of the TBs-related cracking at the atomic level of gamma ''-strengthened Ni-based superalloys in a hydrogen containing environment, and provide practical methods to mitigate the adverse effect of TBs on the performance of these alloys.