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Refractory multiprincipal element alloys (MPEAs) are promising materials to meet the demands of aggressive structural applications, yet require fundamentally different avenues for accommodating plastic deformation in the body-centered cubic (bcc) variants of these alloys. We show a desirable combination of homogeneous plastic deformability and strength in the bcc MPEA MoNbTi, enabled by the rugged atomic environment through which dislocations must navigate. Our observations of dislocation motion and atomistic calculations unveil the unexpected dominance of nonscrew character dislocations and numerous slip planes for dislocation glide. This behavior lends credence to theories that explain the exceptional high temperature strength of similar alloys. Our results advance a defect-aware perspective to alloy design strategies for materials capable of performance across the temperature spectrum.

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a b s t r a c tOrowan equations predicting the strengthening of non-basal slip systems in hexagonal crystals are derived for rationally distributed, shear resistant precipitates of typical morphologies and orientations. These equations may be employed for any hexagonal crystal, but application is made specifically to hexagonal close packed Mg, where alloy development is presently quite active. Particular focus is placed on discerning the effect on 〈 + 〉 c a dislocations, and generally speaking, 〈 + 〉 c a slip is most potently strengthened by prismatic plate shaped precipitates, as was shown previously for basal and prismatic slip. If strengthening is the primary goal, prismatic plate shaped precipitates appear ideal. Because the motion of 〈 + 〉 c a dislocations has been repeatedly emphasized as crucial for preserving ductility, it may be of interest to consider.

The mechanical response of rare earth containing Mg alloy, WE43, plates is found to be more isotropic, as compared to conventional alloys like AZ31, despite a moderately strong texture. In order to understand the grain-level deformation mechanisms which are responsible, the elastoplastic self-consistent (EPSC) polycrystal plasticity code, including the recently developed twinning-detwinning (TDT) model, is used to describe the homogeneous plastic flow of WE43-T5, plate at quasistatic and dynamic strain rates. Latent hardening of the slip modes is based on a recent discrete dislocation dynamics study in order to reduce the number of empirical fitting parameters without sacrificing model fidelity. The approach accounts for the presence of the initial texture and its evolution during deformation. The observed flow stress, strain, and strain hardening anisotropies and asymmetries are well-described. A single set of parameters was used to fit the entire set of results, at a given strain rate, thus enabling determination of strain rate sensitivities of individual deformation modes. Basal slip and extension twinning are rateinsensitive, within the strain rate regime examined, whereas the prismatic and slip exhibit strain rate sensitivities of 0.008 and 0.005, respectively. Various strengthening mechanisms such as precipitation, grain refinement and solid solution hardening effects on each individual deformation modes are assessed. The softer modes, basal slip and extension twinning, are greatly strengthened in this alloy, as compared to the harder modes such as prismatic and slip, which renders this material more isotropic, even at the grain-level, as compared to conventional Mg alloys.

The interaction between dislocations and twins appears to play an important role in the strain hardening behavior of Mg. Detailed transmission electron microscopy study was performed to investigate the concept of dislocation "transmutation" across twin boundaries. A previously proposed dislocation transmutation reaction is confirmed. For twins, the transmutation reactions involve or matrix dislocations resulting in dislocations, which populate the vicinity of the twin boundary. No other slip systems are observed within the twins, despite the fact that the dislocations have similar or lower resolved shear stress, as compared to other slip systems. This suggests the slip mode is source limited, since the observed slip systems are the only ones that can result from transmutation. The formation of a unit dislocation in the twin is proposed to involve two consecutive reactions, necessitating dislocation pileup in the matrix, and the associated dislocation configurations are evaluated in terms of elastic strain energy considerations. Dislocation reactions are proposed which could explain the presence of basal stacking faults of either or type inside twin. At the low stress levels typical of twinning dominated flow of textured polycrystals, the observed dislocations are most likely sessile. However, this could serve as a source mechanism for later deformation, and as a forest hardening mechanism against other slip systems. The interfacial reaction products could result in a dragging effect on twin boundary advancement, and establish a basis for subsequent rapid hardening.

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