Solid solution strengthening in single-phase Mo alloys

In body-centered cubic metals and alloys, screw dislocations are considered to control the strength due to their high critical stress. However, recent experimental and theoretical works on multicomponent solid solutions indicate a similar critical stress
for edge dislocations in these alloys. With increasing atomic misfits due to different atomic sizes, the critical stress increases for edge dislocations, until a transition from screw to edge dislocation-controlled strength is achieved. While individual alloys
have been identified as either screw or edge dislocation-controlled, the transition has not been observed in a systematic study yet. Consequently, the prerequisites to achieve this transition are not yet known. While in multicomponent systems superimposed strengthening contributions from precipitation or local atomic ordering might occur, an investigation of binary solid solutions precludes potential problems from chemical complexity. Here, Mo-Ti and Mo-Nb solid solutions were investigated systematically. Both systems cover a similar range of lattice parameters, however, while the lattice parameter in Mo-Ti solid solutions increases strongly non-linearly with Ti content, the one in Mo-Nb changes almost linearly. ... mehrAccordingly, the former presents a system where both small and large atomic misfits are realized within a single system and the latter serves as reference system with an intermediate misfit value. Mechanical testing from the nanometer to the millimeter scale revealed no significant strength contributions from grain boundaries or oxides potentially formed at grain boundaries. The combination of several chemical analysis methods revealed a significant amount of O dissolved in Ti-rich Mo-Ti solid solutions. As the O impacts the total yield strength, it is corrected for, consistent to the applied strengthening models. The remaining substitutional solid solution strengthening is compared to the models by Labusch, Suzuki as well as Maresca and Curtin to identify the strength-controlling dislocation types. While the strength in both systems can be described as controlled by screw dislocation motion, when appropriate energy parameters are used in the models, the parameter-free model for edge dislocation-controlled strength by Maresca and Curtin indicates competitive strengthening in both systems when certain misfit thresholds are surpassed. These threshold values, when taking the shear modulus into account, allow for a
comprehensive screening of binary and multicomponent solid solutions for candidate systems with edge dislocation-controlled strength to aid future model-guided alloy design.