A recent literature review predicts a transition from screw dislocation to edge dislocation-controlled strength in binary BCC solid solutions when the lattice misfit (as defined by Varvenne et al.) surpasses a certain threshold. While this result falls in line with recent experimental data from HEAs and theoretical work by Maresca and Curtin, it is not expected within established models for the strength of BCC alloys. So far, the correlation between lattice misfit and a possible transition from screw to edge dislocation-controlled strength has not been investigated systematically.
Mo-Ti alloys form BCC solid solutions between 0-80 at% Ti. The lattice parameter changes strongly non-linear, leading to a large change of lattice misfit within the system. This allows to investigate a possible transition from screw to edge dislocation-controlled strength systematically within a single material system: Mechanical properties are characterized across several length scales to isolate solid solution strengthening by using nanoindentation, Vickers hardness and compression testing. This allows the application of the Maresca-Curtin models. An extension of the Maresca-Curtin model corrects for additional interstitial strengthening due to unavoidable O uptake during synthesis, which was assessed by atom probe tomography and hot carrier gas extraction. For comparison, Mo-Nb solid solutions were also characterized: While the lattice parameter range covered in this system is similar to Mo-Ti, the lattice parameter increases almost linear. This leads to a smaller lattice misfit, below the thresholds defined in Ref. [2] and found in Mo-Ti. However, modelling reveals similar screw and edge dislocation-controlled strength. This result points towards a combination of lattice misfit and shear modulus that determines the strength-controlling dislocation type.