The Yield Strength Anomaly in Co–Ni Design Space

A new computational approach to model precipitate compositions and properties in the CoNi-design space for yield strength anomaly prediction is presented. The antiphase boundary (APB) energies on {111} and {010} and the degree of elastic anisotropy are known to influence the yield strength anomaly. APB energies were estimated by a diffuse multi-layer fault (DMLF) model using the structural energies of proximate structures: L1₂, γ_a′, and D0₂₃. The elastic moduli were calculated via the energy-strain approach using density functional theory calculations. (Ni_1−xCo_x)₃(Al_1−yW_y) was chosen as the model system, and Yoo’s criterion is evaluated over the entire range of compositions to identify regions that exhibit the yield strength anomaly. (Ni_0.65Co_0.35)₃(Al_0.5W_0.5) has the maximum APB (111) and any two-phase alloy (γ + γʹ) in Ni–Co–Al–W system with this precipitate composition might exhibit higher strength upon shearing by a matrix dislocation. ThermoCalc was employed to identify stable L1₂ regions in (Ni_1−xCo_x)₃(Al_1−yW_y). The effect of Co on the yield strength anomaly was investigated experimentally in three (Ni_1−xCo_x)₃Al alloys with L1₂ structure. All three alloys exhibited the yield strength anomaly, validating the computational approach. The addition of Co provides solid solution strengthening to Ni₃Al at room temperature; however, this contribution to the overall strength diminished as a function of temperature. CoNi-alloys displayed strengths similar to Ni₃Al at elevated temperatures with Co addition resulting in a marginal increase in strength at the peak temperature. The present study elucidates that APB energies from a DMLF model combined with elastic moduli can be employed to predict yield strength anomaly using Yoo’s criteria.