The Yield Strength Anomaly in Co–Ni Design Space

Abstract

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, γa′, and D023. The elastic moduli were calculated via the energy-strain approach using density functional theory calculations. (Ni1−xCox)3(Al1−yWy) 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. (Ni0.65Co0.35)3(Al0.5W0.5) 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 (Ni1−xCox)3(Al1−yWy). The effect of Co on the yield strength anomaly was investigated experimentally in three (Ni1−xCox)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.