Chemical short range order strengthening in BCC complex concentrated alloys

Atomistic methods are used to anneal two body-centered cubic (BCC) chemically complex alloys (CCAs) in order to assess the effect of chemical short-range order on alloy strength. The two alloys, a model quaternary Co16.67Fe36.67Ni16.67Ti30 and ternary Nb33.33Ti33.33Zr33.33, are represented using simple Zhou interatomic potentials. Chemically random cells are annealed at temperatures 65% of the average melting temperature of the individual elements using a Monte Carlo approach, and the critical stress required to move a/2[111] screw dislocations in the initial and annealed cells are estimated using molecular dynamics simulations. It is shown that annealing leads to a softening of the BCC quaternary alloy relative to the random state, especially for deformation at low temperatures. On the other hand, short-range order has minimal effect on solid solution strengthening in the ternary alloy. These results are modeled using an extension of the Suzuki model of substitutional solid solution strengthening developed for BCC chemically complex alloys. Here the model is modified to account for the effects of chemical short-range order. Good agreement is shown between the model results and direct atomistic simulation data. The model presented should be useful in predicting BCC CCAs with adequate high temperature strength, which will accelerate development of high temperature, high strength BCC alloys for aerospace applications.