Understanding the interactions between interstitial and substitutional solutes in refractory alloys: The case of Ti-Al-O

Abstract Refractory metals such as titanium can dissolve significant quantities of interstitial oxygen. The interactions between dissolved interstitial species and substitutional alloying elements remain poorly understood in refractory metals, but are increasingly recognized as being of importance, especially in high entropy refractory alloys. We performed a first-principles statistical mechanics study of the interactions between substitutionally dissolved Al and interstitial O in hcp Ti. A systematic analysis of phase stability in the Ti-Al-O composition space predicts the low temperature stability of a new ternary Ti6Al2O compound. In spite of a well-known affinity between Al and O, extensive first-principles calculations using density functional theory predict a strong nearest-neighbor repulsion between Al and O when both are dissolved in hcp Ti. The repulsion has its origin in the unique electronic structure of the Ti host. The combination of a cluster expansion Hamiltonian and Monte Carlo simulations shows that the strong short-range interaction between dissolved Al and O has important consequences for phase stability and the degree of order at elevated temperature in Ti-Al-O alloys. Monte Carlo simulations predict that an increase in the dissolved oxygen concentration leads to a reduction of the order-disorder transition temperature of α-Ti3Al. The results of this study provide guidance as to how the solubility and degree of ordering of interstitial species in refractory metals can be tailored by different alloying strategies.