Impurity Effects on Titanium Alloys
Titanium combines formability with corrosion resistance and the highest strength-to-density ratio of any elemental metal, and as a result commercial purity titanium alloys are popular for aerospace and naval applications. A stronger titanium alloy that improves on the formability of existing titanium alloys would result in considerable cost, weight and production savings in these applications.
Structural alloys are often strengthened through the addition of solute atoms. However, given that solute atoms interact weakly with the elastic fields of screw dislocations, it has long been accepted that solution hardening is only marginally effective in materials with mobile screw dislocations. By using transmission electron microscopy and nanomechanical characterization, we report that the intense hardening effect of dilute oxygen solutes in pure α-Ti is due to the interaction between oxygen and the core of screw dislocations that mainly glide on prismatic planes. First-principles calculations reveal that distortion of the interstitial sites at the screw dislocation core creates a very strong but short-range repulsion for oxygen that is consistent with experimental observations. These results establish a highly effective mechanism for strengthening by interstitial solutes. (See the paper in Science describing this discovery here.)
Currently, in a collaborative effort with the Asta and Minor groups, we are delving more deeply into the role that oxygen plays in mechanical properties of Ti, with Max Poschmann leading the theoretical developments.