ELECTRONIC STRUCTURE AND PHASE STABILITY OF TITANIUM ALLOYS.

Abstract

The alloying effect has been investigated on the electronic structure of both bcc Ti and hcp Ti by the DV-X alpha cluster method. The two alloying parameters, which are the metal d-orbital level (Md) and the bond order between atoms (Bo), were determined theoretically. The Md is related to the electronegativity and metallic radius of elements. The Bo is a measure of the strength of the covalent bond between titanium and alloying elements. Both the Md and the Bo change remarkably with alloying elements. A new approach based on the electronic theory has been developed for understanding the phase stability of titanium alloys. With using Md and Bo parameters, titanium binary phase diagrams are interpreted consistently, and various commercial alloys are classified into the three groups of the alpha , alpha plus beta and beta alloys. The beta transus and the Ms temperature for the martensite transformation are predicted from alloy compositions. Furthermore, the activation energy for the impurity diffusion in bcc Ti is estimated in terms of the Bo.