Letter abstract
Nature Materials 4, 129 - 133 (2005)
Published online: 23 January 2005 | doi:10.1038/nmat1292
Subject Categories: Metals and alloys | Structural materials | Computation, modelling and theory
Impurities block the 
to 
martensitic transformation in titanium
Richard G. Hennig1, Dallas R. Trinkle1, Johann Bouchet2, Srivilliputhur G. Srinivasan2, Robert C. Albers2 & John W. Wilkins1
Impurities control phase stability and phase transformations in natural and man-made materials, from shape-memory alloys1 to steel2 to planetary cores3. Experiments and empirical databases are still central to tuning the impurity effects. What is missing is a broad theoretical underpinning. Consider, for example, the titanium martensitic transformations: diffusionless structural transformations proceeding near the speed of sound2. Pure titanium transforms from ductile 
to brittle 
at 9 GPa, creating serious technological problems for 
-stabilized titanium alloys. Impurities in the titanium alloys A-70 and Ti–6Al–4V (wt%) suppress the transformation up to at least 35 GPa, increasing their technological utility as lightweight materials in aerospace applications. These and other empirical discoveries in technological materials call for broad theoretical understanding. Impurities pose two theoretical challenges: the effect on the relative phase stability, and the energy barrier of the transformation. Ab initio methods4, 5 calculate both changes due to impurities. We show that interstitial oxygen, nitrogen and carbon retard the transformation whereas substitutional aluminium and vanadium influence the transformation by changing the d-electron concentration6. The resulting microscopic picture explains the suppression of the transformation in commercial A-70 and Ti–6Al–4V alloys. In general, the effect of impurities on relative energies and energy barriers is central to understanding structural phase transformations.
- Department of Physics, Ohio State University, Columbus, Ohio 43210, USA
- Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
Correspondence to: Richard G. Hennig1 e-mail: rhennig@mps.ohio-state.edu
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