Ternary transition metal aluminide alloy formation

Research Objectives

To understand and predict the stability and properties of metallic alloys.

Computational Approach

First-principles local density calculations, using the full-potential linearized augmented Slater-type orbital (FLASTO) method, provide a means of obtaining the heats of formation of competing alloy phases, both for observed and unobserved structures. In this approach, the electronic structure problem is cast into a set of effective one-particle equations that must be solved self-consistently. The resulting single-particle wave functions are expanded in a set of Slater-type orbitals in the interstitial, augmented by numerical solutions of the effective potential in spheres surrounding each atom. To get a consistent set of heats of formation, all the structural parameters (volume, c/a ratios, internal atomic positions, etc.) need to be optimized. The large number of calculations needed to adequately investigate the competing alloy compositions, different crystal structures, and structural parameters were done on the cluster of Cray J90s at NERSC.

Accomplishments

The bonding of aluminum with the transition metals is of scientific and technological concern. Despite this interest, the properties of ternary aluminides have received relatively little theoretical attention. Almost 50 ternary transition metal aluminides have been reported in the ordered antiphase Heusler BiF₃ structure, representing roughly half of the observed ordered ternaries. To investigate the possible occurrence of other aluminides in this structure, the heats of formation of 38 ordered M₂NAl ternaries, most not reported to occur, have been calculated. While all but one is stable relative to the elemental metals, the test for ternary stability requires comparison of the ternary heat with competing two- and three-phase mixtures of binary phases.

As an example of the competition among different phases, the heats of formation of the ordered Fe₂ZrAl alloy (red filled circle) and various two phase mixtures are compared in the accompanying figure. (The corresponding lines connecting binary phases in the ternary triangle are shown in the inset.) Although the ordered ternary is predicted to be stable relative to the two-phase mixtures, a three-phase mixture of FeAl-Fe₂Zr-ZrAl₂ (square) is even more binding and will suppress the Fe₂ZrAl alloy. Similar investigations for the other 37 alloy systems have been done.

Significance

Of the BiF₃ ternaries hitherto unreported, 9 are estimated to occur, while 18 are found to be unstable. The advantage of having theoretical predictions of the stability of the alloys is the possibility of discerning which of the unreported phases should not occur, as opposed to those that are yet to be observed. The pattern of occurrence obtained from experiments and calculations taken together suggests that on the order of another 20 compounds beyond those predicted here might exist. Thus, this important ordered phase is more pervasive than previously suggested by experiment.

Publications

Watson, R. E., Weinert, M., and Alatalo, M. N. d. Ternary transition metal aluminide alloy formation: The BiF₃ structure. Phys. Rev. B, in press.

Heats of formation of the ordered Fe₂ZrAl alloy compared with various two-phase mixtures.