Abstract
In this paper a model is presented for calculating the site occupancies in ternary B2 compounds. The model is based on a simple approach involving chemical rate theory for representing equilibrium in ordered compounds. Competing exchange reactions between different combinations of the three elements located on two different sublattices in a ternary B2 compound determine the equilibrium ordering scheme. The enthalpy change involved in these reactions is represented within the framework of the Bragg-Williams mean-field approximation using binary interaction parameters. The site occupancies are represented in a simple manner using the ordering tie line (OTL) construction. This model has been used to compute the site occupancies in ternary B2 compounds in Nb-Ti-Al alloys using values of binary interaction parameters available in the literature. The results are in reasonable agreement with experimentally determined OTLs for these alloys. The values of the binary interaction parameters have been identified as significant contributors in determining the accuracy of predictions afforded by the model and have therefore been optimized in order to obtain better agreement between the calculated and experimentally determined OTLs. The order-disorder transformation temperature, predicted using the model, is in excellent agreement with the experimental results for the Nb-40at.%Ti-15at.%Al alloy. Finally, the model also predicts secondary ordering effects in those Nb-Ti-Al alloys which have an average composition close to the Heusler composition A(2)BC, in agreement with results of electronic structure calculations available in the literature.
| Original language | English |
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| Pages (from-to) | 777-795 |
| Number of pages | 19 |
| Journal | Philosophical Magazine A: Physics of Condensed Matter, Defects and Mechanical Properties |
| Volume | 81 |
| Issue number | 4 |
| Publication status | Published - 1 Apr 2001 |