Effect of Ternary Alloying Elements Addition on the Order-Disorder Transformation Temperatures of B2-Type Ordered Fe-Al-X Intermetallics

The effect of alloying element additions on B2↔A2 order-disorder phase transformation temperatures of B2-type ordered Fe 0.5 (Al 1− n X n ) 0.5 intermetallics (X = Cr, Ni, Mo, Ta, Mn, Ti, and W) that readily form single-phase solid solution for X = 1 at. pct were investigated experimentally. It was shown that the type of the ternary substitutional alloying elements have a profound effect on the variation of order-disorder transition temperature of Fe 0.5 (Al 1− n X n ) 0.5 alloys. Based on the magnitude of partial ordering energies of the Al-X and Fe-X atomic pairs, predicted normalized transition temperatures, ∆ T / T o , were verified experimentally. Besides the normalized transition temperature, the relative partial ordering energy (RPOE) parameter, β , was also defined to estimate the extent of variation in B2↔A2 order-disorder phase transformation temperatures upon ternary alloying additions. The RPOE parameter, β , takes into account both the effects of magnitude of partial ordering energies of Al-X and Fe-X atomic pairs and also the lattice site occupation preferences of X element atoms over B2-type ordered Fe-Al sublattices. The alloying elements, which are preferentially distributed Fe sublattice sites, β > 0, and owing to β >> 1, are more effective in increasing order-disorder transformation temperature in Fe-Al (B2) intermetallics. On the contrary, alloying elements having β < 1 tend to decrease the transition temperature slightly relative to the binary FeAl intermetallic. The experimentally determined B2↔A2 order-disorder transition temperatures are in good qualitative or semiquantitative agreement with theoretical predictions for all X ternary alloying elements. Accordingly, the present experimental results confirm the validity of the theoretical model and calculations proposed in our previous study on the B2↔A2 order-disorder transition temperatures of single-phase Fe 0.5 (Al 1− n X n ) 0.5 intermetallics.