Order—disorder phase transitions in ternary alloys Cu3–xMnxAl

Abstract

The Bragg-Williams approximation is the basis of the model of the order-disorder transitions in Heusler alloys. The formulae for the critical temperatures of the transitions are derived. The quantitative results of the calculations of the critical temperatures are obtained after the application of the Lennard-Jones type model of the pair potential of atoms in a crystalline lattice. The calculated values of the critical temperatures are compared with experimental results and the reasons of the discrepancy between the model and the experiment are discussed.

Order-disorder transitions of Cu-Al-Mn shape-memory alloys

“…20͒ and also in recent published measurements on order-disorder transitions in Cu 3 Al͑Be͒; 21 in addition, published results for Cu-Zn-Al 22 show that, as in Cu-Al-Mn, both B2 A2 and L2 1 B2 transitions join in a single transition on reducing the Zn content down to 0. Nevertheless, x-ray diffraction results have been published 10 showing two order-disorder transitions in Cu 3 Al. As it was discussed in detail in Ref.…”

“…When the metastable disordered bcc ␤ phase is retained on cooling, it successively undergoes two ordering transitions. 10 In the first one, nearest-neighbor ordering is developed, and the ␤ 2 phase appears; it has a B2 ordered structure ͑space group, Pm3m). In the second transition, next-nearest-neighbor ordering is developed and, depending on composition, the resulting phase is a solid solution based on the DO 3 structure of Cu 3 Al (␤ 1 phase͒ or the L2 1 structure of the Heusler alloy Cu 2 AlMn (␤ 3 phase; in both ␤ 1 and ␤ 3 cases the space group is Fm3m).…”

Phase Equilibria in Binary and Ternary Systems with Chemical and Magnetic Ordering

“…Moreover, they confirmed its existence in binary Al-Cu alloys in the vicinity of the Cu 3 Al stoichiometric composition. For the same alloy composition Soltys [29] reported a continuous cascade A2 M B2 M D0 3 . It seems that under experimental conditions both scenarios may take place, perhaps depending on minor composition variation, impurity level, cooling rate and thermal treatment state.…”

Magnetic phase separation in ordered alloys

“…23 As well as the phase transitions mentioned above, the system exhibits, at low temperatures, a spinodal decomposition along the line Cu 3 Al-Cu 2 AlMn. 11,12,24 We will center our attention on this two-phase region and denote the Cu-rich portion of the phase diagram of interest in this paper by Cu 3Ϫx AlMn x , with 0 рxр1. In Fig.…”

“…It should be mentioned that this effect was not found in other earlier studies. 11 Other important information, not yet available, refers to the different atomic distributions for the nonstoichiometric L2 1 structure. Some assumptions on this matter will be required in order to perform a theoretical study.…”