Long Period Superlattice Phases in Cu-Al-Zn Alloys
The phase diagram of the Cu-Al and Cu-Al-Zn systems shows a rich variety of different equilibrium phases and, as will be shown in this paper, several metastable phases. Among the stable phases occuring in the composition range used for this study are the face centered cubic α-phase (short range ordered), the cubic α2-phase (fcc with long period anti-phase boundary lattice) and the γ2-phase (complex cubic with 52 atoms per unit cell). (Fig. 1.) Upon a quench from the high temperature β-phase (body centered cubic) the alloy orders (DO3-type of order) and transforms martensitically towards a close packed structure; depending on the electron to atom ratio this structure is 3R (α’-martensite), 9R (β’-martensite) or 2H (γ’-martensite). Since the martensitic transformation is by definition diffusionless the product phase inherits the DO3-order from the parent β-phase, as well as all the defects. The industrial application of these alloys in ‘two-way shape memory devices’ is seriously obstructed by the fact that martensite is a metastable phase; when kept for some time at a temperature below the reverse transformation temperature the system will try to lower its free energy, thereby becoming more stable with respect to the high temperature phase. This so-called ‘stabilisation’ was first observed through an increase of the reverse transformation temperature (1) with time and a strong change of the damping capacity (2).
KeywordsMartensite Plate Close Packed Structure High Voltage Electron Microscopy ANNNI Model Alloy Phase Stability
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