COST 507 - Repositório Aberto da Universidade do Porto

and have been independently described by different sublattice models in the
first round of the COST-507 project, Table 3.
The presence of a non-first order transition from D83 to D82 seems necessary
to make compatible both structures, although there is lack of experimental data
to corroborate it. Thus, it may be essential to make systematic measurements of
Cp along the γ-phase region in order to determine the possible order-order
transition boundary.
Table 3. Structures of the binary γ-phases and sublattices used to describe them.
γ phase
Structure
Sublattice
model
in binary Al-Cu
D8 3 (cP52), Ρ 4 3m
(Al) 16 :(Al,Cu) 4 :(Cu) 32
in binary Cu-Zn
D8 2 (cI52), 14 3m
(Cu,Zn) 8 :(Cu,Zn) 8 :(Cu) 12 :(Zn) 24
From the point of view of the sublattice models, two alternatives are possible.
Either to build an „hypothetical intermediate" sublattice to connect both
structures or to modify the binary descriptions of the binary γ-phases. The first
alternative has some advantages, such as its rapid implementation and the lack
of necessity for a change in the binary descriptions already in use. Its main
disadvantage is related to the complications it introduces in the phase
description, which will increase for higher order systems. Thus, even if this
„provisional" solution is first used, a modification of the binary descriptions
seems to be inevitable in the long term. The new model should account for the
experimentally observed structures and atomic occupancies for the whole range
of the γ-phase.
Experimentally, eight crystallographically different positions are observed,
which in a simplified model could be reduced to four. In this case a possible
sublattice model for describing the γ-phase would be:
(Al,Cu,Zn) 8 : (Cu) 8 : (Cu,Zn) 12 : (Al,Cu,Zn) 24
The ternary Al-Cu-Zn system also displays one ternary phase, τ, with a large
range of solubility. This phase is present below 740°C at compositions close to
Cu4oZn 2 oAl34. Its main feature is that at low temperatures its range of
existence is split into two non-interconnected zones. In one of these regions,
denoted as τ', the experimental results show that a superstructure is created by
the presence of structural vacancies, fig. 11. The vacancies produce a loss of
symmetry in this τ' phase (rombohedral) with respect to the high temperature τ
phase (B2).
Crystallographically, both τ and τ', could be described as a single phase using a
two sublattice model containing vacancies: (Al,Zn)i : (Cu,Va)i
However, such a model, although simple, would not be compatible either with
the present descriptions of the B2 structures or with the description of the A2-
B2 ordering transition. Thus, the actually compatible model should be
(Al,Cu,Zn,Va)i : (MCaZn^Va)!
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