The equilibrium solid phases of the Al-Cu system are (1) (Cu) and (Al), the terminal fcc solid solutions; (Cu) is often designated a; hence the low-temperat
ure ordered phase based on the fcc structure is designated a2; (2) b, the disordered bcc solid solution; b1, the ordered bcc phase, which occurs metastably; and b0, a high-temperature phase of unknown structure; (3) phases with structures based on g brass, g0, g1, and d; (4) the equiatomic phases, h1 and h2, and near-equiatomic phases, z1 and z2, with structures related to the h structures; and (5) q, or Al2Cu, and metastable transition phases q?and q?formed from supersaturated (Al) before the [q + (Al)] equilibrium is reached. The solidus is drawn as a smooth curve joining the eutectic reaction to the melting point of Cu; the width of the two-phase [L + (Cu)] field is consistent with the thermodynamic properties of Cu. The assessed phase diagram for the Al-Cu system is based primarily on review of the work of [Hansen], [22Sto], [24Sto], [25Taz], [26Dix], [33Sto], [34His], [36Aue], [37Dow], [53Tho], [58Vig], [71Fun], [72Lin], [76Tas], and [80San] and was obtained using thermodynamic modeling. The intermediate composition range of the diagram is inaccessible to thermodynamic modeling because of the absence of thermodynamic information on the high-temperature solid phases and the multiplicity of complex solid-state reactions. The calculated diagram agrees well with the available experimental data in all regions within experimental accuracy. The bcc b solid solution is stable as a high-temperature phase at 70.6 to 82 at.% Cu; b melts congruently at 1049 ?1 癈. A two-phase [(Cu) + b] field exists between the eutectic temperature and the eutectoid reaction b ?g1 + (Cu) at 567 ?2 癈. The solubility of Al in (Cu) is 19.7 at.% Al at the eutectic temperature. Solubility decreases below the peritectoid temperature. The a2 phase exists in equilibrium with (Cu) and g1 below a
peritectoid reaction (Cu) + g1 ?a2. The a2 phase has an ordered fcc structure. Placement of the (Cu) solvus is uncertain. It is possible that in stable equilibrium the two-phase [a2 + (Cu)] field may be broader than in the present diagram. The intermetallic compounds in the Al-Cu system are very difficult to work with, because they are hard and brittle. The structures of phases at 58 to 70 at.% Cu, g1, d, and g0, are based on the g brass structure. The g1/[g1 + (Cu)] boundary is accurately delineated, as in the single-phase d region. The existence of the high-temperature phase g0 of unknown structure was demonstrated by thermal analysis, but the g0 + g1 region cannot be delineated metallographically. [71Fun] observed g1 and d in equilibrium in diffusion couples annealed at 500 癈. [80San] compared quenched and slowly cooled structures. The quenched alloys revealed a long-period superlattice structure, probably g2. Alloys slowly cooled from 700 癈 did not reveal any new structure, but contained two phases, (g1 + d). Thus, it appears that g2 exists as a metastable transition phase, but not as an equilibrium phase. The homogeneity range of z2 is 56.6 to 57.9 at.% Al. The equiatomic phases h1 and h2 are closely related structurally to the z1 and z2 phases. q crystallizes by peritectic reaction at 590 ?1 癈. The composition range is 31.9 to 32.9 at.% Cu at the eutectic temperature. The supersaturated (Al) solid solution decomposes to form the metastable precipitates, Guinier-Preston (GP) zones, q? and q? Aging treatments to prepare the desired precipitate usually are based on hardness data as a function of composition, aging temperature, and aging time. There is evidence that the formation of GP zones below 130 癈 proceeds by a spinodal mechanism. At room temperature, the initial decomposition is into the solute-rich and solute-lean areas. GP zones form continuously from the modulated microstructure. The q?structure has been reported to contain three (001) layers that consist of essentially pure Cu. The structure is tetragonal, not because of ordering, but because of the constraint of coherency with the matrix. The structure is thus identical to that of the GP zones, except for the thickness of the precipitate. The structure of q?is a tetragonal distortion of the CaF2 structure. Because of the sluggishness of the eutectoid reaction b ?(Cu) + g1, the b phase can be retained metastably. During quenching, metastable b alloys undergo a martensitic transformation to a b?phase at low Al content or a g?phase at higher Al content. The ordering reaction b ?b1 precedes the martensitic transformation. Thus, three martensitic phases actually appear: b? b? , and g? There are two metastable three-phase equilibria: the eutectoid reaction b ?b1 + (Cu) and the peritectoid reaction b + g1 ?b1. At intermediate cooling rates (air quenching), phase separation into b + b1 occurs before the martensitic transformation. The b ?(Cu) massive transformation is initiated when a b alloy is quenched into a single-phase (Cu) region. With increasing Al content, the massive transformation is interrupted or ruled out by the martensite transformation, and the maximum composition at which massive product is observed is 20.8 at.% Al. According to the present thermodynamic calculations, the temperature interval between the eutectic and the minimum of the fcc/L T0 curve is only about 40 癈. From this, the formation of uniform solid solutions is far more probable than the retention of the amorphous state. Extended single-phase solid solutions have been reported, with a maximum extension of about 16 or 17 at.% Cu. The decomposition of these supersaturated solutions proceeds by the same sequence observed in solid-quenched alloys. A new phase, Al3Cu2, isotypic with Al3Ni2 was
discovered by splat quenching of 45 and 50 at.% Cu alloys [74Ram]. Metastable phases and extended solubilities also can be obtained in thin films produced from the vapor phase. In films produced by cosputtering, [76Can] observed fcc solid solution between 0 and 28.5 at.% Cu, an anomalous fcc solution between 28.5 and 41.7 at.% Cu, and disordered bcc between 47.6 and 60 at.% Cu. [77Dhe] produced the equilibrium phases g1, h2, and q by vapor evaporation, in addition to the extended solid solutions. The (Cu) solid solution was extended to about 78 at.% Cu, g1 was found between 65 and 70.5 at.% Cu, and h2 at 58 at.% Cu. 22Sto: D. Stockdale, J. Inst. Met., 28, 273-286 (1922).