θ-Al2Cu Phase Research Articles

Influence of Cu additions on the morphology of GeSi precipitates in an Al-Ge-Si alloy

Combined additions of Ge and Si to Al are known to produce higher precipitation hardening than that which occurs in the constituent binaries, when the total amounts of alloying atoms are the same for all the alloys investigated. In the resultant Al-Ge-Si alloys, the diamond cubic precipitates contain both Ge and Si and are designated as GeSi. During artificial aging at 160 °C, the GeSi precipitates are commonly present in three forms, i.e., equiaxed, 〈100〉Al lath, and triangular plate. The equiaxed form is the dominant one of the three. This article examines the influence of varying amounts (i.e., 2 to 4 wt pct) of Cu additions on the morphology of GeSi precipitates formed in an Al-2.6 wt pct Ge-1.04 wt pct Si alloy during artificial aging at 160 °C. It is shown that Cu additions have the remarkable effect of maximizing the nucleation frequency of the 〈100〉Al lath form and simultaneously suppressing the nucleation of the equiaxed and the plate forms of the GeSi precipitates. Increasing Cu additions also increase the homogeneity and cause refinement of the 〈100〉Al laths. These results are discussed in light of (1) the critical requirement of vacancies for the nucleation and growth of GeSi precipitates having an atomic volume larger than Al and (2) the crystallographic nature of the negative dilation strains that develop locally in the Cu-rich regions of the Al matrix. It is further shown that, in the alloys containing increased levels (i.e., exceeding about 2.5 wt pct) of Cu, the precipitation of ϑ′ (metastable ϑ-Al2Cu) phase occurs, and that the nucleation of Cu-rich ϑ′ precipitates occurs upon the 〈100〉Al laths of GeSi. The latter effect is discussed in terms of the attainment of both the nucleation site and the necessary solute supersaturation at the 〈100〉Al GeSi/α-Al interfaces.

Effect of Fe Substitution on the Stabilization of the Icosahedral Phase in Al-Cu-V alloys

The effect of Fe substitution on the formation and stabilization of icosahedral phase in Al-Cu-V alloys has been investigated by employing transmission electron microscope and X-ray diffraction. In the as quenched Al75-x Cu15+x Fe5V5 alloys with x = 0, 5 and 10 it has been found that grain size of icosahedral phase increases from 156nm to 0.53μm with increasing x. All the three alloys exhibit the formation of α-Al and θ-Al2Cu phases along with the i-phase. The nanoquasicrystalline and α-Al phases in Al75Cu15Fe5V5 alloy transform to Al7Cu2Fe phase by annealing at 550°C for 4 hrs by peritectic type of reaction. A curious type of pseudo-twelve fold electron diffraction pattern devoid of any diffuse scattering has been observed in some local regions of Al65Cu25Fe5V5 alloy. The angles and details of twelve sectors are varying alternatively. A plausible explanation in terms of microtwin domains of θ-Al2Cu structure in [10] orientation has been given to explain the characteristic features of the pseudo-twelve fold pattern.

Compositional characterization of Cu-rich phase particles present in as-cast Al-Cu-Mg(-Li) alloys containing Ag

The effects of small additions of Ag on the aging of wrought Al-Cu-Mg(-Li) alloys, involving widespread nucleation of Cu-rich Ω (and T1) phase precipitates, are known. This article examines the influence of small additions of Ag on the nature of Cu-rich θ-Al2Cu, S-Al2CuMg, and T1-Al2CuLi phases present in appropriate as-cast Al-Cu, Al-Cu-Mg, and Al-Cu-Li-Mg alloys. Using a combination of light microscopy, scanning electron microscopy (SEM), and electron probe microanalysis (EPMA), it is shown that neither independent nor combined additions of Ag and Mg to the binary Al-Cu alloy alter the composition of the θ phase; however, there are differences in the ways the θ phase is evolved in Al-Cu-Mg alloys with and without Ag. Ag additions to the Al-Cu-Mg alloy result in the formation of an Al-Cu-Mg-based ternary phase, having a Cu content similar to that of the θ phase and containing small amounts of Mg; rapid rates of cooling cause the retention of this phase in the as-cast alloy. Relatively large amounts of both Ag and Mg are always located in the peripheral regions of such a phase. This phase is readily replaced by θ phase upon annealing at 450 °C. The S phase of the ternary Al-Cu-Mg system is identified in this alloy and is found to dissolve small amounts of Ag. In the case of the Al-Cu-Li-Mg-Ag alloy, two major changes are observed: both Ag and Mg are always present in relatively large amounts in the peripheral regions of the T1 phase, and the S-phase particles are once again found to dissolve small amounts of Ag. These results are discussed in light of the known compositional features of the precipitates formed in the artificially aged Al-Cu-Mg(-Li)-Ag alloys, to reveal that examination of phases present in the as-cast microstructure is a contributory step toward determining the locations of trace alloy additions in the phase precipitates of interest.

The effect of Cu content on the level of microporosity in Al-Si-Cu-Mg casting alloys

In this study, AlSiMgMnCu alloys with different Cu and Mg contents were fabricated by high vacuum die casting (HVDC) technique. The phase formation and their microstructure evolution are quantitatively studied by serial block-face scanning electron microscopy (SBFSEM) at nano-scale in three-dimensions. Through analyzing their three-dimensional (3D) microstructure, effects of the spatial distribution, volume fractions and morphology of the intermetallic phases on the alloy’s mechanical properties are comprehensively discussed. It is found that, with higher Cu content and lower Mg content, the nucleation and growth of θ-Al2Cu phase would have advantage over Q-Al5Cu2Mg8Si6 phase. In contrast, decreasing Cu/Mg ratio by increasing Mg content is in favor of the nucleation and growth of the Q-Al5Cu2Mg8Si6 phase. The uniformly distributed fine θ-Al2Cu phase provides the highest contribution to the strengthening of the HVDC AlSiMgMnCu alloys. For the Cu/Mg ratio to be 3, an as-cast HVDC Al-10Si-0.2Mg-0.5Mn-0.6Cu alloy with optimized ultimate tensile strength, yield strength and elongation of 330 MPa, 162 MPa and 10.2%, respectively, had been produced.

Microstructure of Passivated Al-Cu and Al-Si-Cu Conductor Lines: Interaction of Precipitates, Defects and Mechanical Stresses

AbstractIn this paper the results of our investigations of passivated, narrow lines of both Al-Cu and Al-Si-Cu (0.4 at.% Cu, 1.0 at.% Si) by transmission electron microscopy will be presented. Thin alloy films have been deposited on either oxide or refractory-metal films. During the standard anneal treatment of 0.5 h at 400 °C the atomic mobility is relatively high. At this temperature the binary Al-Cu films are one-phase alloys. Depending on the cooling rate, transformation occurs into a two-phase mixture of (initially) finely distributed, Cu-enriched particles and Al. The precipitates eventually form the equilibrium θ-Al2Cu phase, which tends to form large conglomerates upon further annealing. In contrast, the ternary Al-Si-Cu films are two-phase alloys already at 400 °C: at this temperature Si precipitates are pre:sent in the Al matrix, whereas the onset of Cu precipitation is not far below this temperature.

Composition and stability of Ω phase in an Al-Cu-Mg-Ag Alloy

Transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS) were employed to determine the stability and composition of the Ω phase in an Al-Cu-Mg-Ag alloy. It is shown that the Ω. phase can nucleate at isothermal aging temperatures up to 470 °C in the α + θ phase region of the Al-Cu phase diagram. Furthermore, Ω precipitates formed by aging solution-treated and quenched samples at 130 °C were found to exist after reversion treatments at temperatures close to the θ-phase solvus. The TEM/EDS analysis of relief-etched Ω, precip-itates shows that the composition of Ω phase is statistically the same as that of θ-Al2Cu phase in a binary Al-Cu alloy, with Ag and Mg below the limits of detectability. These results indicate that the Ω, phase is probably a metastable {111} variant of the equilibrium θ phase.

The effect of solidification microstructure on the corrosion behavior of a columnar aluminum-copper alloy

The morphological and kinetic nature of corrosion of directionally solidified aluminum-4.5 wt pct copper alloy in the as-cast, solutionized and solutionized-and-aged conditions in air-saturated aqueous 3.5 wt pct NaCl solution were evaluated. In the solutionized and solutionized-and-aged conditions the intergranular attack and pitting are similar to those occurring in solutionized wrought alloys; the extent of attack at long times increases with increasing severity of solidification rate. The as-cast alloy exhibits a cored dendritic structure with significant formation of interdendritic nonequilibrium eutectic. Extensive inter dendritic corrosion of the α-phase containing more than 3.2 wt pct copper is seen; α containing less than 3.2 wt pct copper and the θ-Al2Cu phase are cathodic. Corrosion of the as-cast alloy is parabolic with time and increases with increasing severity of solidification rate in proportion to the amount of nonequilibrium second phase.