Elsevier

Acta Materialia

Volume 52, Issue 12, 12 July 2004, Pages 3671-3679
Acta Materialia

Trace element effects on precipitation in Al–Cu–Mg–(Ag, Si) alloys: a computational analysis

https://doi.org/10.1016/j.actamat.2004.04.021Get rights and content

Abstract

In order to optimize mechanical properties of age-hardenable Al–Cu–Mg–(Ag, Si) alloys, the nucleation of the strengthening precipitate Ω must be enhanced while competing phases such as S and θ (or θ″/θ) must be suppressed. One strategy is to promote the formation of the precursors, i.e., certain solute clusters that aid in Ω nucleation, using trace element additions such as Ag, and/or the elimination of deleterious impurities, e.g., Si. This paper describes the use of computational analysis tools to identify those elements that are either helpful or harmful. Using a multi-component quasi-chemical model, short-range-order parameters were calculated based on CALPHAD databases to indicate cluster formation in Al–Cu–Mg–(Ag, Si) solid solutions. To anticipate the orientation of the plate-like clusters that controls the habit-plane selection of the subsequent precipitates, the anisotropy of the associated strain was considered using the second order elastic tensors obtained from first principle total energy calculations.

Introduction

Al–Cu–Mg–X alloys were the earliest developed and remain one of the most advanced age-hardenable light metal systems. There has been a continuous effort to improve this system for aerospace structural applications [1]. Since precipitation hardening provides the majority of alloy strength, it is the strength of second phase precipitates, as well as the precipitate structures (morphology and distribution), that essentially control the yield strength, creep resistance and fracture toughness. For mostly plate-like precipitates, dislocation slip simulations [2] have shown that a fine and uniform dispersion of precipitates having different habit planes, e.g., {111}αΩ and {100}αθ″/θ in Al–Cu–Mg (with high Cu:Mg ratio) alloys, is desired for optimum mechanical properties. The high thermal stability of the Ω precipitates [3], [4], [5] presents an opportunity to develop new Al–Cu–Mg–X alloys for moderate or high temperature (>150 °C) application, e.g., for the fuselage of Mach 2.2–2.4 aircraft.

In this paper, the effects of trace additions of Ag and Si on precipitation of the Ω phase in Al–Cu–Mg alloys with high Cu:Mg ratios, are examined. The metastable “phase diagrams” are calculated to demonstrate the trace element thermodynamic effects on the formation of Ω (or θ) and S phases based on CALPHAD models of alloy systems containing FCC-A1, S and Ω (or θ). Using a quasi-chemical model for higher order systems, the short-range order parameters are calculated based on the CALPHAD databases to predict possible cluster formation in Al–Cu–Mg, Al–Cu–Mg–Ag and Al–Cu–Mg–Si solid solutions. By considering the strain anisotropy associated with the embedded clusters in the matrix, attempts are made to distinguish the predicted clusters as being either precursors or non-precursors to the plate-like and coherent precipitates. The linear elastic strain energy is calculated based on the second order elastic tensors that are obtained through the first principle total energy approach. The ultimate goal is to establish simplified computational analysis tools to identify trace elements that may be either helpful or harmful in Al–Cu–Mg alloys being developed for moderate temperature applications.

Section snippets

Strengthening phases in Al–Cu–Mg and trace element effects

Experimental investigations of Al–Cu–Mg alloys with a Cu:Mg ratio >1 that lie in the Al-rich corner of the phase diagram indicate that the stable second phases are θ (Al2Cu), S (Al2CuMg) and T (Al32(CuMg)49) [6]. Details of the phase equilibria, shown in Fig. 1, can be obtained using experimentally assessed CALPHAD models [7], [8]. Accordingly, alloy compositions can be selected to achieve the desired second phase(s), and avoid undesired one(s).

The presence of stable equilibrium phases, which

CALPHAD models for Al–Cu–Mg–(Ag, Si) alloys

The thermodynamic models for Al–Cu–Mg–(Ag, Si) are available in the CALPHAD databases and some have been published in the literature [7], [8], [26]. The stable phases of concern here are FCC-A1, S and θ (Fig. 1). The metastable phase Ω, possessing similar chemistry and crystal structure to equilibrium θ, has not been modeled. In order to compare Ω with θ energetically, a first principle total energy calculation was performed using the Vienna ab initio simulation program (VASP) [27] together

Clustering in solid solutions of Al–Cu–Mg–(Ag, Si)

As a result of atomic interactions, the individual clusters (clustering) or co-clusters (short-range-ordering) between elements can be formed locally in an otherwise homogeneous solid solution. The cluster variation method (CVM) [29] and Monte Carlo (MC) simulation [30] based on the first principle approach offer the most comprehensive methods to predict the deviation from randomness for cluster formation or of SROs. Basically the equilibrium Boltzmann distribution of atomic configurations are

Strain energy analysis

As discussed above, plate-like precipitates select habit planes in the supersaturated solid solution during their nucleation stage in order to minimize activation energies. The habit plane selection depends on local strain orientation of the precursors, or clusters, since subsequent precipitate nucleation must accommodate their own lattice misfit strain. On the other hand, the orientation of platelet-like clusters is controlled by the anisotropy of their lattice-misfit induced strain. The

Discussion

The simplified SRO analysis used the CALPHAD databases that are consistent with the equilibria of the phases, e.g., FCC-A1, S and the metastable Ω phases in Al–Cu–Mg–(Ag, Si) alloys. The Ω phase was treated to be equivalent with the equilibrium θ since both phases have similar chemical compositions, crystal structures and formation enthalpies as indicated by the first principle calculation. The phenomenological thermodynamic models are essentially under the point approximation for the mixing

Conclusions

Using a quasi-chemical model for multi-component systems, a short-range order analysis based on the CALPHAD approach was conducted to predict the formation of clusters or co-clusters in Al–Cu–Mg, Al–Cu–Mg–Si and Al–Cu–Mg–Ag solid solutions. A strain energy analysis related to cluster formation was employed to predict the orientation of the plate-like clusters. The results correctly predicted that Cu functions to form Cu plate-like clusters on {100}α that are precursors to the θ″/θ

Acknowledgements

The authors are indebted to the financial support of the Air Force Office of Scientific Research under Grant No. S49620-01-1-0090, Dr. Craig S. Hartley, Program monitor.

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