Effect of solution treatment on precipitation behaviors and age hardening response of Al–Cu alloys with Sc addition

Abstract

Influences of solution treatment on precipitation behaviors and age hardening response of Al–2.5 wt% Cu–0.3 wt% Sc alloys were investigated, in comparison with Sc-free one. The Al₃Sc dispersoids, formed during homogenization, were either survived or dissolved to become Sc solute atoms in solution treatment, depending on the solution temperature. When the temperature for solution treatment is 873 K, most of the Al₃Sc dispersoids were dissolved and a significant enhancement in the uniform precipitation of finer θ′-Al₂Cu particles was achieved in following aging treatment, causing a noticeable increase in peak-aging hardness by about 90% compared to Sc-free alloys. The enhanced age hardening effect was quantitatively related to the remarkable reduction in effective inter-particle spacing of the plate-shaped θ′-Al₂Cu precipitates. When the temperature for solution treatment is 793 K, however, most of the Al₃Sc dispersoids were survived after solution treatment and facilitated the heterogeneous precipitation of θ′-Al₂Cu plates directly on the {1 0 0} facets of dispersoids in following aging treatment. Concomitantly, the uniform precipitation of θ′-Al₂Cu plates was greatly suppressed, resulting in a reduced age hardening response. The age hardening responses were quantitatively assessed by using a modified strengthening model that is applicable to the plate-shaped precipitates. The calculations were in good agreement with experimental results. The present results show the importance of controlling solution treatments to achieve significant promotion effect of Sc addition on the precipitation hardening in heat-treatable aluminum alloys.

Introduction

The research on scandium (Sc) addition in aluminum (Al) alloys has been received increasing attention over the last decade, because of their interesting benefits. Most of the benefits are related to the formation of Al₃Sc particles, including Al₃Sc dispersoids and Al₃Sc precipitates [1]. Here, the Al₃Sc dispersoids are specially referred to the Al₃Sc particles formed in high temperature processing of the alloy, such as homogenization, hot-rolling or extrusion, while the Al₃Sc precipitates referred to those formed during aging process after solution treatment.

The well-known contribution of Al₃Sc dispersoids is to stabilize the grain/subgrain structure of Al alloys through Zener-drag action and hence enhance recrystallisation resistance [2], [3], [4]. Because of excellent thermal stability and high number density, the Al₃Sc dispersoids are very effective in preventing recrystallisation up to temperatures of about 350–400 °C [5]. By adding Sc and Zr in combination, the recrystallisation resistance of Al alloys can be further improved due to the formation of more stable core–shell Al₃(Sc_xZr_1−x) dispersoids [6], [7], [8], [9], [10]. The Al₃Sc precipitates, coherent with the Al matrix, can produce a significant strength increase in the Al alloys. It has been claimed [11], [12] that Sc is a more effective precipitation hardening element per atomic fraction added than the element commonly used for precipitation hardening. However, the Al₃Sc precipitates that can be decomposed from supersaturated solid solution are rather limited, the absolute hardness/strength increase induced by precipitation hardening is relatively low [1].

The addition of Sc in pure aluminum or non-heat-treatable Al alloys (e.g., Al–Mn and Al–Mg alloys) has been extensively investigated [13], [14], [15], [16], [17], [18], mainly due to two advantages. The first one is to strengthen these non-heat-treatable Al alloys by forming Al₃Sc precipitates, making them heat-treatable and creep-resistant. The second is to use Al₃Sc precipitates as model particles for studying basic principles in physical metallurgy, including coarsening behaviors [13], [14], [15] and interaction with dislocations [16], [17], [18]. In comparison, there have been much fewer studies on the Sc addition in heat-treatable Al alloys [19], [20]. A major reason is that it is unfeasible for Sc addition to further strengthen the heat-treatable Al alloys by additionally forming Al₃Sc precipitates. The Al₃Sc precipitates are hardly coexisted with other strengthening precipitates, because the temperature regime for aging treatment of Al₃Sc is between the temperature regimes for aging treatment and solution treatment of common heat-treatable Al alloys [1].

Effect of minor Sc addition on the precipitation behaviors and resultant aging response of heat-treatable Al alloys is far from clear. In some limited reports, contradictory information was presented: the Sc addition has been found to (i) improve [21], [22], (ii) have no effect [23], [24], or (iii) even suppress [25], [26] the precipitation of other strengthening particles in heat-treatable Al alloys. No attempt has been made to explain the discrepancy in the reported effect of Sc addition. In this paper, we show through systematic investigations that the effect of Sc addition on the aging hardening of Al–Cu alloys is closely dependent on the existing forms of added Sc, which is modulated by the solution treatment.

Heat treatments of Al alloys are complicated by three steps of homogenization, solution, and aging. The last step of aging treatment is actually relied on the former two steps, and an appropriate cooperation of the three steps is required to achieve excellent mechanical properties in heat-treatable Al alloys. Usually, the homogenization temperature for common heat-treatable Al alloys falls in the temperature range for Al₃Sc precipitation [1], [14], Al₃Sc dispersoids are expected to form during the homogenization treatment if the addition of Sc is enough. The Al₃Sc dispersoids can be survived or partially dissolved to become solute atoms in subsequent solution treatment, depending on the solution temperature. The existing forms of added Sc, either Al₃Sc dispersoids or Sc solute atoms, should have different influences on the final aging response. Of special interest to note is the possible interaction between the Al₃Sc dispersoids and other precipitates. The nanoscale Al₃Sc particles have an equilibrium shape of Great Rhombicuboctahedron [14], with a total of 26 facets on the {1 0 0}, {1 1 0} and {1 1 1} planes (Fig. 1(a) and (b)). In further evolution at growth process, coarsened Al₃Sc particles still have some clear plane facets [14] (Fig. 1(a)). When the plane of these facets fits well with the habit plane of some non-spherical precipitates, such as the {1 0 0}-Al₂Cu plates in Al–Cu alloy (Fig. 1(c) and (d)), the Al₂Cu plates will have a strong interaction with the early-formed Al₃Sc dispersoids.

The purpose of present work was to examine the influence of solution treatment on the precipitation behaviors and age hardening response of Al–Cu–Sc alloy, in comparison with Sc-free one. The present results show that the solution treatment is crucial for achieving a significant strengthening effect in the heat-treatable Al alloys with Sc addition.