Length-scale dependent microalloying effects on precipitation behaviors and mechanical properties of Al–Cu alloys with minor Sc addition

Abstract

Heat-treatable Al alloys containing Al–2.5 wt% Cu (Al–Cu) and Al–2.5 wt% Cu–0.3 wt% Sc (Al–Cu–Sc) with different grain length scales, i.e., average grain size >10 μm ( defined coarse grained, CG), 1–2 μm (fine grained, FG), and <1 μm (ultrafine grained, UFG), were prepared by equal-channel angular pressing (ECAP). The length scale and Sc microalloying effects and their interplay on the precipitation behavior and mechanical properties of the Al–Cu alloys were systematically investigated. In the Al–Cu alloys, intergranular θ-Al₂Cu precipitation gradually dominated by sacrificing the intragranular θ′-Al₂Cu precipitation with reducing the length scale. Especially in the UFG regime, only intergranular θ-Al₂Cu particles were precipitated and intragranular θ′-Al₂Cu precipitation was completely disappeared. This led to a remarkable reduction in yield strength and ductility due to insufficient dislocation storage capacity. The minor Sc addition resulted in a microalloying effect in the Al–Cu alloy, which, however, is strongly dependent on the length scale. The smaller is the grain size, the more active is the microalloying effect that promotes the intragranular precipitation while reduces the intergranular precipitation. Correspondingly, compared with their Sc-free counterparts, the yield strength of post-aged CG, FG, and UFG Al–Cu alloys with Sc addition increased by ~36 MPa, ~56 MPa, and ~150 MPa, simultaneously in tensile elongation by ~20%, ~30%, and 280%, respectively. The grain size-induced evolutions in vacancy concentration/distribution and number density of vacancy-solute/solute–solute clusters and their influences on precipitation nucleation and kinetics have been comprehensively considered to rationalize the length scale-dependent Sc microalloying mechanisms using positron annihilation lifetime spectrum and three dimension atom probe. The increase in ductility was analyzed in the light of Sc microalloying effect and the strength contributions by different strengthening mechanisms was quantified as well.

Introduction

The heat-treatable Al alloys, as the utmost important structural materials, which are strengthened by the formation of precipitates, are extensively used in the automotive and aviation industries [1]. The binary Al–Cu alloys are one of the most studied precipitation-strengthened alloy systems, because it forms the basis for a wide range of age-hardening alloys, i.e., the so-called 2XXX series [2]. The precipitation sequence observed on aging these alloys, supersaturated solid solution (SSSS)→Guinier–Preston I zones (GPI zones)→θ″(GPII zones)→θ′→θ, is often used as a model system for describing the fundamentals of precipitation process and precipitate hardening [2].

The addition of trace amounts of selected microalloying elements to age hardenable Al alloys causes changes in the nature of clustering and precipitation process that can have a marked effect on mechanical properties [3], [4], [5], [6], [7]. Such microalloying effects have been also applied to the Al–Cu alloys [4], [5], [6], [7], [8], [9], [10], [11], [12]. The well-known example is that trace addition of single or combined Sn, Cd, In, Mg, Si, Ge or Sc in coarse grained (CG) Al–Cu alloy can enhance both the rate and extent of age hardening at elevated temperature, by promoting the formation of θ′ [5], [6], [7], [8], [9], [10], [11], [12]. The proposed microalloying mechanisms mainly focus on two aspects. One is that the trace elements facilitate heterogeneous nucleation of θ′ either directly at Sn (Mg, Si, Cd, In or Sc)-rich particles/clusters, or indirectly at the dislocation loops present in the as-quenched microstructure [5], [8], [9], [10]. The other is that the Si (Mg, Ge, Cd, In or Sc) atoms segregate to the precipitate/matrix interface and somewhat lower the interfacial energy [6], [11], [12]. The solute atom segregation will change the interfacial conditions (e.g., interface structure, chemistry composition, and energies) and cause a series of evolutions in both precipitation behaviors and hardening responses, including precipitate nucleation and concomitantly number density and driving force for precipitate coarsening [6], [13].

In heat-treatable CG Al alloys, the conventional way to approach high strength is through the controlling of intragranular precipitation of nanosized particles [2]. Severe plastic deformation (SPD) (the most studied being equal-channel angular pressing (ECAP)) is now a widely used way to create materials with ultrafine grained (UFG)/nanostructured (NS) microstructures [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37]. Therefore it is not surprising that the precipitation behaviors in the UFG/NS Al alloys are being attracted growing attention [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], with the aim of (i) achieving even higher strength derived from both the small grain sizes and the precipitates (a yield strength of 1 GPa and a uniform elongation of ~5% have been achieved in an Al–Zn–Mg–Cu alloy with a nanostructured hierarchy [36]) and (ii) improving thermal stability as well as ductilization effect by precipitates pinning [16], [20], [25], [26], [29], [30], [31], [32], [33], [34], [35], [36]. Here, it is worth noting that UFG/NS alloys usually have strikingly improved strength but disappointingly low ductility at ambient temperatures due to insufficient dislocation accumulation capability [14], [15], [16], [17], [20], [29], [30], [31], [32], [33], [34], [35], [36]. Dispersing nanosized particles in the grain interior has been proposed to improve ductility of such alloys owing to the fact that the dislocations are forced to accumulate while intersecting or by passing the second-phase particles, leading to strain hardening [16], [20], [25], [26], [29], [30], [31], [32], [33], [34], [35], [36]. In particular, this is the simplest but best approach that has been proved to be applicable for the UFG/NS heat-treatable Al alloys [16], [20], [29], [30], [31], [32], [33], [34], [35], [36], i.e., Al–Cu–Mg-based 2000 series [29], [30], Al–Mg–Si-based 6000 series [31], [32], [33], and Al–Zn–Mg-based 7000 series [34], [35], [36] UFG/NS Al alloys.

Precipitation behaviors of heat-treatable Al alloys are also strongly sensitive to the initial microstructure, besides the microalloying elements. Most recently, the length-scale-dependent precipitation and related phenomena have received some attentions [17], [18], [19], [21], [22], [23], [24], [25], [34], [37]. It has been found that, in the UFG/NS alloys, precipitation occurred much faster (at a rate from one to several orders of magnitude faster) than that in their CG counterparts [7], [17], [23], [24], [37]. In many cases the intermediate metastable phases were skipped so that the equilibrium phase was directly formed on defects (mainly grain boundaries (GBs)) at aging temperatures much lower than the conventional ones or even at natural aging treatment [7], [17], [23], [24], [37]. For example in UFG Al–Cu alloys, extensive equilibrium θ-Al₂Cu phases instead of metastable θ′ were precipitated at grain boundaries after a few weeks stay at room temperature. Similarly in the UFG Al–Zn–Mg–Cu alloys, the precipitation of equilibrium phases at grain boundaries was also observed [17]. These indicate [17], [25] that there exist notable differences in the size, chemistry and spatial distribution of precipitates between the UFG and CG alloys. The precipitations in UFG alloys preferentially occurred at grain boundaries [7], [17], [18], [19], [22], [23], [24], [25], [37], due to the predominant effect of very small grains (high fraction of GBs area) or so-called “non-equilibrium” high angle grain boundaries (HAGBs) [7], [14], [15], [16], [23]. The UFG Al–Cu alloys suffer most from the intergranular precipitation [7], [23], [24]. The intergranular hard particles introduce stress concentration at the grain boundaries and tend to localize strain and cracking near them, resulting in intergranular fracture and unresolved low ductility [7], [38]. The key to prolong ductility of UFG Al alloys is, therefore, to suppress the intergranular precipitation and promote the intragranular precipitation. Our latest results [7] showed that minor Sc addition into UFG Al–Cu alloys can result in a full intragranular precipitation, and hence improve the strength/ductility combination.

In spite of the limited investigations mentioned above, the length-scale dependent precipitation behaviors in heat-treatable Al alloys are still far from clear. Precipitation at the UFG/NS scales is a complex topic since the microstructures and defects (i.e. dislocation density, grain boundaries, solutes/vacancy concentrations) are quite different from those well-studied CG alloys. In addition, the vast majority of published studies on microalloying effects are focused on the CG Al alloys. The microalloying effects at different length scales are unknown.

In the present paper, we performed systematic studies, trying to answer the following two questions. Firstly, whether the microalloying effects on precipitation behaviors and mechanical properties of Al alloys dependent on length-scale? If it is true, how the grain size-dependent defect structures, such as vacancies, dislocations, and grain boundaries, etc., exert influences on the microalloying effects? Secondly, at which length scale the microalloying effect is the most significant? How to develop new materials with enhanced mechanical properties utilizing the most significant microalloying effect? For these purposes, Al–Cu and minor Sc-added Al–Cu–Sc alloys with different grain length scales, i.e, average grain size >10 μm (here we define them coarse grained alloys, CG), 1–2 μm (fine grained, FG), and <1 μm (ultrafine grained, UFG), were respectively prepared by equal-channel angular pressing (ECAP). This allows us to investigate the grain length scale-dependent Sc microalloying effects on the precipitation behaviors and concomitantly on mechanical properties.