Optimizing the strength and ductility of fine structured 2024 Al alloy by nano-precipitation

Abstract

Alloys with grain or subgrain structures refined down to 1 μm or below usually have high strength, but often inadequate tensile ductility. Past efforts in improving their ductility have usually led to a sacrifice of strength. We have developed an effective approach in achieving both high strength and high ductility in a 2024 Al alloy. The approach involves solution-treatment to partially dissolve T-phase particles, cryo-rolling to produce a fine-structure containing a high density of dislocations and submicrometer subgrains and aging to generate highly dispersed nano-precipitates. It was found that the remnant T-phase particles made it very effective in accumulating dislocations during cryo-rolling, which in turn promoted the precipitation of nanosized S′ precipitates with an interparticle spacing of only 10–20 nm. Such a high density of S′ precipitates enabled effective dislocation pinning and accumulation, leading to simultaneous increases in strength, work-hardening ability and ductility.

Introduction

There has been a strong push to refine the grains or subgrains of metals into the submicrometer and nanocrystalline regimes. Such metals typically have high strength, but their inadequate ductility has become a major issue in their practical application. In recent years, a number of methods have been developed to improve the tensile ductility [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]. While most of the earlier approaches improved ductility at the expense of yield strength (e.g., Ref. [3]), a few most recent approaches have succeeded in simultaneously achieving high strength and ductility. One such successful approach involves nano-precipitation hardening in 7075 Al alloy [15]. The processing steps in this approach include: (1) solution-treating the Al alloy to dissolve all second-phase particles and to produce an oversaturated solid solution; (2) cryo-rolling at liquid nitrogen temperature to produce ultrafine-grained (UFG) microstructures, and (3) aging to produce highly dispersed second-phase nano-sized particles. It should be noted that similar precipitation approaches were also attempted earlier and produced good combinations of high strength and ductility [17], [18], [19], [20], [21], [12], although no simultaneous increases in strength and ductility were achieved or verified. For instance, equal-channel angular pressing (ECAP) has been applied to different Al alloy systems to obtain high strength and ductility [19], [20], [21], [12], [22]. Kim et al. reported that by combining solid solutioning, ECAP and post-ECAP low-temperature aging, the yield strength of 2024 Al alloy was improved to ∼630 MPa while maintaining a respectable ∼15% elongation to failure. However, their study did not explore much on the mechanism [21]. Horita et al. studied the effect of post-ECAP low-temperature aging on tensile behavior of Al–10.8 wt.%Ag alloy [10]; by high-resolution transmission electron microscopy (HRTEM), they revealed that work hardening can be improved by producing fine “η-zone” precipitate particles with sizes <50 nm dispersed in the grain volume.

In all of the foregoing investigations, the second-phase particles were mostly or completely dissolved into the matrix to produce a supersaturated solid solution. This may not be desirable for some Al alloys. For example, the high concentrations of alloying elements in the 2024 Al alloy render the alloy very brittle if all of the second-phase (T-phase) particles are dissolved into the matrix, making it very difficult or impossible to refine its grains by plastic deformation methods such as cryo-rolling or ECAP. In addition, if one leaves some second-phase particles undissolved, they could play a positive role in promoting dislocation accumulation and grain refinement during the severe plastic deformation step. This, if proved true, may in turn promote the precipitation of nano-sized second-phase particles during aging, which may further enhance both the strength and ductility by resisting dislocation slip and trapping dislocations.

It was the objective of this study to develop an improved procedure for simultaneously enhancing the strength and ductility of age-hardening Al alloys. In this approach, we aimed to only partially dissolve the second-phase particles during the solution treatment to improve the processability and to enhance the precipitation of nano-sized second-phase particles. We used 2024 Al-alloy in this study because it represents a type of alloys that are brittle if the second-phase particles are completely dissolved. In addition, our investigation on the 2024 Al alloy systematically adjusted the processing parameters to obtain the optimized strength and ductility.