Bimodal microstructure and deformation of cryomilled bulk nanocrystalline Al–7.5Mg alloy

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Abstract

The microstructure, mechanical properties and deformation response of bimodal structured nanocrystalline Al–7.5Mg alloy were investigated. Grain refinement was achieved by cryomilling of atomized Al–7.5Mg powders, and then cryomilled nanocrystalline powders blended with 15 and 30% unmilled coarse-grained powders were consolidated by hot isostatic pressing followed by extrusion to produce bulk nanocrystalline alloys. Bimodal bulk nanocrystalline Al–7.5Mg alloys, which were comprised of nanocrystalline grains separated by coarse-grain regions, show balanced mechanical properties of enhanced yield and ultimate strength and reasonable ductility and toughness compared to comparable conventional alloys and nanocrystalline metals. The investigation of tensile and hardness test suggests unusual deformation mechanisms and interactions between ductile coarse-grain bands and nanocrystalline regions.

Introduction

In recent years, nanocrystalline (nanostructured or ultrafine-grained) metals have become the topic of numerous scientific and technological studies because these materials exhibit remarkable improvements in strength and create the possibility of weight savings. However, nanocrystalline materials generally possess insufficient ductility and a reduced toughness compared to their coarse-grained conventional counterparts. Attempts to address the loss of ductility of nanocrystalline metals include: inducing the formation of a bimodal grain microstructure with grains sufficiently large to facilitate dislocation activity [1] and annealing to promote grain growth [2]. The annealing approach results in strength degradation due to normal and abnormal grain growth of the nanocrystalline grains. In a recent report, a non-uniform bimodal grain size distribution with micrometer-sized grains embedded inside a matrix of nanocrystalline and ultrafine (<300 nm) grains was formed in pure Cu by cold rolling followed by annealing to achieve a high tensile ductility with relatively high strength [2]. In other reports, Legros et al. [3] and Tellkamp et al. [1] also observed that incorporation of coarser grains improved the ductility of nanocrystalline Cu and Al 5083 alloy, respectively. The coarser grains were formed by recrystallization during warm compaction [3] or abnormal grain growth during consolidation by hot isostatic pressing (HIPing) and extrusion [1]. In both cases, the uniaxial tensile fracture strains were higher than typical nanocrystalline metals, while considerable strengthening was achieved relative to the properties of conventional materials.

Previous findings suggest that the presence of coarser grains within the nanocrystalline matrix may enhance the ductility of nanocrystalline materials [4], [5], [6]. However, in practice, selective grain growth and achievement of target grain size and phase of embedded coarse-grains may be difficult or impossible to control by thermal treatment. Furthermore, this method has inherent limitations in the design and control of the specific microstructure, which is closely dependent on balanced properties of ductility and strength. In contrast, a bimodal (or multi-scale) grain structure can be achieved by consolidation of blended powders. In this process, cryomilled nanocrystalline and coarse-grained powders (and sometimes, intermediate grains and dispersoids) can be combined in specific proportions, enabling the design and manufacture of materials that achieve the desired balance of enhanced strength with acceptable ductility and toughness. A schematic comparison of these two methods is illustrated in Fig. 1. Moreover, for the issue of structural applications, mechanical alloying allows the production of nanocrystalline metal samples of a sufficient size with the enhanced mechanical properties, while several other techniques have been limited to the fabrication of small samples to explore the intrinsic properties of the materials [7], [8], [9].

In the present work, ductile-phase toughening in bimodal structured Al–7.5Mg was achieved by deliberate blending of unmilled coarse-grained powders with cryomilled nanocrystalline powders in select proportions. The microstructures and deformation mechanisms were investigated using scanning and transmission electron microscopy (SEM and TEM) and optical microscopy. The stress–strain behavior was measured using uniaxial tensile tests, and microhardness measurements were performed on extruded samples to reveal the interactions between coarse-grain and nanocrystalline regions.

Section snippets

Experimental procedures

Prealloyed Al–7.5%Mg (in weight percent) powders were used to produce materials for this work. Nanocrystalline powders were prepared using low-energy mechanical attrition at a cryogenic temperature (cryomilling) with a stainless steel vessel and milling balls with a diameter of 6.4 mm. The ball-to-charge ratio was 36:1, with stearic acid added at 0.25% of the powder weight to moderate the cold welding process. The cryomilling apparatus was operated at 180 rpm for 8 h, and maintained at a

Results and discussion

The chemically etched sections of as-extruded bimodal samples are shown in Fig. 2. The alloy containing 0% unmilled (i.e. 100% cryomilled) powder exhibited uniform nanocrystalline grains with a few percent of residual coarse-grains. In contrast, the samples containing 15 and 30% unmilled powders revealed light coarse-grain (CG) regions and darker nanocrystalline (NC) regions in the images. Coarse-grain regions formed elongated bands that extended along the extrusion direction, while the

Conclusions

Bimodal structures of Al–7.5Mg composed of nanocrystalline grains and elongated coarse-grains were produced by consolidation of cryomilled powders, resulting in ductile-phase toughening. Bimodal bulk nanocrystalline Alsingle bondMg alloys showed balanced mechanical properties, including enhanced yield and ultimate strength, and reasonable ductility and toughness compared to comparable conventional alloys and with materials comprised of nanocrystalline grains only.

The present work presents what may be a

Acknowledgement

Support from the Office of Naval Research (contract ONR00014-03-1-0149 and ONR00014-03-C-0163) is gratefully acknowledged.

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