Al–Mg alloy engineered with bimodal grain size for high strength and increased ductility
Introduction
The development of nanostructured metals for structural applications must address issues related to the both fabrication of bulk samples and overall material performance. For the first task, several techniques have proved successful for fabricating nanocrystalline metals, including inert gas condensation, rapid solidification and mechanical attrition e.g., [1], [2], [3], [4]. In many cases, however, the techniques have yielded small samples for research but are not conducive to production of bulk forms required for structural applications. Regarding material performance, nanocrystalline metals generally exhibit increased strength in comparison to their coarser-grained counterparts, but at the expense of ductility [5], [6]. This observation has been attributed to limited dislocation activity, and thus absence of work hardening, leading to mechanical instability.
Ductility enhancements have been achieved in nanostructured metals through the incorporation of larger grains in a fine-grained matrix. Legros et al. [7] and Tellkamp et al. [8] observed this in tensile tests of copper and Al 5083 alloy, respectively. In both cases, considerable strengthening was achieved relative to conventional counterparts, with a yield strength of 535 MPa for Cu and 334 MPa for Al 5083. The tensile fracture strains were 2.1% and 8.4%, respectively, both of which are higher than typical nanocrystalline metals. The formation of larger grains was achieved by recrystallization during warm compaction [7] or bimodal grain growth during consolidation by hot isostatic pressing (HIPping) and extrusion [8].
A similar phenomenon was reported for Al–Zr alloys [9]. In Al specimens having Zr concentrations of 0, 0.6 and 6.4 wt.%, tensile elongation was inversely correlated with both grain size and yield stress. In Al–0.6Zr, however, a bimodal grain size distribution was noted in which larger (>100 nm) Al grains were found within the nanostructured matrix. The bimodal grain structure was attributed to individual grain boundaries that were not stabilized by an inhomogeneous distribution of Zr. Similarly, inhomogeneous solute distribution and residual porosity associated with inert gas condensation was surmised to cause abnormal grain growth at room temperature in nanocrystalline Cu [10].
In these studies, the presence of large grains within the nanostructured matrix could be considered a detrimental, as the microstructure is neither uniform nor nanocrystalline. Nevertheless, the enhanced ductility associated with such materials prompted some authors to suggest that a bimodal microstructure might be desirable for engineering purposes [7], [8]. This approach was demonstrated by the thermomechanical treatment of nanocrystalline Cu sheet to deliberately grow larger grains, producing dramatic improvements in strength and ductility [11]. In this approach, the inherently non-equilibrium nature of the microstructure was exploited to generate a heterogeneous microstructure.
In this paper, we describe the fabrication of a nanostructured Al–Mg alloy engineered for high strength with enhanced ductility. Unlike the cited work on nanocrystalline Cu [11], the increased ductility is achieved by the deliberate addition of a coarser-grained fraction. Our work represents a fundamentally different approach to engineering nanostructured materials for structural applications. Furthermore, the bulk nanostructured alloys are produced by common powder metallurgy techniques. Thus, the nanostructured materials described herein constitute prototypes for larger-scale extrusions.
Section snippets
Experimental
Nanocrystalline powder was produced using low-energy mechanical attrition at a cryogenic temperature (cryomilling). Spray atomized Al–7.5Mg alloy powders were cryomilled in a modified Union Process 01-HD attritor with a stainless steel vessel and 6.4 mm milling balls. 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 attritor was operated at 180 rpm, and maintained at −190 °C using flowing liquid nitrogen,
Results
The cryomilled powder had a grain size of ∼20 nm, as determined by X-ray diffraction and TEM. In addition, X-ray diffraction data indicated that despite the high weight percent of Mg, an Al(Mg) fcc solid solution formed during cryomilling [12]. The cryomilled powder demonstrated unusual thermal stability. A grain size of <80 nm was maintained during isothermal annealing at 400 °C for 6 h [12]. The thermal stability may be compared with that of cryomilled pure Al powders [13]. While the
Discussion
The tensile yield strengths for the nanostructured Al–Mg specimens are 3–5 times higher than conventional Al 5083. This is consistent with the increases in yield strength and hardness for other nanostructured materials relative to coarser-grained counterparts [4], [5]. The increase in strength is often discussed in terms of the Hall–Petch relationship between grain size and yield strength, and whether this relationship extends to the smallest grain sizes. In the case of nanostructured metals
Conclusions
Nanostructured Al–7.5Mg powders were produced by cryomilling and subsequently consolidated and extruded, yielding high-strength material with a grain size of 100–300 nm. Blending of unmilled alloy powder of the same composition with the cryomilled powder followed by the same consolidation and extrusion steps increased the elongation to failure several-fold, with a decrease in yield strength of 15% or less. These results illustrate a novel approach to engineering nanostructured materials with
Acknowledgements
This work is supported by the Office of Naval Research under Contract Number N00014-01-C-0384, under the supervision of Rodney Peterson.
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