Investigation of aluminum-based nanocomposites with ultra-high strength
Introduction
Particulate-reinforced metal matrix composites (MMCs) have the potential to provide tailored mechanical properties, for example, high specific stiffness and specific strength and creep resistance [1], [2], [3], which render them attractive for applications in the aerospace, defense and automotive industries to name a few [4], [5], [6]. Among the various MMCs, Al-based composites are of interest because of their low density and good formability [7], [8]. These properties, in combination with recent interest in the high strength of nanostructured (NS) Al alloys [9], [10], [11] have prompted efforts aimed at using NS Al alloys as matrices in MMCs. These efforts have met with only limited success, partly as a result of the fact that the high strength in NS Al alloys is often accompanied with significantly diminished ductility [12], [13], [14]. A number of strategies have emerged in an effort to improve the poor ductility of NS materials [15], [16], [17], [18], [19], [20]. In reference to these various strategies, numerous experiments have verified that the introduction of a bi/multi modal grain size distribution represents an effective approach to improve ductility while retaining a moderate strength level [17], [21], [22], because the NS microconstituent provides high strength while the coarse-grained (CG) microconstituent facilitates plasticity.
On the basis of these results, the novel concept of a tri-modal composite consisting of three phases: coarse-grained matrix, ultrafine- or nano-grained matrix and ceramic reinforcement was recently demonstrated [23]. B4C was selected as the ceramic reinforcement, because it ranks third in hardness, just after diamond and cubic boron nitride, and possesses a low density of 2.51 g/cm3 (which is even less than that of Al) [24]. Al 5083 was selected as a matrix material, given its importance in many applications [25]. This tri-modal composite, when tested in compression, exhibited extremely high strength (up to 1065 MPa), with a compressive strain-to-failure value of 0.8% [23]. Although the level of plasticity in this material is still quite low, it should be noted that without the addition of the coarse-grained material, the consolidated cryomilled Al 5083 plus B4C failed in a brittle mode without any yielding [26]. Equivalent conventional and nanocrystalline materials exhibit significantly lower strengths (∼225 MPa [27] and ∼470 MPa [28], respectively), motivating the need to better understand the microstructural features that lead to this extremely high strength for an aluminum metal matrix composite.
There are two strengthening mechanisms that are typically associated with conventional MMCs: direct strengthening resulting from load transfer from the metal matrix to the reinforcing particle [29], [30] and indirect strengthening resulting from the influence of reinforcement on matrix microstructure or deformation mode [31], such as dislocation strengthening induced by the deformation mismatch between the reinforcement and the matrix. In the case of the tri-modal composite, one needs to understand the individual roles of the UFG and CG microconstituents [29] and the accompanying grain refinement, Orowan (e.g., secondary phase dispersoids) and Taylor (dislocation based) strengthening mechanisms [32], [33], [34], [35]. Published results show that the strength of a tri-modal Al 5083 based composite, as calculated from the Hall–Petch relationship, and invoking an Orowan strengthening mechanism and the rule-of-mixtures, was 792 MPa [23], which is about 273 MPa lower than the experimental value. This discrepancy, in addition to the lack of published studies on this material, suggests that the microstructural origins of the strengthening behavior require further study.
In this work, we have performed systematic microstructure studies on the UFG and CG Al 5083 matrix in the tri-modal Al 5083 based composite specifically aimed at characterizing the following: (1) grain size and distribution, (2) composition and distribution of secondary phase dispersoids by scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDX), (3) dislocation density and configuration by high resolution electron microscopy (HREM), and (4) interface structures between the CG and UFG microconstituents and between the B4C and the Al 5083 matrix. These factors are discussed in terms of the underlying microstructural mechanisms, and their possible contributions to the measured strength values.
Section snippets
Sample preparation
The bulk tri-modal Al 5083 based composite was synthesized by cryomilling, blending, degassing, cold isostatic pressing (CIP) and hot extrusion, as described in previous studies [23], [36], [37].
Microstructural characterization
Scanning electron microscopy (SEM) images were taken in a Philips XL 30 FEG scanning electron microscope with a voltage of 15 kV. The TEM specimens were prepared by mechanically grinding the bulk materials to a thickness <30 μm, then dimpling from both sides to a thickness of approximately 10 μm. Further
Microconstituents identification and distribution
The UFG, CG and B4C microconstituents in the Al 5083 based composite were revealed by bright-field TEM imaging, as shown in Fig. 1. The B4C particles, primarily distributed within the UFG regions, have an angular morphology containing straight and sharp interfaces with the Al 5083 matrix. The morphology of the B4C particles is consistent with that of the starting powder. The B4C particles were enclosed in NC Al 5083 powder during cryomilling and therefore had no direct contact with the CG
Microstructure contributions to strength
The above results provide systematic information on the various microstructural microconstituents of the tri-modal composite, the results of which are summarized in Table 3. In the sections that follow we use this information to provide insight into the relative contributions of individual strengthening mechanisms, on the basis of available theories. We then look at select approaches to aggregate the strength contributions so that overall strength can be estimated.
Summary
In this work, an in-depth microstructural analysis was coupled with existing mechanics models to quantitatively and qualitatively examine a bulk tri-modal Al 5083 based composite and the likely mechanisms that govern its strengthening behavior. The results may be summarized as follows.
- 1.
In addition to Hall–Petch strengthening in the CG and UFG Al 5083 matrix, we also found dispersoids and dislocations, which further increase the strengths of the UFG and CG microconstituents to about 875 MPa and 550
Acknowledgements
The authors would like to acknowledge financial support from Army Research Laboratory Cooperative Agreement No: W911NF-08-2-0028. The authors also are grateful to Prof. S. P. Joshi, Department of Mechanical Engineering at National University of Singapore, for providing the algorithm for the M–T model and for useful technical discussions.
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