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Because the mechanical performance of precipitation-hardened alloys can be significantly altered with temperature changes, understanding and predicting the effects of temperatures on various mechanical properties for these alloys are important. In the present work, an analytical model has been developed to predict the elastic modulus, the yield stress, the failure stress, and the failure strain taking into consideration the effect of temperatures for precipitation-hardenable Al-Mg-Cu-Si Alloys (Al-A319 alloys). In addition, other important mechanical properties of Al-A319 alloys including the strain hardening exponent, the strength coefficient, and the ductility parameter can be estimated using the current model. It is demonstrated that the prediction results based on the proposed model are in good agreement with those obtained experimentally in Al-A319 alloys in the as-cast condition and after W and T7 heat treatments.

Metal matrix nanocomposites (MMNCs) synthesized by the inexpensive and scalable method of stir mixing have received relatively little attention due to the perceived difficulty of dispersing nanoparticles (NPs) in molten metal. However, matrix/reinforcement reactions may be useful in deagglomeration of the particles. A review of previous experimental studies shows that solid solution, Orowan, and grain boundary (GB) strengthening are most likely to influence the strength of reactive stirmixed Al-Mg-[Al.sub.2][O.sub.3np] MMNCs. Matrix/reinforcement reactions, porosity in stir-mixed MMNCs, and NP incorporation on grain size are also discussed. Analysis of variation of grain size with NP concentration shows that the MMNCs of this study do not follow the trend of MMNCs strengthened primarily by GB strengthening, and transmission electron microscope shows that individual NPs and agglomerates were present within the aluminum alloy matrix. This evidence coupled with strength beyond what would be expected from solid solution and GB strengthening indicate that Orowan strengthening is likely present--though significant gas porosity in low Mg concentration MMNCs is often detrimental.

Metal matrix syntactic foams are promising materials for energy absorption; however, few studies have examined the effects of hollow sphere dimensions and foam microstructure on the quasi-static and high strain rate properties of the resulting foam. Aluminum alloy A380 syntactic foams containing [Al.sub.2][O.sub.3] hollow spheres sorted by size and size range were synthesized by a sub-atmospheric pressure infiltration technique. The resulting samples were tested in compression at strain rates ranging from [10.sup.-3] [s.sup.-1] using a conventional load frame to 1720 [s.sup.-1] using a Split Hopkinson Pressure-bar test apparatus. It is shown that the quasi-static compressive stress-strain curves exhibit distinct deformation events corresponding to initial failure of the foam at the critical resolved shear stress and subsequent failures and densification events until the foam is deformed to full density. The peak strength, plateau strength, and toughness of the foam increases with increasing hollow sphere wall thickness to diameter (t/D) ratio. Since t/D was found to increase with decreasing hollow sphere diameter, the foams produced with smaller spheres showed improved performance. The compressive properties did not show measurable strain rate dependence.

Particle-reinforced metal matrix nanocomposites (MMNCs) have been lauded for their potentially superior mechanical properties such as modulus, yield strength, and ultimate tensile strength. Though these materials have been synthesized using several modern solid- or liquid-phase processes, the relationships between material types, contents, processing conditions, and the resultant mechanical properties are not well understood. In this paper, we examine the yield strength of particle-reinforced MMNCs by considering individual strengthening mechanism candidates and yield strength prediction models. We first introduce several strengthening mechanisms that can account for increase in the yield strength in MMNC materials, and address the features of currently available yield strength superposition methods. We then apply these prediction models to the existing dataset of magnesium MMNCs. Through a series of quantitative analyses, it is demonstrated that grain refinement plays a significant role in determining the overall yield strength of most of the MMNCs developed to date. Also, it is found that the incorporation of the coefficient of thermal expansion mismatch and modulus mismatch strengthening mechanisms will considerably overestimate the experimental yield strength. Finally, it is shown that work-hardening during post-processing of MMNCs employed by many researchers is in part responsible for improvement to the yield strength of these materials.

Aluminum Alloy AA-7034 is a high strength wrought alloy with reasonable ductility containing 10–12 wt% Zn, 2–3 wt% Mg, and 0.8–1.2 wt% Cu. This work investigates the effect of varying the concentration of Zn (10–12 wt%) and Cu (0.8–1 wt%) on the solutionizing and aging behavior of squeeze cast AA-7034 samples. The same behaviors were investigated when Mg content was increased beyond 3 wt%. The solutionizing heat treatment dissolved much of the macroscopic second phases present in the as-cast AA-7034 alloys, but a significant amount of second phases remain after solutionizing in alloys with >3 wt% Mg. The behaviors of the various Al-Zn-Mg-Cu alloys are compared to squeeze cast Al-A206 casting alloy heat-treated to the T7 condition. All Al-Zn-Mg-Cu alloys obtained higher hardness values than those obtained by Al-A206-T7.

Self-healing in inorganic materials is a relatively new area in materials science and engineering that draws inspiration from biological systems that can self-repair damage. This article reviews the preliminary attempts to impart self-healing behavior to metals. Several challenges yet exist in the development of metallic alloys that can self-repair damage, including surface bonding issues, such as liquid/solid contact angle (wetting) and oxidation, and practical issues, such as capillary pressure for delivery of a liquid metal to a damaged area or crack, and the overall mechanical properties of a composite system. Although the applied research approaches reviewed have obtained marginal success, the development of self-healing metallic systems has the potential to benefit a wide range of industrial applications and thus deserves greater investment in fundamental research.

Metal matrix syntactic foams are promising materials with high energy absorption capability. To study the effects of matrix strength on the quasistatic compressive properties of syntactic foams using SiC hollow particles as reinforcement, matrices of Al-A206 and Mg-AZ91 were used. Because Al-A206 is a heat-treatable alloy, matrix strength can be varied by heat treatment conditions, and foams in as-cast, T4, and T7 conditions were tested in this study. It is shown that the peak strength, plateau strength, and toughness of the foams increase with increasing yield strength of the matrix and that these foams show better performance than other foams on a specific property basis. High strain rate testing of the Mg-AZ91/SiC syntactic foams showed that there was little strain rate dependence of the peak stress under strain rates ranging from 10 −3 /s to 726/s.

Magnesium alloys have considerably lower density than the aluminum alloy matrices that are typically used in syntactic foams, allowing for greater specific energy absorption. Despite the potential advantages, few studies have reported the properties of magnesium alloy matrix syntactic foams. In this work, Al2O3 hollow particles of three different size ranges, 0.106–0.212 mm, 0.212–0.425 mm, and 0.425–0.500 mm were encapsulated in Mg-AZ91D by a sub-atmospheric pressure infiltration technique. It is shown that the peak strength, plateau strength and toughness of the foam increases with increasing hollow sphere wall thickness to diameter (t/D) ratio. Since t/D was found to increase with decreasing hollow sphere diameter, the foams produced with smaller spheres showed improved performance—specifically, higher energy absorption per unit weight. These foams show better performance than other metallic foams on a specific property basis.