Opportunities for aluminum-based nanocomposites

Abstract

Abstract. High performance aluminum alloys are conventionally made by heat treating alloys containing a variety of alloying elements in solid solution. Key performance attributes are controlled at the microstructural level by tailoring sizes and morphology of nano-sized second phases. This enabled the successful development of aluminum alloys having properties optimized in strength, damage tolerance and corrosion resistance. However, this process is naturally limited by the solubility of alloying elements in the aluminum matrix. In real world products, significant effort is deployed to achieve a homogeneous distribution of the alloying elements both at the macro and micro scales. Despite these efforts, heat treatable alloys can exhibit chemical gradients at grain boundaries, resulting in sub-optimized properties. Additionally, due to the very nature of the strengthening mechanisms, the properties of heattreatable alloys are decreasing when exposed to elevated temperatures. To step outside the boundaries given by the solubility of alloying elements in the aluminum matrix, the extrinsic addition of nano-sized particles to the aluminum matrix is being evaluated. IntroductionImproving the properties of aluminum alloys has a long history. A combination of the established metallurgical mechanisms, cold work, solid solution strengthening and precipitation strengthening, has enabled highly engineered materials to be developed for demanding customer requirements. These requirements typically focus on strength and damage tolerance, while at the same time being of low cost enabling wide spread applications. Many text books cover the underlying mechanisms and how they can be utilized to tailor a materials performance to an application.For a material to be successful commercially, it needs to fulfill a spectrum of property requirements as well as meet a target cost point. Results of new materials or processes are frequently being reported as achievements in tensile strength; however, a successful material for most technical applications must meet not only strength but other properties such as damage tolerance [1] and/or formability. To date, aluminum alloys used for aerospace structures have driven most of the major advancements in material properties. With improvements to manufacturing process and cost structure, these alloys are now finding additional uses in other applications such as transportation. It is not the objective of this paper to review the state of advanced aluminum alloys.Over the years, attempts have been made to step outside of the spectrum enabled by precipitation strengthening and other basic metallurgical methods by using a composite approach. This means adding an extrinsic material in small or large volume fractions to the aluminum matrix to not only improve strength but to also modify the modulus. This material class is commonly referred to as Metal Matrix Composites (MMCs) [2]. When MMCs were developed, most second phase materials that were combined with the aluminum matrix where particles with sizes ...