Additive manufacturing (AM), where a part is built layer-by-layer, is a promising approach for creating near-net shapes and is challenging the dominance of conventional manufacturing processes for products with high complexity and greater material efficiency 1 . However, achieving good mechanical properties in the as-produced part, given the variation in solidification conditions including the control of defects in AM, is challenging. In particular there are limited opportunities for post processing to further control the microstructure/properties. Therefore, further metallurgical research on materials for AM is required to accelerate the maturity of AM technology for structural components. 3D-printed titanium alloys have been used in numerous applications, including the biomedical and aerospace industries. However, the 3D-printing of many conventional titanium alloys usually results in a microstructure comprised of coarse columnar grains, which often leads to undesirable anisotropic mechanical properties. In contrast to other common engineering alloys, such as aluminium, there is no commercial grain refiner, containing potent inoculants that can survive in liquid Ti, able to control microstructure effectively. To address this challenge, we have developed a novel technique for AM by using Ti-Cu alloys with a high constitutional supercooling capacity that overrides the negative effect of a high thermal gradient in the melt pool during AM. Through this approach, it is shown that an as-printed Ti-Cu alloy specimen is comprised of fully equiaxed, fine grained microstructure without any special process control or additional subsequent treatment. The new AM Ti-Cu alloys also display promising mechanical properties, compared to conventional alloys under similar processing conditions, due to the formation of an ultrafine eutectoid microstructure by taking full advantage of the high cooling rates and multiple thermal cycles in the AM process. We anticipate that this approach will be equally applicable to other eutectoid forming alloy systems. MainMetal based 3D printing or additive manufacturing (AM) is enabling mass customization of manufactured parts. The intrinsic high cooling rates and high thermal gradient in the metal AM process often leads to a very fine microstructure and a tendency towards almost exclusively columnar grains particularly in Ti-based alloys 1 . Such columnar grains in AM Ti components can cause anisotropic mechanical properties and hence are not desirable 2 . Numerous attempts to optimise the processing parameters of AM have shown that it is extremely difficult to alter the conditions such that equiaxed growth of prior β-Ti grains is promoted 3 . According to the Interdependence Theory 4 , the key factors controlling grain Affiliations

In additively manufactured (AM) metallic materials, the fundamental interrelationships that exist between composition, processing, and microstructure govern these materials’ properties and potential improvements or reductions in performance. For example, by using AM, it is possible to achieve highly desirable microstructural features (e.g., highly refined precipitates) that could not otherwise be achieved by using conventional approaches. Simultaneously, opportunities exist to manage macro-level microstructural characteristics such as residual stress, porosity, and texture, the last of which might be desirable. To predictably realize optimal microstructures, it is necessary to establish a framework that integrates processing variables, alloy composition, and the resulting microstructure. Although such a framework is largely lacking for AM metallic materials, the basic scientific components of the framework exist in literature. This review considers these key components and presents them in a manner that highlights key interdependencies that would form an integrated framework to engineer microstructures using AM.

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High entropy alloys represent a unique class of metal alloys, comprising nominally five or more elements in near equiatomic proportions. High entropy alloys have gained significant interest on the basis that the high configurational entropy of such alloy systems is purported to result in a single-phase solid solution structure. While such a single-phase structure can occur in unique systems, it is now appreciated that the definition of high entropy alloys can be broader, with systems comprising only four elements possible of forming single phases, and most five (or more) element systems actually being multi (>2) phases. To this end, the notion of compositionally complex alloys is a more general description, with the concise review herein focusing on the corrosion of compositionally complex alloys (inclusive of high entropy alloys). It is noted that generally, in spite of complex compositions and in many cases complicated microstructural heterogeneity, compositionally complex alloys are nominally corrosion-resistant. This is discussed and aspects of the status and needs are presented.

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