The literature on grain refinement of magnesium alloys is reviewed with regard to two broad groups of alloys: alloys that contain aluminum and alloys that do not contain aluminum. The alloys that are free of aluminum are generally very well refined by Zr master alloys. On the other hand, the understanding of grain refinement in aluminum bearing alloys is poor and in many cases confusing probably due to the interaction between impurity elements and aluminum in affecting the potency of nucleant particles. A grain refinement model that was developed for aluminum alloys is presented, which takes into account both alloy chemistry and nucleant particle potency. This model is applied to experimental data for a range of magnesium alloys. It is shown that by using this analytical approach, new information on the refinement of magnesium alloys is obtained as well as providing a method of characterizing the effectiveness of new refiners. The new information revealed by the model has identified new directions for further research. Future research needs to focus on gaining a better understanding of the detailed mechanisms by which refinement occurs and gathering data to improve our ability to predict grain refinement for particular combinations of alloy and impurity chemistry and nucleant particles.

Despite the extensive literature on grain refinement, there is not a consensus on the mechanism of grain refinement in aluminum alloys. Recently, there has been a shift in understanding of the grainrefinement paradigm from purely being concerned with the nucleation event, called here the "nucleant paradigm," to also being concerned with the effect of solute elements, or, the "solute paradigm," on the final grain structure. This article is divided into two parts. In Part I, the literature underpinning both paradigms is explained, and the validity of the paradigm shift toward the solute paradigm as a more complete understanding of grain refinement is presented. Part II experimentally confirms the validity of the solute paradigm and details a mechanism which explains the need for both effective nucleants and a solute of a good segregating power in order to obtain grain refinement.

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To be able to determine the grain size obtained from the addition of a grain refining master alloy, the relationship between grain size (d), solute content (defined by the growth restriction factor Q), and the potency and number density of nucleant particles needs to be understood. A study was undertaken on aluminium alloys where additions of TiB 2 and Ti were made to eight wrought aluminum alloys covering a range of alloying elements and compositions. It was found from analysis of the data that . From consideration of the experimental data and from further analysis of previously published data, it is shown that the coefficients a and b relate to characteristics of the nucleant particles added by a grain refiner. The term a is related to the maximum density of active TiB 2 nucleant particles within the melt, while b is related to their potency. By using the analysis methodology presented in this article, the performance characteristics of different master alloys were defined and the effects of Zr and Si on the poisoning of grain refinement were illustrated. d ϭ a 1 3 pct TiB 2 ϩ b Q

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

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