Novel ternary Zn–Ca–Cu alloys were studied for the development of absorbable wound closure device material due to Ca and Cu's therapeutic values to wound healing. The influence of Ca and Cu on the microstructure, mechanical and degradation properties of Zn were investigated in the as-cast state to establish the fundamental understanding on the Zn–Ca–Cu alloy system. The microstructure of Zn-0.5Ca-0.5Cu, Zn-1.0Ca-0.5Cu, and Zn0.5Ca-1.0Cu is composed of intermetallic phase CaZn
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distributed within the Zn–Cu solid solution. The presence of CaZn
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phase and Cu as solute within the Zn matrix, on the one hand, exhibited a synergistic effect on the grain refinement of Zn, reducing the grain size of pure Zn by 96%; on the other hand, improved the mechanical properties of the ternary alloys through solid solution strengthening, second phase strengthening, and grain refinement. The degradation properties of Zn–Ca–Cu alloys are primarily influenced by the micro-galvanic corrosion between Zn–Cu matrix and CaZn
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phase, where the 0.5% and 1.0% Ca addition increased the corrosion rate of Zn from 11.5 μm/y to 19.8 μm/y and 29.6 μm/y during 4 weeks immersion test.
Recent research and a simulation of heat transfer and solidification during acoustically generated convection showed that the location of the coolest liquid, and thus the place where the first grains are expected to form, is under the sonotrode. Further, the generated vigorous convection produces a very flat temperature gradient in the bulk of the melt facilitating the formation of a refined equiaxed structure throughout the casting. This study validates these findings through a series of experiments on an Al -2 wt.% Cu alloy, that evaluate grain formation under the sonotrode over time and relates this to the formation of the macrostructure of a cast ingot. Analysis of the results confirms the predictions of the simulation and shows, for the conditions applied, that most grains nucleated in the cavitation zone are swept into the melt by acoustically generated convection and over a period of 70 seconds the number of grains increase and they grow with spherical and globular morphology gradually filling the casting with refined equiaxed grains. It was found that the macrostructure of each casting is made up of three microstructural zones. A fine grained equiaxed zone forms from the bottom of the casting below the sonotrode due to settling of grains during and after termination of ultrasonic treatment (UST), which increases in size with increasing duration of UST. Above this zone a coarse grained structure is formed due to depletion of UST-generated grains on termination of UST. At the top of the casting a zone of columnar grains growing from the top surface of the melt is formed. The latter two zones decrease in size with increasing UST duration until 80 seconds where the macrostructure consists entirely of the equiaxed zone.
Research on ultrasonic treatment (UST) of aluminium, magnesium and zinc undertaken by the authors and their collaborators was stimulated by renewed interest internationally in this technology and the establishment of the ExoMet program of which The University of Queensland (UQ) was a partner. The direction for our research was driven by a desire to understand the UST parameters that need to be controlled to achieve a fine equiaxed grain structure throughout a casting. Previous work highlighted that increasing the growth restriction factor Q can lead to significant refinement when UST is applied. We extended this approach to using the Interdependence model as a framework for identifying some of the factors (e.g., solute and temperature gradient) that could be optimised in order to achieve the best refinement from UST for a range of alloy compositions. This work confirmed established knowledge on the benefits of both liquid-only treatment and the additional refinement when UST is applied during the nucleation stage of solidification. The importance of acoustic streaming, treatment time and settling of grains were revealed as critical factors in achieving a fully equiaxed structure. The Interdependence model also explained the limit to refinement obtained when nanoparticle composites are treated. This overview presents the key results and mechanisms arising from our research and considers directions for future research.
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