The high-pressure torsion processing technique was used to consolidate and process magnesium-based chips. Chips were prepared by machining commercially pure magnesium and a magnesium alloy AZ91 separately. Optical microscopy and microhardness measurements showed good consolidation of pure magnesium. The magnesium alloy continued to exhibit the boundaries between the chips even after 5 turns of HPT suggesting poor bonding. The results show that soft chips are easier to consolidate through HPT than harder alloys.
Herein, the relationship between the grain size and the strength in the magnesium alloy ZK60 in the ultrafine‐grained region is clarified. Discs are processed by high‐pressure torsion to refine the grain size and some samples are subjected to annealing to increase the grain size. Microhardness and indentation creep experiments are used to determine the strength and strain rate sensitivity. Also, the grain structure below the tip of an indentation creep test is compared with its counterpart before testing to show that deformation‐induced grain growth is very limited in this alloy. The results show different trends in grain refinement hardening at different grain size ranges. Grain refinement hardening is observed in the coarse‐grain (>1 μm) region, there is a change in slope in the ultrafine grain range and negligible hardening and possible grain refinement softening in the range of ≈100 nm. It is also shown that samples with finer grain sizes display higher strain rate sensitivities, suggesting a change in the deformation mechanism. The current results agree with a model of grain boundary sliding deformation.
Recent studies show significant advances in improving the mechanical properties of magnesium and its alloys. While many papers deal with different alloy compositions, it is apparent that grain size plays a key role in the mechanical behavior of these materials. The ability to produce samples with very fine grain sizes leads to observations of high strength and/or high elongations. There are recent reports of exceptional elongations of over 100% in pure magnesium and a few alloys. These recent findings are critically reviewed in the present study. The experimental data from over 300 papers are collected, and trends between flow stress, elongation, strain rate sensitivity, and grain size are identified. The role of alloy content is examined.The data clearly shows a transition in the flow stress vs. grain size relationship which is attributed to a change in deformation mechanism from twinning controlled in coarse grained to slip controlled in fine and ultrafine grained samples. The slip controlled deformation agrees with the model of grain boundary sliding, which has shown good agreement with multiple metallic materials. It is shown that the elongations display a maximum in the grain size range in which there is a transition in the deformation mechanism. Three strategies are described for achieving high strength, high ductility, and good strength-ductility combination.
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