Conventional ultrafine-grains can generate high strength in Mg alloys, but significant tradeoff of corrosion resistance due to inclusion of a large number of non-equilibrium grain boundaries. Herein, an ultrafine-grain structure consisting of dense ultrafine twins is prepared, yielding a high strength up to 469 MPa and decreasing the corrosion rate by one order of magnitude. Generally, the formation of dense ultrafine twins in Mg alloys is rather difficult, but a carefully designed multi-directional compression treatment effectively stimulates twinning nucleation within twins and refines grain size down to 300 nm after 12-passes compressions. Grain-refinement by low-energy twins not only circumvents the detrimental effects of non-equilibrium grain boundaries on corrosion resistance, but also alters both the morphology and distribution of precipitates. Consequently, micro-galvanic corrosion tendency decreases, and severe localized corrosion is suppressed completely. This technique has a high commercial viability as it can be readily implemented in industrial production.

In this work, the degradation behavior of AZ91 magnesium alloy in five solutions including phosphate buffering solution (PBS), 0.9% NaCl solution, simulated body fluid (c-SBF), Hank’s solution, and Dulbecco's modified eagle medium (DMEM) are systematically investigated. Our results show that AZ91 magnesium alloy exhibits a dramatically different degradation behavior in the five solutions. The concentration of buffering agents and the content of hydrocarbonates in test solutions primarily affect the degradation performance, especially the degradation behavior during the early stage. In DMEM and c-SBF with higher contents of buffering agents, the sample shows much more negative corrosion potentials compared to those in Hank’s solution, 0.9% NaCl solution, and PBS. The initial degradation rates in DMEM and c-SBF are similar and much higher than those in PBS, 0.9% NaCl, and Hank’s solution. Rapid reaction of <mml:math display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mtext>OH</mml:mtext></mml:mrow><mml:mo>−</mml:mo></mml:msup></mml:mrow></mml:math>OH− with the large amount of buffers in DMEM and c-SBF dramatically promotes transformation from Mg to <mml:math display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mtext>Mg</mml:mtext></mml:mrow><mml:mrow><mml:mn>2</mml:mn><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:mrow></mml:math>Mg2+ , constituting the main reason for the high degradation rates. Distinct corrosion morphology and corrosion products are present on the samples exposed to the five test solutions. Therefore, the proper selection of test solution with similar buffering capability and similar ingredients as biological fluids is imperative to the investigation of the degradation behavior of biomedical magnesium alloys.

In the present study, a coarse grained AZ31 plate was refined by "Equation missing"<!-- image only, no MathML or LaTex --> twin boundaries (TBs) and grain boundaries (GBs), respectively. A comparative study about the different effects of grain refinements by GBs and by TBs on tension-compression yield asymmetry was performed. Our results show that both the refinements by GBs and by TBs increase the tensile and compressive yield strengths, but to a different degree. "Equation missing"<!-- image only, no MathML or LaTex --> TBs are more effective to harden "Equation missing"<!-- image only, no MathML or LaTex --> twinning, but yield a lower strengthening against prismatic slip and a much lower tension-compression yield asymmetry is thus obtained. Both the differences in boundary coherence and misorientation between GBs and TBs affect the hardening. The misorientation of TBs provides a lower geometric compatibility factor (a higher hardening) for both prismatic slip and "Equation missing"<!-- image only, no MathML or LaTex --> twinning than that of GBs, which in detail is the result of the much higher angle between c-axes of the two sides of TBs (about 86°) than GBs (0–50°). It is found that, for hardening of prismatic slip, boundary coherence plays a more important role than misorientation. With regard to "Equation missing"<!-- image only, no MathML or LaTex --> twinning, the different misorientation of TBs from GBs mainly accounts for their different hardening effects.