Effects of dynamic recrystallization and strain-induced dynamic precipitation on the corrosion behavior of partially recrystallized Mg-9Al-lZn alloys

The corrosion susceptibility of recrystallized and un-recrystallized grains in equal channel angular pressed(ECAPed)Mg-9Al-lZn(AZ91)alloys immersed in chloride containing media was investigated through immersion testing and an electrochemical microcell technique coupledwith high resolution techniques such as scanning Kelvin probe force microscopy(SKPFM),transmission electron microscopy(TEM),andelectron backscatter diffraction(EBSD).During ECAP,dynamic recrystallization(DRX)and strain-induced dynamic precipitation(SIDP)simultaneously occurred,resulting in a bimodal grain structure of original elongated coarse grains and newly formed equiaxed fine grainswith a large volume fraction ofβ-Mg17Al12 precipitates.Corrosion preferentially initiates and propagates in the DRXed grains,owing tothe greater microchemistry difference between theβ-Mg17Al12 precipitates formed at the DRXed grain boundaries and the adjacentα-Mgmatrix,which induces a strong microgalvanic coupling between these phases.Additionally,the weaker basal texture of the DRXed grainsalso makes these grains more susceptible to electrochemical reactions than the highly textured un-DRXed grains.The influence of dynamicrecrystallization and dynamic precipitation was also studied in ECAPed alloys with differenl levels of deformation strain through corrosion andelectrochemical techniques.Increasing the strain level led to a more uniform corrosion with a shallow penetration depth,lower corrosion ratevalues,and higher protective ability of the oxide film.Furthermore,higher levels of strain resulted in greater hardness values of the ECAPedalloys.The superior corrosion resistance and strength of the ECAPed alloys with increasing strain level was attributed to the combination ofsmaller DRXed grain size,higher DRX ratio,and higher volume fraction of uniformly distributed fineβ-Mg17Al12 precipitates.c 2020 Published by Elsevier B.V.on behalf of Chongqing University.

Corrosion behavior of Mg-Zn-Zr-RE alloys under physiological environment-Impact on mechanical integrity and biocompatibility

Benefits of RE addition on Mg alloys strength and corrosion resistance are widely reported but their effects on biodegradability and biocompatibility are still of concern.This paper investigates the effect of RE additions on biodegradability of Mg-Zn alloys under simulated physiological conditions.In this context,two commercial Mg-Zn-Zr-RE alloys,namely ZE41 and EZ33,with same RE addition but different concentrations are studied in Hank’s Balanced Salt Solution(HBSS)at 37℃and with pH of 7.4.Weight-loss,hydrogen evolution,real-time insitu drop test,electrochemical impedance spectroscopy(EIS)and potentiodynamic polarization are deployed to study corrosion characteristics.The mechanical integrity of both alloys is assessed by mechanical testing post immersion.Furthermore,in vitro biocompatibility is evaluated by indirect cytotoxicity tests using NIH3T3 cells.Results reveal that although both alloys showed similar microstructure,size and distribution of precipitates played a significant role on its corrosion response.EIS and open circuit potential results show stable film formation on EZ33,while ZE41 showed passive layer formation followed by its deterioration,over the analyzed time period.Using real-time drop test,it was shown in ZE41 alloy that both T-phase and Zr-rich precipitates acted as micro cathodes,resulting in an unstable surface film.In EZ33,Zr-rich regions did not influence corrosion response,resulting in better corrosion resistance that was corroborated by post-immersion surface morphology investigations.The higher degradation observed in ZE41 alloy resulted in higher drop in flexural and tensile strength compared to EZ33 alloy.In addition,cytotoxicity tests on NIH3T3 cells revealed that cell viability of EZ33 increased with increasing incubation time,contrary to ZE41,owing to its lower biodegradation behavior and despite higher concentrations of REs.Present results show that an increase in RE concentration in EZ33,relative to ZE41,had a positive effect on corrosion rate that subsequently controlled alloy mechanical integrity and biocompatibility.