Microstructure,mechanical Properties And Corrosion Behaviors Of Mg-Yb Binary Alloys

Magnesium(Mg)alloys have attracted extensive attention due to their advantages in high specific strength,low density,energy saving,environmental and biological friendliness.However,its low strength,poor room-temperature plasticity and corrosion resistance seriously restrict its widespread application.Available investigations indicate that a trace amount of rare-earth element alloying can effectively improve the mechanical properties and corrosion resistance of Mg alloys.In our previous studies,it was demonstrated that a trace amount of Yb addition significantly improved the tensile properties of ZK60 alloys and prevented the occurrence of local corrosion.However,due to the combined influence of Zn and Zr elements,the modification effect of Yb on the Mg matrix alone is not clear yet.To clarify this,the pure Mg and Mg–x Yb binary alloys with different Yb concentrations(x=0.5,1.0,2.0,wt.%)were prepared.The alloying concentration of Yb on the dominant deformation mechanism,microstructure development,mechanical properties,and in-vitro degradation behaviors of the Mg matrix during hot extrusion and subsequent annealing were systematically investigated and key conclusions were drawn as follows:The addition of Yb can effectively refine the as-cast columnar grain morphology.With the increase of Yb content,the columnar width was decreased from?384?m in the case of pure Mg to?296?m,?245?m and?209?m in the cases of Mg–0.5 Yb,Mg–1.0 Yb,and Mg–2.0 Yb alloys,respectively.However,the columnar morphology did not change evidently with increasing Yb concentration,which should be attributed to the limited constitutional supercooling induced by the addition of Yb alone.In other words,the constitutional supercooling effect induced by 2.0 wt.%Yb alloying alone can only play a role in hindering the growth of original columnar grains rather than promoting the formation of a nucleus.Accordingly,it is suggested to incorporate other powerful nucleating agents such as Zr or Al to improve the modification effect of Yb in magnesium alloys.When extruded at 300?,the dynamic recrystallization(DRX)process was significantly inhibited in companion with much refined DRX grain size.With increasing Yb concentration,the average grain size and the DRX fraction decreased from?18.14?m and 98.9%in the case of pure Mg alloy to?9.97?m and 97.3%,?3.35?m and 52.1%,?2.22?m and 46.7%in the cases of 0.5,1.0,and 2.0 wt.%Yb alloyed samples,respectively.The in-grain misorientation axes(IGMA)analysis combined with TEM observation indicated that non-basal slips operated with increasing Yb concentration.Specifically,the prismatic<a>slip should be robustly activated in Mg–1.0 Yb extrudate,promoting the formation of the texture with{10-10}plane normal to the extrusion direction(ED),while for the Mg–2.0 Yb counterpart,the increased activity of pyramidal<c+a>slip and the relaxation of basal/<c+a>dislocations generated an ED-tilted texture component.Favored by the grain refinement,the Mg–2.0 Yb extrudate exhibited a high tensile yield strength(TYS)of 304±3.5 MPa.Contrarily,Mg–0.5 Yb extrudate exhibited better elongation to failure(EL)when tension along the extrusion direction(ED),which should be related to its lower density of residual dislocation after extrusion and the softening in prismatic slip caused by a trace amount of Yb addition,retarding the occurrence of fracture.After annealing at 400?,the preferred grain growth dominated the texture development of Mg–Yb binary alloys.The results show that Mg–0.5 Yb alloy exhibited a typical oriented grain growth towards the<-12-10>direction,which was analogous to the pure magnesium,suggesting that a trace amount of Yb addition exerts limited influence on the preferential growth of matrix.However,when higher concentrations of Yb are alloyed,the specific growth orientations change from<-12-10>to<-24-23>and<-12-13>in the cases of Mg–1.0 Yb and Mg–2.0 Yb samples,respectively.It can be concluded that the texture weakening/altering in Mg–Yb binary alloys during high-temperature annealing was caused by grains with the basal orientation vanished and with the non-basal orientations intensified due to an increase in Yb addition.The tensile strength of Mg–Yb binary alloy at room temperature decreased with the increase of Yb concentration,but the ductility increased significantly.The annealed Mg–2.0 Yb sample present a favorable elongation to failure of 14.8±1.2%,which mainly due to the homogeneous grain structure,weak ED-tilted texture,and dissolution of coarse phases after high-temperature annealing.Furthermore,the in-vitro degradation behavior of Mg–Yb binary alloy was investigated by electrochemical test and constant temperature immersion.The results of Nyquist and Tafel curves show that the charge transfer resistance of as-extruded Mg–1.0Yb alloy in SBF solution is the highest,followed by Mg–0.5 Yb,and Mg–2.0 Yb counterparts.The corrosion rates calculated from the data of in-vitro immersions are well consistent with those of electrochemical results.The P_i and₍(2) values of as-extruded Mg–0.5 Yb,Mg–1.0 Yb and Mg–2.0 Yb alloys were?10.30 and?9.66 mm·year^-1,?5.59 and?3.54 mm·year^-1,?14.42 and?11.54 mm·year^-1,respectively.Microstructure characterization show that the compact film was uniformly covered on the corrosion surface of Mg–1.0 Yb sample,effectively preventing the penetration of ions in the SBF into the matrix.This should be the reason for the lowest corrosion rate of extruded Mg–1.0 Yb extrudate.After annealing,the corrosion resistance of Mg–Yb binary alloys was improved due to the dissolution of blocky phases,but the corrosion mechanism remains unchanged.The P_i and₍(2)values of as-annealed Mg–0.5 Yb,Mg–1.0 Yb,and Mg–2.0Yb alloy were?5.74 and?4.62 mm·year^-1,?2.32 and?1.66 mm·year^-1,?8.43 and?7.22mm·year^-1,respectively.The as-annealed Mg–1.0 Yb sample still exhibited the best corrosion resistance.