Size Effects Of AZ31 Mg Foils And The Full-field Crystal Plasticity Modeling

Lightweight metals like magnesium(Mg)alloys are of great interest in lightweight and part miniaturization due to their attractive properties like the high specific strength,high specific stiffness as well as excellent biodegradability and biocompatibility.With the decrease of the feature size of micro-components,the effect of the asymmetry and the anisotropy of single grain increase significantly.Meanwhile,the size effect on the micro-forming processing is more complicated than other cubic metals.Thus,it is of great significance for the part miniaturization of Mg alloy to clarify the size effect on the deformation mechanisms of Mg and establish the suitable numerical simulation method.In this study,based on the commercial AZ31Mg alloys the influence of grain sizes((9?)and specimen thicknesses(t)on the micro-forming of Mg alloys was investigated via the grain number(?)in thickness.Physical experiments,e.g.equal channel angular pressing(ECAP)operations with a multi-temperature step-down technology,micro-tensile tests,metallographic observations,SEM and ex-situ EBSD combined with full-field crystal plasticity simulations were employed to capture the size effects on the mechanical properties,fracture characteristics and micro-deformation mechanisms of AZ31 Mg foils.What's more,a full-field crystal plasticity model considering the interaction of dislocation slips and twinning was established.ECAP operations with a multi-temperature step-down technology was carried out to refine the grains and restructure the texture of AZ31 Mg alloys.In the 1^st to 4^th passes,the grains are refined via the strong shear strain and dynamic recrystallization(DRX).However,some coarse grains and necklace kind of structure still exist.In 5^thto 8^th passes,as the decrease of deformation temperature and the accumulation of deformation the DRX is tend to be completed,the grains are fully refined and a new texture that the basal plane parallel to shear plane is obtained.The samples refined by ECAP operations were annealed under different heat treatments to obtain the samples with different grain sizes but the same texture.The annealed samples were cut into the micro-tensile specimens with different thicknesses.The micro-tensile tests show that there is an obvious size effect on the deformation of Mg foils.Besides the effect of“fine-grained strengthening”,the flow stress curves change from typical concave down shape to convex up shape with the decrease of?.Meanwhile the effect of extension twins on the mechanical behaviors in the early deformation stage is gradually weakened.The fine grained specimens show the size effect that “smaller being weaker” with the decrease of specimen thicknesses.However,in coarse grained specimens the effect of specimen thicknesses on mechanical behaviors is more complex,the deformation behavior of an individual grain may directly affect the deformation behavior of the samples.With the increase of?,the fracture characteristics change from brittle cleavage fracture to ductile shear fracture and dimple fracture.The EBSD results of the fracture region show that there are more double twins in coarse grain specimens.As the double twins will develop into the crack sources,the premature facture generally appears in coarse grain specimens.The extension twin boundary as the high angle grain boundaries will cause the accumulation of dislocations near the extension twin boundaries,and then evolve into the propagation path of cracks.The results of ex-situ EBSD and full-field crystal plasticity simulation show that the activation and expansion of extension twins are significantly suppressed with increase of?.The grain number in thickness becomes the key factor that affects the competition and coordination between dislocation slips and deformation twins.The evolution of the fraction of extension twins and the contribution ratio of slip systems were obtained from the results of crystal plasticity simulation.During the micro-plastic deformation of Mg alloys,the effect of extension twins on the mechanical properties increases significantly and the resistance of Basinski mechanism to slip is further strengthened with the decrease of ?.