Deformation Compatibility Behavior Of HCP Crystal Structure Alloys At Room Temperature

It has been reported that many Mg alloys with rare earth additions(Mg-Gd,Mg-Y,Mg-Ce)can weaken the deformed texture and achieve higher elongation,which are even higher than most of the Al alloys.But the increase in elongation does not mean the great improvement in formability and ductility.Current application status suggests that the formability and ductility of Mg alloys is still poor compared with Al alloys due to the deformation anisotropy in Mg alloys.The deformation behavior differences come from the different crystal structure from mesoscale to macroscale.The deformation behavior of HCP crystal structure alloys is strong dependent on the loading stress,grain orientation and limited slip systems at room temperature,which results in the local strain heterogeneous.Micro-crack initiation and the final fracture of HCP crystal structure alloys are commonly recognized resulting from the heterogeneous intergranular deformation.By contrast,FCC crystal structure alloys have enough slip systems and are not sensitive to the loading stress,thus the formability is good.Most current researches on deformation mechanism of FCC and HCP crystal structure alloys are based on the deformation behavior from intragranular scale to macroscopic scale directly,which lacks the discussions in the surrounding effect.In-situ SEM,in-situ EBSD,FIB and DIC were used to describe the relationship of local strain and neighboring grain effect and compare the deformation behaviors differences of pure Al and GW83 alloys in mesoscale.Besides,micro-cracks initiation were also followed and predicted quantitatively in Mg-2%Gd alloys.As the macro strain increases,the {111}<110>slip systems are the dominant deformation mechanism in pure Al;the strain distribution in pure Al were homogeneous without much local strain concentrations.Besides,there were not obvious differences between the average intragranular strain and the average intergranular strain by statistical result.As such,the intragranular slip systems could contribute enough strain;the average Schmid factor in pure Al was 0.45 and the intragranular strain increases with the value of the Schmid factor;the local strain concentration in pure Al did not correlate with the surrounding grain orientations.As the macro strain increases,grain rotation and basal slip systems provide the main deformation in GW83 alloys and the extension twinning only contributes a little to the strain compatibility;the average intergranular strain was found much higher than the average intragranular strain in GW83 alloys.Such inhomogeneous strain distribution did not result from the shear band but correlates with basal slip systems and extension twinning nucleation;Schmid Law was not applicable for predicting the strain amplitudes in GW83 alloys;the strain compatibility was influenced by the neighboring grain effect in GW83 alloys and the neighboring grain orientation related geometric compatibility parameter M could well evaluate the intergranular deformation behaviors in GW83 alloys:(1)the grain boundaries with high M value and high Schmid factor of corresponding slip systems in adjacent grains could accommodate large local strain.Slip-slip was the dominant strain transfer method at grain boundaries.(2)When the slip-slip could not well accommodate the intergranular strain,the slip-twinning was an effective supplementary mechanism to accommodate local strain.This phenomenon was easy to happen in the “soft” and “hard” grain pairs.The basal slip systems in the “soft” grains easily activated the neighboring twinning nucleation in the “hard” grains.Schimid factor could not effectively predict the twinning variant type.The inhomogeneous deformation in Mg alloys resulted in the micro-cracks initiation at grain boundaries.Extension twinning in the larger grain sized Mg alloys did not accelerate the fracture and contributed to the ductility improvement.The fracture initiation parameter F1 factor including grain orientations and applied stress could well predict the micro-cracks initiation at grain boundaries.F1 factor contains three terms: the first term defines the largest shear deformation arising from twinning or slip band near the grain boundaries;the second term defines the grain boundary strength under the tensile stress;the third term defines the how well the shear deformation could be accommodated by dislocations.According to statistical analysis for the selected grain pairs,the grain boundaries with larger F1 factor were more likely to cause micro-crack initiation.Thus,the predictive factor F1 defined for the low ductility materials with limited slip systems could also be a new definition of slip band and twinning activation near the grain boundaries of Mg alloys...