Intergranular Deformation Behavior And The Microstructure Evolution And Mechanical Properties Of The Mg-Gd-Y-Zn Alloy Processed By Multi-directional Forging/Rolling

Magnesium has the lowest density 1.7 g/cm~3 of all the structural metals,approximately 23%that of steel and 66%that of aluminum alloys,and is best suited for lightweight application.Magnesium and its alloys are relatively new engineering materials that possess an extraordinary combination of propertie(e.g.,high specific strength,specific modulus,and good machining properties).However,magnesium with its hexagonal-close-packed(HCP)has few active slip systems and poor ductility,which greatly limit its application.Compared with the conventional magnesium,ample works reported that the addition of rare earth elements can improve the ductility of magnesium alloys.In addition,the solubility of rare-earth elements in magnesium is high,and the alloy can be strengthened after the precipitation heat treatment.Therefore,it is necessary to investigate strengthening and deformation mechanism in Mg-RE alloys.This study aims to study the effect of hot working on the mechanical properties of Mg-Gd-Y-Zn alloy by multi-directional forging and hot rolling.The as-received ingot is coarse-grained because the absence of Zr before casting.As a result of recrystallization,the wrought alloy has a variety of grains size after forging and rolling,and the crystallographic orientation of the original and recrystallized grains is different during hot working.Thus,the microstructure of this wrough magnesium alloy is similar to that of the laminated composite.By controlling the hot working parameters such as the forging passes,hot-rolling strain,rolling speed,and heat treatment time,a rare-earth magnesium sheet is fabricated with a tensile strength of295 MPa and elongation of about 19%.In addition,the effect of the recrystallization ratio on the mechanical and texture of the sheet is also analysed in this work.It is found that the large grains in the unrecrystallized state are more likely to rotate during rolling,which generate strong basal texture,and these large grains are hard-oriented for basal slip.However,heterogeneous deformation and premature failure are induced by the presence of these large unrecrystallized grains.Therefore,the ductility of the Mg-RE sheet can be balanced by controling the recrystallization ratio:it is found that the Mg-RE sheet has optimal performance when the billet is forged by 6 passes.In this paper,the strain accomodation mechanism in Mg-RE alloy is investigated by in-situ tensile testing from three aspects:(1)slip transfer across grain boundaries;(2)the orientation dependence of individual GND type in polycrystalline Mg-RE alloy;(3)the evolution of local stress during deformation twinning.The detrimental decomposition of<c+a>dislocation in magnesium can be avoided by the addition of rare-earth elements.Moreover,the presence of rare-earth elements can reduce the critical shear stress difference between basal slip and pyramidal dislocation.Thus,the deformation behavior of rare-earth magnesium is different from the conventional magnesium alloy.Slip transfer of basal-basal and basal-pyramidal type is observed for the forged Mg-Gd-Y-Zn alloy with weak texture.The product of m′factor and the sum of the Schmid factors demonstrates that the threshold for basal-basal slip may exist.However,there is significant scatter in the calculated results of m′timesΣSF for basal to pyramidal slip,indicating complex stress tensor within the grains to initiate pyramidal slip.The GND calculation results indicate that at the early stage of deformation,the basal<a>dislocations followed the Schmid’s law and tended to accumulate in the soft grains.With straining,the<a>type basal dislocations preferred to assemble in grains with small Schmid factors.Whereas,for<c+a>type dislocation,there is a weak correlation between GND density and grain orientation.In-situ EBSD tensile test shows that the geometric compatibility factor,m′,plays an important role in determining the twin variant,and all of the twin variants are correlated with high Luster-Morris m′values in this research.The shear stress evolution on the twin plane is a dynamic process.Negative shear stress may be seen during the propagation stage,which is associated with the twin-induced stress reversal.The local Schmid factor evolves with deformation twinning.Compared with the unactivated variants,the initiated twins are correlated with the highest local Schmid factor before twinning.The slip transfer parameter,which is taken as the m′factor multiplied by the local Schmid factor of a twin variant,may reveal a threshold for the slip-induced twinning process.