Deformation Mechanisms And Mechanical Properties Of Nanograined HCP Metals Prepared By Plastic Deformation

Dislocation slip and deformation twinning are two major plastic deformation mechanisms for polycrystalline metals,the activation and competition of which affect the mechanical properties decisively.Compared to face-centered cubic(FCC)metals,the deformation behaviors of hexagonal close-packed(HCP)metals are more complex due to the low crystallography symmetry.The main deformation capacity of HCP metals comes from basal slip system with Burgers vectors b=1/3<1120>.To achieve macroscopic plasticity,deformation along c-axis through twinning or(c+a)pyramidal slip is also needed.It has been widely reported that deformation mechanisms of HCP metals,especially the twinning tendency,exhibit a strong dependence on grain size.However,there is a lack of systematic investigation of the grain size effect on deformation mechanisms in hcp materials in the nanograined size range(<100 nm).In this work,the grain size dependences of deformation mechanisms of pure Co were systematically investigated by using gradient nanograined(GNG)samples.The GNG samples were prepared by using surface mechanical grinding treatment at liquid nitrogen temperature(SMGT).The grain size dependences of deformation mechanisms of pure Mg were studied by using Mg with different grain sizes prepared by high pressure torsional(HPT).The structural characteristics and deformation modes of the two metals with different grain sizes were studied by transmission electron microscope,electron backscatter diffraction and transmission Kikuchi diffraction.In addition,the microstructure of Co with extremely-fine nanograins prepared by HPT was also observed.The main results are as follows:After LNT-SMGT,a typical gradient microstructure was formed on the surface layer of the pure cobalt sample with the grain sizes increasing from tens of nanometer at the topmost layer to several tens of micrometers.Typical microstructures from the top surface to the deep matrix in the SMGT Co sample can be defined as:nanograined structure(NG),ultrafine grained structure(UFG)and deformed twinning structure(DT).The grain size of the topmost nanograin is about 54 nm.In the deformed twinning region,{1012}<101>tensile twinning dominates.With strain increasing,the density of deformation twins increased greatly,and various compression twins and secondary twins were activated in the coarse grain.Twin-twin intersections can promote the fragmentation of coarse grain.Different dislocation types were characterized by twobeam diffraction imaging,and it was found that(a)dislocations presented a predominant type.In the ultrafine grained structure region,there were still abundant twins and the morphology was more complex.The dislocation density of(a)type dislocation increased greatly,and gradually formed high-density low-angle grain boundaries(LAGBs).In the nanograin structure region,the density of deformation twinning decreased greatly and the dislocation density increases greatly,especially for(c+a)dislocations.The(c+a)dislocation slip can not only coordinate the strain in the c-axis direction,but also form low-angle grain boundaries(LABs),promoting the further refinement of nanograins.In addition,the Hall-Petch relation slope changes consequently with the transition of deformation mechanism.Extremely-fine nanograined Co with grain size of 11 nm was successfully prepared by HPT.The phase composition of pure cobalt samples with different grain sizes was analyzed by selected area electron diffraction(SAED)and XRD.It was found that with decreasing grain size to 11 nm,most Co HCP grains transformed to FCC structures.Thermodynamic calculation showed that the increase of free energy caused by decreasing the grain size is enough to provide the energy required for phase transformation.Atomic scale analysis by HAADF-STEM showed that as the grain size decreases to nanoscale,the stacking fault(SF)density increases greatly.The HCP-FCC transformation can be accomplished by the sliding of partial dislocations on(0001)planes,resulting in the change in stacking from…ABABAB…to…ABCABC…The orientation relationship between the two structures adopts<1120>HCP//<110>FCC and{0001}HCP//{111}FCC.In addition,high density SFs and deformation twins emitting from GBs enhanced the GB relaxation process,increasing thermal stability of extremely fine nanograins.Ultrafine grained Mg samples with different grain sizes were prepared by HPT.It is found that as the grain size decreases to submicron scale,deformation twinning decreased greatly,and the grains presented distinct morphologies of dynamic recrystallization:the grain interior is clean,with low dislocation density.After high pressure torsion deformation for 8 passes,the grain size of pure Mg is about 234 nm.It is difficult to further refinement via increasing strain.Indentation tests showed that decreasing the grain size of Mg to submicron scale,the hardness decreases abnormally rather than increases.The strain rate sensitivity value of pure Mg samples with different grain size was calculated by indentation tests under different holding time.When the grain size is about 10 μm,the strain rate sensitivity value is about 0.062,increasing to 0.434 as reducing the grain size to 234 nm,indicating that the main deformation mechanism of ultrafine grained Mg is grain boundary sliding.Grain coarsening caused by grain boundary sliding makes it difficult to refine grain size to nanoscale.