Deformation Twinning Of Low-symmetry Metal Titanium And Uranium

Titanium has been widely used in medical equipment,aviation,navigation and automobile industry because of its high specific strength,excellent corrosion resistance and good biocompatibility.Uranium has important applications in military and nuclear industry due to its high density and excellent nuclear properties.However,titanium and uranium are low symmetry metals,which have hexagonal closed-packed(hcp)and base-centered orthorhombic lattice,respectively.Therefore,twinning plays an important role in the deformation modes for titanium and uranium.Since uranium can be considered as a type of distorted hcp lattice,the twinning behavior in titanium and uranium has certain relevance.In this paper we focused on the similarities and differences in twinning behaviors between the hexagonal close-packed metal titanium and the base-centered orthorhombic metal uranium.As for titanium,the high strain rate(-2600 s-1)compression was conducted on split Hopkinson pressure bar(SHPB)under varied temperatures.As for uranium,the quasi-static compression,tension and high strain rate compression were carried out using mechanical testing machine,tensile device mounted on XRD sample stage and SHPB,respectively.Strains of samples deformed by high strain rate compression were controlled by strain-stop rings arround samples.Deformed microstructue was characterized by scanning electron microscope(SEM),electron backscatter diffraction(EBSD),X-ray diffraction(XRD)and transmission electron microscope(TEM).Uranium single crystal micropillars were prepared using focused Ga+ ion beam(FIB)device,then uniaxial compression conducted using flat punch equipped on nanoindentation device.Four types of twins were observed in titanium deformed by high strain rate compression,namely extension twins {1012}(E1),{1121}(E2)and contraction twins {1122}(C1),{1124}(C3).The twin volume fraction of E1 and C1 in the deformed microstructure is larger than E2 and C3,and has a significant effect on the texture evolution.The E2 twins have wavy boundaries which become the twin nucleation locations of E1.C1 twins were generated in the matrix between E2 twin bands,and E3 appeared at the intersection of C1 and E2.By analyzing the Schmid factor(SF)and the twinning shear gradient tensor(DGTs)of all twin variants,it is shown that most of the twin variants activated under high strain rates do not follow the Schmid law,and 73%of these can be explained by DGTs.The stress-strain curves in high strain rate compressed titanium showed staged hardening phenomenon.The yield strength and twinning density decreased with the increase of temperature,and the c-axes of the titanium lattice gradually turn to compression direction as deformation temperature increase.Long band shaped {130},’{172}’ and {112} twins exist in some as-cast uranium usually,among which the {130} twins may disappear or continue to grow in subsequent tensile or compressive deformation,depending on the local stress state.Four types of twinning of {130},’{172}’,{112} and ’{176}’ were observed in uranium deformed under quasi-static compression,and all the activated variants obey the Schmid law.Twin pairs of’(176)’-’(172)’with the geometric matching factor 0.933 were observed at local grain boundary.Only three types of twinning of {130},’{172}’and {112} were observed in uranium deformed under quasi-static tension,and part of the activated variants do not follow the Schmid law.The fracture of as-cast uranium deformed by quasi-static tension has the ductile-brittle characteristics,and there is a large amount of {130} twinning and slip interaction near the fracture.It is observed that the second generation of {130} twinning produced within the primary ’(172)’ twins near the fracture.Four types of twinning of {130},’{172}’,{112} and ’{176}’were observed in uranium deformed under high strain rate compression,among which approximately 20%of the activated variants do not follow the Schmid law.Kink bands appeared in the SHPB compressed uranium when strains over 15%,which usually have their[100]directions oriented 45°to the[100]directions of the matrix.The kink bands have complex interactions with twinning.The interface of some kink bands become the active sites for {130} and {112}twins nucleation.The {130},’{172}｝ or {112} twinning may happen inside some kink bands.In some {112} twins within the kink bands,secondary {130} twinning was observed.The deformation zone become loose and micro-voids or cracks occur when tensile strain reaches 25%.Among single crystal uranium micro-pillars with three different orientations,(100),(001)and(3 24 13),the(001)oriented pillars have the largest modulus and hardness,while the(324 13)are the smallest.In micro-pillars of the same size,(100)oriented pillars show the highest compressive strength,and the(3 24 13)ones lowest.The mechanical responses of all the pillars are strongly size-dependent,the smaller in pillars size,the stronger in the compression.