Tem Investigation On The Deformation Twin In Deformed Hexagonal Close-packed Materials

As typical representatives of hexagonal close-packed(hcp)materials,magnesium,titanium and their alloys are widely used in aerospace and automobile industries.Due to the limited number of slip systems,deformation twin as an important deformation mechanism plays a crucial role during the plastic deformation of hcp materials at room temperature,also deformation twin is one of the main factors that can influence the mechanical properties of hcp materials.Therefore,a much more comprehensive and profound knowledge of the structure features,formation mechanism of deformation twins as well as the interaction between twins and other crystal defects,including crystal dislocations and other twinning modes,is of theoretical and practical important to design,improve and develop hcp materials with superior properties.In this research,by means of transmission electron microscopy(TEM)and high-resolution transmission electron microscopy(HRTEM),we focus on three kinds of typical hcp materials,including AZ31 magnesium alloy,pure cobalt and pure titanium and systematically investigate the structure features of boundaries of deformation twins at the atomic scale and the interactions between the deformation twins and other crystal defects,including crystal dislocations and other twin variants in detail.In addition,based on the traditional dislocation theory,we also study and discuss the formation mechanism of these interfacial features and the evolution of microstructure during the plastic deformation.The main conclusions are summarized below:(1)The boundaries of {10-12} tension twin consist of coherent twinning boundaries({10-12}CTB),which are parallel to the theoretical(10-12)pyramidal planes,and basal-prismatic(BP or PB)facets.For the boundaries of {10-12} twin tip,it even can consist of only BP and PB interfaces.The BP or PB interface is not also entirely incoherent,there exist interfacial defects with step structure on it.The existence of BP or PB interfaces is suggested to be responsible for the deviation of {10-12} twinning boundaries from the theoretical one.The degree of deviation depends on the length of BP,PB and {10-12}CTB,and the amount of them.Boundary deviation is an intrinsic attribute of {10-12} twin,which is independent of special experimental materials and deformation condition.(2)The boundaries of {10-13} contraction twin consist of coherent twinning boundaries({10-13}CTB),which are parallel to the theoretical(10-13)pyramidal planes,and basal-pyramidal(BPy or PyB)facets.The existence of BPy or PyB interfaces is suggested to be responsible for the deviation of {10-13} twinning boundaries from the theoretical one.The degree of deviation depends on the length of BPy,PyB and {10-13}CTB,and the amount of them.The similar deviation phenomenon is also observed in {10-11} twin system.(3){10-11} and {10-13} twinning boundaries can coexist in one twinning mode.This may be ascribed to that within the framework of traditional twinning theory,{10-11} and {10-13} twinning modes are conjugate to each other.If the {10-11} twinning plane is considered as the first undistorted plane,the {10-13} twinning plane is the corresponding second undistorted plane,and vice versa.Therefore,it is reasonable to believe that these two types of twinning boundaries can coexist in one twinning mode in some suitable situation.(4)The interfacial structural features of twin-twin boundaries(TTBs)caused by the interaction between two different {10-12} twin variants sharing the same [11-20] zone axis can be identified: 1)TTB is nearly parallel to the basal planes of interacting twins,referred to as basal-basal(BB)interface.2)TTB is nearly parallel to the prismatic planes of interacting twins,referred to as prismatic-prismatic(PP)interface.3)TTB is nearly parallel to the(10-1-1)twinning planes of interacting twins,referred to as TB14 interface.4)TTB consists of a series of crystal planes with more varied in possible orientations.At the atomic scale,the TTB connects with the BP and PB interfaces of the interacting twins directly.The occurrence of these interfacial feature of TTBs may be ascribed to the interaction between the defects lying on the BP and PB facets and the reconstruction process of interfacial defects during the further plastic deformation.(5)Two kind of basal stacking faults(I1 and I2)can be found within the deformation twins,including {10-12} tension twin and {10-11} contraction twin.The density of the stacking faults in the interior of twin is higher than that within the corresponding matrix.One or both of the ends of the basal stacking faults always connect(s)with the twinning boundaries.The formation of such high density of basal stacking faults within the twin may be related to the interaction between the dislocation in the matrix and the twinning boundaries and the subsequent migration of the twinning boundaries.(6)The actual misorientation angles in twinning modes,including {10-12} tension twin,{10-11} contraction twin and {10-11}-{10-12} double twin,can deviate significantly from the theoretical ones.Such phenomenon is thought to be induced by the reorientation of twin lattices associated with the dislocation slips with the twin.As a consequence,the corresponding twinning planes belonging to the twin and the matrix do not coincide any longer.