Z. Jasieński

Shear band microtexture formation in twinned face centred cubic single crystals

Abstract The formation of brass-type shear bands (SB) in twinned microstructures of medium and low stacking fault energy (SFE) metal single crystals (copper, copper–2 wt.% aluminium and silver), with initial orientation {112}〈111〉 has been investigated after channel-die deformation at 77 and 293 K. The microstructures and local orientations were characterised over a wide range of scales by convergent beam electron diffraction in a transmission electron microscope, electron backscattered diffraction in a scanning electron microscope and optical metallography. For all metals it is shown that slip initially causes a general rotation to D(4 4 11)[11 11 8 ], which is then followed by partial twinning to the DT(26 26 5)[ 5 5 52] orientation. Subsequent shear banding of the unstable, layered, twin structure is responsible for the development of the strong Goss {110}〈100〉 orientation within the bands. A minor group of components is observed near {114}〈221〉, arising from the near primary matrix orientation. The intensity of mechanical twinning, and therefore, the relative amounts of primary matrix and twinned material, influences the SB internal microstructure. Alloys with low SFE such as Cu–2 wt.% Al deformed at 77 K twin almost completely whereas Cu at 77 K and Ag at 293 K retain significant quantities of (re-oriented) matrix and hence a relatively strong secondary microtexture component within the brass-type bands.

Crystallographic aspects of the early stages of recrystallisation in brass-type shear bands

Abstract The orientations of recrystallisation nuclei and their adjacent, as-deformed regions have been characterised in deformed single crystals of different metals (Cu, Ag and Cu–2%Al) in which twinning and/or shear banding occur. The {112}〈111〉 oriented crystals of these metals have been plane strain compressed to different strains then lightly annealed and the crystallographic aspects of the recrystallisation process along shear bands (SB) examined by TEM/CBED and SEM/EBSD. The results clearly show the existence of a well-defined crystallographic relation between the local deformation substructure and the first recrystallised areas of uniform orientation. The first-formed nuclei always exhibit near 30 ∘ 〈111〉 type misorientations, in the direction of highest growth with respect to one of the two main groups of the deformation texture components. The 〈111〉 rotation axis can be correlated with one of the slip plane normals of high activity within the SB. As recrystallisation proceeds, recrystallisation twinning develops strongly and facilitates rapid growth; the first and higher generations of twins then tend to obscure the initial primary crystallographic relation between the SB and recrystallisation nuclei.

Local and Global Effects in Texture and Microstructure Observed after Channel-Die Compression of Copper Single Crystals and after Cross-Rolling of Copper Sheets

The change of the deformation path leads to destabilization of the substructure and affects the texture of the deformed metal. The observed changes of texture and microstructure are, as a rule, significant and their characteristics depend on the geometry of the deformation process. Previous investigations on copper (and copper alloy) samples after deformation by rolling and channel-die compression were based on X-ray pole figure measurements and on observations in the light microscope. Hereby only global texture and structural characteristics have been obtained. The present study is mainly based on measurements of individual crystal orientations performed by ACOM (Automated Crystal Orientation Measurement, “Automated EBSD”) in the SEM which enables a precise local analysis of the investigated phenomena. For the channel-die experiments, (1 1 2)[1 1 -1] and (1 1 2)[1 -1 0] oriented copper single crystals have been used. After pre-deformation, a second deformation step has been carried out in transverse direction. The {1 1 2}<1 1 0> orientations are destabilized by channel-die compression, and clusters of layers develop which are composed of complementary {1 1 0}<1 1 2> components. The deformation process in polycrystalline sheets after rotating the rolling direction leads again to a distinct disintegration of the microstructure and destabilization of the b fiber. This process of microstructure reorganization after pre-deformation is fast and of high dynamics.

Texture of Al-Cu Alloys Deformed by ECAP

Materials of ultra-fine grained microstructure (sub-micrometer grain size) exhibit large strength, hardness and ductility and also the increased toughness in comparison with conventional coarse-grained ones. In these materials also the super-plastic flow at lower temperatures is observed. This behaviour may be interesting when aluminium alloys like AlCuZr, used in superplastic forming, are considered. In the paper, the methods of preparing such materials by equal-channel angular pressing (ECAP) is proposed and the texture analysis, based on neutron diffraction pole figure measurements and calculated orientation distribution function of two alloys AlCu4SiMn and AlCu5AgMgZr is discussed. The influence of short time recrystallization is discussed in relation with TEM and SEM observations.

The Effect of Shear Bands on the Evolution of Rolling and Recrystallization Texture in Cold-Rolled Direct Chill Cast Strips of Brass

The texture of cold rolled and annealed cast strips of 70/30 brass was investigated. A delayed development of the main {110} component of brass type rolling texture was attributed to the intense deformation twinning which is associated with appearance of the strong {211} texture component at 50% reduction. The predominace of the {011} texture component in the range of 70%-90% reductions was related to the extensive shear banding. As for the rolling textures, the delay in development of the {236} component of brass type recrystallization texture was found.

Strain Induced Fracture in Textured Al-Li Alloys

Metallurgical as well as geometrical factors may be responsible for severe in-plane anisotropy of mechanical properties and abnormal fracture behaviour of aluminium-lithium alloys sheets. The crystallographical texture is considered to explain the observed anisotopy of tensile strength and the coefficient anisotropy (r-value). Morphology of the grain is additionally taken into account when discussing the fracture behaviour.