Henryk Paul

Shear banding and recrystallization nucleation in a Cu–2%Al alloy single crystal

Abstract High-purity Cu–2%Al single crystals with initial C{112}〈111〉 orientation have been deformed by channel-die compression at 77 and 293 K to develop fine shear bands in a twin-matrix (T-M) substructure. The microstructural and microtextural changes during deformation and the early stages of recrystallization have been studied in detail by TEM/CBED and SEM/EBSD. Local orientations measurements indicate that the scattering of the original T-M orientations within shear bands towards two, twin-related positions is due to the localised lattice rotation around the TD ‖ 〈011〉 axis. The starting points for the occurrence of new, recrystallized grains are the {110}〈100〉 and {114}〈221〉 components of the as-deformed microtexture inside the shear bands. As recovery and recrystallization proceed, the orientations of the primary nuclei systematically evolve to adopt simple boundary misorientation relations of ∼30°〈111〉 type with respect to one of the two components of the deformed matrix. The results of the present study clarify the respective roles of all three basic mechanisms, i.e. oriented nucleation, growth (defined here as ∼30°〈111〉) and recrystallization twinning which are involved in the initial stages of recrystallization texture development in this low stacking fault energy alloy.

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.

Microstructure and Phase Constitution Near the Interface of Explosively Welded Aluminum/Copper Plates

The microstructure changes and the phase constitution within the layers close to the bonding interface strongly influence the properties of bimetallic strips. In this work, the layers near the interface of explosively welded aluminum and copper plates were investigated by means of microscopic observations, mostly with the use of transmission electron microscopy (TEM) equipped with energy dispersive spectrometry (EDX). The study was focused on the identification of the intermetallic phases, the possible interdiffusion between the copper and the aluminum, and the changes in the dislocation structure of the parent plates. In macro-/mesoscale, the interfaces were outlined by a characteristic sharp transition indicating that there was no mechanical mixing between the welded metals in the solid state. In micro-/nanoscale, the layers adhering to the interface show typical deformed microstructure features, i.e., structure refinement, elongated dislocation cells, slip bands, and microtwins (in copper plate). The internal microstructure of the intermetallic inclusion is composed mostly of dendrites. The electron diffractions and TEM/EDX chemical composition measurements revealed three crystalline equilibrium phases of the γ-Al4Cu9, η-AlCu, and Θ-Al2Cu type (the last one was dominant). However, most of the observed phases of the general CumAln type (also crystalline) do not appear in the equilibrium Al-Cu phase diagram. Inside the intermetallic inclusions, no significant regularity in the phase distribution with respect to the parent sheets was observed. Therefore, it was concluded that the processes occurring in the melt determined their local chemical composition.

Microstructure and interfacial reactions in the bonding zone of explosively welded Zr700 and carbon steel plates

Abstract The microstructure of an explosive cladding joint formed between parallel Zr700 alloy and carbon steel plates was examined with the use of scanning and transmission electron microscopes equipped with energy dispersive spectrometry. The study focused on near-the-interface microstructural changes and possible interdiffusion between the plates. At the macro-scale, the interfaces were outlined by a characteristic sharp transition, indicating that there was no mechanical mixing between the welded metals in the solid state. At the micro-scale, the melted zones often showed non-uniform swirl-like areas of a similar contrast. The nano-scale analysis revealed that the melted areas were composed of mixed amorphous and nano-crystalline phases. The bonding was always achieved by way of surface melting of the joined materials, which might be invisible for observation methods other than transmission electron microscopy.

Mechanisms of New Orientation Formation during Recrystallization of Old Deformed Aluminium Bicrystals

The crystallography of recrystallization nucleation has been investigated in channel-die deformed pure aluminium bicrystals with {100}<011>/{110}<001> and {100}<001>/{110}<001> orientations. The new grain orientations and misorientations were followed by systematic local orientation measurements using SEM and semi-automatic measurements in a TEM. In particular, orientation mapping combined with in-situ sample heating was used to investigate the formation and growth of new grains and their crystallographic orientation changes at very early stages of recrystallization. Grain boundary migration and ‘consumption’ of the as-deformed areas was always favoured along directions parallel to the traces of the {111} slip planes that had been most active during deformation. The orientations of the first formed nuclei were misoriented with respect to the orientations identified within the neighbouring deformed areas by α(<111>, <112>, or <100>)relations.

Recrystallization Nucleation in Some Channel Die Deformed, High Symmetry Aluminium Bicrystals

The crystallography of recrystallization nucleation in high purity aluminium bicrystals, with boundary planes situated parallel to the compression plane, has been characterized in detail by microtexture analyses. The experiments were carried out on samples with 2 types of bicrystal orientations composed of the stable Goss{110}<001> with an unstable cube{100}<001> or shear{100}<011> orientation. The bicrystals were cold deformed in channel-die compression up to true strains of 1.5. During recrystallization annealing, the deformation bands of the unstable orientations are the privileged sites for the formation of new grains. These nuclei were misoriented with respect to the orientations identified within the neighbouring deformed areas by α(<111>, <112> or <100>) relations. Growth of the new grains is always favoured along directions parallel to the traces of the {111} slip planes as shown in the stable and structurally homogeneous Goss {110}<001> oriented grains. The grain boundary can play a role in recrystallization similar to that of the deformation bands.

Interfacial Reactions during Explosive Bonding

The layers near the interface of explosively welded plates were investigated by means of microscopic observations, mostly with the use of transmission electron microscopy (and Focus Ion Beam technique for the thin foils preparation) equipped with energy dispersive spectrometry. The metal compositions based on steels and Ti, Zr, Ta or Cu, were analyzed. The study was focused on the identification of the intermetallic phases inside the melted zones, the possible interdiffusion between the bonded metals and the changes in the dislocation structure.

The Formation of New Orientations during Recrystallization of Silver Single Crystals with {112} Initial Orientation

The recrystallization mechanisms in high purity Ag crystals with C{112}<111> initial orientation, deformed by channel-die compression, have been studied by local orientation measurements using TEM and SEM/EBSD. The microtexture analysis clearly indicates the importance of a simple relation of 25-40o (<111> or <112>) type, which is frequently observed during the early stages of recrystallization between isolated nuclei of uniform orientation and one of the as-deformed groups of components. As recrystallization proceeds, recrystallization twinning increases radically. In C-oriented silver single crystals this latter mechanism also plays a decisive role in the formation of the cube orientation.

Crystallographic aspects of nucleation and growth during primary recrystallization in stable single crystals of Al and Al-1%Mn alloy

The early stages of recrystallization have been characterized in stable, single crystals of Al and an Al-1%wt. Mn alloy to rigorously quantify the orientation relations between nuclei and simple deformation structures. Goss{110} and brass{110} oriented samples were deformed in a channel-die to develop a homogeneous structure composed of two sets of symmetrical microbands and then lightly annealed. SEM/EBSD analyses demonstrate a strong relation between as-deformed orientations and the limited number of recrystallized grain orientations. The misorientation angles across the recrystallization front are mostly grouped in the ranges of 25-55° around axes located near, but not at, the normals of all four {111} planes. In some cases the orientation of the growing grain transforms through the formation of a first generation twin with the twinning plane normal lying near the rotation axis. The intensity of new grain nucleation in near-surface areas is significantly greater than those observed in central parts of the sample. Many new grains grow from the surface into the sample centre mostly along the {111} planes. However, irrespective of the section new grains possess the same type of orientation relationship with respect to the deformed state.