Microstructure and microtexture evolution during strain path changes of an initially stable Cu single crystal

The microstructure and microtexture evolution in a deformed Goss oriented crystal were characterized after a sample rotation and consequent change in strain path, over a range of scales by optical microscopy, high resolution scanning electron microscopy equipped with field emission gun and electron packscattered diffraction facilities and transmission electron microscopy orientation mapping. High purity copper single crystals with initial Goss{1 1 0}〈0 0 1〉 orientation were channel-die compressed 59% to develop a homogeneous structure composed of two sets of symmetrical primary microbands. New samples with ND rotated orientations of Goss{1 1 0}〈0 0 1〉, brass{1 1 0}〈1 1 2〉, M{1 1 0}〈1 1 1〉 and H{1 1 0}〈0 0 1〉, were then cut out and further compressed in channel-die by a few per cent. The change in flow stress could be correlated with the change in dislocation substructure and microtexture, particularly along shear bands initiated by the strain path change. In the H{1 1 0}〈0 1 1〉 and M{1 1 0}〈1 1 1〉 orientations, the flow stress increased by Taylor factor hardening then decreased by intense macroscopic shear band (MSB) formation. In the softer brass orientation and in the absence of Taylor factor hardening, more diffuse MSB formation occurred. The local rotations in the band were used to deduce the possible local slip systems initiated during the strain path change.     

Texture evolution by shear on two planes during ECAP of a high-strength aluminum alloy

The evolution of texture was examined during equal-channel angular pressing (ECAP) of an Al–Zn–Mg–Cu alloy having a strong initial texture. An analysis of the local texture using electron backscatter diffraction demonstrates that shear occurs on two shear planes: the main shear plane (MSP) equivalent to the simple shear plane, and a secondary shear plane which is perpendicular to the MSP. Throughout most regions of the ECAP billet, the MSP is close to the intersection plane of the two channels but with a small (5°) deviation. Only the {1 1 1}〈1 1 0〉 and {0 0 1}〈1 1 0〉 shear systems were activated and there was no experimental evidence for the existence of other shear systems. In a small region at the bottom edge of the billet that passed through the zone of intersection of the channels, the observed textures were fully consistent with the rolling textures of Copper and Goss.     

Orientation dependence of local lattice rotations at precipitates: Example of κ-Fe3AlC carbides in a Fe3Al-based alloy

Local lattice rotations and in-grain orientation gradients at κ precipitates in matrix grains with orientations near the 45° rotated cube {0 0 1}〈1 1 0〉 (RC) and the γ-fiber components {1 1 1}〈1 1 2〉 were investigated in a Fe₃Al alloy warm-rolled to reductions of between 10% and 60%. Near-RC grains showed larger local lattice rotations at precipitates than γ-fiber grains. In RC-oriented grains the local lattice rotations about the transverse direction (TD) were dominant at low reductions, but rotations about the rolling direction (RD) also occurred at higher strains. In the γ-fiber grains the axes of the in-grain lattice rotations were scattered between TD and RD. The rotations around the particles and their orientation dependence were analyzed using 3-D crystal plasticity finite-element simulations of a spherical inclusion in a plane strain deformed matrix of different orientations, namely RC, {1 1 1}〈1 1 2〉 and {1 1 1}〈0 1 1〉.     

Contribution of non-octahedral slip to texture evolution of fcc polycrystals in simple shear

The contribution of non-octahedral {1 0 0}〈1 1 0〉 slip to texture evolution under simple shear in face-centred cubic (fcc) polycrystals was studied. It was found that, by adding the {1 0 0}〈1 1 0〉 slip system family to the usual {1 1 1}〈1 1 0〉, the ideal orientations remain the same. However, the stability of the ideal orientations, the rotation field and the rate of change of the orientation density function were affected by the non-octahedral slip activity. The stress state, the slip distribution and the form of the equipotential functions were also examined along the ideal fibres. Finally, the texture evolution in pure aluminium during equal channel angular extrusion was simulated and analysed.     

Orientation selection process during the early stage of cubic dendrite growth: A phase-field crystal study

Using the phase-field crystal model, we investigate the orientation selection of the cubic dendrite growth at the atomic scale. Our simulation results reproduce how a face-centered cubic (fcc) octahedral nucleus and a body-centered cubic (bcc) truncated-rhombic dodecahedral nucleus choose the preferred growth direction and then evolve into the dendrite pattern. The interface energy anisotropy inherent in the fcc crystal structure leads to the fastest growth velocity in the 〈1 0 0〉 directions. New {1 1 1} atomic layers prefer to nucleate at positions near the tips of the fcc octahedron, which leads to the directed growth of the fcc dendrite tips in the 〈1 0 0〉 directions. A similar orientation selection process is also found during the early stage of bcc dendrite growth. The orientation selection regime obtained by phase-field crystal simulation is helpful for understanding the orientation selection processes of real dendrite growth.     

Mechanical properties and determination of slip systems of the B2 YZn intermetallic compound

Single crystal specimens of YZn (B2) were tested in tension at room temperature. Specimens with a [1 0 1] tensile axis orientation exhibited {0 1 1}〈1 0 0〉 primary slip and an ultimate tensile strength of 365 MPa at 3.7% elongation. Specimens with [0 0 1] and [1 1 1] tensile axis orientations showed no slip lines and fractured at a stress of 180 MPa at 3.3% and 130 MPa at 2.9% elongation, respectively. Transmission electron microscopy (TEM) examination of the Burger’s vector of dislocations in tensile tested specimens revealed 〈1 0 0〉-type dislocations. TEM analysis suggested that a secondary slip system, {0 0 1}〈1 0 0〉, may be active. Banded features with a {0 2 1} orientation were observed in deformed YZn; these may be slip traces produced by the cross-slip of 〈1 0 0〉 dislocations. Acting together, {0 1 1}〈1 0 0〉 and {0 0 1}〈1 0 0〉 slip provide only three independent slip systems, and no extra independent systems are provided by the cross-slip. This finding is consistent with the low ductility of YZn.     

Spatial characterisation of the orientation distributions in a stable plane strain-compressed Cu crystal: A statistical analysis

Single copper crystals of the stable Goss orientation {0 1 1}〈1 0 0〉 were deformed in plane strain compression and the deformation-induced dislocation structures were investigated by high-resolution electron backscattered diffraction. Although the orientation maps exhibited an anisotropic dislocation boundary structure it was shown that the mean disorientation angle between point pairs saturated and became isotropic if their spacing was large enough (typically >30 μm). This saturation behaviour was interpreted as being a consequence of the anti-correlations between nearby dislocation boundaries and is discussed in terms of recent stochastic models of boundary formation. It was found that the disorientation boundaries, which were considered as being formed at relatively low strains, underwent rigid body-like rotations during deformation.     

Observation and modeling of the through-thickness texture gradient in commercial-purity aluminum sheets processed by accumulative roll-bonding

The texture evolution in commercial-purity aluminum (AA1070) processed by accumulative roll-bonding (ARB) is investigated with the aid of X-ray diffraction and crystal plasticity modeling. The experimental results indicate strong texture gradients through the sheet thickness, from rolling-type textures with orthorhombic symmetry at the center to shear-type textures with monoclinic symmetry near the surface. The experimental textures are reproduced well by polycrystal plasticity modeling carried out with deformation histories from finite element simulations. The observations of a relatively strong {4 4 11}〈11 11 8〉 component at the center and a {0 0 1}〈1 1 0〉 component at the surface are attributed to their higher orientation stability than the other rolling- and shear-type orientations. Examination of the average through-thickness textures suggests that the ARB technique may not be an effective means to develop apparent {1 1 1}〈u v w〉 components and thus to enhance the normal anisotropy of plasticity of the bulk sheet materials.     

Determination of slip systems and their relation to the high ductility and fracture toughness of the B2 DyCu intermetallic compound

DyCu single crystals with CsCl-type B2 structure were tensile tested at room temperature. Slip trace analysis shows that the primary slip system in DyCu with a tensile axis orientation of 〈1 1 0〉 is {1 1 0}〈0 0 1〉 and the critical resolved shear stress for {1 1 0}〈0 0 1〉 slip is 18 MPa. Slip traces were also observed from a secondary slip system, {1 1 0}〈1 1 1〉, and this slip system appears to be a key contributor to the previously reported high ductility and high fracture toughness of polycrystalline DyCu. Transmission electron microscopy determinations of the Burgers vectors of dislocations in tensile tested specimens revealed 〈1 0 0〉 and 〈1 1 1〉 dislocations, with 〈1 0 0〉-type dislocations being more abundant. The implications of these findings for the understanding of the mechanical properties of DyCu and the large family of ductile rare earth B2 intermetallics are discussed.     

Deformation-induced grain coalescence in an electrodeposited pure iron sheet studied by in situ neutron diffraction and electron backscatter diffraction

The plastic deformation behavior of an ultrafine-grained electrodeposited pure iron sheet with a strong {1 1 1}〈h k l〉 texture was studied by in situ neutron diffraction during tensile deformation at room temperature and by electron backscatter diffraction (EBSD). The combination of volume-averaged crystallographic orientation changes determined by neutron diffraction and the local orientation relationship determined by EBSD reveals a texture change to {1 1 1}〈1 1 0〉 and corresponding microstructural changes with tension deformation. Related to such grain rotation, grain coalescence on deformation was found using semi in situ EBSD. The results obtained are explained using a characteristic slip model, which also gives a reason for the ultrahigh Lankford value of this material.     

Cyclic twin nucleation in tin-based solder alloys

The microstructure and Sn crystal orientations of lead-free solder alloys such as near-eutectic SnAgCu have a significant influence on the mechanical response of a solder joint to service conditions. Thus solidification processes were examined in SnAgCu solder joints. Distinct evidence of sixfold cyclic growth twinning of Sn during solidification from the melt was observed in Sn–Ag, SAC and Sn–Cu solders. Three orientations of Sn grains, each having a common 〈1 0 0〉 direction, were found in each of these systems, though the morphologies of these cyclic twinned microstructures differed. Analysis of dendrite arm spacing in cyclically twined structures with a beach ball morphology implies that the common 〈1 0 0〉 axis intersects with the region of the nucleation event. Models are presented for two pseudo/metastable hexagonal unit cells based upon {1 0 1} or {3 0 1} twins that introduce the cyclic twinning structure at the nucleation stage. Formation of these hexagonal unit cells may be facilitated by the presence of alloy elements. Subsequent epitaxial growth of the tetragonal unit cell on this nucleus can account for all three types of morphologies observed in microstructures of Sn-rich solder alloys.     

Texture memory effect in heavily cold-rolled Ni3Al single crystals

In Ni₃Al the cold-rolled Goss texture changed to a complicated one after primary recrystallization and returned to the original Goss during the subsequent grain growth, which can be referred to as the texture memory effect. In this study, we examined the evolution of grain orientations during the grain growth using the electron backscatter diffraction (EBSD) method. It was found that just after the primary recrystallization most of the grains had a 40°〈1 1 1〉 rotation relationship to the Goss texture, the remaining grains being Goss and other textures. The formation of the 40°〈1 1 1〉 rotated grains can be explained by a multiple twinning mechanism. In the grain growth, the Goss grains, which were surrounded by the 40°〈1 1 1〉 rotated grains, grew preferentially due to the high mobility of the 40°〈1 1 1〉 grain boundaries, leading to the texture memory effect.     

Microstructure,texture and magnetic properties of strip casting Fe–6.2 wt%Si steel sheet

An Fe–6.2 wt%Si strip with equiaxed grains and mild {0 0 1}〈0 v w〉 fiber texture was produced by twin-roll strip casting process. Then the as-cast strip was treated with or without the hot rolling prior to the warm rolling and annealing. When the hot rolling was not introduced, a fine and heterogeneous warm-rolled microstructure was produced and led to a fine recrystallization microstructure and very weak {0 0 1}〈0 v w〉 fiber texture in the annealed sheets. When the hot rolling was introduced, a coarse and homogeneous warm-rolled microstructure was produced and led to a very coarse recrystallization microstructure and much stronger {0 0 1}〈0 v w〉 fiber texture in the annealed sheets. The annealed sheets with hot rolling showed a higher magnetic induction and a higher core loss than those without hot rolling.     

Deformation of wrought uranium: Experiments and modeling

The room temperature deformation behavior of wrought polycrystalline uranium is studied using a combination of experimental techniques and polycrystal modeling. Electron backscatter diffraction is used to analyze the primary deformation twinning modes for wrought alpha-uranium. The {1 3 0}〈3 1 0〉 twinning mode is found to be the most prominent twinning mode, with minor contributions from the ‘{1 7 2}’〈3 1 2〉 and {1 1 2}‘〈3 7 2〉’ twin modes. Because of the large number of deformation modes, each with limited deformation systems, a polycrystalline model is employed to identify and quantify the activity of each mode. Model predictions of the deformation behavior and texture development agree reasonably well with experimental measures and provide reliable information about deformation systems.     

Effect of crystalline anisotropy and forming conditions on thinning and rupturing in deep drawing of copper single crystal

Copper single crystal has excellent electrical properties and ductility, and it has a peculiar application in micro-manufacturing. In this paper, a specific mold was designed and made in order to conduct deep drawing of copper single crystal and evaluate the crystalline orientation effect to the local thinning and rupturing in the process. As a contrast, a finite element subroutine (VUMAT in ABAQUS) based on the crystal plasticity theory was developed to simulate the deep drawing process according to the experimental configurations. The results show that the (1 1 0) blank has better deep drawing performance than (0 0 1) blank; The crack of (0 0 1) blank originates at 〈1 0 0〉 orientation in the plane because 〈1 0 0〉 orientation has poor plasticity; friction is a crucial factor to the forming quality in a small scale deep drawing process; the simulations are in good agreement with the experiments, which also indicates the crystal plasticity model is very necessary to study the plastic forming of metallic material, especially the crystalline characteristics should be considered in the research.     

Effect of deformation and annealing on the formation and reversion of ε-martensite in an Fe–Mn–C alloy

Microstructure and texture evolution during cold rolling and subsequent annealing were studied in an Fe–22 wt.% Mn–0.376 wt.% C alloy. During rolling the deformation mechanisms were found to be dislocation slip, mechanical twinning, deformation-induced ε-martensite transformation and shear banding. At higher strains, the brass-type texture with a spread towards the Goss-type texture dominated. A decrease in the Cu- and S- components was attributed to the preferential transformation to ε-martensite in Cu- and S-oriented grains. The texture of ε-martensite was sharp and could be described as {1 1 2 9}〈3 3 6 2〉. The orientation relationship {1 1 1}^γ//{0 0 0 1}^ε and 〈110〉^γ//〈1 1 –2 0〉^ε between ε-martensite and austenite was observed but only certain variants were selected. On subsequent annealing, the ε-martensite transformed reversely to austenite by a diffusionless mechanism. Changes in length along rolling, normal and transverse directions on heating were anisotropic due to a combination of volume expansion and shape memory effects. The S-texture component increased significantly due to transformation from the ε-martensite.     

Deformation mechanisms in a Ru–Ni–Al ternary B2 intermetallic alloy

Deformation mechanisms in a B2 Al50Ni5Ru45 alloy have been studied in compression over the temperature range 298–1323 K. The alloy exhibited a low temperature sensitivity of the flow stress over the temperature range 298–973 K. The strain rate sensitivity below 973 K was relatively low, similar to binary RuAl-based alloys. Dislocation analyses after room temperature compression indicate the presence of 〈1 0 0〉 and 〈1 1 0〉 dislocations on {1 1 0} planes, with the 〈1 0 0〉 dislocations present with slightly higher densities. Compression creep tests at stress levels between 300 MPa and 500 MPa revealed exceptional creep strength in the temperature range investigated. The predominant dislocation substructure after creep deformation consisted of uniformly distributed, cusped 〈1 0 0〉-type screw dislocations on {1 1 0} planes. The deformation behavior and creep mechanisms are discussed in comparison with other high melting temperature B2 intermetallics.     

Micromechanisms of fracture in NiAl studied by in situ straining experiments in an HVEM

The room temperature brittleness of NiAl constitutes a major problem for technical applications. In order to investigate the micromechanisms of fracture in NiAl, we have carried out in situ tensile straining experiments on stoichiometric NiAl single crystals in a high-voltage electron microscope. According to our observations, crack propagation always involves dislocation activity around the crack tip, even in the hard orientation at room temperature. The Burgers vectors and the typical arrangements of the dislocations, as well as the extension of the corresponding plastic zone vary with the loading direction and the orientation of the microcrack versus potential glide systems. We observe that local concentrations of slip leads to irregular deviation of the cleavage plane from the {1 1 0} facets one usually observes at the macroscopic level. The results of our experiments help to understand why the mode I fracture toughness of NiAl is significantly larger for 〈1 0 0〉 loading directions than for non-〈1 0 0〉 directions.