Microstructural evolution and mechanical properties of Al–40 wt.%Zn alloy processed by equal-channel angular extrusion

The Al-based Al–40 wt.%Zn alloy was subjected to multi-pass equal-channel angular extrusion (ECAE) via route-A and route-B_C. Before and after ECAE processing, microstructural evolution, the tensile properties, impact toughness and fracture behavior of the alloy were investigated.ECAE processing caused to elimination of the as-cast dendritic microstructure and formed a structure consisting of elongated, ribbon shaped α-phase via route-A and mostly equiaxed α-phase via route-B_C. ECAE processing also caused plastic instability as necking at early onset of deformation. As a result of more effective mechanical mixing via route-B_C, softening and necking occurred more apparently. The tensile and yield strength of the alloy increased just after first pass and then slightly decreased with increasing number of passes. On the other hand, its elongation to failure and impact toughness increased with increasing number of passes in both routes. The increase obtained via route-A is more pronounced in both properties. Fracture behavior of the as-cast alloy changed from brittle to ductile mode after multi-pass ECAE. Elimination of dendritic as-cast structure with reduction of porosities and deformation-induced homogenization by the effect of ECAE processing increased the ductility and impact toughness of the alloy and caused formation of a fracture surface consisting of micro-voids and dimples which indicates of ductile fracture. Attained experimental results indicate that multi-pass ECAE processing is very effective in improving the tensile elongation and impact toughness of binary Al–40 wt.%Zn alloy.     

A modified die for equal channel angular pressing

Tensile stress occurs in the vicinity of upper surface of the specimen in the severe plastic deformation zone, which increases the cracking and fracture tendency of the specimen and impedes the further ECAP processing. In this paper, the conventional ECAP die (Ψ = 16° and Φ = 90°) was modified to eliminate the tensile stress and enhance the compressive stress in the severe plastic deformation zone, therefore reducing the cracking and fracture tendency of the specimen. Finite element analysis demonstrated that the stress state changes from tensile to strongly compressive when using the modified die. A modified die was made and employed to extrude the commercially pure aluminum to verify its effectiveness experimentally. The billet was successfully extruded for 20 passes without obvious surface defects with the modified die, compared to 13–14 passes at most for the conventional die. Consequently, much more fine and uniform microstructure was obtained with the average grain size of 200–300 nm, while the average grain size is ～500 nm in the case of using the conventional die.     

ECAP with back pressure for optimum strength and ductility in aluminium alloy 6016. Part 2: Mechanical properties and texture

The simultaneous increase in strength and ductility of aluminium alloy 6016 processed by equal channel angular pressing (ECAP) was investigated. A complete study of microstructure, texture and mechanical properties after ECAP processing with and without back pressure was carried out for the O temper. The simultaneous increase in strength and ductility of AA6016-O with number of ECAP passes was explained by the use of back pressure during ECAP. A maximum ductility of ～100% was obtained at the temperature of 200 °C and strain rate of 10^?4 s^?1, which is a significant improvement on the ductility exhibited by AA6016 (～89%) after a conventional thermomechanical treatment at a much higher temperature of 500 °C. The mechanical behaviour was interpreted in the context of the textures developed in the material. A significant amount of texture rotation due to applied back pressure was found.     

High-cycle fatigue behavior of severe plastically deformed binary Zn–60Al alloy by equal-channel angular extrusion

High-cycle fatigue properties, mainly the fatigue strength and fatigue life, of two-phase Zn–60Al alloy subjected to severe plastic deformation (SPD) via multi-pass equal-channel angular extrusion (ECAE) using two different routes, route-A and route-B_C, were studied. The effect of ECAE processing parameters on the fatigue behavior of the alloy was investigated, and the stress amplitude versus number of cycles to failure (S–N) curves of the alloy were determined in the as-cast and multi-pass ECAE-processed conditions. The results showed that the fatigue performance of the alloy improved significantly through multi-pass ECAE. The fatigue endurance limit of the as-cast alloy reached to 146 MPa from 79 MPa. On the other hand, the fatigue properties of the ECAE-processed alloy were affected by ECAE parameters, and showed sensitivity to number of cycles and stress levels. Correlation of fatigue data showed that Basquin's law can be used to express the fatigue behavior of the multi-pass ECAE-processed alloy. Also, the ECAE processing changed the nature of fracture characteristics of the as-cast Zn–60Al alloy.     

Texture,microstructure and mechanical properties of equiaxed ultrafine-grained Zr fabricated by accumulative roll bonding

The texture, microstructure and mechanical behavior of bulk ultrafine-grained (ufg) Zr fabricated by accumulative roll bonding (ARB) is investigated by electron backscatter diffraction, transmission electron microscopy and mechanical testing. A reasonably homogeneous and equiaxed ufg structure, with a large fraction of high angle boundaries (HABs, ∼70%), can be obtained in Zr after only two ARB cycles. The average grain size, counting only HABs (θ > 15°), is 400 nm. (Sub)grain size is equal to 320 nm. The yield stress and UTS values are nearly double those from conventionally processed Zr with only a slight loss of ductility. Optimum processing conditions include large thickness reductions per pass (ε ∼ 75%), which enhance grain refinement, and a rolling temperature (T ∼ 0.3T_m) at which a sufficient number of slip modes are activated, with an absence of significant grain growth. Grain refinement takes place by geometrical thinning and grain subdivision by the formation of geometrically necessary boundaries. The formation of equiaxed grains by geometric dynamic recrystallization is facilitated by enhanced diffusion due to adiabatic heating.     

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.     

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.     

Microstructure–mechanical property relationships in ultrafine-grained NbZr

The present paper reports on the microstructure–mechanical property relationships in an ultrafine-grained (UFG) niobium–1 wt.% zirconium (NbZr) alloy, a potential biomedical material, severe plastically deformed at room temperature utilizing equal channel angular extrusion (ECAE). Monotonic tensile and low-cycle fatigue (LCF) experiments were carried out on the NbZr samples processed along ECAE routes 8B_C and 16E, along with extensive microstructure analysis. The important finding is that the NbZr alloy processed along ECAE routes that lead to a higher volume fraction of high-angle grain boundaries (HAGBs) exhibits a stable cyclic deformation response in the LCF regime. This stands in good agreement with prior studies on other materials, such as UFG interstitial-free steel, in which the stable fatigue behavior was associated with the dominance of HAGBs. The current results provide a venue for utilizing the UFG NbZr alloy in biomedical applications that require a combination of long-term durability, high strength and very good biocompatibility, where the latter is not altered by ECAE processing. Furthermore, for the first time, we present guidelines for optimizing processing parameters that define the microstructure–cyclic stability relationship in UFG alloys.     

Characterization of the texture evolution during thermomechanical processing of an Fe–30 at% Al–6 at% Cr alloy

Texture and mesotexture analyses of samples obtained from an Fe–30 at% Al–6 at% Cr alloy during different steps of a thermomechanical processing route of hot and intercalated warm-rolling/isochronal annealing steps indicate a complex microstructural evolution depending on the strain and temperature paths that are taken. A strong 〈hkl〉 {111} fiber is obtained in warm-rolled samples, while almost random texture is obtained in recrystallized samples, independent of recrystallization temperature (20 min, 1073 K < T < 1372 K). The mesotexture of the recrystallized samples, however, shows the incidence of “clusters” of neighboring recrystallized grains with similar orientation, which are also present in the hot-rolled samples. Evidences are presented for the formation of these complex microstructures by a grain boundary rotation mechanism, at least for high temperature deformation conditions.     

Microstructure and texture development in hydrostatically extruded Mg–Al–Zn alloys during tensile testing at intermediate temperatures

Tensile testing of hydrostatically extruded round bars of AZ31 and AZ61 has been performed to analyse the flow behaviour as well as the microstructure and texture development as a function of temperature (175–225 °C) and strain rate (0.0001–0.01 s^?1). The post-testing microstructure is a result of dynamic recrystallization with varying significance of different texture components. In some cases the resulting textures are found to be similar to those textures that typically develop during extrusion of rare-earth-containing magnesium alloys. Dynamic recrystallization (DRX) and grain boundary sliding (GBS) are considered as the mechanisms that generate the changes in texture. Precipitates can exert a grain boundary pinning effect limiting grain growth. These different mechanisms contribute differently to the texture development if the testing parameters are changed.     

Microstructural evolution in copper subjected to severe plastic deformation: Experiments and analysis

The evolution of microstructure and the mechanical response of copper subjected to severe plastic deformation using equal channel angular pressing (ECAP) was investigated. Samples were subjected to ECAP under three different processing routes: B_C, A and C. The microstructural refinement was dependent on processing with route B_C being the most effective. The mechanical response is modeled by an equation containing two dislocation evolution terms: one for the cells/subgrain interiors and one for the cells/subgrain walls. The deformation structure evolves from elongated dislocation cells to subgrains to equiaxed grains with diameters of ∼200–500 nm. The misorientation between adjacent regions, measured by electron backscatter diffraction, gradually increases. The mechanical response is well represented by a Voce equation with a saturation stress of 450 MPa. Interestingly, the microstructures produced through adiabatic shear localization during high strain rate deformation and ECAP are very similar, leading to the same grain size. It is shown that both processes have very close Zener–Hollomon parameters (ln Z ∼ 25). Calculations show that grain boundaries with size of 200 nm can rotate by ∼30° during ECAP, thereby generating and retaining a steady-state equiaxed structure. This is confirmed by a grain-boundary mobility calculation which shows that their velocity is 40 nm/s for a 200 nm grain size at 350 K, which is typical of an ECAP process. This can lead to the grain-boundary movement necessary to retain an equiaxed structure.     

Separate contributions of texture and grain size on the creep mechanisms in a fine-grained magnesium alloy

The influence of texture and grain size on the creep behavior of a fine-grained magnesium alloy, over the temperature range 423–723 K was investigated. Equal channel angular pressing and rolling were used to produce samples with different textures. Two deformation regimes could be distinguished by their stress exponents. A stress exponent close to 2 and activation energy of 91 kJ mol⁻¹, close to that for grain boundary diffusion, were found at the lower strain rates. In this range, there is no detectable effect of texture. In the high stress exponent regime, within the range 3 < n < 12, a noticeable effect of texture and grain size has been found. The texture effect is related to the orientation of the basal planes. The influence of grain size distribution on flow stress is satisfactorily explained by modeling the deformation as a combination of grain boundary sliding and slip creep.     

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–property relationship in textured ZnO-based varistors

The relationship between microstructure texturing and electrical characteristics of a ZnO-based varistor system was investigated in comparison with a varistor system having the same chemical composition but conventional microstructure. Highly textured ZnO-based varistors were produced via the templated grain growth (TGG) technique. Stereological analysis, electron back-scattered diffractometry (EBSD) and X-ray diffractometry (XRD) were conducted to analyze texture development and orientation distribution. The degree of orientation, r, calculated from the (0 0 0 1) EBSD pole figure, was 0.34; the texture fraction, f (Lotgering factor), calculated from the XRD data, was 0.98 for the samples produced via TGG. The threshold voltages were found to be anisotropic, consistent with the observed morphological texture. The non-linear coefficients, α, did not exhibit a significant difference as a function of direction, even in the highly textured samples. However, different types of grain boundary characteristics depending on the direction were identified with 0.42, 0.69 and 1.14 eV Schottky barrier heights.     

Thermal behavior of Ni (99.967% and 99.5% purity) deformed to an ultra-high strain by high pressure torsion

Polycrystalline Ni of two purities (99.967% (4N) and 99.5% (2N)) was deformed to an ultra-high strain of ε_vM = 100 (ε_vM, von Mises strain) by high pressure torsion at room temperature. The 4N and 2N samples at this strain are nanostructured with an average boundary spacing of ～100 nm, a high density of dislocations and a large fraction of high angle boundaries (>15°) of 0.68–0.74, as determined by transmission electron microscopy, and 0.8–0.83, as determined by electron backscattering diffraction. The deformed samples were annealed isochronally for 1 h at temperatures from 100 to 600 °C, and the evolution of the structural parameters (boundary spacing, average boundary misorientation angle and the fraction of high angle boundaries), crystallographic texture and hardness were determined. Based on microstructural parameters the energy stored in the deformed state was estimated to be 14 MPa and 24 MPa for 4N Ni and 2N Ni, respectively. The isochronal annealing leads to a drop in hardness in three stages: a relatively small decrease at low temperatures (recovery), followed by a rapid decrease at intermediate temperatures (recrystallization) and a slow decrease at high temperature (grain growth). Both recovery and recrystallization of the 2N Ni are strongly retarded by the presence of impurities reducing the mobility of boundaries. In the recrystallization stage, changes in hardness, microstructure and texture show that the 4N Ni recrystallizes discontinuously, in spite of a large fraction of high angle boundaries in the deformed state. This finding contradicts previous experiments and theory, which suggest that recrystallization is continuous when the fraction of high angle boundaries is high. In the 2N Ni, the observations suggest that some structural coarsening (continuous recrystallization) may take place simultaneously with discontinuous recrystallization. The findings emphasize the importance of alloying, which can delay the process of recovery and recrystallization and thereby enable tailoring of the microstructure and properties through an optimized annealing treatment.     

Texture memory and variant selection during phase transformation of a zirconium alloy

The texture evolution of cold-rolled Zircaloy-2 sheets during the α → β → α phase transformation was characterized in situ using high-energy synchrotron X-ray diffraction. It was found that the strong rolling texture is initially modified during heating, evidencing recrystallization, after which it is weakened by a complete phase transformation. Hardly any variant selection was observed during α → β phase transformation. When the material was rapidly cooled back from above the β-transus, the α texture exhibited moderated variant selection. In a study of the effect of maximum heating temperature on the inherited α texture, it was found that incomplete transformations, even with peak temperatures nearing the β-transus, resulted in a perfect texture memory in spite of dramatic morphological changes in the microstructure.     

Enhancing room temperature mechanical properties of Mg–9Al–Zn alloy by multi-pass equal channel angular extrusion

Influence of equal channel angular extrusion on room temperature mechanical properties of cast Mg–9Al–Zn alloy was investigated. The results show that room temperature mechanical properties of Mg–9Al–Zn alloy, such as yield strength, ultimate tensile strength and elongation, can be improved heavily by equal channel angular extrusion. Processing routes, processing temperature and extrusion passes have important influence on room temperature mechanical properties of processed Mg–9Al–Zn alloy by equal channel angular extrusion. The optimum room temperature mechanical properties such as yield strength of 209 MPa, ultimate tensile strength of 339 MPa and elongation of 14.1%, can be obtained when Mg–9Al–Zn alloy was processed by equal channel angular extrusion for 6 passes at route B_C at 498 K. Large bulk materials of Mg–9Al–Zn alloy with average grain size of 4 μm and high mechanical properties can be prepared.     

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.     

Highly textured and twinned Cu films fabricated by pulsed electrodeposition

This paper reports the influence of substrates and peak current densities on: crystallographic textures; twin density, thickness, spacing and twin boundary orientations; and electrical transport and mechanical properties of Cu films fabricated by pulsed electrodeposition. Films of {3 1 1}, {1 0 0}, {1 1 0} and {1 1 1} out-of-plane textured Cu with a high density of {1 1 1} twins were synthesized. With increasing twin density and change of texture by selection of the peak current densities and substrates, the ultimate tensile strength, hardness and elongation monotonically increase. This paper also discusses the mechanisms for manipulation of texture and twin densities via changing both the peak current densities and substrates, and the correlation among the microstructure, electrical resistivity and mechanical properties.     

Modelling texture development during equal channel angular extrusion of aluminium

The deformation textures that develop in aluminium during ECAE (without rotation of the billet) have been investigated experimentally and modelled using the FC-Taylor approach, for two different die angles (90 and 120°), by using actual deformation histories measured from scribed marker grids. This has shown that the deformation during ECAE can best be described in terms of streamline coordinates and involves a simple shear parallel to the streamline, which becomes aligned with the final extrusion direction, and a plane strain tension and compression component that develops as the material enters and leaves the dies deformation zone. The textures observed were similar to those found following torsion straining and had the main components {001}〈110〉 and {112}〈110〉 along a B partial fibre. However, in the case of ECAE, the positions of maximum intensity were rotated by ~15–20° about the transverse direction (TD). Similar textures were seen for even and odd numbers of extrusion passes, suggesting that the TD rotation is not caused by alignment of the fibre direction with the die’s ‘shear plane’, as has been previously reported. In contrast, texture simulations showed that this rotation occurs as a consequence of the additional plane strain compression component in ECAE deformation.