Search

The effects of Zn/Mg ratios on microstructure and mechanical properties of Al-Zn-Mg-Cu alloys aged at 150 °C have been investigated by using tensile tests, optical metallography, scanning electron microscopy, transmission electron microscopy and atom probe tomography analyses. With increasing Zn/Mg ratios, the ageing process is significantly accelerated and the time to peak ageing is reduced. T′ phase predominates in alloys of lower Zn/Mg ratios while η′ phase predominates in alloys with a Zn/Mg ratio over 2.86. Co-existence of T′ phase and η′ phase with a large number density is beneficial to the high strength of alloys. Such precipitates together with narrow precipitate free zones cause a brittle intergranular fracture. A strength model has been established to predict the co-strengthening effect of T′ phase and η′ phase in Al-Zn-Mg-Cu alloys, including the factors of the grain boundary, solid solution and precipitation.

Compression experiments were designed to examine the intergranular deformation behaviors of 7xxx aluminum alloy at a micro-scale. Band structures formed in grains upon deformation were characterized by electron backscatter diffraction technique, and the local stored energy was analyzed in a quantitative way. In addition, the deformation behaviors of the six pairs of adjacent deformed grains show that, the dislocation slips are transmissible between some pairs of grains but terminated in front of other grain boundaries. Such slip transmission is influenced by the Schmid factor, the strain coordination factor mʹ, and the degree of strain concentration between grains, according to the analysis of kernel average misorientation.

Excellent comprehensive mechanical properties including good formability, high strength and high ductility are prior demands for Al-Mg-Si-Cu alloys. This study utilizes calculation of phase diagram (CALPHAD) to simplify the alloy design and meet these demands. Specifically, CALPHAD was used to finely tune the Mg/Si atomic ratio in solid solution and accurately control the type and content of second phases, especially to avoid the formation of the harmful constituent phase β-AlFeSi. Constituents and dispersoids of only α-AlFeMnSi phase were found in the alloy prepared. An optimized microstructure with fine grains, micro scale constituents, densely distributed submicron scale dispersoids and extremely dense nano precipitates provides effective impediment to dislocation gliding and induces transgranular fracture. Therefore, the designed alloy has better comprehensive mechanical properties than other 6xxx series aluminum alloys, including excellent formability, strength and ductility. The low T4P strength of 149 MPa as well as the high elongation of 26.1% implies the alloy's applicability to automobile body panel forming. The yield strength was rapidly improved from 149 MPa to 277 MPa during the paint bake ageing, because the number density of precipitates is twice as high as that of some other 6xxx alloys. Meanwhile, the elongation was kept at a high level of 20.0%.

The flow behavior and microstructural evolution of aluminum alloy 7055 after different hot thermo-mechanical processes (HTMP) were investigated. The effect of cooling rates between solution treatment and hot compression on the flow behavior was studied for the understanding of deformation softening mechanism. For the HTMP sample deformed under the same condition, the value of peak stress decreases with the increasing cooling rates. Discontinuous dynamic recrystallization (DDRX) tends to operate in the sample deformed at high strain rates (≥1s−1), while continuous dynamic recrystallization (CDRX) is the main mechanism at lower strain rates (≤0.1s−1). Compared with the microstructure of normal thermo-mechanical process (NTMP) sample, there are more “necklace” structures (higher degree of DDRX) in the HTMP sample due to a more homogeneous deformation.

Although grain boundary (GB) exhibits crucial influence on grain subdivision and work hardening of tantalum (Ta), GB characteristics and the role it plays in deformation have not sufficiently investigated yet. By means of electron backscatter diffraction analysis and Vickers hardness measurements, we systematically analyzed microstructure and stored energy at GBs deformed under unidirectional rolling (UR) and 135° clock rolling (CR), respectively. Geometrical necessary dislocation densities of GB regions were calculated based on kernel average misorientation. GB deformation mechanisms were deduced from Schmid factor analysis. Results show that in UR-Ta {111}〈uvw〉 (〈111〉//normal direction) and {100} 〈uvw〉(〈100〉//normal direction) grains tend to be deformed in uniserial and multiple slipping, respectively, while only multiple slipping is activated in CR-Ta. Hence, in UR-Ta, uniserial slipping induces dense intragranular micro-shear bands (MSBs), 23°–45° relative to rolling direction, which penetrate through adjacent {111} grains, while in {100} grains multiple slipping introduces orientation-gradient cells instead of similar MSBs observed in {111} grains. Compared to orientation-dependent grain subdivisions in UR-Ta, the multiple slipping in CR-Ta generates only cell structures in both {111} and {100} grains. Interaction effects between {100}-{100},{100}-{111} and {111}-{111} grains are therefore varies with varying rolling paths.

In order to improve the toughness of W, a series of W/Ti multilayer composites are prepared by spark plasma sintering ranging from 1000 °C to 1400 °C. The relation between the microstructure and the properties is systematically investigated. The W/Ti multilayer laminates have a better comprehensive property when the sintering temperature is between 1000 °C and 1200 °C. Under those conditions, the elongated grains in the W layers are maintained. Meanwhile, the interface microstructure of the laminates can be divided into three layers, including β-W(Ti) solid solution with lamellas, β-Ti(W) solid solution and nanosized martensite with a β-matrix and α phase in basketweave microstructure. The orientation relationship between the α-Ti variants and the β-Ti matrix is determined to be the Burgers orientation relationship. The intervariants boundaries between different α colonies are 60°/[11 2 ¯ 0]. The fracture mechanism of the composites shows a mixed facture mode and debonding of the interfaces. The fracture of the W layers is transgranular cleavage fracture and lamellas fracture, and the fracture of the Ti layers is ductile fracture with dimples.

The addition of Ag significantly increases the early ageing response, overall ageing response and peak age-hardening of an Al-Zn-Mg-Cu alloy. Ag is the most segregated of the solutes during ageing and a positive correlation between Ag and Cu exists in clusters and larger aggregates throughout the precipitation sequence (1–120 min).

The combined effect of adding Sn and natural aging (NA) time on the precipitation and age-hardening rate of the Al-0.4Mg-1.0Si alloy was investigated by a hardness test, electrical resistivity test, differential scanning calorimetry (DSC) analysis, 3-D atom probe (3DAP) analysis, and transmission electron microscopy (TEM). The results showed that the NA of the Al-0.4 Mg-1.0Si alloy was suppressed by adding Sn. This suppression was ascribed to the suppression of the formation of clusters. The short NA time (less than 5 days) had a positive effect on the subsequent artificial aging (AA) in the Sn-added alloys, i.e., a fast AA-hardening rate was achieved. The positive effect of the NA on the AA in the Sn-added alloys increased as the amount of Sn increased, but decreased as the NA time increased. The AA was negatively affected by the NA when the NA time was increased to 14 days. The accelerated AA precipitation of the Sn-added alloys after a short NA time was responsible for the fast AA-hardening rate. This accelerated precipitation was related to the increased amount of retained quenched-in vacancies according to their NA resistivity curves and similarity of the chemical composition between the Sn-containing clusters and β" phases.

Atom probe tomography was used to study the distribution of alloying elements near grain boundaries (GBs) in Zr-1.0Sn-0.3Nb-0.3Fe-(0.1Cu) alloys at the early stage of aging after β quenching. Cu was observed to segregate at GBs for the first time. Zr3Fe, Zr3(Fe,Cu) and Zr(Nb,Fe,Cr)2 are precipitated on or near GBs. In addition, the impurity Si is mainly precipitates in the form of second-phase particles (SPPs), forming Zr3(Fe,Si) and Zr3(Fe,Cu,Si).

The flow behavior of an Al-Cu-Li-Mg-Ag alloy was studied by thermal simulation tests at deformation temperatures between 350 °C and 470 °C and strain rates between 0.01–10 s−1. The microstructures of the deformed materials were characterized by electron backscattered diffraction. Constitutive equations were developed after considering compensation for strains. The processing maps were established and the optimum processing window was identified. The experimental data and predicted values of flow stresses were in a good agreement with each other. The influence of deformation temperature and strain rates on the microstructure were discussed. The relationship between the recrystallization mechanism and the Zener–Hollomon parameter was investigated as well.

Anisotropic Sb2Se3 is an emerging earth-abundant photocathode for photoelectrochemical water splitting. However, controlling the growth of the Sb2Se3 film with optimal [001] crystallographic orientation is still the most challenging issue. Here, we successfully synthesized [001]-oriented Sb2Se3via a reliable and facile method. The [001]-oriented Sb2Se3 film could provide an excellent carrier-migration efficiency. Consequently, we achieved a record-high photocurrent density of -20.2 mA cm-2 at 0 VRHE and a very high half-cell solar-to-hydrogen efficiency of 1.36% under 1-sun simulated solar illumination in a TiO2/[001]-Sb2Se3 photocathode. This work provides an effective strategy and important guidelines for rationally designing optoelectronic devices based on the [001]-oriented Sb2Se3 film.

To identify the size effects on the mechanical behavior of microparts is essential for micro-forming. Here, the size effect on the tensile properties of commercial pure titanium foils with thickness ranging from 5 µm to 200 µm has been investigated. The tensile strength, yield strength and work-hardening rate reach a minimum value at the thickness of 30 µm because the main deformation microstructures including dislocations, twins and deformation induced face-centered cubic titanium (fcc-Ti) are different in different foils. The density of deformation twins decreases as the thickness decreases. However, the density of fcc-Ti increases and reaches a maximum value at the thickness of 30 µm. The dominant deformation mechanism is dislocation slip when the thickness is below 20 µm. The yield strength decreases with the decrease in thickness because of the strengthening effect of fcc-Ti being smaller than that of twins when the thickness is above 30 µm. When the ratio of the thickness to grain size is below 5, the yield strength increases with the decrease in thickness due to the large volume fraction of oxides layer on the surface. Based on these, two kinds of constitutive equations have been well established in consideration of the grain size or the ratio of the thickness to grain size.

The precipitation behaviors of an Al-Cu-Li-Mn-Zr alloy at different ageing temperatures (120, 160 and 200 °C) were investigated using Vickers hardness measurements and transmission electron microscopy (TEM) characterization. Age hardening curves show an increase in precipitation kinetics with increasing ageing temperature. The results of TEM show that for the samples peak aged at 120 °C, the amount of δ' (Al3Li), GP zones/θ' (Al2Cu) and χ (Al5Cu6Li2) phases is obviously higher than that of T1 (Al2CuLi) precipitates; while the samples peak aged at 160 and 200 °C are usually dominated by T1 phase with a minor fraction of GP zones/θ' and δ', and the χ phase almost does not form. In addition, quantitative analysis on the T1 platelets demonstrates that the samples peak aged at 200 °C have larger plate diameter and smaller area fraction of T1, as compared to the samples peak aged at 160 °C. Correspondingly, the possible reasons for such phenomena are discussed.

We describe here the aging characteristics and strengthening behavior of a low-carbon medium-Mn Cu precipitation-strengthened steel. Atom probe tomography (APT) was employed to characterize the evolution of Cu-rich precipitates in terms of mean radius, number density and volume fraction. Aging at 500 °C and 550 °C for 1 h resulted in substantial coherent body-centered cubic (bcc) Cu-rich precipitates with mean radius of 1.35 and 2.59 nm, respectively. The precipitation strengthening mechanism for these two aging conditions was shearing mechanism and the corresponding strengthening contribution was ~ 266 and ~ 312 MPa, respectively. Here, coherency strengthening and modulus strengthening played a major role, while the contribution of chemical strengthening was relatively small. With increased aging temperature to 600 °C, the precipitates grew and coarsened to elongated shape with incoherent face-centered cubic (fcc) structure, and the strengthening mechanism was Orowan mechanism with a contribution of ~ 232 MPa. Increasing the aging temperature also facilitated the formation of retained austenite, which was of great benefit to plasticity without pronounced deterioration on precipitation strengthening. Ultra-high yield strength of 1020 MPa with superior total elongation of 25.8% was obtained in the sample aged at 600 °C for 1 h. The excellent mechanical properties derived from the combination of precipitation strengthening by Cu-rich precipitates and plasticity effect of retained austenite can be considered as a design principle to simultaneously optimize strength and ductility.

The ND ( //normal direction)( ND) grains in micro-shear bands in high-purity tantalum were investigated using electron backscatter diffraction and X-ray line profile analysis. The generation of the ND grains and their subdivision and rotation behaviors upon the subsequent deformation were characterized by multi-scale analysis methods based on information about the slip systems, misorientation angle/axes and stored energy. The obtained results show that in the transverse plane, ND grains are oriented at angles of 15°–25° to the adjacent deformed matrices in the 60% rolled specimen, and at angles of 25°–35° in the 87% rolled specimen. The ND grain provided strain accommodation during the shear deformation. Moreover, the energy of the ND grains in the 87% rolled specimen is approximately three times larger than that in the 60% rolled specimen, indicating that the role of strain accommodation is enhanced with the increase in the micro-shear stress concentration in a local region in tantalum.

The characteristics of shear bands in cold-rolled Ta-2.5W alloy are systematically investigated at different mechanical reduction levels. With increasing strain, the shear bands can appear as in-grain shear bands, trans-granular shear bands and wavy bands. At low strain the crystallographic planar in-grain shear bands are aligned close to the highly stressed slip planes at low strain. As strain increases, the shear bands rotate around the or direction to smaller inclination angles with respect to the rolling direction (RD). Meanwhile, new groups of shear bands are developed. Two groups of in-grain shear bands with a specific fishbone structure appear in the orientations with larger Taylor factor at medium strain. The trans-granular shear bands are formed when the adjacent grains get the continuity of slip planes. Therefore, the frequency of trans-granular shear bands increases with the enhancing of local texture. The wavy bands appear to be a result of sequential necking in the harder γ-fiber and a cooperative flow in the softer α-fiber. Meanwhile, the formation model of the wavy bands is also proposed based on the laminar composites.

The recrystallization behavior of tantalum was investigated under two different annealing regimes, i.e. long-time annealing at low temperatures (LALT) and short-time annealing at high temperatures (SAHT), aiming to elucidate how annealing schedule arouses not only different texture evolution but grain shape as well. The texture and grain morphologies at each recrystallization stage were revealed by Electron Back-Scatter Diffraction (EBSD) technique. LALT was observed to enhance the {100} texture (〈100〉//ND (normal direction)) while simultaneously suppressing the {111} texture (〈111〉//ND); whereas a reverse trend of texture evolution was observed under SAHT. In terms of grain shape, different from the near-equiaxed grains introduced by LALT, SAHT resulted in lots of elongated grains with random orientations (i.e. orientations except for {111}〈uvw〉 and {100}〈uvw〉). The formation mechanism of elongated grains was established. It is anticipated that results in this study would provide guidelines for microstructural optimization of Ta sputtering targets.

The effect of pre-recovery on the recrystallization kinetics of a supersaturated Al-Mn alloy annealed at 530 °C was investigated. The results show that the pre-recovery treatment can lead to faster recrystallization kinetics at the initial stage but cause sluggish recrystallization at the later stage. This different recrystallization behavior is explained by the formation of precipitates-free zones and the consumption of driving force during the pre-recovery treatment. The promoted recrystallization at the early stage can be interpreted by the promotion of recrystallization nucleation through sub-grain growth to a critical size inside precipitates-free zones. A simple model is presented to describe the nucleation of recrystallization with pre-recovery treatment in the supersaturated Al-Mn alloys.

Two variants of face-centered cubic (fcc) Ti phase appear in ultra-thin commercial pure Ti foils with hexagonal closed packed (hcp) structure during tensile deformation at room temperature. The orientation relationship between fcc-Ti phase and hcp-Ti matrix is determined as 〈0001〉HCP // 〈001〉FCC, 〈11 2 ¯ 0 〉HCP // 〈1 1 ¯ 0 〉FCC and 〈01 1 ¯ 0〉HCP // 〈110〉 FCC. The fcc-Ti phase with a habit plane of {01 1 ¯ 0}HCP usually nucleates at grain boundary and grows along the direction of 〈11 2 ¯ 0 〉HCP. The thickness of fcc-Ti phase with a shape of thin plate ranges from 10 nm to 200 nm. Dislocations, steps and stacking faults with a space of 0.58 nm can be seen in fcc-Ti. The formation of a five-layer fcc-Ti can be accomplished by the formation of stacking faults and gliding of 1/6[11 2 ¯ 0 ] Shockley partial dislocations on the prismatic plane of hcp-Ti. The 1/3[11 2 ¯ 0 ] dislocations in hcp-matrix can cross the fcc-Ti phase because the Burgers vector of these dislocations is coincidence with 1/2[110] dislocations in the fcc-Ti.

The effects of recovery on the subsequent recrystallization of tantalum were investigated via two-step annealing at two temperatures. Transmission electron microscopy, electron back-scattered diffraction, and X-ray diffraction were employed to determine the respective microstructures and textures after various annealing regimes. The results show that many large grains with {111} orientations are developed when heating at 1573 K without pre-recovery, while pre-recovery can introduce a homogeneous fine microstructure and weaken the texture. This difference can be attributed to the recovery-induced change in nucleation mechanisms. Elongated grains stretch along grain boundaries or locate in the interior of deformed grains, mainly due to the heterogeneous distribution of stored energy. Dislocations characterized by submicron bands in the deformed state evolve into sub-grains during pre-recovery and grow continuously, indicating a homogeneous distribution of dislocation or stored energy after pre-recovery, which can significantly influence subsequent nucleation and grain growth.

The work presents a detailed investigation of mechanical properties and deformation mechanism of ultrathin commercial pure Ti foils subjected to uniaxial tensile. The initial microstructure of the annealed Ti foils is fine equiaxed grains, with an average grain size of 3.3 µm. The main types of texture in pure Ti foils are {10 1 3} and {11 2 4}. Dimples, tear ridges and slip traces are found on the fracture surface, and the elongation reaches 16.7%. The large tensile elongation of pure Ti foils can be attributed to the slip of dislocations, twinning and phase transformation of HCP-Ti to FCC-Ti, which is confirmed by both X-ray diffraction and transmission electron microscope. The orientation relationship of the two phases is determined to be: HCP // FCC, 2 0 >HCP // 1 0 >FCC and {01 1 0}HCP // {110}FCC. According to this relationship, the HCP to FCC phase transformation expands the volume of the unit cell by about 10.5%. Furthermore, the volume fraction of FCC-Ti is determined to be about 3.44%. Therefore, the transformation is expected to contribute about 3.6% to the applied strain.

The formation mechanism of T 1 (Al 2 CuLi) precipitate in an Al-Cu-Li alloy was investigated by using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging, atom probe tomography (APT) and first-principles energy calculations. The HAADF-STEM images reveal the existence of the T 1 precursor phase, which is termed as T 1p in this study. The T 1p phase has a lattice structure identical to the Al matrix, and consists of a stacking of Li-rich and Cu-rich layers along the {111} Al planes. APT analysis further confirms the existence of T 1p phase, in which the concentrations of Li and Cu are significantly lower than those in T 1 plate. Based on the first-principles energy calculations, the structural evolution from T 1p phase to T 1 phase is proved to be energetically favorable. Correspondingly, one possible evolution process from T 1p phase to T 1 phase involving the change of stacking sequence is proposed and discussed.

Unidirectional rolled (UR) and clock rolled (CR) high purity tantalum sheets were analyzed with an emphasis on the microstructural difference in surface layers. Misorientation characteristics of deformed grains with different orientations are analyzed in detail by visualizing the misorientation angle based on an electron backscatter diffraction dataset. {100}( // normal direction (ND)) grains were found to associate with long-range cumulative orientation changes in CR sample while {111} ( // ND) grains were found to contain many micro-shear bands and microbands in UR sample. Then, micro-shear bands and micro-bands were detailedly characterized by transmission electron microscope, and the analysis based on Schmid factor suggested that the primary slip system activated in {111} grains leads to the formation of micro-bands during UR process. Band contrast values were used to evaluate the energy stored in {100} and {111} grains and results showed that the gap of energy between them was narrowed by CR process. Additionally, significant dispersion degree of hardness values indicates the inhomogeneous deformation in UR sample, while different degree in annealed stage indicates the different recrystallization kinetic for UR and CR samples. Upon annealing, nucleation prefers to occur along {111} deformed matrices or {111}-{100} boundaries in UR sample and recrystallization grains are large in size. While nucleation tend to take place in intersected regions in CR sample and recrystallization grains are small, which contributes to the appearance of fine grains in fully recrystallized CR sample.

To develop an improved approach in achieving an excellent combination of high strength and ductility, the solutionized Al-Cu-Li plates were subjected to rolling at cryogenic and room temperatures, respectively, to a reduction of 83%, followed by aging treatment at 160 °C. The results indicate that Al-Cu-Li alloys through cryogenic rolling followed by aging treatment possess better mechanical properties. Rolling at cryogenic temperature produces a high density of dislocations because of the suppression of dynamic recovery, which in turn promotes the precipitation of T 1 (Al 2 CuLi) precipitates during aging. Such high density of T 1 precipitates enable effective dislocation pinning, leading to an increase in strength and ductility. In contrast, room temperature rolled alloys after aging treatment exhibit lower strength and ductility due to low density of T 1 precipitates in the grain interior and high density of T 1 precipitates around subgrain boundaries.

In this work, two different intermediate annealing processes, single-step annealing (SSA, 530 °C/15 h) and two-step annealing (TSA, 450 °C/5 h + 530 °C/15 h), were used to tailor microstructure before cold-rolling and annealing of the final twin-roll cast Al-Mn foils. The recrystallization behavior and mechanical properties during annealing of severely cold-rolled foils were systematically studied. Our results show that discontinuous recrystallization occurs in SSA-foils during annealing at 150–310 °C, in contrast with continuous recrystallization in TSA-foils. The continuous recrystallization develops much finer grains (∼1.35 μm) in the TSA-foils than those by discontinuous recrystallization in the SSA-foils (∼14.7 μm). The texture components in cold-rolled TSA-foils hardly change (retained rolling-textures) after continuous recrystallization, while those in the cold-rolled SSA-foils mainly transform into a strong cube component {001} 〈100〉 after discontinuous recrystallization. Finally, a maximized elongation to fracture of ∼23.9% was achieved in the TSA-foil, much higher than that of the SSA-counterparts, ∼8.3%. The relationships between the microstructure and mechanical properties were discussed.

Al-cladded Al–Zn–Mg–Cu sheets were compressed up to 70% reduction on a Gleeble–3500 thermo-mechanical simulator with temperatures ranging from 380 to 450 °C at strain rates between 0.1 and 30 s−1. The microstructures of the Al cladding and the Al–Zn–Mg–Cu matrix were characterized by electron back-scattered diffraction (EBSD) and X-ray diffraction (XRD). The microstructure is closely related to the level of recovery and recrystallization, which can be influenced by deformation temperature, deformation pass and deformation rate. The level of recovery and recrystallization are different in the Al cladding and the Al–Zn–Mg–Cu matrix. Higher deformation temperature results in higher degree of recrystallization and coarser grain size. Static recrystallization and recovery can happen during the interval of deformation passes. Higher strain rate leads to finer sub-grains at strain rate below 10 s−1; however, dynamic recovery and recrystallization are limited at strain rate of 30 s−1 due to shorter duration at elevated temperatures.

In 6xxx aluminium alloys for automotive panel applications, pre-ageing is normally employed to reduce the negative effect of natural ageing on the paint-bake response of the materials. Pre-ageing can also influence the stretch formability, although the mechanism is still unclear. In this paper, the effect of a pre-ageing treatment on the stretch formability of 6xxx alloys for the levels of Mg/Si ratio ranging from 2.5 to 0.4 was studied by using tensile testing, forming limit diagram (FLD) tests, and three dimensional atom probe (3DAP) investigations. In particular, the 3DAP results were used to explain the work hardening behaviour of samples. The results showed that pre-ageing deteriorates the stretch formability of all the 6xxx alloy samples studied, regardless of the magnitude of the Mg/Si ratio, due to a reduction in both the work hardening and the strain-rate hardening capabilities. Furthermore, it was demonstrated that the paint-baked hardness is inversely correlated with the plane strain stretch formability of the alloys.

In order to show the formation mechanism of {001}〈210〉 orientation, a Ta-2.5W alloy was cold rolled to a large strain when many microbands are aligned with rolling direction. The {001}〈210〉 orientation is usually developed in a deformation band with normal direction//〈001〉 next to {111}〈110〉 orientation. The {001}〈210〉 and {111}〈110〉 orientations share a common rotation axis of 〈111〉 and a highly stressed slip system of {110}〈111〉. The orientation relationship between {001}〈210〉 and {111}〈110〉 orientations could be a twin relationship (60°/〈111〉). The {001}〈210〉 orientation is probably developed by the rotation of intra-grain deformation bands, which is related with the shear banding and rigid body rotation. When the strain reaches 90%, deformation bands are crossed by shear bands and the orientations are changed. However, the {001}〈210〉 orientation can still be seen. The {001}〈210〉 oriented grains grow larger during subsequent annealing at 1223 K to 1323 K attribute to the coincident site lattice interfaces.

The effect of pre-ageing (PA) on dynamic strain ageing (DSA) in Al-Mg-Si alloys with different Mg/Si ratios has been studied by using hardness measurements, tensile testing, three dimensional atom probe (3DAP), and scanning electron microscopy (SEM). The results show that a pre-ageing step prior to one week of natural ageing promotes DSA and gives rise to a reduced strain rate sensitivity (SRS) of the flow stress. Mechanisms for enhanced DSA are discussed in terms of the 3DAP results. It is concluded that the most likely mechanism for the observed decrease in the SRS is associated with the presence of excess vacancies promoted by pre-ageing. The excess vacancies may reside in vacancy-solute complexes (e.g. after pre-ageing at 100 °C for 5 min) or be trapped in clusters (e.g. after pre-ageing at 100 °C for 2 h).

A two-step annealing treatment consisting of a pre-recovery process (450 °C/5 h) and a following recrystallization process (530 °C/15 h) was developed for a cold-rolled twin-roll cast 3003 alloy (Al-1.13Mn-0.45Fe-0.1Si in wt%) sheet. The effects of pre-recovery on recrystallization behavior and mechanical properties were systematically studied. Our results show that fine quasi-equiaxed grains together with very weak ND-rotated cube (001) texture was successfully obtained using the proposed two-step annealing treatment. It is found that pre-recovery stimulates the formation of precipitates-free zones around constituent particles. Within such precipitates-free zones, substantial random oriented sub-grains can readily grow to critical size for nucleation, which contribute to enhanced nucleation without preferential orientation. This is the reason for the formation of fine grains as well as a weak texture in the two-step annealed sheet. The sheets that were subject to this two-step annealing also exhibits an extremely weak mechanical anisotropy with a Δ r value 0.03, much lowe r than that of sample (0.31) subjected to a single-step annealing at 530 °C for 15 h.

The evolution of dislocation microstructure in electron beam melted Ta-2.5W alloy was investigated by transmission electron microscope (TEM). Long straight dislocations and dislocation loops are formed in Ta-2.5W alloy cold-rolled by 5%. A set of long, continuous extending planar boundaries (EPBs) are formed when the reduction reaches 20%. In the early stage of development, EPBs are fragmented, diffused and curved, which are connected by non-crystallographic cells boundaries to maintain their continuity. The straight segments of EPBs are usually parallel with the trace of {110}, and incline at about 25–35° to the rolling direction (RD). Two groups of EPBs are formed in a grain when the reduction is larger than 30%. The dislocations within EPBs tend to rearrange themselves with increasing strain in a sequence, from tangled dislocations, followed by parallel long straight screw dislocations and finally into dislocation nets, which a re composed by 1/2 and [100] type dislocations. The relaxation process of dislocations and the interaction of dislocations with EPBs make EPBs appear wavy and deviate from the trace of slip planes.

The hot deformation behavior of Al-Mg-Si alloys was investigated by isothermal compression tests in a temperature range from 400 °C to 550 °C and a strain rate range from 0.001 to 1 s−1 with a true strain of 1.2. Flow behaviors were investigated and processing map was established. The microstructure of deformed samples was characterized by electron back scattered diffraction (EBSD) techniques. Geometrically necessary dislocation (GND) density and grain orientation spread (GOS) maps were calculated in order to investigate the dynamic recrystallization (DRX) mechanisms. It was found that the deformed microstructure strongly depends on the Zener-Hollomon (Z) values. Both sub-grain size and DRX fraction increased with decreasing ln Z. Both dynamic recovery (DRV) and DRX happened during deformation at elevated temperatures. DRV dominated at high ln Z, while DRX dominated at low ln Z. Three different DRX mechanisms, continuous dynamic recrystallization (CDRX), discontinuous dynamic recrystallization (DDRX) and geometric dynamic recrystallization (GDRX) were observed in the experimental materials. The main DRX mechanism depended on ln Z, i.e., GDRX dominated at low ln Z, CDRX dominated at middle ln Z, while DDRX dominated at high ln Z.

Effects of Ag addition on the age hardening and precipitation behavior of aluminum alloy 7075 were investigated. The transmission electron microscopy (TEM) and atom probe tomography (APT) were used to study the aggregate distribution and identify the role of Ag in precipitation hardening of the alloy. The hardness and strength of the Ag–containing alloy were increased by more than 10% due to a higher number density of fine aggregates, which was 2.5 times that of the Ag–free alloy 7075. Both T–type and η–type precipitates existed in the alloys, and two precipitation sequences occurred simultaneously as: SSS → η–type cluster → GP zone → η′ → η → T, and SSS → T–type cluster → GP zone→ T′ → T. Particularly, Ag promoted the formation of T–type clusters due to the interaction among Ag, Mg and Zn atoms as well as vacancies at the early stage of aging. The Mg–Ag, Zn–Ag, and vacancy–Ag co–clusters acted as nucleation sites for the subsequent precipitates (η′, T′, etc.). This also caused the increase of Mg concentration in T–type clusters at the beginning of aging and the narrowing of the precipitation free zone (PFZ) near grain boundaries at the peak aging.

The ageing precipitates and strengthening effects of Al-Zn-Mg-Cu alloys with different Zn contents were investigated. The results showed that increasing Zn contents can significantly improve the age-hardening response and influence the type of precipitates as well. The low Zn-containing alloy (1.70 at.%) is strengthened by nano-scale T′ phases in the peak aged condition. However, the microstructure of the high Zn content (2.34 at.%) alloy in the peak aged condition consists of a higher density of globular T′ phases and a small proportion of plate-like η′ phases. The results of atom probe tomography revealed that clusters formed at the early stage of ageing with a Zn/Mg atomic ratio higher than 1.3 transform into η′ phases and these clusters with a lower Zn/Mg ratio transform into T′ phases. The composition of a T′ phase varies with its size, where the concentrations of Zn and Mg increase with its increasing radius, while the concentration of Al presents an opposite trend.

The effect of quenching rate on strengthening behavior of an Al-Zn-Mg-Cu alloy 7085 during natural ageing has been investigated by hardness, tensile testing, differential scanning calorimetry, transmission electron microscopy, high resolution transmission electron microscopy and three dimensional atom probe. Quench-induced Y phase forms in the air-cooled condition and contributes a remarkable 100 MPa to the as-quenched strength. The air-cooled condition continues to exhibit higher hardness and strength than for the rapid, water-cooled condition during natural ageing for up to about 30000 h. With the increase of natural ageing time, the hardness difference tends to decrease slowly within the first 0.5 h, then sharply until about 20 h, and finally approaches a constant, small difference. The reasons for this unusual quench sensitivity are discussed based on the different precipitation features arising during quenching and natural ageing.

The effects of trace Er on the microstructural evolution and mechanical properties of newly designed Al–Zn–Mg–Cu–Zr alloys for drilling pipes were investigated. After homogenization, most of the eutectics dissolved into the matrix and some primary phases remained, including Al8Cu4Er, Al12(Mn,Cr), and Al3Ti. Dispersed nano-sized Al3(Er,Zr) precipitates were formed in the Er-containing alloys after homogenization. Following extrusion, solution and retrogression re-aging (RRA) treatment, the hardness and yield strength of the 0.10 wt% Er-containing alloy are increased by 14% (25.4 HV) and 12.7% (70 MPa) compared to the alloy without Er. This improvement could be ascribed to the precipitation strengthening of fine Al3(Er,Zr) particles, which complemented the conventional η' phase precipitation strengthening. The intergranular corrosion resistance of the RRA-treated alloy with the addition of 0.10 wt% Er was improved by 65% due to a lower amount of high angle grain boundaries. The fracture and intergranular corrosion mechanisms in alloys with and without Er addition were linked to the respective grain structures.

The influence of ageing temperatures on mechanical properties, corrosion behavior and precipitation features of aluminum alloy 7085 have been investigated by using tensile testing, intergranular corrosion tests, polarization curve measurements and transmission electron microscopy (TEM) observations. The results show that the precipitate sequence of aluminum alloy 7085 is different at different ageing temperatures. GP II is the main strengthening precipitate in the alloy peak aged at 90 °C, while η′ contributes most to the strength of the alloy peak aged at 150 °C. Similar ultimate yield strength can be obtained under either peak ageing conditions, but the work-hardenability at 90 °C is much better than that at 150 °C due to the predominance of GP II zones. Alloy 7085 peak aged at 150 °C shows better corrosion resistance than at 90 °C.

The flow behavior and the microstructural evolution of aluminum alloy 7055 in two heat treatment conditions (homogenized vs. pre-rolled, solution treated, stretched and naturally aged (T3)) were investigated for a height reduction of 60% with deformation temperatures ranging from 370 &deg, C to 450 &deg, C and strain rates ranging from 0.01 s&minus, 1 to 10 s&minus, 1. Flow stress decline ratio maps as a function of deformation temperature and strain rate were produced along with processing maps at a strain of 0.8 to reveal optimum hot-working conditions for deformation at strain rates of 0.01 s&minus, 1 to 0.1 s&minus, 1. The results showed that the stress drop ratio during deformation is higher for the homogenized condition than for the pre-rolled, T3 condition. A higher degree of recrystallization after deformation was observed in the pre-rolled, T3 condition due to finer second phase particles, smaller grain size, and more numerous sub-grains. The mechanism for deformation softening is discussed in the context of grain boundary characteristics.

In this paper, the effect of Mg/Si ratio and Cu content on the stretch formability of aluminium alloys of the 6xxx series was investigated using scanning electron microscopy, hardness tests, tensile testing and forming limit diagram measurements. The results show that with increasing Mg/Si ratio, the formability of Al–Mg–Si alloys decreases due to a decrease in the work hardening and strain-rate hardening capability. The addition of Cu enhances the work hardening capacity, but slightly decreases the strain-rate hardening capability. In particular, it was found that the addition of 0.3 wt% Cu improves the stretch formability of alloys with Mg/Si ratios greater than one, but has little or no effect in alloys with a large excess of Si. Furthermore, the work hardening behaviour of the alloys was modelled using the Kocks–Mecking–Estrin model.

The mechanical behavior and microstructure evolution of a single-crystal aluminum alloy 6063 were studied and visualized by in-situ micro-compression experiments. The crystallographic orientation significantly impacts the deformation behavior, and the fluctuation on the stress-strain curve corresponds well to the variation of deformed microstructure. The deformation mechanism was analyzed based on the Schmid factor as well.

The microstructure, hardness and corrosion resistance of aluminum alloy 7085 after different retrogression and re-aging (RRA) treatments were investigated. The results showed that the properties of alloy 7085 are sensitive to the temperature and dwell time of the retrogression treatment. Desirable mechanical property and corrosion resistance can be obtained for the alloy pre-aged at 120 °C for 24 h, followed by a retrogression at 160 °C for 1.5 h and re-aging at 120 °C for 24 h. After the optimized RRA treatment, the hardness of alloy 7085 is improved by 10.2% as compared with the peak-aged one, which is mainly due to the distribution of dispersed rod-like η′ precipitates in the matrix; while the improved corrosion resistance is mainly attributed to the discrete and coarse η phase with higher Cu content, and narrow precipitate-free zone about 45–50 nm in width along the grain boundaries.

The effect of trace Sn on the age hardening response and corresponding precipitation of Al-0.4Mg-1.0Si alloy were investigated by the hardness test, transmission electron microscopy (TEM) and 3-D atom probe (3DAP) to reveal the corresponding mechanism. It is shown that the natural aging (NA) is suppressed by the addition of trace Sn, which is ascribed to the suppression of the formation of clusters. The artificial aging (AA) hardening response of the natural aged alloy is slowed down by the addition of trace Sn when the aging temperature is low, i.e., 180 °C/195 °C, which is ascribed to the decreasing density of precipitates. When the natural aged alloy is artificial aged at 210 °C/225 °C, the age hardening response is accelerated due to the formation of high density precipitates. Based on the 3DAP and TEM results, a schematic model about the evolution of clusters in the Sn-free and Sn-added alloys during NA and AA is proposed to reveal the effect of Sn on the age hardening response at different aging temperatures.

The effect of cold rolling on the hardness and microstructure of alumina dispersion strengthened copper (ADSC) alloys and oxygen free pure copper was investigated. With increased cold rolling deformation, a work softening phenomenon can be observed in ADSC alloys. The higher the alumina content is, the more difficult it is to observe the work softening phenomenon. This phenomenon is not observed in oxygen free pure copper, even after cold rolling 90%. The microstructural changes of the ADSC alloys were analyzed by TEM as a function of deformation. Initially, with an increase of the cold rolling deformation, a large number of dislocation cells are formed. Then, with further increased deformation, both a decrease of dislocation density and the formation of the smaller dislocation cells (subgrains) within the larger dislocation cells or the elongated bands are observed. Finally, as deformation attains a certain value, the smaller dislocation cells (subgrains) begin to coalesce, and the work softening phenomenon appears. All of these changes are the result of the interaction between the alumina particles and the dislocation line segments. In order to analyze the work softening mechanism for the ADSC alloys, a model is introduced in the paper.