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The composition of precipitates in peak-aged Al-Zn-Mg-Cu alloys with various Zn/Mg ratios has been investigated by atom probe tomography (APT). The results show that the η′ and the T′ phases have d...

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.

The hot deformation and dynamic recrystallization (DRX) behaviors of 7055 aluminum alloy were studied at temperatures of 390−470 °C and strain rates of 0.01−1 s−1. A low DRX fraction between 1% and 13% was observed by using EBSD technique. A modified JMAK-type DRX model was proposed for such low DRX fraction problems. The model was used together with commercial FEM software DEFORM-3D to simulate the hot compression of 7055 aluminum alloy. There was a good agreement between experimental and predicted DRX fractions and grain size with an average absolute relative error (AARE) of 13.7% and 6.3%, respectively. In order to further verify the validity of the proposed model, the model was also used to simulate DRX in industrial hot rolling of 7055 aluminum alloys. The results showed that the distribution of DRX fraction was inhomogeneous, and agreed with experimental observations.

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 tensile properties and deformation microstructure of commercial high pure tantalum foils with thickness ranging from 30 μm to 200 μm have been investigated. The origin foils show a large texture intensity of γ fiber. The yield strength and ultimate tensile strength gradually increase with the decrease of thickness due to the change of the scale factor of the surface grains and the increase of volume fraction of passivation film. The constitutive model based on the surface layer theory is well established. Meanwhile, the anomalous yield point phenomenon can be seen. The deformation microstructure is closely related with the orientations. Two sets of microbands are developed in most of the grains. The density of microbands is the largest in the grains with tensile direction // 〈110〉, which belong to α fiber. Microbands are usually concentrated near the grain boundaries, which frequently incline at 25–45° with respect to the tensile direction. The trace of microbands is usually consistent with the slip planes of {110} or {112} with largest or second largest Schmid factor. Therefore, the appearance of wavy slip traces on the fracture is attributed to the dislocation cross-slip on these different kinds of slip planes.

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 W/Ti multilayer composites are prepared by spark plasma sintering. The original constituents are cold-rolled W foils with elongated grains and annealed Ti-1.5Al foils with fine equiaxed grains. After diffusion bonding, both elongated grains and fine equiaxed grains can be seen in the W layers. The diffusion rate of W in Ti is much faster than that of Ti in W due to different methods of diffusion. The detailed microstructure of W/Ti laminates can be divided into four types at different regions including pure W, mixture of α-W and β-Ti with light and dark lamellas, pure β-Ti and Ti-rich Widmanstaten α-β structure with alternative light and dark lamellas. Compared with the microhardness of original materials, the microhardness of W layers slightly decreases and the microhardness of Ti layers increases dramatically.

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.

Simultaneous improvement in yield strength, ductility and impact toughness has always been a challenge for structural steels. In the attempt to achieve this goal, we proposed a Cu-precipitation-strengthened low-carbon medium-Mn steel in the present study. Upon aging at 600 °C for 1 h, a considerable amount of Cu-rich precipitates with average radius of 6.3 nm was uniformly distributed in the tempered martensitic matrix. An ultra-high yield strength, therefore, was attained by the strong interaction of Cu-rich precipitates with dislocations. In addition to the precipitation strengthening effect, the introduction of Cu also dramatically promoted the dissolution of cementite particles and formation of retained austenite at triple junctions or other high-angle grain boundaries. Compositional analysis of cementite and retained austenite was conducted by atom probe tomography. Austenite formation removed Mn segregation from the grain boundaries, consumed brittle cementite particles, and hence eliminated intergranular fracture, leading to a remarkable improvement in toughness. Moreover, a volume fraction of 23.2% retained austenite with appropriate mechanical stability also enabled the steel to be more ductile. The optimal mechanical properties, yield strength of 1005 MPa, tensile strength of 1070 MPa, total elongation of 24.3% and room temperature impact energy of ~ 73 J were obtained, through the cumulative contribution of Cu-rich precipitates and retained austenite.

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 combined strengthening effects of high pressure torsion (HPT) and age hardening on a recently developed 3rd generation Al-Cu-Li alloy was investigated. Solution treated samples were processed through HPT at room temperature, followed by low temperature artificial ageing (i.e. T4-HPT-AA). A micro-hardness of ~240 Hv was achieved on ageing at 110°C/60h after HPT. A further improvement in the hardness to ~260 Hv was accomplished by a pre-ageing 110°C/24h before HPT in combination with a post-HPT ageing process at 110°C for 180h (i.e. T6-HPT-AA). These novel multi-stage processes give rise to an increase in hardness by a factor of 2 as compared to the T4 condition (~120 Hv). After HPT the grain size was dramatically refined to the ultrafine-grained (UFG) structure, accompanied by a large amount of dislocations. No long-range ordered precipitates were observed after HPT and subsequent ageing treatments. Instead, atom probe tomography (APT) provided clear evidence that Cu-Mg co-clusters were homogeneously distributed in the matrix of T4 and T6 processed samples and they segregate strongly to the grain boundaries (GBs) during HPT. Further ageing treatment after HPT leads to the segregation of clusters to the dislocations. A strengthening model that incorporates dislocation hardening, grain boundary hardening, solid solution strengthening and a new short-range order strengthening mechanisms was used to predict the yield strength of the alloy. This model indicates that the combined effect due to all three types of Cu-Mg clusters (clustering in matrix, clustering at GBs and at dislocations) is dominant for the strength in all conditions.

Pre-ageing and pre-stretching before artificial ageing can significantly affect the precipitation during the subsequent artificial ageing in Al–Zn–Mg–Cu alloys, and their effects are closely relevant to the alloy composition and Zn/Mg ratio due to the different predominate precipitates. The diverse effects of pre-ageing and pre-stretching on the mechanical properties and microstructures of Al–Zn–Mg–Cu alloys with five different Zn/Mg ratios have been systematically studied with transmission electron microscopy, hardness measurements and tensile tests. The results reveal that pre-ageing can effectively enhance the strength due to the improvement of number density of precipitates. Different pre-aging time causes similar improvement of strength but different elongation values in peak-aged alloys A2 (T′ phase strengthened) and A5 (η′ phase strengthened), which can be explained by different size and density of precipitates and PFZ width. The effects of pre-stretching on age-hardening behavior of the alloys with various compositions are significantly different. It results in a reduction in hardness in the lowest Zn/Mg ratio alloy A1 (T′ phase strengthened) due to accelerated coarsening of precipitates; on the contrary, it results in a remarkable improvement of the hardness of the highest Zn/Mg ratio alloy A5. Pre-stretching causes different strengthening effect in alloy A5, and the maximum enhancement takes place with 2% pre-stretching because the precipitates become denser and smaller. Additionally, both pre-ageing and pre-stretching are effective ways to reduce PFZ width by promoting the precipitation near the grain boundary.

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 hot deformation behaviors of Al-Mg-Si alloys were studied at temperatures of 400–550 °C and strain rates of 0.001–1 s−1. The microstructure of the compressed samples was characterized by electron back scattered diffraction (EBSD). It was found that the strain distribution was inhomogeneous in the hot compress samples, which caused inhomogeneous dynamic recrystallization fraction (XDRX) at different position of the sample. The geometrically necessary dislocation (GND) density was calculated from EBSD results by a MATLAB toolbox MTEX. GND densities increased with the increase of strain rate and strain, and the decrease of temperature. A constitutive equation was obtained from experimental stress-strain curves. The parameters for a dynamic recrystallization (DRX) kinetic model were determined considering the inhomogeneous XDRX at different position of the sample. The experimentally acquired constitutive relation and DRX model were imported to commercial software DEFORM-3D to simulate the hot compression tests, and the results agreed well with experimental observations. Finally, the industrial hot extrusion were simulated to further validate the DRX model.

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.