Search

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.

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.

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 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.

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.

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.

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.

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 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.

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 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.

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 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.

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.

Specially prepared 6xxx series aluminium alloys (Al–Mg–Si) with different Mg/Si ratios and Cu contents were investigated in the naturally aged, under-aged, peak-aged and over-aged conditions. Potentiodynamic polarisation and immersion testing in 0.1 M NaCl were used to investigate the corrosion behaviour of such alloys in a general sense. Results reveal a significant dependence of the corrosion rate and morphology on the ageing conditions and solute content. Whilst contributing to broader understanding of 6xxx alloy corrosion, we note that corrosion rate is lowest in the naturally aged condition and is principally influenced by Cu and subsequently by the Mg/Si ratio.

To well characterize alumina dispersion strengthened copper (ADSC) alloys, much work has been performed, while the effect of cold working methods on their microstructures and tensile fracture behaviors in the different directions, and corresponding fracture mechanism have not been studied systematically. In order to carry out such a systematic study, unidirectional and tandem rolling techniques were used to the ADSC alloys. The differences of tensile fracture behaviors between longitudinal and transverse directions for unidirectional rolled ADSC alloys are significant. Their mechanical properties and microstructures in longitudinal and transverse directions were studied. The C–G fracture mechanism was used to explain a sudden reduction of overall stress level in the transverse tensile curves for the unidirectional rolled ADSC alloys. Two fracture models were introduced to describe fracture processes. To reduce mechanical property anisotropy of ADSC alloys, a tandem rolling technology was proposed. Experimental results show this technology can significantly reduce the anisotropy.

The annealing characteristics of the forged and upset pure molybdenum bar were investigated by bending measurement, optical microstructure (OM) observation, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results show that the modified forge and upset technique is effective to refine the grains and interpenetrate the fiber structures, which largely improve the transverse ductility of the pure molybdenum bar. For the bar deformed by 85%, recovery is the main behavior at the annealing temperature of 1073 K for 1 h; at 1253–1323 K for 1 h, recrystallization begins; over 1423 K, further recrystallization continues and results in the decrease of transverse ductility. For the pure molybdenum bar with good transverse ductility, its fracture is mainly a transcrystalline fracture type accompanied by little intergranular cracks and some dimples.

Bulk copper alloys with nano TiB2 particles were prepared by a new technology that combining in situ reaction with rapid solidification. Appropriate reactor shape and procedure parameters are as follows: S0/(S1 + S2) = 0.8–1.2, L0/D0 = 10–15, θ = 60–90°, melt temperature 1350–1450 °C, air pressure 0.2–0.3 MPa. A special water-cooled copper mold can satisfy the need of rapid solidifying Cu–TiB2 alloy melt. The average size of TiB2 particle in the copper matrix synthesized by using optimal parameters is about 20 nm.

An expanded model for the thermodynamics of co-clusters and their strengthening is presented, and the model is applied to predict co-cluster formation and strengthening in Al-Mg-Si alloys. The models are tested against data on a wide range of Al-Mg-Si alloys aged at room temperature. The strengthening due to co-clusters is predicted well. The formation of the co-clusters is studied in an Al-0.5at%Mg-1at%Si alloy using three-dimensional atom probe analysis. The results correspond well with the model. It is shown that in general, (short-range) order strengthening due to co-clusters will be the main strengthening mechanism in these alloys. Apart from the co-clusters, Si clusters also form, but due to their low enthalpy of formation, they contribute little to the strength.