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The effects of strain rate on mechanical properties, microstructural evolution and underlying deformation mechanisms of two kinds of interstitial CrMnFeCoNi high entropy alloys (HEAs), (CrMnFeCoNi) 99.5 C 0.5 and (CrMnFeCoNi) 99.0 C 1.0 , are investigated under quasi-static and dynamic tension. Dynamic loading is carried out via a split Hopkinson tension bar system. The pre- and post-deformation microstructures are characterized with electron back scatter diffraction and transmission electron microscopy. Both yield strength and work hardening increase remarkably with increasing strain rate. Plastic deformation in both alloys under quasi-static and dynamic loading are dominated by dislocations, stacking faults, kink bands and deformation twins. Twinning is activated easier under dynamic loading, and twin density decreases with increasing carbon content due to the increased stacking fault energy. The Khan-Liu constitutive model can describe the experimental results for these two interstitial HEAs in a wide strain range. • Both volume fraction and size of the carbides increase with the increase of C content. • The increase of C content and strain rate lead to the increase in σ Y and σ UT. • Twin density decreases with increasing C content due to the increased SF energy. • The Khan-Liu constitutive model is used to describe the deformation. [ABSTRACT FROM AUTHOR]

A combination method of beneficial wrinkle and preform shape is proposed to solve the problem of non-uniform thickness distribution during hydroforming of a variable diameter tubular component. The effect of loading path for the hydroform part and the preform shape are simulated by LS-DYNA. At the same time, the wall thickness deviation of the maximum and minimum of the hydroformed part is considered as the optimal target and the loading path for the preform shape is optimized to obtain the beneficial wrinkles by response surface methodology. Simultaneously, hydroforming experiments were conducted to verify the simulation results. Study shows that the loading path plays an important role in the forming of the beneficial wrinkles, which influence the cracking and the wrinkling defects. Only the loading path is appropriate can the beneficial wrinkles be formed and the wrinkles are flattened in the subsequent calibrating process. Response surface methodology is an effective way to obtain the optimal loading path, and the desired loading path can be achieved by controlling the relationship of the axial feeding and the internal pressure. [ABSTRACT FROM AUTHOR]

The microstructures arrays are widely used and have good prospects in different applications because of their special and unique engineering functions. Micro V-grooves array is one of the most common micro structures used for optical and biomedical functions. Precision grinding is one of the micro mechanical machining technologies for a wide range of hard non-ferrous materials to fabricate micro structures array. In this paper, a three-axis computer numerical control (CNC) grinding machine tool is designed for the grinding of micro V-grooves array on hard and brittle materials. Material removal and machining strategy are studied for the grinding of V-grooves array. A machining strategy design (MSD) system is established to generate NC program, simulate the tool paths and machining process as well as predict the total machining time for the grinding of V-grooves array in the developed machine tool. Experiments are designed to grind flat surface and micro V-grooves array on optical glass. The experimental results show that the grinding machine tool can produce high quality surface and micro V-grooves array with sub-micrometer form accuracy. [ABSTRACT FROM AUTHOR]

Si3N4 ceramics and 304 stainless steelwere joined by the Cu/Ag-Cu/Ti laminated fillermetal. Interfacialmicrostructure of brazed joint and effect of brazing temperature and thickness of Cu foil on mechanical properties were studied in this paper. Research results showed that the interfacial microstructure of the brazed joint might be 304 stainless steel/TiFe2/Ag-Cu eutectic + Cu(s,s)/Cu(s,s)/Cu(s,s) + Ag-Cu eutectic/Cu3Ti + TiN/Si3N4 ceramics. With the increasing of the brazing temperature, four-point bending strength of the brazed joint initially increased, then decreased. The bending strength reached the maximum value of 53MPa at 1153K when the thickness of Cu foil was 500 µm. The bending strength reached the maximum value of 57MPa with 1mmthickness Cu interlayer under the brazing temperature of 1153 K. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: The high temperature compression behavior of TiAl–Ru alloys was studied at different temperatures and strain rates. Ru was found to have a strong strengthening effect on TiAl alloys. However the Ru addition amount was limited by its low solubility in γ-TiAl and α2-Ti3Al, and the detrimental effect of excessive ternary phase precipitation. Furthermore, the melting temperature decreases when Ru ≥0.6 at.% as the alloy composition approaches a ternary eutectic point. The strengthening mechanism is discussed and two separate mechanisms are proposed, viz. solid solution strengthening and refined colony strengthening. Intergranular cracks were found in the alloys with low Ru or no Ru addition, but were barely detected as Ru content increased to above 0.6 at.%. It was suggested that Ru showed a beneficial effect on both strength and ductility of TiAl alloys due to the refined colony size. Three-point bend test results showed that the Ru addition can also improve the room temperature ductility of TiAl alloys. Hot workability was increased according to the compression tests. Thermal-mechanically treated TiAl–Ru had much smaller grain size than the heat-treated samples due to dynamic recrystallization. But it did not show superior strength in the compression test compared to the heat-treated samples. The Zener–Hollomon parameter was calculated from the compression strength of heat-treated TiAl–Ru alloys. Its relationship with dynamic recrystallization and hot work is discussed. The mechanical properties of TiAl–Ru alloys are compared with TiAl–Nb samples and demonstrate a promising combination of strength and ductility. [Copyright &y& Elsevier]

The effects of the main process variables on the stir zone microstructure in friction stir welds were investigated for Ti–6Al–4V. Welds were produced by employing varying welding speeds under a constant rotation speed or different rotation speeds at a constant welding speed. The stir zone microstructure was examined by optical microscopy and transmission electron microscopy. It was found that the stir zone microstructure was determined by the parameters controlling temperature and deformation history during the friction stir welding. A bimodal microstructure characterized by primary α and transformed β with lamellar α + β or a full lamellar microstructure composed of basket-weave α + β lamellae could be developed in the stir zone. The microstructural evolution mechanism in the stir zone was discussed. [ABSTRACT FROM AUTHOR]

The formation and the thickness growth of the (Bi,Pb)2Sr2Ca2Cu3O10 phase in Ag-sheathed tapes have been investigated by scanning electron microscopy on samples sintered at 840°C in a flow of 8.5% O2 (rest N2) and quenched in air after sintering for 1 to 50 h. The thickness of the (Bi,Pb)2Sr2Ca2Cu3O10 grains was measured statistically after different sintering times. The experimental results show that the average thickness of these grains increases during the entire sintering period, while the average thickness growth rate decreases exponentially with sintering time. The volume fractions of the various phases present during the heat treatment were also measured from micrographs. Detailed studies of the microstructure and phase formation kinetics support the view that the formation of the (Bi,Pb)2Sr2Ca2Cu3O10 phase proceeds via a nucleation and growth process. Based on the present observations, a model describing the microstructure evolution is presented. [ABSTRACT FROM AUTHOR]

Commercial purity aluminium has been cold-rolled to reductions from 40 to 99% or true thickness strains (ϵt) from 0.5 to 5.0. Applying transmission electron microscopy techniques the microstructural evolution has been followed and structural parameters such as spacing between dislocation boundaries and misorientation angles across these boundaries have been measured and analyzed. This analysis shows that the microstructural evolution does not saturate at large strains. In parallel, the flow stress and strain hardening behavior has been determined by uniaxial tensile testing of rolled speciments. These stress–strain curves do not saturate in the strain range examined, even when changes in the Taylor M-factor are considered. This agrees with the evolution in the boundary spacing and misorientation angle, which are considered to be the strength determining parameters based on the operation of two mechanisms, dislocation strengthening and grain boundary strengthening. Following this description, the individual strength contributions are calculated and their addition leads to flow stress values and strain hardening behaviour in good agreement with those determined experimentally. [ABSTRACT FROM AUTHOR]

Typical α-sialon starting compositions, of formula Ln0.33Si9.3Al2.7O1.7N14.3, were densified by hot-pressing using Ln2O3 as sintering additives, where Ln = Nd, Dy, and Yb. The as-sintered materials were heat-treated at 1450°C for 96 hours and then re-sintered at 1800°C for 1 hour to observe the overlapping effects of both Ln2O3 and multiple heat-treatment on thermal stability of the Ln-α-sialon phase and also the change in microstructure. The kinds of grain boundary phases which occurred also affected the results. The hardness, fracture toughness and flexural strength of the materials were evaluated using indentation and three-point bending tests, respectively. Mechanical tests and detailed microstructural analysis have led to the conclusion that a multiple-mechanism is involved, with debonding, crack deflection, crack bridging, and elongated grain pull-out all making a significant contribution towards improving the fracture toughness. Nd-containing specimens were tough with a highest indentation fracture toughness K1C of 7.0 MPa m1/2. In contrast, Dy- and Yb-containing specimens were hard and brittle with a highest Vickers hardness HV10 of 18.0 GPa. All re-sintered specimens underwent β→α transformation to some degree, leading to a degradation of mechanical properties as a consequence. [ABSTRACT FROM AUTHOR]

Al–1%Mn single crystals of {0 0 1}〈1 1 0〉 orientation, rolled to reductions of between 18% and 90%, were examined using transmission and scanning electron microscope techniques, in order to examine the crystal subdivision pattern over both macroscopic and microscopic length scales. Deformation resulted in a macroscopic subdivision across the specimen thickness into three types of region – matrix regions, transition regions, and a central region consisting of two sharply alternating, highly misoriented, orientations. Significant differences, both in the microstructure and local crystallography, were observed in each of the three types of region. A model linking the location-dependent distortion tensor to the crystal rotation via the slip amplitudes was able to account for the observed pattern of crystal rotations, except for in the central region of each sample. The assumption used in the model of imbalanced slip on two sets of coplanar systems was supported by an analysis, using the Frank formula, of dislocation boundary content. [Copyright &y& Elsevier]

Abstract The Al-Mg-Si alloys with and without pre-aging treatment were subjected to cryorolling, warm rolling and subsequent aging. Mechanical behavior and microstructural evolution were investigated by tensile tests, differential scanning calorimetry, X-ray diffraction and transmission electron microscopy. Tensile results showed that the strength and ductility of the cryorolled alloy were simultaneously enhanced by warm rolling and aging. After final aging, the pre-aged alloy exhibited better combination of strength and ductility than the alloy without pre-aging. Microstructural results demonstrated that pre-aging treatment was beneficial for the formation of strengthening phases during warm rolling and final aging, which accounted for the improved strength. The presence of finely dispersed nano-precipitates as well as a decrease in dislocation density played an important role in good ductility. Highlights • The mechanical properties of a cryorolled Al-Mg-Si alloy are improved. • Pre-aging accelerates the precipitation during post-cryorolled treatment. • Microstructure containing nano-precipitates and ultrafine subgrains is achieved. [ABSTRACT FROM AUTHOR]

Novel ultrafine lamellar (α + β) microstructures comprising ultrafine (∼200–300 nm) α-laths and retained β phases were created via promoting in situ decomposition of a near α′ martensitic structure in Ti–6Al–4V additively manufactured by selective laser melting (SLM). As a consequence, the total tensile elongation to failure reached 11.4% while maintaining high yield strength above 1100 MPa, superior to both conventional SLM-fabricated Ti–6Al–4V containing non-equilibrium acicular α′ martensite and conventional mill-annealed Ti–6Al–4V. The formation and decomposition of α′ martensite in additively manufactured Ti–6Al–4V was studied via specially designed experiments including single-track deposition, multi-layer deposition and post-SLM heat treatment. The essential SLM additive manufacturing conditions for Ti–6Al–4V including layer thickness, focal offset distance and energy density, under which a near α′ martensitic structure forms in each layer and then in situ transforms into ultrafine lamellar (α + β) structures, were determined. This is the first fundamental effort that has realized complete in situ martensite decomposition in SLM-fabricated Ti–6Al–4V for outstanding mechanical properties. [ABSTRACT FROM AUTHOR]

135° clock rolling, which denotes that the rolling direction clockwise rotates 135° after per pass, was utilized to develop a desired cold-rolled texture distribution through the thickness of Ta plate. Meanwhile, the influence of starting texture and microstructure was also investigated in detail. Texture from surface to center of the initial and rolled Ta plates was characterized by X-ray diffraction (XRD). In order to explore the correlation between initial microstructure and the evolution of rolling texture, through-thickness microstructure of the initial Ta plate was observed by optical microscope (OM). Results showed that 135° clock rolling was effective to mitigate the through-thickness texture gradients often existing in cold-rolled tantalum plate. It was found, furthermore, that texture and microstructure prior to rolling play a particular role in the formation of rolling texture. [ABSTRACT FROM AUTHOR]

The bulk stored energy of cold rolled tantalum (Ta, 87% thickness reduction) was measured by differential scanning calorimetry, and related microstructure evolution during the stored energy release was studied by electron backscatter diffraction method. In addition, transmission electron microscopy was employed to reveal the substructure of the deformed Ta. Results showed that the deformed Ta consisted of {1 1 1} texture (<1 1 1> // ND) and {1 0 0} texture (<1 0 0> // ND), and its substructure was characterized by dislocation cells or dense dislocation walls. The total stored energy of as-rolled Ta was about 30.6J/mol, most of which was released by recovery. The release of the stored energy suffered a complicated process and was orientation dependent. Correlative microstructure evolution path during heating referred to (i) recovery of all grains, (ii) primary recrystallization of {111} grains and recovery of {100} grains, and (iii) recrystallization of {100} grains. [ABSTRACT FROM AUTHOR]

Highlights: [•] Investigated the influence of different processing routes on the mechanical anisotropy. [•] Compared the effect of annealing on mechanical anisotropy of AZ31 sheets produced by three routes. [•] Annealing treatment could decrease the mechanical anisotropy of the three samples. [Copyright &y& Elsevier]

Effects of heating rate on the phase and microstructure evolution in Bi-2223/Ag/Ni superconducting tapes were studied. It was found that 30°C/h, the lowest heating rate, was the most suitable to obtain microstructure improved tapes. The volume fraction of Bi-2223 phase increased and the amount and size of secondary phases decreased with the heating rate reducing. Meanwhile, the optimal texture formed earliest in the samples heated at 30°C/h. The size of Bi-2212 grains is the largest in the samples heated at 30°C/h, which may lead to the largest size of Bi-2223 grains eventually. [ABSTRACT FROM AUTHOR]

Abstract: Columnar grained Ni is used as a model material allowing simultaneous non-surface investigations of the evolution of crystallographic orientations and deformation microstructures within individual grains as a function of rolling strain up to ε =0.7. Electron channelling contrast and electron backscattered diffraction are used to visualise microstructures and crystallographic orientations. It is found that both the microstructural and the textural development depend strongly on the initial grain orientation. A grain size effect is observed on the deformation-induced orientation scatter within the grains. Large grains have microstructure and orientation scatters similar to those observed in single crystals of similar orientation. The observations are interpreted based on a slip system analysis, considering the relative effects of the initial grain orientation and the interaction between neighbouring grains as well. [Copyright &y& Elsevier]

The microstructural evolution during cold rolling of IF-steel up to reductions of 90% has been investigated using scanning and transmission electron microscope techniques. The deformation microstructures consist of similar features to those already identified in several FCC metals, namely cell blocks showing a pattern of subdivision on two levels. An important difference compared to FCC metals is an increased prevalence of strain localization, seen most directly by highly localized micro-shearing. An orientation dependence of the deformed microstructure is observed in that nearly all the extended dislocation boundaries and micro-shear traces are crystallographic, coincident either with {1 1 0} or {1 1 2} slip planes. A sharp fall in ductility seen between rolling reductions of 30% and 50% is attributed to the onset of localized micro-shearing. A flow stress calculation, based on linearly additive contributions from dislocation and grain boundary hardening, leads to flow stress values and a strain hardening behavior in good agreement with experimentally determined values. [Copyright &y& Elsevier]