Nickel with multimodal grain size distribution achieved by SPS: microstructure and mechanical properties

Using spark plasma sintering, bulk polycrystalline nickel samples are created from high-purity multimodal nickel powder. The resulting compacts yield relative mass densities of 95–97 %, depending on the sintering temperature. Microstructural investigations of the processed samples show a composite-like microstructure made of the following elements: multicrystalline agglomerates (5 < d < 120 μm), isolated single crystalline and small aggregates (1 < d < 5 μm) and an ultrafine-grained (UFG) component (d < 1 μm). Adjusting the processing parameters allows full control of the microstructural characteristics, such as the average grain size and the volume fraction of the UFG component. The mechanical properties of the processed samples are investigated by compression tests conducted at room temperature at a strain rate of 2 × 10⁻⁴ s⁻¹. The flow stress of the multimodal micrometre-sized nickel increases from ~100 to 400 MPa, depending on the microstructural characteristics, whereas the monomodal nickel has a flow stress of 395 MPa. Stress–strain plots reveal that there is a strong but brief strain hardening, followed by a plateau and/or a strain softening. The evolution of grain misorientation across boundaries after straining suggests the following: in the UFG components, deformation occurs mainly via a grain rotation mechanism, while the coarser microstructures experience deformation by a classical dislocation-based plasticity. Strain softening is more pronounced for samples with a large fraction of UFG components because the rotations of the grains may induce deformation incompatibilities, leading to microcracking at grain boundaries.     

Tensile properties and fracture behaviour of an ultrafine grained Ti–47Al–2Cr (at.%) alloy at room and elevated temperatures

An ultrafine grained (UFG) Ti–47Al–2Cr (at.%) alloy has been synthesized using a combination of high energy mechanical milling and hot isostatic pressing (HIP) of a Ti/Al/Cr composite powder compact. The material produced has been tensile tested at room temperature, 700 and 800 °C, respectively, and the microstructure of the as-HIPed material and the microstructure and fracture surfaces of the tensile tested specimens have been examined using X-ray diffractometry, optical microscopy, scanning electron microscopy and transmission electron microscopy. The alloy shows no ductility during tensile testing at room temperature and 700 °C, respectively, but very high ductility (elongation to fracture 70–100%) when tensile tested 800 °C, indicating that its brittle to ductile transition temperature (BDTT) falls within the temperature range of 700–800 °C. The retaining of ultrafine fine equiaxed grain morphology after the large amount of plastic deformation of the specimens tensile tested at 800 °C and the clear morphology of individual grains in the fractured surface indicate that grain boundary sliding is the predominant deformation mechanism of plastic deformation of the UFG TiAl based alloy at 800 °C. Cavitation occurs at locations fairly uniformly distributed throughout the gauge length sections of the specimens tensile tested at 800 °C, again supporting the postulation that grain boundary sliding is the dominant mechanism of the plastic deformation of the UFG TiAl alloys at temperatures above their BDTT. The high ductility of the UFG alloy at 800 °C and its fairly low BDTT indicates that the material a highly favourable precursor for secondary thermomechanical processing.     

Thermal stability of ultrafine grained aluminium alloy prepared by large strain extrusion machining

Abstract

In the present study, ultrafine grained (UFG) Al alloy chips with average grain sizes of ～200 nm were successfully prepared by large strain extrusion machining (LSEM) process using a combined cutting tool with rake angle of 10° and chip compression ratio of 1.0. The tests showed that the Vickers hardness of the UFG Al alloy is significantly improved due to grain size reduction. To understand effect of heat on the microstructure and mechanical properties, the UFG chips were subjected to heat treatment at different temperatures and different annealing time durations. When annealed <100°C, most of fine grains within the UFG chips were found to be replaced by elongated grains whose grain sizes increased with a significant increase in the aspect ratio as the annealing time increased. Despite such increase in grain size, the Vickers hardness was not reduced as expected because of the precipitation of secondary phases. When annealed at temperatures up to 200°C, recrystallisation occurred, along with grain growth, but the Vickers hardness did not deteriorate because of precipitation of secondary phases, as before. However, annealing at temperatures of 300°C and above resulted in significant reduction in hardness of the chips due to dominance of grain growth over secondary precipitation. These results indicated that UFG Al alloy chips have a good thermal stability at temperatures <200°C.     

Effect of grain size and Cu-rich phase on the electric properties of CaCu3Ti4O12 ceramics

The CaCu₃Ti₄O₁₂ ceramics were prepared by the traditional solid-state reaction method under different sintering conditions. The XRD patterns show that crystal structures of the samples are basically single-phase pseudo-cubic, except little second phases of CuO and Cu₂O in the samples sintered in air at 1050 and 1100 °C, respectively, for 12 h. The SEM results indicate that the pellet sintered at 1100 °C for 12 h possess larger grain size and more Cu-rich phases at the grain boundaries than the pellet sintered at 1050 °C for 12 h. It is interesting that the pellet sintered at 1050 °C under the pressure of 5 Gpa for 3 h shows smaller grain size (~1 μm) and no Cu-rich phases due to the higher pressure during the sintering process. The results show that the grain size has a reverse effect on the values of the permittivity and the values of breakdown electric field (E _b) and nonlinear coefficient. The pellet sintered at 1100 °C for 12 h exhibits a higher permittivity, but with a lower breakdown electric field (E _b) and a lower nonlinear coefficient due to larger grain size. The pellet sintered at 1050 °C under the pressure of 5 Gpa for 3 h exhibits a lower permittivity, but with a higher breakdown electric field (E _b) and a higher nonlinear coefficient due to smaller grain size. The Cu-rich phases at grain boundaries can raise the resistance of the grain boundary leading to the lower dielectric loss tangent, which has been supported by the results of impedance spectroscopy analysis.     

Effect of humidity on the conduction processes of Li3VO4

Polycrystalline Li₃VO₄ was prepared by solid-state reaction method at 950 °C for 5 h. The X-ray diffraction pattern of the material shows the formation of single phase orthorhombic βII-type structure with no impurities. TGA measurements reveal 1.0 wt% loss in the temperature range between 25 and 120 °C attributed to humidity desorption. Impedance spectroscopy measurements were carried out on pellets of polycrystalline Li₃VO₄ at temperatures from 25 to 500 °C at 25 °C steps. An anomalous behavior of the conductivity attributed to the presence of water in the temperature range between 25 and 120 °C is observed. The activation energy values of the conduction process within the grains (bulk conductivity) and across grain boundaries were found to be 1.00 and 0.87 eV, respectively.     

Microstructure investigation and thermal stability of 99.1% aluminum processed by equal channel angular extrusion

Microstructure evolution of 99.1% aluminum after equal-channel angular extrusion (ECAE) and subsequent heat-treatment was investigated. After deformation the samples were annealed at different temperatures. The deformed and annealed states were characterized by Transmission Electron Microscopy (TEM), X-ray diffraction (XRD), and microhardness tests. It was shown that the observed microstructure changes during subsequent annealing have to be associated with recovery and cells formation. The initial stages of recovery were investigated using weak-beam technique. The microstructure obtained after annealing for 1 h at 100 °C consists of some arrangements of the dislocations into sub-grain boundaries within the wide preexisting grains. Annealing at 300 °C led to the appearance of a duplex microstructure consisting of bands of slightly coarsened grains associated with refined grains. No growth of dislocation cells was observed up to 400 °C. In XRD measurements, the lattice parameter increase with subsequent heating. This indicates a continuous grain growth during annealing. This is due to the important increase of coherency length, D observed parallel to a substantial decrease of rms-strain, ε.     

Fabrication of dense β-calcium orthophosphate with submicrometer-sized grains and its high-temperature superplastic deformation

High-density β-calcium orthophosphate (β-Ca₃(PO₄)₂, also called β-tricalcium phosphate: β-TCP) ceramics with submicrometer-sized grains were fabricated using a pulse-current pressure firing route. The maximum relative density of the β-TCP compacts was 98.7% at 1050 °C and this was accompanied by a translucent appearance. The mean grain size of the β-TCP compacts increased slightly with temperature to reach 0.78 μm at 1000 °C. However, upon further increasing the firing temperature to 1050 °C the mean grain size increased significantly to 1.6 μm. The extent of plastic deformation during tensile testing was examined at temperatures between 900 and 1100 °C using a strain rate in the range 9.26 × 10⁻⁵ to 4.44 × 10⁻⁴ s⁻¹. The maximum tensile strain achieved was 145% for a test temperature of 1000 °C and strain rate of 1.48 × 10⁻⁴ s⁻¹ and this was attributed to the relatively high density and small grain size.     

The role of plastic deformation in compaction and decompaction processes in sintering materials based on wurtzitic boron nitride

The evolution of the microstructure of wurtzitic boron nitride-based polycrystals with sintering time (τ) increasing from 15 to 240 s at p = 7.7 GPa and T = 1800°C has been TEM examined. It has been concluded that the decisive role in compaction below 90 s has been played by the plastic deformation of initial particles caused by the basal slip and irregular, i.e. non-crystallographic, rotation. A dispersion of particles into disoriented fragments without distortion of their continuity, which defines their further deformation due to slipping along the fragment boundaries, is promoted by the deformation via the irregular rotation. The strengthening of the orientation factor for the development of basal slip and, hence, the activation of the wBN transition into the cubic phase is facilitated by the fragment disorientation. A decompaction of polycrystals in sintering for τ > 90 s is caused by the evolution of plastic deformation via the creep initiated by the formation of areas of the microstructure of a material based on the perfect singlephased (cBN) grains of sizes below 1 μm with well-developed (with no relief) boundaries between them. The creep process is assumed to proceed owing to intergranular slipping combined with diffusion atom-by-atom mass transfer in grain boundaries.     

The comparative influences of structural ordering, grain size, Li-content, and bulk density on the Li+-conductivity of Li0.29La0.57TiO3

The lattice and total Li⁺-ionic conductivity of Li_0.29La_0.57TiO₃ ceramic (LLTO) sintered at 1200 °C were determined as functions of powder calcination temperature and sintering duration, and these results were correlated with the relative degrees of Li⁺-ordering, Li-content, grain size, and bulk density to assess the relative impact of these parameters on material performance. Under all conditions, LLTO formed with a high degree of tetragonal superstructure to its perovskite related framework, and the lattice conductivity closely followed the relative amounts of the superstructure, as evaluated via determination of the sample ordering parameter from X-ray diffraction data. LLTO powders that were calcined at 900 °C for 1 h and sintered at 1200 °C for 6 h gave lattice conductivity values (~1.14 × 10⁻³ S cm⁻¹) comparable within the highest ranges reported in the literature. This coincided with the lowest degree of tetragonal superstructure formation, and it was also found to be largely independent of the values of Li-content measured on sintered ceramic despite significant Li₂O volatilization at longer sintering times (up to 23 % after 12 h at 1200 °C). Samples of LLTO powder that were calcined at 1100 °C and sintered at 1200 °C for 12 h resulted in the highest total Li-ion conductivity value ~6.30 × 10⁻⁵ S cm⁻¹. The total conductivity of LLTO varied inversely with grain size when the grains were <20 μm but was insensitive to that parameter above that size threshold. The strongest influence on total conductivity was primarily the bulk ceramic density. It was estimated from measured values that as the bulk ceramic density approached the full theoretical value for LLTO the total conductivity could near the lattice conductivity of ~1.2 × 10⁻³ S cm⁻¹.     

High temperature creep of ultrafine-grained Fe-doped MgO polycrystals

Creep deformation in ultrafine-grained (0.1 to 1μm) Fe-doped magnesia polycrystals is studied in compression, at temperatures of 700 to 1050° C, and constant loads of 50 to 140 MPa. The stress exponent observed to be nearly unity and the strong grain size sensitivity (ė∼d ^−2.85) suggest that diffusional creep mechanisms dominate the deformation. In the grain size range of the present study the grain boundary diffusion contribution is significantly more important than lattice diffusion. Magnesium is tentatively identified as the rate-controlling species along grain boundaries from an analysis of the diffusivities inferred from the present work and from other authors for Fe-doped magnesia. Associated with the CNRS.     

Annealing temperature dependence of effective piezoelectric coefficients for Bi3.15Eu0.85Ti3O12 thin films

The effects of annealing temperatures 600, 650, 700, and 750 °C on microstructure, chemical composition, leakage current, ferroelectric, dielectric, and piezoelectric properties of Bi_3.15Eu_0.85Ti₃O₁₂ (BET) thin films prepared by metal–organic decomposition were studied in detail. The largest spontaneous polarization 2P _s (98.7 μC/cm² under 300 kV/cm), remnant polarization 2P _r (81.7 μC/cm² under 300 kV/cm), dielectric constant ε_r (889.4 at 100 kHz), effective piezoelectric coefficient d ₃₃ (46.7 pm/V under 260 kV/cm), and lowest leakage current (1.3 × 10⁻⁶ A/cm² under 125 kV/cm) of BET thin film were obtained with annealing at 700 °C. The mechanisms concerning the dependence of the enhancement d ₃₃ are discussed according to the phenomenological equation, and the improved piezoelectric performance could make BET thin film a promising candidate for piezoelectric thin film devices.     

In situ TEM investigation of microstructural behavior of superplastic Al–Mg–Sc alloy

Dynamic changes in microstructure of the superplastic ultrafine-grained Al–3Mg–0.2Sc (wt.%) alloy refined by equal-channel angular pressing (ECAP). were observed by in situ transmission electron microscopy at temperatures up to 300 °C (annealing and tensile deformation) in order to simulate the initial stages of superplastic testing. It was found that the microstructure changes significantly during the preheating before the superplastic deformation, which was accompanied by decreased microhardness. During the deformation at 300 °C, high dislocation activity as well as motion of low-angle grain boundaries was observed while high-angle grain boundaries did not move due to the presence of scandium in the alloy.     

Hot deformation and annealing of cube oriented aluminium single crystals

Abstract

Single crystals of the {001}〈100〉 orientation of an Al–0.05Si single phase alloy have been deformed in plane strain compression at temperatures of 300–500°C and strain rates of 0.5–50 s^-1, and annealed in a salt bath at temperatures of 300–450°C. Quantitative texture measurements by electron backscatter diffraction (EBSD)show that, in agreement with previous work, the cube orientation is stable at lower strain rates and higher temperatures (lower Zener–Hollomon parameter Z), whereas this orientation is unstable at higher values of Z. During annealing of the deformed crystals there is a competitive migration of subgrain boundaries of a wide range of orientations, and recrystallisation starts preferably at deformation bands of high orientation gradient. Measurement of subgrain growth has enabled the dependence of the mobility of low angle grain boundaries on misorientation to be determined. The results are in accord with those obtained for lower angle (<6°)boundaries in the same material.     

Effect of strain-modified particles on the formation of fined grains and the properties of AA7050 alloy

With a new two-pass deformation, a fine-grained AA7050 alloy was obtained owing to small particles which can affect the grain refinement. The banded structures were produced in the elongated grain interiors after the 1st-pass deformation at 300 °C. And deformation bands containing dislocation arrays and small spherical particles were obtained. A few new fined grains appeared along the elongated grain boundaries. After the 2nd-pass deformation at 430 °C, isolated chains of new fine grains were developed in the elongated grain interiors. The boundary glide and the increase of grain boundary misorientation due to cumulative strain could refine the elongated grains. The pinning effect of the particles accelerated the formation of deformation bands. The increase of deformation temperature promoted the rapid evolution of grain refinement during the deformation. The strength of the fine-grained AA7050 alloy was enhanced while the ductility was decreased.     

Microstructure evolution and hardness variation during annealing of equal channel angular pressed ultra-fine grained nickel subjected to 12 passes

The microstructure, thermal stability and hardness of ultra-fine grained (UFG) Ni produced by 12 passes of equal channel angular pressing (ECAP) through the route Bc were studied. Comparing the microstructure and hardness of the as-ECAPed samples with the published data on UFG Ni obtained after 8 passes of ECAP through the route Bc reveals a smaller average grain size (230 nm in the present case compared with 270 nm in 8-pass Ni), significantly lower dislocation density (1.08 × 10¹⁴ m⁻² compared with 9 × 10¹⁴ m⁻² in 8-pass Ni) and lower hardness (2 GPa compared with 2.45 GPa for 8-pass Ni). Study of the thermal stability of the 12-pass UFG Ni revealed that recovery is dominant in the temperature range 150–250°C and recrystallisation occurred at temperatures >250 °C. The UFG microstructure is relatively stable up to about 400 °C. Due to the lower dislocation density and consequently a lower stored energy, the recrystallisation of 12-pass ECAP Ni occurred at a higher temperature (~250 °C) compared with the 8-pass Ni (~200 °C). In the 12-pass Nickel, hardness variation shows that its dependence on grain size is inversely linear rather than the common grain size^−0.5 dependence.     

The role of low-angle grain boundaries in multi-temperature equal channel angular pressing of Mg–3Al–1Zn alloy

Equal channel angular pressing was used to process an AZ31B magnesium alloy (nominally Mg–3Al–1Zn in wt%) at temperatures decreasing from 200 to 150 °C. The resulting microstructure was characterized by electron backscattered diffraction to reveal the role of low-angle grain boundaries in grain refinement. It was found that low-angle grain boundaries with misorientation angles lower than 5° are surrounded by regions of increased strain gradients, which can stimulate the generation of non-basal slip dislocations during the equal channel angular pressing at temperatures of approximately 150 °C. The strain gradients in the vicinity of the grain boundaries with misorientation angles in the range of 5°–10° were less frequent or were completely absent for high-angle grain boundaries with misorientation angles higher than 10°. This article also discusses the importance of low-angle grain boundaries for the generation of non-basal 〈c+a〉 dislocations needed for successful equal channel angular pressing of AZ31B at temperature of 150 °C.     

Effect of annealing on precipitation, microstructural stability, and mechanical properties of cryorolled Al 6063 alloy

The effect of annealing on precipitation, microstructural stability, and mechanical properties of cryorolled Al 6063 alloy has been investigated in the present work employing hardness measurements, tensile test, XRD, DSC, EBSD, and TEM. The solution-treated bulk Al 6063 alloy was subjected to cryorolling to produce ultrafine grain structures and subsequently annealing treatment to investigate its thermal stability. The CR Al 6063 alloys with ultrafine-grained microstructure are thermally stable up to 250 °C as observed in the present work. Within the range of 150–225 °C, the size of small precipitate particles is <1 μm. These small precipitate particles pin the grain boundaries due to Zener drag effect, due to which the grain growth is retarded. The hardness and tensile strength of the cryorolled Al 6063 alloys have decreased upon subjecting it to annealing treatment (150–250 °C).     

Characterization of Bi1.6Pb0.4Sr2Ca2Cu3O y ceramic superconductor prepared via coprecipitation method at different sintering time

The Bi(Pb)-2223 superconductor has been prepared via coprecipitation (COP) method from solutions of metal acetates and 2-propanol solution of oxalic acid at low temperature (0–2 °C). The metal oxalates powder was subjected to precalcination of 12 h at 730 °C, followed by 24 h calcination at 845 °C. The pelletized powder was sintered for 24, 48 and 100 h at 850 °C. The dominance of high-T _C phase was observed for all samples as evidenced in the single step transition of (R–T) curves. The T _{C(R = 0)} for samples sintered at 24, 48 and 100 h were 102, 102 and 104 K, respectively. XRD data showed the tetragonal structure for all samples followed by the enhancement of the 2223 phase as sintering time increased. Ac magnetic susceptibility measurements showed the improvement of the grain connectivity as sintering time increased. SEM micrographs showed large flaky grains of ∼7 μm in size and randomly distributed, which belong to 2223 phase. The degree of grains alignment increased as the sintering time increased.     

Tensile and compressive test in nanocrystalline and ultrafine carbon steel

Plastic deformation behavior was investigated in near fully dense nanostructured and ultrafine-grained bulk samples of carbon steel (0.55 wt% C) under compression and tension tests. The specimens were obtained by hot pressure from mechanically milled powder at 400 and 500 °C. Subsequent heat treatments at temperatures going from 600 to 900 °C produced samples with ferrite grain sizes from 30 nm to 17 μm. Nanocrystalline grained steel samples presented very high strength with low ductility. Once, in the ultrafine range, as the ferritic grain size was increased, the strength was decreased and the ductility was improved. The porosity and carbon atoms within the structure were analyzed in order to explain the results of strength and strain obtained.     

Microstructural studies of self-supported (1.5–10 μm) Pd/23 wt%Ag hydrogen separation membranes subjected to different heat treatments

The microstructure of self-supported 1.5–10-μm thick Pd/23 wt%Ag membranes grown by magnetron sputtering have been studied after heat treatment and hydrogen permeation tests using electron microscopy and synchrotron X-ray diffraction. After hydrogen flux stabilization and permeance measurements at 300 °C, the membranes were annealed in air at 300 °C or in N₂/Ar at 300/400/450 °C for 4 days and then tested for hydrogen permeation. The permeation results show that changes in permeability depend on the treatment atmosphere and temperature, as well as membrane thickness. Air treatment at ~300 °C generally induced a positive effect on permeation in the thickness range of 1.5–10 μm. Significant microstructural changes, including grain growth, strain relief, void formation, and growth of nodules occurred in the membranes. The changes in microstructure are more severe for the thinner membranes, and may be attributed mainly to the oxidation processes at or near the surface. For samples annealed in N₂/Ar, enhanced permeation was only obtained with treatment at ~450 °C for 5 and 10 μm. The changes in the microstructure generally increased with heat-treatment temperature, and decreased with membrane's thickness. The membrane with enhanced permeation was accompanied by significant grain growth, strain relief, and surface roughening. For all the membranes, the relative changes in the microstructure were substantially more prominent on the permeate surface than on the feed surface. Details of the analysis are presented and discussed.