Evolution of dislocation density distributions in copper during tensile deformation

The evolution of dislocation storage in deformed copper was studied with cross-correlation-based high-resolution electron backscatter diffraction. Maps of 500 μm × 500 μm areas with 0.5 μm step size were collected and analysed for samples deformed in tension to 0%, 6%, 10%, 22.5% and 40% plastic strain. These maps cover ～1500 grains each while also containing very good resolution of the geometrically necessary dislocation (GND) content. We find that the average GND density increases with imposed macroscopic strain in accord with Ashby’s theory of work hardening. The dislocation density distributions can be described well with a log-normal function. These data sets are very rich and provide ample data such that quantitative statistical analysis can also be performed to assess the impact of grain morphology and local crystallography on the storage of dislocations and resultant deformation patterning. Higher GND densities accumulate near grain boundaries and triple junctions as anticipated by Ashby’s theory, while lower densities are rather more spread through the material. At lower strains (?6%) the grain-averaged GND density was higher in smaller grains, showing a good correlation with the reciprocal of the grain size. When combined with a Taylor hardening model this last observation is consistent with the Hall–Petch grain size effect for the yield or flow stress.     

Electrically conductive ZTA–TiC ceramics: Influence of TiC particle size on material properties and electrical discharge machining

Processing of highly abrasive materials via powder injection molding or extrusion requires mold materials with high wear resistance to increase the durability of the tools and to sustain a high quality of the manufactured products. High performance ceramics which exhibit high hardness, bending strength and toughness show the perfect combination of properties for these applications. However they also have the usual drawback that they cannot be economically customized in complex shapes and low quantities, as they are required for tool and mold design. Recent material development enabled EDM of electrically conductive oxide ceramics, the most widespread machining process for machining of hard materials, as an alternative to conventional ceramic manufacturing and hard machining technologies.This study focuses on the influence of TiC particle sizes on material properties and EDM machinability of ZTA–TiC ceramics with 24 vol.% TiC, 17 vol.% ZrO₂ and 59 vol.% Al₂O₃. Fracture toughness, bending strength and electrical conductivity were analyzed for samples produced from TiC powders with particle sizes varying from 0.43 μm to 2.54 μm. Surface integrity of wire cut samples and feed rate during machining were investigated. It was shown that reducing the size of electrical conductive grains strongly increases the electrical conductivity and slightly decreases mechanical properties. Therefore also the machining characteristics are influenced by TiC grain size. The feed rate increases with decreasing particle size to a maximum at d₅₀ = 1–1.3 μm. Reduction of TiC particle size also leads to significantly decreasing surface roughness after the main cut. Additionally the necessary number of trimming steps to achieve a distinct surface roughness is also minimized for low particle sizes.     

Creep deformation mechanisms in fine-grained niobium

Creep rates in fine-grained Nb were measured at 600 °C using free-standing Cu/Nb polycrystalline multilayered foils. For specimens with layer thicknesses ranging from 0.5 to 5 μm and Nb grain sizes ranging from 0.43 ± 0.05 to 1.87 ± 0.13 μm, two distinct regimes were observed. At high stresses, the stress dependence, grain size dependence and activation energy for creep are consistent with power-law creep, with an average stress exponent of 3.5. At low stresses, creep rates exhibited a linear dependence on stress and an inverse linear dependence on grain size. A model is presented for a vacancy generation-controlled creep mechanism, whereby deformation rates are controlled by the rate of vacancy generation at or near grain boundaries, not by their diffusion. The proposed model is consistent with experimental observations of stress and grain size dependence, as well as the measured activation energy for creep.     

Cyclic deformation of ultrafine-grained aluminum

Cyclic deformation of ultrafine-grained (UFG) Al with different grain sizes has been studied. It was found that UFG Al had shorter fatigue life than its coarse-grained counterparts. For UFG Al, the fatigue life decreases with decreasing grain size. Shear bands (SBs) shorten fatigue life. SBs are always inclined at 45° to the loading axis, and extend across the whole specimen. A SB is a thin sheet of tangled dislocations that have different Burgers vectors; its thickness is much less than the grain size. The strain–stress field inside a SB is very high. SBs produce shear steps, but not surface extrusions/intrusions, on the specimen surface. Thick shear bands (TSBs), about 200–300 μm, were found in the 6.36 μm grain size specimens, which also inclined 45° to the loading axis. TSBs consist of dislocation cells. The formation of TSBs does not reduce the fatigue life.     

Grain-size effect on plastic flow in nanocrystalline cobalt by atomistic simulation

Molecular dynamics simulation is employed to investigate the plastic flows in nanocrystalline (nc) hexagonal close-packed cobalt under uniaxial tensile deformation. In nc-Co samples modeled by a semi-empirical tight-binding potential, different deformation behaviors such as nucleation and growth of disordered atom segments (DAS) inside grains, deformation-induced hexagonal close-packed to faced-centered cubic transformation, partial dislocation activities are identified at different grain sizes (4–12 nm). At high stresses (1.2–3.2 GPa) and low temperatures (77–470 K), growth of DAS and their interaction with stacking faults are found to dominate the deformation process, even when the grain size is as small as 4 nm. A model for plastic flow generated by DAS inside grains is proposed. The strain rates and the inverse Hall–Petch-like behaviors in nc-Co with sub-10 nm grain sizes can be well described by the DAS plastic-flow model.     

Size effects on yield strength and strain hardening for ultra-thin Cu films with and without passivation: A study by synchrotron and bulge test techniques

We present a systematic study of the mechanical properties of different Cu, Ta/Cu and Ta/Cu/Ta films systems. By using a novel synchrotron-based tensile testing technique isothermal stress–strain curves for films as thin as 20 nm were obtained for the first time. In addition, freestanding Cu films with a minimum thickness of 80 nm were tested by a bulge testing technique. The effects of different surface and interface conditions, film thickness and grain size were investigated over a range of film thickness up to 1 μm. It is found that the plastic response scales strongly with film thickness but the effect of the interfacial structure is smaller than expected. By considering the complete grain size distribution and a change in deformation mechanism from full to partial dislocations in the smallest grains, the scaling behavior of all film systems can be described correctly by a modified dislocation source model. The nucleation of dissociated dislocations at the grain boundaries also explains the strongly reduced strain hardening for these films.     

Peculiar size effect in nanocrystalline BaTiO3

Dense BaTiO₃ ceramics with grain sizes of 35 nm to 5.6 μm were prepared, and the electrical properties investigated in the temperature range 500–700 °C by means of impedance spectroscopy. Charge carriers (oxygen vacancies and holes) are depleted in the space charge regions at the BaTiO₃ grain boundaries. When the grain size is ?250 nm, the width of the space charge region was determined to be ～40 nm. Therefore, the depletion regions were expected to overlap when the grain size decreases to 35 nm; in such a situation, charge carriers would be depleted over the entire grain, resulting in depressed conductivity. However, the conductivity of the 35 nm grain size sample was measured to be one to two orders of magnitude higher than those of the microcrystalline samples, and the activation energy markedly lower. Moreover, we determined a width of ～7 nm for the space charge regions in the 35 nm grain size sample; therefore, the space charge regions do not overlap. The enhanced conductivity is ascribed to a reduced oxidation enthalpy in nanocrystalline BaTiO₃, and the distorted grain boundaries in nanocrystalline BaTiO₃ are believed to be the atomic level origin of the reduced oxidation enthalpy.     

Effect of hot rolled grain size on the precipitation kinetics of nitrides in low carbon Al-killed steel

The precipitation of nitrides plays a general role in the industrial processing of deep drawing quality Al-killed low carbon steels. In this paper, the effect of hot rolled grain size on the precipitation of nitrides has been analysed. To evaluate the effect of grain size on the nitride precipitation kinetics, thermoelectric power based investigations have been performed on hot and cold rolled specimens.In the hot rolled state, the precipitation of nitrides occurs more intensively in the fine grain size microstructure (average grain size = 9 μm) than in the large grain size microstructure (average grain size = 23 μm) until the precipitated fraction of nitrides reaches about 70%. In the cold rolled state the effect of grain size is much less significant; probably the precipitation process occurs simultaneously at the grain boundaries and along dislocations. According to the simulation results, significant differences can be found between the precipitated fraction of nitrides in fine and large grain size sheets coiled in the temperature range 550–650 °C. In this interval, the precipitated nitride fraction is about two times larger in a fine grain microstructure (9 μm) than in sheets with 23 μm average grain size. The local position in the coil also affects significantly the precipitated fraction of nitrides. In the outer ring of the coil, less than 20% precipitated fraction is predicted in coiling temperature range 550–700 °C. However, in the middle ring of a hot rolled coil, the precipitated fraction changes from 5% to 85% with increasing coiling temperature from 550 to 700 °C.     

Laser compression of nanocrystalline tantalum

Nanocrystalline tantalum (grain size ～70 nm) prepared by severe plastic deformation (high-pressure torsion) from monocrystalline [1 0 0] stock was subjected to shock compression generated by high-energy laser (～350–850 J), creating pressure pulses with initial duration of ～3 ns and amplitudes of up to ～145 GPa. The laser beam, with a spot radius of ～1 mm, created a crater of significant depth (～135 μm). Transmission electron microscopy revealed few dislocations within the grains and an absence of twins at the highest shock pressure, in contrast with monocrystalline tantalum. Hardness measurements were conducted and show a rise as the energy deposition surface is approached, evidence of shock-induced defects. The grain size was found to increase at a distance of 100 μm from the energy deposition surface as a result of thermally induced grain growth. The experimentally measured dislocation densities are compared with predictions using analyses based on physically based constitutive models, and the similarities and differences are discussed in terms of the mechanisms of defect generation. A constitutive model for the onset of twinning, based on a critical shear stress level, is applied to the shock compression configuration. The predicted threshold pressure at which the deviatoric component of stress for slip exceeds the one for twinning is calculated and it is shown that it is increased from ～24 GPa for the monocrystalline to ～150 GPa for the nanocrystalline tantalum (above the range of the present experiments). Calculations using the Hu–Rath analysis show that grain growth induced by the post shock-induced temperature rise is consistent with the experimental results: grains grow from 70 to 800 nm within the post-shock cooling regime when subjected to a laser pulse with energy of 684 J.     

Spatial characterisation of the orientation distributions in a stable plane strain-compressed Cu crystal: A statistical analysis

Single copper crystals of the stable Goss orientation {0 1 1}〈1 0 0〉 were deformed in plane strain compression and the deformation-induced dislocation structures were investigated by high-resolution electron backscattered diffraction. Although the orientation maps exhibited an anisotropic dislocation boundary structure it was shown that the mean disorientation angle between point pairs saturated and became isotropic if their spacing was large enough (typically >30 μm). This saturation behaviour was interpreted as being a consequence of the anti-correlations between nearby dislocation boundaries and is discussed in terms of recent stochastic models of boundary formation. It was found that the disorientation boundaries, which were considered as being formed at relatively low strains, underwent rigid body-like rotations during deformation.     

Influence of Pb segregation on the deformation of nanocrystalline Al: Insights from molecular simulations

Molecular dynamics straining simulations using a two-dimensional columnar model were run for pure Al with grain sizes from 5 to 30 nm, and for 10 nm grain size Al–Pb alloys containing 1, 2 and 3 at.% Pb. Monte Carlo simulations showed that all the Pb atoms segregate to the grain boundaries. Pb segregation suppresses the nucleation of partial dislocations and twins during straining. At 3 at.% Pb, no dislocations or twins are observed throughout the straining history. It also appeared that Pb tends to segregate to the same locations in grain boundaries that were favorable for partial dislocation emission. Grain boundaries with Pb segregates were very robust against dissociation during straining compared to pure Al. The yield stress determined from stress–strain curves showed a decrease with increasing Pb content, supporting a similar observation for the hardness change measured on nanocrystalline Al–Pb alloys.     

Effect of initial particulate and sintering temperature on mechanical properties and microstructure of WC–ZrO2–VC ceramic composites

Owing to improving the mechanical properties of cemented carbides in high speed machining fields, a new composite tool material WC–ZrO₂–VC (WZV) is prepared from a mixture of yttria stabilized zirconia (YSZ) and micrometer VC particles by hot-press-sintering in nitrogenous atmosphere. Commercial WC, of which the initial particle sizes are 0.2 μm, 0.4 μm, 0.6 μm and 0.8 μm, is mixed with zirconia and VC powder in aqueous medium by following a ball mill process. The sintering behavior is investigated by isostatic pressing under different sintering temperature. The relative density and bending strength are measured by Archimedes methods and three-point bending mode, respectively. Hardness and fracture toughness are performed by Vickers indentation method. Microstructure of the composite is characterized by scanning electron microscopy (SEM). The correlations between initial particles, densification mechanism, sintering temperature, microstructure and mechanical properties are studied. Experimental results show that maximum densification 99.5% is achieved at 1650 °C and the initial particle size is 0.8 μm. When temperature is 1550 °C and particle size is 0.4 μm, the optimized bending strength (943 MPa) is obtained. The best hardness record is 19.2 GPa when sintering temperature is 1650 and particle size is 0.8 μm. The indention cracks propagate around the grain boundaries and the WC particles fracture, which is associated with particle and microcrack toughening mechanism.     

Nanoscaled grain boundaries and pores,microstructure and mechanical properties of translucent Yb:[LuxY(1−x)O3] ceramics

Translucent ceramics of Yb:[Lu_xY_(1?x)O₃] system doped by ZrO₂ was sintered from nanopowder synthesized by laser evaporation. The relative density of the ceramics was 99.97%, residual pores had sizes from 8 nm to 20 nm, Young modulus was 200 GPa at the applied load of 2000 mN, the microhardness was 12.8 GPa. The grains of ceramics had sizes 1–10 μm, but the thickness of grain boundaries was about 1 nm. The transcrystalline type of the crack propagation was detected in the specially broken ceramics. The results indicated high strength of grain bonds and good perfection of grain boundaries in the studied ceramics but an increased content of pores (higher than 10^?3 vol.%) and stoichiometry deviation (Lu:Y:O = 0.21:0.79:3) from the required one (Lu:Y:O = 0.25:0.75:3).     

Attenuation of ultrasound in severely plastically deformed nickel

Ultrasound attenuation was measured in nickel specimens of about 30 mm diameter prepared using the high pressure torsion technique. The cold working process produced an equivalent shear strain increasing from zero at the center up to 1000% at the edge of the specimen. The fragmentation of the grains due to multiple dislocations led to an ultrafine microstructure with large angle grain boundaries. The mean value of the grain size distribution gradually decreased from ～50 μm at the center to 0.2 μm at the edge. Laser pulses of 5 ns were employed for the excitation of broadband ultrasound pulses covering the spectral range of 0.1–150 MHz. The ultrasound pulses were measured from the opposite side of the specimen by means of an optical interferometer and a piezoelectric foil transducer in two experimental setups. The features of the detected signal forms are discussed. The absolute value of the attenuation decreases from the center to the edge of the specimen showing nearly linear frequency dependence. The variation of the phase velocity was measured in a 6 mm-thick high pressure torsion nickel sample, revealing a velocity increase from the center to the edge.     

Transition of solidification mode and the as-cast γ grain structure in hyperperitectic carbon steels

Formation processes of as-cast γ grain structures during casting of hyperperitectic carbon steels with 0.15–0.45 mass% carbon concentrations have been studied by means of a rapid unidirectional solidification technique. In steels with 0.15–0.41 mass% carbon concentrations, coarse columnar γ grains (CCGs) with a minor axis diameter of 1–3 mm developed along the direction of temperature gradient. In a steel with 0.38 mass% carbon, importantly, columnar γ grains (CGs) whose minor axis diameter is less than 500 μm form before the formation of CCGs and the grain structure changes discontinuously from CG to CCG. The fraction of the CG region increases with an increase in the carbon concentration. In the samples with a carbon concentration higher than 0.43 mass%, the as-cast structure consists of CGs over almost the entire ingots. Analyses of the relation between γ grain and dendrite structures and their crystallographic orientations indicate that the formation of CGs originates from the primary solidification of γ phase instead of δ phase. This is supported by numerical analysis of the dendrite growths.     

Creep response and deformation processes in nanocluster-strengthened ferritic steels

There is increasing demand for oxide-dispersion-strengthened ferritic alloys that possess both high-temperature strength and irradiation resistance. Improvement of the high-temperature properties requires an understanding of the operative deformation mechanisms. In this study, the microstructures and creep properties of the oxide-dispersion-strengthened alloy 14YWT have been evaluated as a function of annealing at 1000 °C for 1 hour up to 32 days. The ultra-fine initial grain size (approx. 100 nm) is stable after the shortest annealing time, and even after subsequent creep at 800 °C. Longer annealing periods lead to anomalous grain growth that is further enhanced following creep. Remarkably, the minimum creep rate is relatively insensitive to this dramatic grain-coarsening. The creep strength is attributed to highly stable, Ti-rich nanoclusters that appear to pin the initial primary grains, and present strong obstacles to dislocation motion in the large, anomalously grown grains.     

Plasma nitridation of a novel Ni–10.8 wt.%Cr nanocomposite

A Ni–10.8Cr nanocomposite (by wt.%), consisting of nanocrystalline Ni matrix (mean grain size: 60 nm) and dispersed Cr nanoparticles (mean particle size: 42 nm), has been synthesized by nanocomposite electrodeposition. The unique structure causes the nanocomposite to form a double-layered nitrided zone during plasma nitridation at 560 °C for 10 h. The outer layer (∼50 μm thick) precipitates nanometer-sized CrN (<100 nm), which increased in size but decreased in number with increasing nitridation depth (following Böhm–Kahlweit’s mode). The inner layer (∼5 μm thick) exhibits larger-coarsened nitride precipitates (100–200 nm) which almost link together. The greatly enhanced nitriding kinetics in the nanocomposite compared to a compositionally similar but microstructurally different Ni–10Cr alloy (mean grain size: 30 μm) is mainly associated with the fact that the numerous grain boundaries dramatically increase the nitrogen permeability, according to the treatment using a classical Wagner’s approach. The nanohardness profile in relation to the microstructure of the nitrided zone in the nanocomposite has also been investigated.     

Microstructural and mechanical properties of cBN-Si composites prepared from the high pressure infiltration method

The grain size dependence of the mechanical properties of cBN-Si composites prepared using the high pressure infiltration method has been investigated. Indentation testing indicates that cBN-Si composites have hardness values of 38–43 GPa, which increase with increasing grain size and are harder than traditional polycrystalline cBN composites (PcBNs). Thermostability analyses display that cBN-Si composites with a grain size of > 9 μm also possess a higher temperature of oxidation, compared to traditional PcBNs, and the thermostability increases with increasing cBN grain size. Fracture toughness tests show that almost no cracks appear on the polished cBN-Si samples when the loading forces are increased to 294 N and the fracture toughness is better than for commercial samples. Scanning electron microscopy illustrates that deformations and close pores occurred easily between coarse BN grains, leading to denser cBN-Si compacts with better mechanical performances.     

Mechanism and dynamics of shrinking island grains in mazed bicrystal thin films of Au

This work investigates the mechanism and dynamics of grain boundary migration driven by capillary forces via in situ electron microscopy, complemented by molecular-dynamics simulations. Using thin films of Au with the mazed bicrystal geometry, the shrinkage of island grains with 90°〈1 1 0〉 tilt grain boundaries was observed by diffraction contrast and high-resolution imaging. The grains remained cylindrical throughout the shrinkage, and there was no measurable grain rotation even at very small sizes. The rate of shrinkage was found to be erratic and inconsistent with parabolic kinetics, accelerating before complete disappearance. Residual defects were found immediately after complete shrinkage, although the type and magnitude of the defects varied from grain to grain. Measurement of the grain boundary shape anisotropy showed a preference for facets on low-index planes of the crystals, including the mirror-symmetry planes of the bicrystal. These facets were also found directly on individual images extracted from high-resolution video recordings of shrinking grains at ～300 °C. The dynamics of boundary motion were found to be limited by nucleation and propagation of steps on these facets. The cylindrical geometry and size of the experimentally observed island grains allow direct comparison with molecular-dynamics simulations on the same length scale, which reproduced many of the experimentally observed features, including non-parabolic shrinkage, absence of systematic grain rotation, step-controlled migration and dislocation debris after complete grain shrinkage. Differences between model and experiment are discussed in terms of the possible role of impurities, surfaces and interfacial steps.