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Abstract: Classical Hall–Petch relation for large grained polycrystals is usually derived using the model of dislocation pile-up first investigated mathematically by Nabarro and coworkers. In this paper the mechanical properties of nanocrystalline materials are reviewed, with emphasis on the fundamental physical mechanisms involved in determining yield stress. Special attention is paid to the abnormal or ‘inverse’ Hall–Petch relationship, which manifests itself as the softening of nanocrystalline materials of very small (less than 12nm) mean grain sizes. It is emphasized that modeling the strength of nanocrystalline materials needs consideration of both dislocation interactions and grain-boundary sliding (presumably due to Coble creep) acting simultaneously. Such a model appears to be successful in explaining experimental results provided a realistic grain size distribution is incorporated into the analysis. Masumura et al. [Masumura RA, Hazzledine PM, Pande CS. Acta Mater 1998;46:4527] were the first to show that the Hall–Petch plot for a wide range of materials and mean grain sizes could be divided into three distinct regimes and also the first to provide a detailed mathematical model of Hall–Petch relation of plastic deformation processes for any material including fine-grained nanocrystalline materials. Later developments of this and related models are briefly reviewed. Prof. Frank Nabarro was a physicist by training, a metallurgist by profession and a genius by nature, blessed with a unique ability to treat everyone as his equal. During his later years he was very much interested in the mechanical properties of nanocrystalline materials. This review on that topic is our contribution to the special issue of Progress in Materials Science honoring him. [Copyright &y& Elsevier]

Abstract: Cobalt content, surface morphology and microhardness of nanocrystalline Ni–Co deposits prepared by pulse plating technique at constant electrodeposition conditions with varied concentration of saccharin and cobalt sulfate in the electrolyte were investigated. It is found that appropriate amount of both additions could lead to finer structure and higher hardness of the deposit and further increase of the concentration could result in decline of the hardness, which is regarded as the result of inverse Hall–Petch relation. The maximum hardness of the Ni–Co alloy deposits is not higher than that of their pure Ni counterparts, indicating that the refinement hardening effect (Hall–Petch relation) is dominant in nanocrystalline Ni–Co alloy deposits. By adding Co ions to the electrolyte, the amount of organic refiner saccharin (responsible for introduction of sulfur and carbon impurities) needed to produce nanocrystalline deposits could be remarkably reduced. [Copyright &y& Elsevier]

Abstract: Metallic multilayers exhibit a very pronounced size effect where the mechanical strength depends on the layer thickness. Traditionally the Hall–Petch relation is used to account for the size effect. However, rigorous application of dislocation pileup theory predicts significant deviation from the Hall–Petch relation due to elastic inhomogeneity, discreteness of dislocations and dislocation source operation. Elastic inhomogeneity leads to anomalous scaling where the scaling exponent deviates from 1/2 of the classical Hall–Petch relation. The discrete dislocation effect is properly accounted for by a piecewise approach that can be applied at all length scales. In this article, a key step in the formulation is taken: the dislocation source characteristics are taken into consideration. Thus, all the three effects are accounted for. Analytic formulas linking yield stress to microscopic interface strength, dislocation source activation stress and other easily obtainable parameters (the Burgers vector, the elastic constants of constituent materials, crystal structure and layer thickness) are provided for all length scales. The model is then applied to Cu/Ni multilayers and the predicted strength is compared with experimental data. [Copyright &y& Elsevier]

Abstract: The present work involves synthesis, characterization and evaluation of hardness and sliding wear resistance of electrodeposited nanocrystalline Ni–W alloys. Crystallite size reduced with an increase in current density due to an increase in the W content. Ni–W alloy with 9.33at.% W plated at 75°C exhibited the maximum hardness of 638HV. Alloys plated at 75°C followed direct Hall–Petch relation. However, alloys plated at 85°C exhibited an inverse Hall–Petch relation below a crystallite size of 15nm. Wear resistance of alloys plated at 75°C increased due to an increase in hardness with a reduction in the crystallite size up to 20nm. It reduced due to brittle fracture of the coating below 20nm. Wear resistance of alloys plated at 85°C increased with a reduction in the crystallite size in the direct Hall–Petch region and decreased in the inverse Hall–Petch region. Ni–W coatings with 6–8at.% W exhibited superior wear resistance. [Copyright &y& Elsevier]

Atomic-scale simulations of nanocrystalline copper with grain sizes from 5 to 50 nm have been performed. The simulations show a clear maximum in the flow stress when the grains are 10–15 nm in diameter. At this grain size, there is a shift in deformation mechanism, from dislocation-mediated plasticity at larger grain sizes to grain boundary sliding at smaller. Above the maximum in hardness, the grain size dependence of the hardness is consistent with the Hall–Petch relation, conventionally explained by the creation of dislocation pile-ups in the grains. It has not been clear if this explanation of the Hall–Petch effect is valid for sub-micrometre grains, but the simulations presented here clearly show the existence of pile-ups in the simulation with average grain size of 50 nm. The dislocation dynamics in the grains is dominated by the grain boundaries, as almost all dislocation nucleation occurs at the grain boundaries, which also act as efficient dislocation sinks. During the plastic deformation, a large number of stacking faults and a much lower number of twin boundaries are created. These do not contribute significantly to the flow stress, as no work hardening is seen whereas the number of stacking faults increase with strain. [Copyright &y& Elsevier]

Phase pure and 25 % Sb incorporated Bi 2 Te 3 compounds with a rhombohedral crystal structure are synthesized by mechanical alloying the stoichiometric mixtures of elemental powders within 3 h under an inert atmosphere. The structure and microstructure of the synthesized samples are revealed by analyzing XRD patterns with Rietveld refinement, FESEM, HRTEM images, and EDX spectra. The increment of lattice parameter 'c' with increasing milling time is associated with the increased [Bi/Sb Te ] type antisite defects. Optical bandgaps of the pure and Sb-alloyed Bi 2 Te 3 samples of different sizes are determined from the FTIR spectra. The transition of undoped n-type to Sb-alloyed p-type semiconducting nature is revealed from the photo responses of the samples. The ac electrical conductivity as well as complex impedance spectra in the temperature range 303–403 K reveal the semi-metallic, non-Debye type behavior of the samples and temperature-dependent relaxation phenomena. The change in electrical conductivity and the corresponding activation energies due to Sb alloying and for different grain sizes are explained in terms of antisite defects and porosity of materials. The temperature variation of the frequency exponent has been demonstrated with the small polaron hopping transaction. The Mott VRH model is used to explain the electrical conduction mechanism of the synthesized samples. The variation of Vickers microhardness with the grain size of milled samples corroborates well with the Hall-Petch relation. The normal indentation size effect (ISE) is depicted from Meyer's law. The photocatalytic activity of 10 h milled Sb-alloyed sample to degrade Rhodamine B cationic dye in wastewater is ∼ 91 %. The correlations of electrical and mechanical properties with the photocatalytic activity of the samples have been established. [Display omitted] • Bi 1.5 Sb 0.5 Te 3 of different sizes have been fabricated via facile mechanical alloying. • A transition from n-type to p-type semiconductor is observed upon Sb inclusion. • Mott's VRH model and SPH model is applied to explain the conduction mechanism. • Grain size and porosity dependent microhardness is measured to find normal ISE. • Improved photocatalytic activities against organic dyes are observed for the first time. [ABSTRACT FROM AUTHOR]

The difference in the mechanical behaviors of dilute solid solutions, complex solid solutions and their corresponding strengthening mechanisms, is an evolving field of study. An understanding of the mechanisms and formulation of theories of strengthening in the complex atomic energy landscapes could eventually lead to a better understanding of the fundamental behavior of condensed matter itself. In this work we attempt to extract the effect of thermo–mechanical processing on the microstructure–mechanical property linkages of complex concentrated alloys (CCAs) by training machine learning (ML) models using processing information / parameters as features. The effect of processing on the phase morphology and the mechanical properties is studied. The stacking fault energy (SFE) predicted based on CCA composition is used as a benchmark to identify deformation mechanisms that are activated based on the arrangement of the component elements within the distorted CCA lattice. This work presents a novel method that attempts to establish ML based process–structure–property (PSP) linkages that could help capture higher order dependencies that may not be adequately captured by existing relations between mechanical properties, phase evolution, composition and processing information. An assortment of Bayesian–learning models are used to create a framework that captures the evolution of phases, their volume fractions, grain sizes and the corresponding change in mechanical properties of a diverse set of CCA compositions as they encounter various processing conditions. The evolution of the mechanical property with grain size is captured as Hall–Petch relations as an example of possible PSP linkage representations. • Effect of thermo-mechanical processing on the microstructure-mechanical properties. • A hierarchical model for determining properties over multiple length scale. • Bayesian learning models for predicting phases and their volume fractions. • Hall-Petch relation is verified as an example of PSP linkage representations. [ABSTRACT FROM AUTHOR]

Abstract The presented numerical model solves the heat and mass transfer equations in the Laser Metal Deposition process and based on the evolution of the thermal field predicts the grainsize, the resulting hardness and evaluates the pores formation probability in an AISI 304 stainless steel. For this purpose, in a first step, the model calculates the shape of the deposited material and the variations of the temperature field. In a second step, and based on the evolution of the thermal field, the model calculates the resulting hardness of the deposited material, the grainsize and the porosity formation probability after the deposition process. Numerical results are experimentally validated, and good agreement is obtained. Consequently, besides predicting the geometry of the resulting part and the evolution of the thermal field, the developed model enables to evaluate the quality of the deposited material. Therefore, the optimum process conditions and strategy when depositing AISI 304 stainless steel can be determined without initial trial-and-error tests. Highlights • A model that simulates the Laser Material Deposition (LMD) process has been developed and validated experimentally. • The model is capable of determining the Dendrite Arm Spacing of the deposited material based on the cooling rate. • The model predicts the hardness of the deposited material based on the Hall-Petch relation and the sensitization effect. • The model predicts the pore formation based on the trapped gas due to a fast solidification and the material shrinkage. • The model enables to optimize the deposition strategy and minimize defects such as pores. [ABSTRACT FROM AUTHOR]

Abstract Coarse-grained high-entropy alloys (HEAs) have such excellent mechanical properties as high hardness and strength. The well-known Hall-Petch relation suggests that this high hardness should be further increased if the crystallite size of HEAs is decreased to nanometer scale. We prepared single-phased nanocrystalline (NC) refractory VNbMoTaW HEAs powders by mechanical alloying (MA). The achieved NC HEAs have a body-centered cubic (BCC) structure and an average grain size of approx. 6 nm. We further used a high-pressure/high-temperature (HPHT) technique to consolidate the NC powders to achieve bulk NC HEAs. The evolution of phases, microstructure and mechanical properties of the consolidated bulk NC HEAs were studied as a function of consolidating temperature. The bulk NC VNbMoTaW HEAs consolidated at an optimized temperature of 1150 °C have an average grain size of ∼30 nm and a hardness of 11.4 GPa, ∼ twice that of coarse-grained refractory HEAs. This ultrahigh hardness has three origins: solid solution, grain boundary and dislocation hardening. Highlights • Single-phased nanocrystalline VNbMoTaW high-entropy alloy powders were prepared. • The powders were consolidated to a nanocrystalline bulk with an ultrahigh hardness of 11.4 GPa. • The ultrahigh hardness results from solid solution, grain boundary and dislocation hardening. [ABSTRACT FROM AUTHOR]

Abstract A model is developed to describe the grain size effect on elastoplastic behavior of ultrafine grained ( ufg ) and nanocrystalline ( nc ) materials using a self-consistent approach. The grain size effect is modeled using two different length scales, the microscale (crystallographic slip system) and the mesoscale (granular level via the grain/matrix interaction law). The difference between the new extension and that previously proposed in Abdul-Latif et al., (2009) is based on the fact that the current extension describes not only the ufg materials with grain size diameter ( d ) range of (100–1,000nm), but also the nc materials having a range of diameters of (limit value-100 nm) which is defined by a lower slope of the linear Hall-Petch relation compared to the ufg regime. Note that such a limit value ( lv ) varies between about 15 nm and 30 nm. As a model limitation, the nc metals with grain sizes below lv which behave in the case of copper ( lv = 25 nm) either as a plateau or as a decrease of the yield strength cannot be described by the model. In this extension, the grain-boundary attribution is assumed to be globally and implicitly described particularly with further grain refinement (i.e., nc materials). The used self-consistent scheme deals with a non-incremental inclusion/matrix interaction law of softened nature. It describes the non-linear elastic-plastic behavior of fcc polycrystals. The overall kinematic hardening effect can be naturally produced by the interaction law. Within the framework of small strain hypothesis, the elastic isotropic behavior is defined at the granular level. The heterogeneous inelastic deformation is locally determined using the slip theory. The model describes fairly well the effect of grain size on the strain-stress responses of copper and nickel. [ABSTRACT FROM AUTHOR]

Nanostructured face-center-cubic/body-center-cubic Cu/(NbMoTaW) multilayers were prepared with equal layer thickness ( h ) spanning from 5 to 100 nm. The hardness was found to increase with reducing h to ~50 nm, following the Hall–Petch relation. Below this critical thickness, a hardness plateau emerged indicative of an interface barrier strengthening mechanism. The high density of misfit dislocations at interfaces facilitates the yielding of high entropy alloy NbMoTaW layers at stresses far less than the strength of Cu/(NbMoTaW) estimated by rule-of-mixture. Compared with reported multilayers, the present Cu/(NbMoTaW) samples show the size-independent hardness at a larger size-range h  < 50 nm. [ABSTRACT FROM AUTHOR]

The effects of grain refinement on the quasi-static compressive behavior of AISI 321 austenitic stainless steel (ASS) were studied. The effect of strain on the final microstructure after compressive deformation was also investigated. The compression tests on steel specimens were conducted at a strain rate of 4.2 × 10 −3 s −1 . Ultrafine-grained (UFG) specimen with the grain size of 0.24 μm exhibits an excellent combination of high yield strength (∼1 GPa) and good strain hardenability. Meanwhile, the coarse-grained (CG) specimen with the grain size of 37 μm exhibits yield strength of ∼0.2 GPa. At 0.53 true strain, UFG and CG specimens exhibit compressive strengths of 5.95 and 4.80 GPa, respectively. The Hall-Petch relation constants, σ o , and K, for the AISI 321 ASS were estimated to be 128 MPa and 478 MPa μm −0.5 , respectively. The strain hardening behavior of both UFG and CG specimens occur in three distinctive stages. CG specimen exhibits higher strain hardening rate than the UFG specimen up to a critical true strain of 0.4, above which strain hardening rate in UFG becomes greater. X-ray diffraction (XRD), electron backscatter diffraction (EBSD) and transmission electron microscope (TEM) techniques were used for microstructural analyses to understand the underlying mechanisms behind the strain hardening behavior. Texture evolution during deformation, orientation relationship between phases and the sequence of martensitic phase transformation were also studied and are discussed in this paper. Visco-plastic self-consistent (VPSC) modeling was employed to decipher the role of deformation mechanisms in macroscopic stress-strain response and also in texture evolution during uniaxial compression loading. [ABSTRACT FROM AUTHOR]

Twin lamellas, were observed in transparent AlON ceramics in our previous work. In this work, the influence of the twin lamellas on mechanical strength was evaluated and its mechanism was investigated by SEM, EBSD, and HRTEM. Both the crystallographic direction and the space direction of a crack will deflect when it propagates across a twin lamella. The dynamic mechanism is that the dislocation propagation in front of the crack tips is blocked by the twin boundaries. Because the fraction of twin boundaries increases with grain size in transparent AlON ceramics, the strengthening effect becomes more notable for large-sized samples resulting in inverse Hall-Petch relation. It is a potentially useful way to advance the mechanical properties of transparent AlON ceramics by controlling the content of twin lamellas. [ABSTRACT FROM AUTHOR]

A micromechanical-based finite element framework is developed to investigate the rolling contact fatigue (RCF) in rails. The microstructure of a representative part of the rail in contact with the wheel during wheel passage is modeled using Voronoi tessellation algorithm. The geometry explicitly represents microstructural grain and grain interface features that represent actual microstructure of rail steel. The grain interface is modeled using cohesive elements. For higher computational efficiency, moving Hertzian load is applied to the rail surface instead of moving the wheel explicitly on top of the rail. The jump-in-cycles approach is employed to efficiently simulate a large number of loading cycles, while the material degradation at the grain interface is represented by an accurate damage evolution law. Additionally, the model is used to evaluate the effects of temperature change, traction coefficient, and grain size on initiation and progression of RCF. The results indicate that colder temperatures increase the progression of RCF, while warmer temperatures do not have any significant effects. Large traction forces at the wheel-rail interface significantly accelerate RCF growth, mainly through migration of subsurface micro-cracks to the surface of the rail. The surface cracks grow into larger ones that lead to loss of rail material. Controlling the grain size can have a positive effect on both the initiation and migration of sub-surface cracks. Changing yield strength of material with the Hall-Petch relation, the model evaluates the effect of refining the grain size. The results indicate that smaller grains can improve RCF properties of the rail. The effect of maximum contact pressure on RCF is studied, and the results show a power relation between maximum contact pressure and RCF life. Also several case studies are simulated and compared with the experimental observations. The simulation results are in close agreement with the experimental observations for the crack pattern and crack depth. [ABSTRACT FROM AUTHOR]

The microstructure evolution in both surface and bulk grains in a pure Fe-19Cr-12Ni alloy has been analyzed using electron backscatter diffraction after tensile testing interrupted at different strains. Surface grains were studied during in situ tensile testing performed in a scanning electron microscope, whereas bulk grains were studied after conventional tensile testing. The evolution of the deformation structure in surface and bulk grains displays a strong resemblance but the strain needed to obtain a similar deformation structure is lower in the case of surface grains. Both slip and twinning are observed to be important deformation mechanisms, whereas deformation-induced martensite formation is of minor importance. Since the stacking fault energy (SFE) is low, ~17 mJ/m 2 , dynamic recovery by cross slip of un-dissociated dislocations is unfavorable. This reduces the annihilation of dislocations which in turn leads to a significant increase of low angle boundaries with increasing strain. The low SFE also favors formation of deformation twins which reduces the slip distance, leading to a hardening similar to the Hall-Petch relation. The combination of a low ability for cross-slip and a reduced slip distance caused by twinning is concluded to be the main reason for maintaining a high strain-hardening rate up to strains close to necking. [ABSTRACT FROM AUTHOR]

Three hot-rolled TWIP steels with different grain size were obtained by changing final rolling temperatures. The results show that the recrystallization microstructure can be obtained at the final rolling temperature range of 966–1083 °C and the grain size can be refined from 17.3 µm to 8.5 µm by decreasing final rolling temperature from 1083 °C to 966 °C. Moreover, the correlation between yield strength and grain size obeys Hall-Petch relation. However, surprisingly, the larger the grain size is, the higher the ultra low-temperature Charpy impact absorbed energy, and the reason may be is that the number fraction of austenite grains occupied by nano-twins formed in two or more twinning systems is relatively high for the steel with larger grain size, thereby leading to stronger dynamic grain refinement effect and heavier plastic deformation. [ABSTRACT FROM AUTHOR]

The microstructure, texture and mechanical properties evolution of the extruded fine-grained Mg-(1, 5) Y (wt%) alloy sheets were investigated during annealing. The as-extruded sheets exhibited a fully dynamic recrystallized (DRXed) microstructure consisting of uniform and fine equiaxed grains (5.7–8.7 µm) and a small amount of YH 2 second phase particles. Both sheets exhibited significant thermal stability at 300 °C annealing and remarkable grain growth at temperatures higher than 400 °C. The measured grain growth activation energy ( Q ) of Mg-1Y at all temperatures tested was 91 ± 3 kJ/mol, suggesting that the growth was controlled by grain boundary diffusion. Meanwhile, for Mg-5Y alloy at lower temperatures (300–400 °C), the Q value (59 kJ/mol) indicated that the grain growth was controlled by grain boundary diffusion while the Q value (175 kJ/mol) at higher temperatures (400–450 °C) implied that lattice self-diffusion controlled the process. A representative rare-earth texture was present for both sheets in the as-extruded condition, and remarkable basal texture weakening was observed with the grain growth during each isothermal annealing process. The texture weakening can be ascribed to the preferential growth of non-basal oriented grains based on the EBSD analysis, which was likely related to the segregation of Y atoms at grain boundaries. The microhardness and grain size relationship can be described well by the Hall-Petch relation for all the annealed sheets, while the yield stress was not the case indicating that the macroscopic strength was more sensitive to the texture than the localized microhardness. [ABSTRACT FROM AUTHOR]

The deformation behavior and amorphization process of polycrystalline and single-crystalline silicon are investigated using molecular dynamics (MD) simulations and compared with the previous nanoindentation experiments. In order to unravel the grain size effect on the deformation mechanism, the indentation simulations of polycrystalline silicon with the different grain sizes are performed, and all grains follow the inverse Hall-Petch relation. The results reveal that Young’s modulus of polycrystalline silicon is much greater than Young’s modulus obtained in the 〈1 0 0〉 direction single-crystalline silicon due to the random crystallographic orientation and strong anisotropy of polycrystalline silicon at nanoscale. The nucleation of amorphization in both single-crystalline and polycrystalline silicon appears always beneath the indenter due to the maximum shear stress, controlling the plastic deformation during the indentation process. The horizontal expansion of amorphization region is faster and larger than the vertical expansion of amorphization region due to two reasons: the limit of indentation stress along the depth direction is compared with along the width direction; the grain boundary is the most possible path of weak connection, and produces the complex stress distribution for the propagation of amorphization region. The smaller grain size leads to the stronger gradient strain, especially in the region of upper surface. [ABSTRACT FROM AUTHOR]

Electrodeposition of Fe-W coatings has been carried out from an environmentally friendly Fe(III)-based glycolate-citrate bath. Samples with tungsten content from 6 to 25 at.% were electrodeposited in a controlled way by changing electrodeposition parameters: current density, pH, and temperature. X-ray diffraction analysis showed that the structure of Fe-W coatings transforms from nanocrystalline to amorphous-like as the W content increases and the crystallite size reduces below 10 nm. However, the peculiarities of the structural transitions are linked not only with the W content. Deposition temperature plays a crucial role, due to the different activation energy of crystallization. Following the direct Hall–Petch relation, a maximum hardness of ~ 10 GPa was found for the alloy with the highest W content, making it comparable to that of electrolytic chromium. The Fe 2 W intermetallic compound forms at higher W concentration as proven by Mössbauer spectroscopy, and contributes to the increased hardness of these alloys. The alloys retain a soft magnetic character within a wide compositional range, although increasing the W content leads to a reduction of the saturation magnetization. Fe-12 at.% W coatings show an optimum combination of mechanical and magnetic properties, thus making these newly developed coatings appealing environmentally-friendly alternative materials for multi-scale technologies. [ABSTRACT FROM AUTHOR]

Molecular dynamics was used to simulate tensile behavior of nanocrystalline titanium with ultra-small grain size ranging from 2.8 nm to 10.2 nm at the strain rate ranging from 10 8  s −1 to 10 10  s −1 . Three dimensional samples with random oriented grains containing no textures were developed by Voronoi tessellation. For all the samples, the yielding is solely controlled by GB mediated process. The inverse Hall-Petch relation between grain size and flow stress was found. The strain rate sensitivity shows increase with the decrease of grain size due to the enhancement of grain boundary (GB) mediated process. Meanwhile, it increases with the applied strain rate due to the local disorder around GBs. Remakeable grain coarsening observed in the 2.8 nm sample causes the slight increase of flow stress. Both GB mediated process and partial dislocation slips play an important role in the plastic deformation of nanocrystalline Ti. With the increase of grain size, rare twins initiated from GBs can be observed. From the size dependent dislocation density analysis, it is concluded that with the increase of grain size, dislocation related deformation contributes more to the plastic strain. [ABSTRACT FROM AUTHOR]

The promising mechanical properties of electrochemically deposited nickel‑cobalt alloys (eNiCo) significantly depend on the resultant grain size. This study describes the influence of bath hydrodynamics (BHD) on grain size and thus material properties of the electrodeposited layers. To study the role of BHD in grain growth, in situ analysis using hydrodynamic galvanostatic electrochemical impedance spectroscopy (HGEIS) and in situ hydrodynamic linear sweep voltammetry (HLSV) techniques are used to correlate the influences of BHD on grain growth kinetics. The HGEIS studies revealed that with increasing BHD velocity from 0 cm/s to 42 cm/s, the charge transfer resistance decreased, while the double layer capacitance increased. The HLSV studies revealed that the limiting current density increased with BHD velocity, whereas the corresponding computed diffusion layer thickness decreased from ca. 115 μm to 96 μm (16.4 % reduction) for eNiCo coatings and from 120 μm to 106 μm (11.6 % reduction) for eNi coatings, respectively. Thorough analysis using ion channelling images from focused ion beam (FIB) cross sections of the deposited films in combination with a qualitative model revealed a strong correlation of BHD with diffusion layer thickness and grain size. Whereas without BHD a predominant columnar growth and only a thin layer formed directly after nucleation with fine grain distribution are observed, at large BHD velocities a fine equiaxed grain growth over the full layer thickness was achieved. The microstructure transformation as function of BHD, consequently resulted in improved micromechanical properties which is exemplified by the microhardness up to 692 HV0.01 of the eNiCo coatings. For eNiCo a strong relation between mean grain size value derived from focused ion beam ion channelling microscopy and Vickers microhardness of the layer was proved. • The increasing BHD velocities decreased the resultant diffusion layer thickness. • In situ analysis revealed charge transfer resistance decreased with increasing BHD velocity. • The decreased diffusion layer thickness supports fresh nucleation. • Using BHD, a remarkable grain growth transformation from columnar to fine-equiaxed occurs. • The resultant grain size refinement showed a good fit with the Hall-Petch relation. [ABSTRACT FROM AUTHOR]

We perform molecular dynamics simulations to explore the effects of grain size, alloy composition, and temperature on the mechanical characteristics of Si 100-x Ge x alloys during the indentation process. The structural evolution, von Mises shear strain, von Mises stress, and dislocation in workpiece are evaluated to explain the transformation of the loading force. Moreover, hardness and Young's modulus are also computed to evaluate the mechanical characteristics of different specimens. The results present that the loading force, hardness, Young's modulus of single crystalline are higher than polycrystalline. Besides, the loading force and hardness reduce as the grain size reduces; this result reveals a reverse Hall-Petch relation. In the indentation process of small grain size workpiece, the dislocation freely migrates to the grain boundary and absorbed, leading the amount of dislocation movement significantly reduces, which will instead promote the migration and sliding at the grain boundaries. It is confirmed that the grain boundaries play an essential role in the deformation behavior of the material. For the purpose of study the effect of various alloy compositions on the mechanical characteristics of the Si 100-x Ge x substrate, a series of various alloy compositions are conducted: Si 20 Ge 80 , Si 40 Ge 60 , Si 50 Ge 50 , Si 60 Ge 40 , and Si 80 Ge 20. The results reveal that the hardness, and Young's modulus of specimens increase as the Si content increases. In addition, the influence of temperature is also investigated in this work. It is found that when the temperature increases, leading to the atomic displacement and shear strain increase, the total dislocation length and the number of defect atoms increase, which leads to the hardness and Young's modulus reduction. • The grain boundaries play an important role in the deformation behavior of the material. • A reverse Hall-Petch relation is observed as the decrease in the grain size. • When the Si content increases, the hardness and Young's modulus of the substrate increase. • The hardness and Young's modulus of the substrate decrease as increasing the temperature. [ABSTRACT FROM AUTHOR]

The yield strength of metals is typically governed by the average grain size through the Hall-Petch relation, and it usually follows the rule of mixture (ROM) for layered composite materials. In the present study, an extraordinarily high yield strength, far beyond the predicted values from the Hall-Petch relation and the ROM, is achieved in a specially designed layered titanium that is characterized by alternating coarse- and fine-grain layers (C/F-Ti) where the grain sizes match the layer thicknesses in both the coarse- and fine-grained layers. The strengthening mechanism of such a layered C/F-Ti is investigated based on detailed experimental characterizations, including nanoindentation tests of local hardness, in-situ synchrotron Laue X-ray microdiffraction (μXRD) measurement of the lattice strain distribution during tensile testing, and transmission electron microscopy (TEM) analysis of the dislocation structure after yielding. In the coarse-grain layers, significantly higher hardness values are observed next to the layer interfaces compared to the layer center regions, and dislocations and densely populated pile-ups of dislocations are exclusively observed in the interface regions after yielding. These experimental results point to an enhanced interface constraint effect on the deformation mechanism in hexagonal close-packed (hcp) materials with a large difference in the critical resolved shear stress between slip and slip. The C/F-Ti combines the strength of fine grains and the ductility of coarse grains, demonstrating a new structural design strategy for property optimization of single-phase hcp materials. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Mechanical properties of multilayer films of aluminum zinc oxide (AZO) and indium zinc oxide (IZO) prepared from precursors obtained by solution and combustion synthesis have been characterized through nanoindentation. The AZO films have been prepared from solution of zinc acetate dehydrate doped by aluminum chloride, while IZO films have been obtained by mixing indium nitrate hydrate and zinc nitrate hexahydrate in various proportions. The preparation of solutions requires use of 2-methoxyethanol as solvent and of urea as fuel. Each film of the multilayer stack has been spin-coated, on glass substrate. Films of AZO and of IZO have been heated according to different recipes. X-rays diffraction measurement showed that AZO films were c-axis oriented with hexagonal wurtzite structure, while IZO films showed cubic bixbyite structure. We observe that, for crystalline films, the mechanical behavior follows normal Hall-Petch relation, while in case of amorphous films the hardness came from dopant atoms with higher atomic radii that stresses the lattice due to the size difference. [ABSTRACT FROM AUTHOR]

Laser remelting was employed to refine the interlamellar spacing (λ) to approximately 30 nm on an arc-cast Al-32.7 wt%Cu eutectic. λ scaled with A ′ (h) - ° 0.25 where h is the distance from the trace bottom, and A ′ is a factor dependent on laser processing conditions. A ′ increased with the laser power (at a constant spot diameter) but decreased with the spot diameter (at a constant laser power). The nanoindentation measured hardness increased up to 4.68 GPa with decreasing interlamellar spacing, following the Hall-Petch relation. The as-cast eutectics showed microcracks around indents, while cracking was suppressed in the laser remelted nanoscale eutectics. A dislocation theory based mechanism is proposed for the enhanced plastic co-deformation of fine scale laser-remelted Al-Al 2 Cu eutectics. [ABSTRACT FROM AUTHOR]

Influence of solution treatment (ST) on microstructure evolution and mechanical behavior of nickel-based UNS N10276 alloy were investigated by a variety of techniques such as optical metallography, scanning electron microscopy, transmission electron microscopy and tensile testing. The average grain size remained almost unaffected by hold time at 1050 °C, resulting from the pinning effect of secondary phase on the grain boundary. A sharp increase in grain size and grain growth rate was observed with increasing solutionizing temperature up to 1150 °C irrespective of hold time, which was mainly related to the complete dissolution of precipitated particles as well as to the enhanced diffusion of solute atoms. The rate of grain growth reduced with extending solutionizing time and decreasing temperature. Flow curve parameters based on Ludwigson equation for the alloy after heat treatment were determined. The dissolution of secondary phase particles, combined with the grain growth, increased the work hardening exponent ( n ) and dramatically improved the tensile ductilities (TD), but reduced the tensile strengths (TS) to large extent. Both TD and n monotonically increased with increasing solutionizing temperature and prolonging hold time as the alloy was solutionized beyond 1100 °C, while the TS and strain hardening rate (SHR) showed a reverse varying trend to that of TD and n . As the heat treatment was performed at 1050 °C, tensile properties were not monotonically changing with hold time, which is associated with the increase in amount of precipitates and with the Ostwald ripening of precipitated particles. On the basis of Hall–Petch relation and experimental data, the mathematical relationships between tensile properties and grain size of the alloy solutionized at temperatures higher than 1150 °C, were obtained by linear regression analysis. The Hall–Petch strengthening coefficient for yield strength of the UNS N10276 alloy was estimated to be 18.35 MPa mm 1/2 . [ABSTRACT FROM AUTHOR]

In this paper, the phase composition and content of Al-30 wt%Mg 2 Si composite prepared by in-situ process have been investigated; and the effect of different addition of La inoculation on microstructure, granularity and mechanical properties of Al-30%Mg 2 Si composite have been studied. Scanning electron microscope (SEM) and X-ray diffraction (XRD) images show that the phase composition and content of Al-30 wt%Mg 2 Si composite agree with pseudo-binary Al-Mg 2 Si phase diagram. The effect of La inoculation on microstructure indicates that La addition can change the morphology of primary Mg 2 Si particles and refine the particle size of primary Mg 2 Si. When the La addition reaches to 0.8 wt%, the refinement effect of Al-30%Mg 2 Si composite is the best. Similarly, the results reveal that La addition can effectively improve the ultimate tensile stress (UTS), breaking elongation (BE) and Vickers microhardness (VM) values. When La addition is over 0.8 wt%, primary Mg 2 Si particles become coarsening and mechanical properties decline. The crystal size was characterized by XRD and deduced by Cauchy and Gaussian approximation. The relationship between crystal size and UTS or VM can be described by Hall-Petch relation. The linear fitting degree of UTS is better. [ABSTRACT FROM AUTHOR]

The microstructure and mechanical properties of high silicon Al9Si0.45Mg alloys without grain refinement and refined by Al5Ti1B and Al3Ti3B master alloys were investigated. The results showed that the primary α–Al grain sizes were 1288 ± 361 µm and 645 ± 146 µm in the non–refined and Al5Ti1B refined alloys respectively, and that was decreased significantly to 326 ± 116 µm in Al3Ti3B refined alloy. After T6 heat treatment, the yield strength, ultimate tensile strength and elongation of the non–refined alloy were 298 ± 2 MPa, 350 ± 2 MPa and 4.5 ± 0.7% separately. When the grain refiner was changed from Al5Ti1B to Al3Ti3B, the yield strength was increased from 304 ± 2 MPa to 311 ± 1 MPa, the ultimate tensile strength was increased from 357 ± 3 MPa to 366 ± 1 MPa, and the elongation was increased significantly by 38.8% from 8.0 ± 1.0% to 11.1 ± 1.2%. The Hall–Petch relation between the yield strength and grain size of the heat–treated alloys was determined as σ y = 285.1 + 470.2· d −1/2 . It was found that β'' phase was precipitated from the α–Al matrix for the peak strengthening of the heat–treated alloys. TiB 2 and TiAl 3 particles were found coexisting in Al5Ti1B master alloy, while TiB 2 and AlB 2 particles were verified coexisting in Al3Ti3B master alloy. The inoculation of AlB 2 particles results in the significant grain refinement under Al3Ti3B. The increase of strength in the Al3Ti3B refined alloy is attributed to the refinement of the primary α–Al grains, while the increase of ductility in the Al3Ti3B refined alloy resulted from the reduced oxides and porosity defects as well as decreased grain size. [ABSTRACT FROM AUTHOR]

A hardening model within the framework of finite-strain Cosserat crystal plasticity is extended to dynamic recrystallization. The new model includes an Avrami equation to account for softening, a level set equation to represent the propagation of the domain walls, and the Ambrosio-Tortorelli approach inherited from image analysis to identify the grain dislocation structure. Numerically, flow curves are computed and the Hall-Petch relation is studied. [ABSTRACT FROM AUTHOR]

Uni-axial tension and compression tests were conducted on commercially pure titanium sheet. Strong anisotropy and tension-compression asymmetry was observed. The work hardening behaviour was found to be strongly dependent on the direction and sign of the acting stress. Specifically, twinning occurred and lead to a dynamic Hall-Petch effect, but only under uni-axial compression in transverse direction. A novel approach to define a very flexible, three-dimensional yield criterion is proposed. Deviatoric stresses are mapped from five-dimensional space into a reduced three-dimensional space (the material is assumed to show an isotropic response under shear loading) and transformed to spherical coordinates. A two-dimensional Fourier series of the angular coordinates is used to define an equivalent stress. It was possible to find a smooth yield surface that could accurately capture all experimental data (yield stresses and Lankford parameters). Directional twinning and the related distortional hardening is modelled by a modified Hall-Petch relation and a dynamic internal variable representing the twin volume fraction. The models were implemented in an explicit FE code and a cup drawing test was simulated. The results of the proposed models showed a clear improvement of accuracy compared to those of two established models, while calculation time was even reduced. [ABSTRACT FROM AUTHOR]

This paper is aimed at investigating the mechanical properties and deformation mechanisms of nanocrystalline copper under nanoindentation. The Voronoi tessellation method is adopted to generate nanocrystalline structures with stochastic grain orientations that mimic those in experiments. Grain-size effect is studied and discussed via molecular dynamics simulations. The results reveal the inversion of Hall-Petch relation about hardness at a grain size of 15.1 nm, which agrees well with previous works in tension. Below the critical grain size, grain boundary sliding, grain growth and grain rotation are easily observed. Grain boundary motion is the dominant deformation with smaller grain size below 15.1 nm while dislocation motion dominates above the critical value. It is noteworthy that the elastic recovery in indentation direction, increases with larger grain size and monocrystalline copper behaves with the strongest elastic recovery. The study further reveals the deformation mechanism of nanocrystalline copper under nanoindentation and accelerates the functional applications of nanocrystalline materials. Material science; computational material. • Nanocrystalline structure is generated by Voronoi tessellation method. • Nanoindentation behaviors of nanocrystalline copper are investigated. • The critical grain size for the inversion of Hall-Petch relation is about 15.1 nm. • Grain boundary sliding, grain rotation and grain growth are easily observed. • The depth elastic recovery is sensitive to grain size. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulation (MD) was performed on the polycrystalline copper with bimodal structures. As our special grain size distribution based on the inverse Hall-Petch relation, the small grain region and large grain region play different roles in the uniaxial tensile tests. The large grain region imparts the bimodal Cu samples high strength for the lattice dislocations activation. The large grain size effect was studied in this paper. Through our MD simulations, an atomic scale view of the interactions between two regions is also presented with the deformation progress. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulation is utilized to study the effects of grain size and temperature on the mechanical properties of quasi-two-dimensional (2D) nanocrystalline Cu with grain sizes of 3–8 nm under uniaxial tensile tests. Our results show that the grain growth is caused by the grain rotation and leads to strain hardening at 300 K for a grain size of <5 nm and at 100 K for a grain size of <4 nm. For a grain size of ≤7 nm, the inverse Hall–Petch relation was observed in the yield stress. The Young’s modulus for the quasi-2D system is higher than that for the three-dimensional (3D) system. [ABSTRACT FROM AUTHOR]

The microstructure and deformation/strengthening behavior of Cu/Ru (face-centered cubic (fcc)/hexagonal close-packed) and Cu/Cr (fcc/body-centered cubic) multilayered films with equal individual layer thickness h ranging from 0.8 to 200 nm were investigated. Based on systematic X-ray diffraction and transmission electron microscopy analyses, as h was varied, interface structure transitions from semi-coherent to fully coherent structures were observed in Cu/Ru rather than Cu/W. These structure transitions were found to play crucial roles in length-scale dependent strengthening mechanisms. Specifically, the multilayer strength derived for relatively large h ( ≥ 50 nm ) followed the classic Hall–Petch relation, while the load-bearing effect was proposed to be the dominant mechanism for relatively small h ( 50 nm ≥ h ≥ 4 nm ). More importantly, as h was further reduced to below 4 nm in Cu/Ru, a strengthening mechanism based on dislocation loop crossing several coherent interfaces was proposed and quantitatively evaluated, The number of layers needed to be crossed by the dislocation loop was found to determine the strength. [ABSTRACT FROM AUTHOR]

The mechanical implications of short-range ordering (SRO) and grain boundary (GB) segregation in the inverse Hall-Petch behavior of nanocrystalline CoCuFeNiPd high-entropy alloys (HEAs) were studied using hybrid Monte Carlo (MC)/Molecular Dynamics (MD) simulations. Results show that the presences of SRO and GB segregation inhibit the grain size softening (inverse Hall-Petch relation), leading to grain-size independence of the high flow stress. We find that the dislocation nucleation at triple junctions is suppressed by the SRO due to the increased stacking fault energy, which attenuates the dislocation activities. Furthermore, the strain localization at GBs is intensified by the GB segregation due to the increased GB energy, which facilitates the GB activities and glass-like deformation. The GB-governed plasticity associated with glass-like deformation leads to suppression of the inverse Hall-Petch effect. These findings may provide useful strategies for the design of high-performance HEAs by tuning the GB composition and SRO structure. [Display omitted] [ABSTRACT FROM AUTHOR]

The influence of the ratio between specimen thickness (t) and grain size (D) on the tensile and fatigue properties of plain and notched aluminium plate specimens has been studied. Grain sizes ranging from 66 μ m to 9.74 mm have been prepared and used in the tests. The observed decrease on tensile properties when D is comparable, or even exceeds, t can be accounted for by adding an additional negative exponential term to the Hall–Petch equation. An exponential decrease has also been found in endurance limits as grain size approaches plate thickness in plain specimens. In notched specimens , however, increasing grain size has two opposing effects, namely: a decrease in the plain endurance limit associated with the decrease in thickness relative to grain size and an increase in notched fatigue strengths brought about by a decrease in notch sensitivity. • Consider difficulties in assessment of fatigue strength of very small components. • Addresses whether use of bulk properties is adequate for very small components. • Exponential decrease in endurance as grain size approaches plate thickness. • Correlated by additional negative exponential term in the Hall–Petch equation. [ABSTRACT FROM AUTHOR]

The effects of hot-rolling on the secondary electron property evolution of activated Cu-2.7Be sheets were investigated in this study. The microstructures of both pristine and activated Cu-2.7Be alloys subjected to different hot-rolling temperatures were characterized. Their phase compositions were found to comprise an α matrix and eutectoid phases (α + γ). Using electron back-scattered diffraction, the initial average grain size (d 0) was found to vary from 6.45 μ m to 15.63 μ m as the hot-rolling temperature increased from 680 °C to 830 °C. Scanning electron microscopy and X-ray photoelectron spectroscopy studies of the activated Cu-2.7Be sheets revealed that the amount of BeO increased with a decrease in the average grain size, resulting in the enhancement of the peak secondary electron emission yield. Scanning Kelvin probe force microscopy measurements confirmed that the main contributor to the BeO enhancement was the α matrix, while transmission electron microscopy analysis showed that BeO formed preferentially from sub-grain boundaries, indicating the importance of grain boundaries for BeO formation. Moreover, the peak secondary electron yield (δ m) exhibited a linear relationship with d 0 −0.5 similar to the Hall-Petch relation. The results in this study will be important for enhancing the secondary electron yield of Cu-Be dynodes used in photomultiplier tubes and electron multipliers. • The peak secondary electron yield (SEY)of Cu-2.7Be alloys after activation are inversely proportional to the rolling temperature. • The lower the deformation temperature is, the more BeO content is after activation. • The matrix contributes more to the increase of SEY than eutectoid structure. [ABSTRACT FROM AUTHOR]

We present a combined experimental and finite element study on the deformation behavior of micron-sized polycrystalline gold particles. This study enables detailed insights into the underlying deformation mechanisms of the particles. Scanning electron microscope supported in situ uniaxial compression experiments of the single spherical polycrystalline gold particles were performed in the size range of 1 μm by using a custom built manipulation device. By testing a large number of particles stress–strain data and information on the particle morphology were obtained with statistical significance. The experimentally observed stress–strain behavior and the geometric shape of the stressed particles were found to be in excellent agreement with the elastic-perfectly plastic finite element model accounting for frictional effects at the contact interfaces. A significantly increased yield strength compared to bulk gold was found — grain size strengthening according to the Hall–Petch relation was identified as the main hardening mechanism. Hardness was found to vary with strain — an effect related to the altering geometric shape of the particles during compression. Comparison to a frictionless finite element model revealed the necessity of considering the effect of friction. These findings are not restricted to gold particles, but should be applicable to a wide range of elastic-perfectly plastic materials. [ABSTRACT FROM AUTHOR]

The mechanical behavior of face centered cubic metals is deeply affected when specimen dimensions decrease from a few millimeters to a few micrometers. At room temperature, a critical thickness ( t ) to grain size ( d ) ratio ( t / d ) c was previously highlighted, under which the softening of mechanical properties became very pronounced both in terms of Hall–Petch relation and work hardening mechanisms. In this work, new experimental results are provided concerning the influence of temperature on this size effect for copper, nickel and Ni–20 wt.%Cr, representative of a wide range of deformation mechanisms ( i.e. dislocation slip character). It is shown that multicrystalline samples ( t / d < ( t / d ) c ) are not deeply affected by an increase in temperature, independently of the planar or wavy character of dislocation glide. For pronounced wavy slip character metals, surface effects in polycrystals ( t / d > ( t / d ) c ) are not significant enough to reduce the gap between polycrystal and multicrystal mechanical behavior when the temperature increases. However, a transition from wavy slip to planar glide mechanisms induces a modification of the polycrystalline behavior which tends toward multicrystalline one with a moderate increase in temperature. This work demonstrates that surface effects and grain size influence can be successfully disassociated for the three studied materials using an analysis supported by the Kocks–Mecking formalism. All these results are supported by microscopic investigations of dislocation substructures and compared to numerical simulations using a strain gradient plasticity model. [ABSTRACT FROM AUTHOR]

We perform MD simulations of the nanoindentation on (001) and (111) surfaces of Ag–Ni multilayers with different modulation periods, and find that both the hardness and maximum force increase with the increase of modulation period, in agreement with the inverse Hall–Petch relation. A prismatic partial dislocation loop is observed in the Ni(111)/Ag(111) sample when the modulation period is relatively large. We also find that misfit dislocation network shows a square shape for the Ni(111)/Ag(111) interface, while a triangle shape for the Ni(001)/Ag(001) interface. The pyramidal defect zones are also observed in Ni(001)/Ag(001) sample, while the intersecting stacking faults are observed in Ni(111)/Ag(111) sample after dislocation traversing interface. The results offer insights into the nanoindentation behaviors in metallic multilayers, which should be important for clarifying strengthening mechanism in many other multilayers. [ABSTRACT FROM AUTHOR]

Electron beam melting (EBM®)-built Ti–6Al–4V has increasingly shown great potential for orthopedic implant and aerospace applications in recent years. The microstructure and mechanical properties of EBM-built Ti–6Al–4V have been systematically investigated in this work. Its microstructure consists of columnar prior β grains delineated by wavy grain boundary α and transformed α/β structures with both cellular colony and basket-weave morphology as well as numerous singular α bulges within the prior β grains. The β phase is found to form as discrete flat rods embedded in continuous α phase and its volume fraction is determined to be ∼3.6%. Moreover, α′ martensite was not observed as it has decomposed into α and β phases. In particular, the α/β interface was studied in detail combined transmission electron microscopy with atom probe tomography. Of note is that graded Ti–6Al–4V microstructure i.e. both prior β grain width and β phase interspacing continuously increase with the build height, was observed, which mainly arises from the decreasing cooling rate. Furthermore, an increasingly pronounced strain hardening effect was also observed as the previously built layers undergo a longer annealing compared to the subsequent layers. As a result, graded mechanical properties of Ti–6Al–4V with degraded microhardness and tensile properties were found. A good agreement with the Hall–Petch relation indicates that the graded property takes place mainly due to the graded microstructure. In addition, this graded microstructure and mechanical properties were discussed based on a quantitative characterization. [ABSTRACT FROM AUTHOR]

In this work, we investigate the mechanical behavior of multi-phase steels using a continuum dislocation dynamic model (CDD) coupled with a viscoplastic self-consistent (VPSC) model that accounts for both the effect of dislocations evolution inside the grain as well as grain–grain interactions. Because the conventional viscoplasticity theory does not capture the grain size effect, we introduce an intrinsic length scale within the concepts of geometrically necessary dislocations (GND) by means of the Nye's dislocation tensor. The effect of the GND density is implemented into the model for the mean free path of dislocations and is shown to contribute to strain hardening. As a validation of this multiscale model, we investigate the mechanical behavior of various dual phase steels. The stress-strain response obtained from this approach is compared to experimental data found in the literature and reveal good agreement between experimental results and predictions. The model also predicts the evolution of dislocation densities in each phase and suggests the connection between underlying deformation mechanisms and macroscopic material hardening. The relation between flow stress and grain size is also investigated. The model predictions follow the Hall–Petch relation of strength versus grain size for grains larger than one micron meter but deviates from this relation for grains in the order of tens of nanometers. [ABSTRACT FROM AUTHOR]

Metal composites have attracted great interest for the combination of high strength and high electrical conductivity. We take the Cu–3 wt.% Cr composite as a model system to reveal the structure–property relations in a large range of drawing strains ( η ), and propose the strengthening mechanism during cold drawing. At small and moderate drawing strains, the incoherent and semi-coherent interfaces are effective barriers for dislocation motion and pile-up. The mechanical behaviors are described well by the Hall–Petch relation. The work hardening at η < 4.5 is the direct consequence of the grain refinement and the increasing density of phase boundaries, which can be well modelled by a linear rule of mixture. Deviation is observed at η > 4.5, when the strength gradually reaches a plateau. Microstructural analysis reveals that further refinement can hardly occur at high drawing strains due to the dynamic recovery process. Another characteristic of the turning point is the structural homogenization, i.e. the mechanical alloy induced inter-mixing and the full coherency. The transition of the interface coherency is clearly identified by both electron diffraction and high resolution transmission electron microscopy. The structural homogenization at the interface may alleviate the interfacial blockage to the dislocation slipping, indicating the existence of an interfacial strength limit. The linear rule of mixture is modified to account for this effect at η > 4.5. The complete mechanical behavior of the Cu–3 wt.% Cr composite is captured by a piece-wise function. [ABSTRACT FROM AUTHOR]

The hardness–composition–grain size relationships of cemented WC–Co with WC particle sizes in the submicrometer range have been investigated. It is found that the Hall–Petch relations for individual WC and Co phases derived from micrometer-sized WC–Co are still applicable to submicrometer-sized WC–Co, suggesting that the deformation mechanism remains the same when WC particle sizes change from micrometers to submicrometers. However, the measured hardness of submicrometer-sized WC–Co is lower than that predicted using the equation derived from micrometer-sized WC–Co and the principle that the applied loads are transmitted through the carbide skeleton and the continuous binder phase (Lee and Gurland (1978) [7] ). It is proposed that this discrepancy can be addressed by the reduced continuity of WC when WC particle sizes decrease from micrometers to submicrometers at a given WC volume fraction. [ABSTRACT FROM AUTHOR]

A commercially pure (CP) Titanium alloy classified as Grade 1, was processed by Equal Channel Angular Pressing (ECAP) up to 4 passes in the temperature range of 450–150 °C. The resulting microstructures were observed by Electron Back-Scattered Diffraction, revealing a bimodal grain size distribution consisting of small recrystallized grains of submicrometer size, with an average value of 0.3 µm, and elongated bands of 1.4 µm with different degree of substructure. Additionally the fraction of restored and deformed grains were evaluated as a function of processing temperature following an internal grain misorientation criterion, leading to an overall fraction of recrystallized grains between 40% and 20% in samples ECAPed at 450 and 150 °C, respectively. The strengthening contributions of the grain size, equivalent oxygen content (O eq ) and Low Angle Grain Boundaries (LAGBs) to the yield stress were identified by the Hall Petch and Taylor equations. The strengthening coefficient k of the Hall–Petch relation was approximately 5 MPa mm −1/2 , with an increment of 0.44 MPa mm −1/2 per 0.1 O eq .-%, while the LAGB strengthening contribution was responsible approximately by half of the experimental yield stress values measured. [ABSTRACT FROM AUTHOR]

Al5086 alloy specimens were irradiated in a vacuum ~10 −6 mbar with 2 MeV proton beam (200 nA) for different exposure times in the range 10–50 min (dose range: 0.35×10 15 –1.75×10 15 p/cm 2 ). Surface hardness was found to increases on irradiation for 10 min, and later on it decreases with the increase in exposure time. The electrical resistivity of the specimens measured by four-point probe technique was also found to follow the same pattern. The observed behavior has been explained in terms of relative contribution of two processes, namely defects formation and heat generation due to proton–material interaction. Structural characterization of the specimens was done by X-ray diffraction technique. Both surface hardness as well as electrical resistivity of un-irradiated and irradiated specimens is found to decrease with the increase in X-ray crystallite size. Moreover, the surface hardness follows the Hall–Petch relation, which indicates that crystallite boundaries progressively impede the motion of dislocations as the crystallite size gets smaller. [ABSTRACT FROM AUTHOR]

The fabrication process of a powder metallurgy (P/M) tantalum product with full density and fine microstructure was developed by using cold and hot isostatic pressing techniques. In order to increase the compact density and make the uniform density distribution, cold isostatic pressing (CIPing) of tantalum powders was conducted. Prior to hot isostatic pressing (HIPing), the CIPed billet was encapsulated and degassed to remove the contaminants in the container. After degassing, HIPing was performed twice and full densification of the tantalum powders was accomplished, regardless of powder size. The effect of processing conditions on the microstructure and mechanical properties of P/M tantalum billets was investigated. As the number of processing steps and temperature increased, the grain size of HIPed tantalum billets increased. Moreover, contrary to the Hall–Petch relation, the mechanical strength was increased in spite of increasing the grain size. This is because the oxygen content of the billets increased with rising in temperature and the number of processing steps. Therefore, in case of tantalum, it is found that the mechanical properties of tantalum may be highly influenced by the amount of interstitial elements, especially oxygen, rather than microstructural properties. [ABSTRACT FROM AUTHOR]