Strain incompatibility and its influence on grain coarsening during cyclic deformation of ARB copper

It is well known that the characteristic length scale in ultra-fine grained and nanocrystalline metals has a significant effect on the mechanical behaviour. The inhibited ability to accommodate imposed strain with conventional dislocation mechanism has led to the activation of unconventional deformation mechanisms. For one, grain coarsening at shear bands has been observed to occur within metals with sub-micron grain size upon cyclic deformation. Such grain coarsening is often linked to the observed cyclic softening behaviour. The purpose of this study was to investigate the relationship between strain localisation associated with shear banding and the observed deformation-induced grain coarsening in ultra-fine grained metals. The investigation was carried out using ultra-fine grained, oxygen-free high conductivity copper processed by accumulative roll-bonding. A close relationship between strain localisation and deformation-induced grain coarsening was revealed. As strain localisation is not only found at shear bands, but also at other places whereby heterogeneous microstructure or geometric discontinuity is present, hence the present study bears a general significance. Such strain localisation sites may also include a hard constituent embedded in a relatively ductile matrix, micro-crack tips and artificial notches. The stress concentration at these sites provides a high input of strain energy for grain boundary motion leading to grain coarsening. Furthermore, when the grain size is very small, the stress gradient leading away from the stress concentration sites is also believed to increase the driving force for grain boundary migration within the affected regions.     

Grain size effects on microstructural stability and creep behaviour of nanotwinned Ni free-standing foils at room temperature

Creep tests were performed on the high stacking fault energy (SFE) nanotwinned (NT) Ni free-standing foils with nearly the same twin thickness at room temperature (RT) to investigate the effects of grain size and loading rate on their microstructural stability and creep behaviour. The grain growth mediated by the twinning/detwinning mechanism at low applied stresses (<800 MPa) and grain refinement via the detwinning mechanism at high applied stresses (>800 MPa) were uncovered in the present NT-Ni foils during RT creep, both of which are attributed to the interactions between dislocations and boundaries. It appears that a higher initial dislocation density leads to a faster primary creep strain rate and a slower steady-state creep strain rate. Unlike the non-twinned metals in which grain growth often enhances the creep strain rate, the twinning/detwinning-mediated grain growth process unexpectedly lowers the steady-state creep strain rate, whereas the detwinning-mediated grain refinement process accelerates the creep strain rate in the studied NT-Ni foils. A modified phase-mixture model combined with Arrhenius laws is put forward to predict the scaling behaviour between the creep strain rate and the applied stress, which also predicts the transition from grain growth-reduced to grain refinement-enhanced steady-state creep strain rate at a critical applied stress. Our findings not only provide deeper insights into the grain size effect on the mechanical behaviour of nanostructured metals with high SFE, but also benefit the microstructure sensitive design of NT metallic materials.     

High-rate deformation of nanocrystalline iron and copper

Stress–strain curves are recorded during a high-speed impact and slow loading for nanocrystalline and coarse-grained iron and copper. The strain-rate sensitivity is determined as a function of the grain size and the strain. It is shown that the well-known difference between the variations of the strain-rate sensitivity of the yield strength with the grain size in fcc and bcc metals can be extended to other strain dependences: the strain-rate sensitivity of flow stresses in iron decreases with increasing strain, and that in copper increases. This difference also manifests itself in different slopes of the dependence of the strain-rate sensitivity on the grain size when the strain changes.     

Thermal stability of grain boundaries in nanocrystalline Zn studied by positron lifetime spectroscopy

Nanocrystalline Zn prepared by compacting nanoparticles with mean grain size about 55 nm at 15 MPa has been studied by positron lifetime spectroscopy. For the bulk Zn sample, the vacancy defect is annealed out at about 350 °C, but for the nanocrystalline Zn sample, the vacancy cluster in grain boundaries is quite difficult to be annealed out even at very high temperature (410 °C). In the grain boundaries of nanocrystalline Zn, the small free volume defect (not larger than divacancy) is dominant according to the high relative intensity for the short positron lifetime (τ₁). The oxide (ZnO) inside the grain boundaries has been found having an effect to hinder the decrease of average positron lifetime (τ_av), which probably indicates that the oxide stabilizes the microstructure of the grain boundaries. This stabilization is very important for the nanocrystalline materials using as radiation resistant materials.     

Influence of the grain size and structural state of grain boundaries on the parameter of low-temperature and high-rate superplasticity of nanocrystalline and microcrystalline alloys

A model has been proposed for calculating the grain size optimum for the deformation of nanocrystalline and microcrystalline materials under superplasticity conditions. The model is based on the concepts of the theory of nonequilibrium grain boundaries in metals. It has been demonstrated that the optimum grain size d _opt can be calculated as the size at which a high level of nonequilibrium of grain boundaries is combined with a high intensity of the accommodation of grain boundary sliding. The dependences of the quantity d _opt on the rate and temperature of the strain and the thermodynamic parameters of the material have been derived. The results obtained have been compared with the experimental data on the superplasticity of nanocrystalline and microcrystalline aluminum and magnesium alloys.     

Competitive effects of metal dissolution and passivation modulated by surface structure: An AFM and EBSD study of the corrosion of alloy 22

Electron backscatter diffraction (EBSD) and atomic force microscopy (AFM) are used to correlate crystallographic grain orientation with corrosion rates of polycrystalline alloy 22 following immersion in 1 and 3 molar (M) hydrochloric acid. For each acid concentration, relative corrosion rates are simultaneously characterized for approximately 50 unique grain orientations. The results demonstrate that the corrosion rate anisotropies are markedly different in the two acid concentrations. In very aggressive acidic environments (3M HCl), where electrochemical impedance spectroscopy and spectroscopic ellipsometry data demonstrate that the passive oxide film of alloy 22 is completely dissolved, alloy dissolution rates scale inversely with the average coordination number of surface atoms for a given grain orientation, where highly correlated surfaces dissolve the slowest. Thus, similar to simple metallic systems, the corrosion rates scale with the surface plane-normal crystallographic orientations as {1 1 1} < {1 0 0} < {1 1 0}. Less intuitively, in milder corrosive environments (1M HCl), where the passive film of the alloy is still intact, the dissolution does not scale inversely with surface atomic density. Rather, corrosion rates scale with crystallographic orientations as {1 1 1} < {1 1 0} < {1 0 0}. This is attributed to the fact that facets most susceptible to corrosion (least coordinated) are also the most able to form protective oxides, so that the dissolution anisotropy is a result of the delicate balance between metal dissolution and oxide growth.     

Magnetic characteristics of HPT deformed soft-magnetic materials

Five sets of soft-magnetic metals, such as pure Fe, pure Ni, Fe-3 wt% Si, Fe-6.5 wt% Si and Fe-17 wt% Co, were subjected to high pressure torsion (HPT) up to strain levels where a saturation of the microstructural refinement is observed. Following HPT at 77, 293 and 723 K, transmission electron microscopy (TEM) was used to study the grain size and grain shape of the severely deformed metals. The coercivity H_C was characterized in a magnetic closed system by using ring shaped samples. Magnetic measurements obtained on ring shaped samples give a much higher accuracy for determining the coercivity. Depending on the material the mean microstructural sizes in the steady state vary from 300 nm at 723 K to 30 nm at 77 K, respectively. The coercivity of the deformed materials first increases with decrease in grain size. Once the crystallite size is far below 100 nm the coercivity shows a strong decrease.     

Investigations and computer simulations of the intergrain diffusion in submicro-and nanocrystalline metals

Investigations of the features of the diffusion along grain boundaries and of the diffusion-controlled processes in submicroscopic and nanocrystalline materials produced by methods of intense plastic deformation are reviewed. To determine the parameters of the diffusion along grain boundaries and triple junctions in metals, which are independent of the results of processing of diffusion experiments based on diffusion models, results of a molecular dynamic investigation of the diffusion in nanocrystalline copper considered as an example are presented. Comparison of the features of grain boundary diffusion in submicroscopic and nanocrystalline materials produced by various methods is performed. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 47–60, April, 2008.     

Mechanical properties of nanocrystalline copper under thermal load

The material properties of nanocrystallines are known to generally have a strong dependence on their nanoscale morphology, such as the grain size. The Hall-Petch effect states that the mechanical strength of nanocrystalline materials can vary substantially for a wide range of grain sizes; this is attributed to the competition between intergranular and intragranular deformations. We employed classical molecular dynamics simulations to investigate the morphology-dependent mechanical properties of nanocrystalline copper. The degradation of material properties under thermal load was investigated during fast strain rate deformation, particularly for the grain size. Our simulation results showed that the thermal load on the nanocrystalline materials alters the grain-size behavior of the mechanical properties.     

Effect of grain size dispersion on the strength and plasticity of nanocrystalline metals

The effect of the dispersion of the grain size distribution on the yield stress, ultimate stress, and uniform strain of nanocrystalline metals is analyzed theoretically. It is shown that, as the grain size dispersion increases, the degree of grain boundary hardening (Hall-Petch effect) of nanocrystalline materials decreases, the onset of the grain boundary softening (inverse Hall-Petch effect) shifts to smaller nanograin sizes, and the uniform strain at which necking occurs increases.     

Investigating the magnetic properties of soft magnetic composites based on mechanically alloyed nanocrystalline Fe-5 wt% Ni powders

In this work, the soft magnetic composites (SMCs) of the nanocrystalline Fe-5 wt% Ni powders coated with phenolic resin were studied. The nanocrystalline powders with an average diameter of 10 nm were obtained by mechanical alloying up to 96 h milling in a high-energy planetary ball mill. The microstructure and magnetic properties of the milled powders were characterized by X-ray diffraction, energy dispersive X-ray spectroscopy and a vibrating sample magnetometer. The results of X-ray diffraction showed that the bcc Fe(Ni) solid solution is formed after 24 h milling. Magnetic measurements indicated that the 96 h milled powders with a steady-state grain size of 10 nm have the highest saturation magnetization and the lowest coercivity. The SMCs based on nanocrystalline powders showed higher electrical resistivity and magnetic permeability up to 1 MHz, as compared with the pure iron-based composites. Besides, the nanocrystalline-based SMCs exhibited higher relaxation frequency and a significantly lower loss factor up to 1 MHz.     

Low temperature resistivity minimum and its correlation with magnetoresistance in La0.67Ba0.33MnO3 nanomanganites

A systematic investigation of structural, magnetic and electrical properties of nanocrystalline La_0.67Ba_0.33MnO₃ materials, prepared by citrate gel method has been undertaken. The temperature-dependant low-temperature resistivity in ferromagnetic metallic (∼50 K) phase shows upturn behavior and is suppressed with applied magnetic field. The experimental data (<75 K) can be best fitted in the frame work of Kondo-like spin-dependant scattering, electron-electron and electron-phonon interactions. It has been found that upturn behavior may be attributed to weak spin disorder scattering including both spin polarization and grain boundary tunneling effects, which are the characteristic features of extrinsic magnetoresistance behavior, generally found in nanocrystalline manganites. The variation of electrical resistivity with temperature in the high temperature ferromagnetic metallic part of electrical resistivity (75K<T<T_P) has been fitted with grain/domain boundary, electron-electron and magnon scattering mechanisms, while the insulating region (T>T_P) of resistivity data has been explained based on adiabatic small polaron hopping mechanism.     

Nanoindentation study of size effect and loading rate effect on mechanical properties of a thin film metallic glass Cu49.3Zr50.7

A binary metallic glass (MG) Cu_49.3Zr_50.7 in the form of thin film was successfully grown on a Si (1 0 0) substrate by magnetron sputtering. The mechanical properties, specifically, hardness and modulus at various peak loads and loading rates were characterized through instrumented nanoindentation. Unlike other metallic glasses showing an indentation size effect (ISE), the composition of this study does not have an ISE, which is phenomenologically the result of the negligible length scale according to the strain gradient plasticity model. The proportional specimen resistance model is applicable to the load-displacement behaviors and suggests that the frictional effect is too small to contribute to the ISE. The occurrence of plasticity depends on loading rates and can be delayed so that the displacement during the load holding segment increases logarithmically. In addition, the hardness and modulus are both dependent on the loading rates as well, i.e., they increase as the loading rate increases up to 0.1 mN/s and then hold constant, which is independent of creep time (≤100 s). These loading-rate-dependent behaviors are interpreted as the result of viscoelastic effect rather than free volume kinetics.     

Optical properties of hydroxyapatite obtained by mechanical alloying

Calcium phosphate based bioceramics, mainly in the form of hydroxyapatite (HA), have been in use in medicine and dentistry for the last 20 years. Applications include coatings of orthopaedic and dental implants, alveolar ridge augmentation, maxillofacial surgery, otolaryngology, and scaffolds for bone growth and as powders in total hip and knee surgery. These materials exhibit several problems of handling and fabrication, which can be overcome by mixing with a suitable binder. In this paper, mechanical alloying has been used successfully to produce nanocrystalline powders of HA using five different experimental procedures. The milled HA were studied by X-ray powder diffraction, infrared and Raman scattering spectroscopy. For four different procedures, HA was obtained after a couple of hours of milling (on an average, 20 h of milling depending on the reaction procedure). The XRD patterns indicate that the grain size is within the range of 29-103 nm. This milling process, used to produce HA, presents the advantage that melting is not necessary and the powder obtained is nanocrystalline with extraordinary mechanical properties. The material can be compacted and transformed in solid ceramic samples. The high efficiency of the process opens a way to produce commercial amount of nanocrystalline HA. Due to the nanocrystalline character of this powder, their mechanical properties have changed and for this reason a pressure of 1 GPa is enough to shape the sample into any geometry.     

Investigation of microstructure thermal evolution in nanocrystalline Cu

The microstructure of nanocrystalline Cu prepared by compacting nanoparticles (50-60 nm in diameter) under high pressures has been studied by means of positron lifetime spectroscopy and X-ray diffraction. These nanoparticles were produced by two different methods. We found that there are order regions interior to the grains and disorder regions at the grain boundaries with a wide distribution of interatomic distances. The mean grain sizes of the nanocrystalline Cu samples decrease after being annealed at 900 °C and increase during aging at 180 °C, which are observed by X-ray diffraction, revealing that the atoms exchange between the two regions. The positron lifetime results clearly indicate that the vacancy clusters formed in the annealing process are unstable and decomposed at the aging time below 6 hours. In addition, the partially oxidized surfaces of the nanoparticles hinder grain growth when the samples age at 180 °C, and the vacancy clusters inside the disorder regions, which are related to Cu₂O, need longer aging time to decompose. The disorder regions remain after the heat treatment in this work, in spite of the grain growth, which will be good for the samples keeping the properties of nanocrystalline material.     

The linear thermal expansion of bulk nanocrystalline Al and SS304 at low temperature

The linear thermal expansions (LTE) of bulk nanocrystalline (NC) Al, 304 stainless steel (SS304) and their conventional coarse-grained poly-crystalline counterparts (CCPC) were studied by the strain gage method in four directions within rolling plane (for bulk NC SS304 and Al) and one direction (for their CCPC) from liquid nitrogen temperature to 300 K. LTE of bulk NC Al and SS304 in four directions were equal to or less than those of their CCPC. This result was different from the fact that the smaller thermal expansions of many other bulk nanocrystalline materials were larger than those of their CCPC. We had to conclude that the less linear thermal expansions of bulk NC SS304 and Al were attributed to their larger defects at grain boundaries and residual strain. However, the larger linear thermal expansions of many other NC materials resulted from two factors.     

Atomistic simulation of the stacking fault energy and grain shape on strain hardening behaviours of FCC nanocrystalline metals

ABSTRACT

Ultra-fine grained copper with nanotwins is found to be both strong and ductile. It is expected that nanocrystalline metals with lamella grains will have strain hardening behaviour. The main unsolved issues on strain hardening behaviour of nanocrystalline metals include the effect of stacking fault energy, grain shape, temperature, strain rate, second phase particles, alloy elements, etc. Strain hardening makes strong nanocrystalline metals ductile. The stacking fault energy effects on the strain hardening behaviour are studied by molecular dynamics simulation to investigate the uniaxial tensile deformation of the layer-grained and equiaxed models for metallic materials at 300?K. The results show that the strain hardening is observed during the plastic deformation of the layer-grained models, while strain softening is found in the equiaxed models. The strain hardening index values of the layer-grained models decrease with the decrease of stacking fault energy, which is attributed to the distinct stacking fault width and dislocation density. Forest dislocations are observed in the layer-grained models due to the high dislocation density. The formation of sessile dislocations, such as Lomer–Cottrell dislocation locks and stair-rod dislocations, causes the strain hardening behaviour. The dislocation density in layer-grained models is higher than that in the equiaxed models. Grain morphology affects dislocation density by influencing the dislocation motion distance in grain interior.     

Modeling the deformation behavior of nanocrystalline alloy with hierarchical microstructures

A mechanism-based plasticity model based on dislocation theory is developed to describe the mechanical behavior of the hierarchical nanocrystalline alloys. The stress–strain relationship is derived by invoking the impeding effect of the intra-granular solute clusters and the inter-granular nanostructures on the dislocation movements along the sliding path. We found that the interaction between dislocations and the hierarchical microstructures contributes to the strain hardening property and greatly influence the ductility of nanocrystalline metals. The analysis indicates that the proposed model can successfully describe the enhanced strength of the nanocrystalline hierarchical alloy. Moreover, the strain hardening rate is sensitive to the volume fraction of the hierarchical microstructures. The present model provides a new perspective to design the microstructures for optimizing the mechanical properties in nanostructural metals.     

Effects of peak current density on the mechanical properties of nanocrystalline Ni-Co alloys produced by pulse electrodeposition

Cobalt content, grain size, microhardness and tensile strength of nanocrystalline Ni-Co deposits produced from a solution containing saccharin and cobalt sulfate at constant electrodeposition conditions (pulse on-time T_on at 1 ms and pulse off-time T_off at 15 ms) but varying the peak current density J_p were investigated. It is found that an increase in J_p makes the deposit Co content lower, colony-like morphology more obvious, grain size smaller, and hardness and tensile strength higher. All of the facts are believed to result from the higher overpotential and nucleation rates caused by the J_p increase. But its further increase could lead to reduction in the hardness and tensile strength. Peak current densities in the range of 100-120 A dm⁻² are recommended for the preparation of nanostructured Ni-Co alloy deposits with grain sizes in the range of 15-20 nm, containing 7-8% Co, possessing hardness of 590-600 kg mm⁻² and tensile strength of 1180-1200 MPa—significantly higher than the strength of pure nickel deposit which is produced by the similar method and gets similar grain size.