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To gain fundamental insight into the relationship between length scales and mechanical behaviour, Ni-Fe multilayered materials with a 5-μm-layer thickness and a modulated grain size distribution have been synthesized by pulsed electrodeposition. Microstructural studies by SEM and TEM reveal the alternating growth of well-defined layers with either nano (d = 16 nm) or coarse grains (d ≥ 500 nm). Room temperature tensile tests have been performed to investigate the mechanical response and understand the underlying deformation mechanisms. Tensile test results and fractographic studies demonstrate that the overall room temperature mechanical behaviour of the multilayered material, i.e. strength and ductility, is governed primarily by the layers containing nanocrystalline grains. The measured properties have been discussed in the context of modulated grain structure of the multilayered sample and contribution of each grain size regime to the overall strength and ductility.

The present study is an accurate estimate of the milling parameters in order to maximize the energy transferred to the synthesized nanopowders. The maximum energy represents the leading stage for minimizing the synthesis time of nanocomposites during high energy ball milling. Accordingly, 271 dataset were collected from the literature and then by a modeling algorithm called gene expression programing (GEP), a mathematical relation between the grain size and milling parameters is developed. Afterwards by an optimization algorithm called Artificial Bee Colony (ABC), the milling parameters including amount of reinforcement, type and amount of process control agent (PCA), type of mill, type of vial, type of ball, vial spinning rate, BPR, milling atmosphere and milling time were optimized in order to achieve minimum grain size. Minimizing the mean grain size is equal to maximize the energy transferred to the nanopowders during high energy ball milling. Experiments were performed at the optimized parameters to proof the validity of the analysis. Given the broad range of the parameters used, it was found that our analysis and model is fully functional to accurately estimate the optimal conditions for ball milling experiments which shows the potential application of these calculations and analysis in materials science and engineering.

The mechanism of nanostructure formation during cryogenic and room-temperature milling of Cu and Cu–3wt%Zn was investigated using X-ray diffraction line profile analysis. For that, the whole powder pattern modeling approach (WPPM) was used to analyze the evolution of microstructural features including coherently scattering domain size, dislocation density, and density of planar faults. It was found that for all sets of experiments, structural decomposition is the dominant mechanism of nanostructure formation during cryomilling. During subsequent RT-milling, grain refinement still occurs by structural decomposition for pure copper. On the other hand, discontinuous dynamic recrystallization is responsible for nanostructure formation during RT-milling of Cu–3wt%Zn. This is attributed to lower stacking-fault energy of Cu–3wt%Zn compared to pure copper. Finally, room temperature milling reveals the occurrence of a detwinning phenomenon.

To provide insight into the influence of the length scale on the kinetics of phase evolution during severe plastic deformation, we studied the microstructure evolution of cryomilled Al and Ti mixture, which is further subjected to high-pressure torsion (HPT). The cryomilled microstructure consisted of elemental Al and Ti, and the subsequent HPT deformation at ambient temperature led to the solid state formation of Al-rich intermetallics. X-ray diffraction peaks originating from TiAl2 and TiAl3 were observed after one revolution of HPT, suggesting a shear strain-assisted formation of the intermetallics. A high resolution transmission electron microscope confirmed the formation of TiAl2 following HPT for one revolution. Further HPT straining led to microstructure refinement and a mixing of the Ti and Al, as well as of any phases formed initially. The solid state formation of the intermetallics and the overall evolution of the microstructure are discussed based on the generation of a high density of lattice defects that evolve under the strain conditions present during HPT.

An Al2O3-TiB2 nanocomposite was successfully synthesized by the high energy ball milling of Al, B2O3 and TiO2. The structures of the powdered particles formed at different milling times were evaluated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Thermodynamic calculations showed that the composite formed in two steps via highly exothermic mechanically induced self-sustaining reactions (MSRs). The composite started to form at milling times of 9–10 h but the reaction was not complete. The remaining starting materials were consumed by increasing the milling time to 15 h. The XRD patterns of the annealed powders showed that aluminum borate is one of the intermediate products and that it is consumed at higher temperatures. Heat treatment of the 6-h milled sample at 1100°C led to a complete formation of the composite. Increasing the milling time to 15 h led to a refining of the crystallite sizes. A nanocomposite powder with a mean crystallite size of 35–40 nm was obtained after milling for 15 h.

The thermal stability of nanocrystalline cartridge brass (Cu–30 at.% Zn) and brass–Zr alloys were investigated. The alloys were produced by cryogenic ball milling and subsequently heat treated to a maximum temperature of 800 °C. The grain size of pure brass was found to be relatively stable in comparison to pure copper, and a high hardness was retained up to 600 °C. When 1 at.% zirconium was alloyed with the brass, the grain size was stabilized near 100 nm even at 800 °C. At the highest temperature, hardness was retained above 2.5 GPa for 1 and 5 at.% zirconium alloys, but the pure brass softened significantly. The stabilization is believed to be dominated by Zn–Zr interactions as a second phase of these two was observed in X-ray diffraction and transmission electron microscopy. Thermodynamic modeling indicates a zero grain boundary energy may be achieved depending on the mixing enthalpy value used (i.e., calculated vs. experimental) under ideal conditions, but microstructural features such as twinning and second phase particles are thought to be the dominant stabilization mechanism. Zr worked well in stabilizing the brass in the nanocrystalline state to nearly 90 % of its melting temperature.

In recent years advanced high strength steels (AHSS) received increased interest for light structures with improved performance, but they are often sensitive to edge cracking during sheet metal forming. In this study mechanically sheared edges were characterized for three dual phase steels (DP600, DP780 and DP980), sheared with three die clearances (5%t, 10%t, 15%t) and along rolling and transverse directions. Microstructures of the materials were provided first, and then the sheared edges were examined by optical microscopy and scanning electron microscopy that reveal the morphology and random feature of the sheared edges. A factorial analysis was performed to reveal the general trends of the processing parameters on four edge zones. A new strain measurement method was used for characterizing strain distribution in the sheared region, which shows the peak strain to be higher than 3. The strain quickly decreases from sheared edge to interior, leaving a shear-affected zone of about 500 μm or 31% of the thickness. The fracture processes and involved mechanisms were discussed.

Nanocrystalline Cu–30% Zn samples were produced by high energy ball milling at 77 K and room temperature. Cryomilled flakes were further processed by ultrahigh strain high pressure torsion (HPT) or room temperature milling to produce bulk artifact-free samples. Deformation-induced grain growth and a reduction in twin probability were observed in HPT consolidated samples. Investigations of the mechanical properties by hardness measurements and tensile tests revealed that at small grain sizes of less than ∼35 nm Cu–30% Zn deviates from the classical Hall–Petch relation and the strength of nanocrsytalline Cu–30% Zn is comparable with that of nanocrystalline pure copper. High resolution transmission electron microscopy studies show a high density of finely spaced deformation nanotwins, formed due to the low stacking fault energy of 14 mJ m –2 and low temperature severe plastic deformation. Possible softening mechanisms proposed in the literature for nanotwin copper are addressed and the twin-related softening behavior in nanotwinned Cu is extended to the Cu–30% Zn alloy based on detwinning mechanisms.

A series of Cu– x Nb ( x = 1–15 at.% 2 ) alloys have been investigated to study the metastable solid solubility extension of Nb in Cu by mechanical alloying. Analysis of X-ray diffraction and Gibbs free energy change confirmed that 7.5% of Nb was metastably dissolved in Cu after 8 h of milling at room temperature although Cu–Nb is a system with positive heat of mixing. The solid solubility could be extended up to 10% after enhancing milling duration to 16 h. Detailed thermodynamic analysis revealed that the additional energy stored during mechanical alloying could overcome the required energy barrier as per Miedema's model for the formation of disordered solid solution. The extended solid solubility has been explained along with the other possible mechanisms. Extensive annealing experiments and structural investigation revealed that the supersaturated solid solution is completely stable up to 400 °C. The matrix grains were stabilized and retained their size, ∼25 nm, even after annealing at 600 °C. Microhardness measurement and grain size analysis show that the dissolution of Nb in Cu has a larger strengthening effect than that of free Nb in the compositions.

The effect of low-temperature on the active deformation mechanism is studied in pure copper. For this purpose, cryogenic wire drawing at liquid nitrogen temperature (77 K) was performed using molybdenum disulfide lubrication. Microstructural investigation and texture analysis revealed severe twin formation in the cryogenically drawn copper, with a broad twin size distribution. The spacing of the observed deformation twins ranges from below 100 nm, as reported in previous investigations, up to several micrometers. The extent of twin formation, which is significantly higher when compared to other cryo-deformation techniques, is discussed with respect to the state of stress and the texture evolution during wire drawing. � 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Pure copper and Cu–12.1 at.%Al–4.1 at.%Zn alloy were subjected to rolling in liquid nitrogen. TEM studies showed that dynamic recovery during the deformation process was effectively suppressed and hence microstructures with dislocation substructure and deformation twins were formed. Mechanical properties were assessed via microtensile testing that shows improved yield strength, 520 ± 20 MPa, and ductility, 22%, in the case of pure copper. Alloying with Al and Zn results in reduction in stacking fault energy (SFE) which can contribute to enhanced strength and good ductility. Physical activation volume obtained via stress relaxation tests is 26 b 3 , and 8 b 3 for pure copper, and Cu–12.1 at.%Al–4.1 at.%Zn, respectively. The effect of SFE on work hardening rate of samples is discussed. Although twinning is observed in the alloy, it is concluded that network dislocation strengthening plays the major role in determining the mechanical properties.

Twinning and dislocation slip are two major and competing modes of plastic deformation in metals and alloys. In addition to controlling the dislocation substructure in coarse grained materials, stacking fault energy (SFE) also affects the propensity to form deformation twins. However, the influence of SFE has not been fully explored in nanocrystalline materials. Here the role of SFE in deformation twinning and work hardening was systematically studied in bulk artifact-free, nanocrystalline (nc) Cu (SFE 55 mJ m −2 ), and a nc Cu–12.1 at.% Al–4.1 at.% Zn alloy (SFE 7 mJ m −2 ). The nc Cu (23 nm) and nc Cu alloy (22 nm) were synthesized using in situ consolidation during cryo and room temperature milling. Both materials showed ultra-high tensile strength, significant strain hardening, and good ductility. The nc Cu alloy exhibits a higher yield strength and lower uniform elongation (1067 ± 20 MPa, 6.5%) than that of nc Cu (790 ± 12 MPa, 14%). The SFE variation played a significant role in strengthening the nc Cu alloy. High resolution transmission electron microscopy analyses revealed that the low SFE of the nc Cu alloy alters the deformation mechanism from a dislocation-controlled deformation, which allows for the higher strain hardening observed in the nc Cu, to a twin-controlled deformation.

Copper alloys with different amounts of zinc were synthesized via high energy ball milling at liquid nitrogen and room temperature. Bulk samples were produced in situ by controlling the milling temperature. It is shown that temperature plays an important role in formation of artifact-free consolidated samples via its effect on defect formation and annihilation during the milling process. The mechanical behavior of Cu–Zn nanocrystalline alloys was examined using Vickers microhardness and tensile tests. The nanostructure of the alloys was investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The hardness results of processed alloys vary as a function of the alloying elements. Considering typical low ductility of nanocrystalline materials, the improved ductility with the high strength observed in these alloys suggests that they are artifact-free and may have several deformation mechanisms, which may include dislocation activity and nano-twinning.

High energy ball milling was performed on a mixture of titanium and aluminum elemental powders with a composition of Ti -48(at.%) Al . Stearic acid was added to this powder mixture as a process control agent (PCA) to study its effect on the microstructure evolution and crystallite size of the milled powder after various milling times. Phase compositions and morphology of the milled powders were evaluated using X-ray diffraction and scanning electron microscopy. Crystallite sizes of milled powders were determined by Cauchy-Gaussian approach using XRD profiles. It was shown that addition of 1wt.% of stearic acid not only minimizes the adhesion of milling product to the vial and balls, but also reduces its crystallite sizes. It has also a marked effect on the morphology of the final product.

1 Department of Materials Science and Engineering, North Carolina State University, 911 Partners Way, Engineering Building I, Raleigh, NC 27695-7907, USA 2Department of Materials Science and Engineering, University of North Texas, Denton, TX 76203, USA 3Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616, USA 4Leibniz Institute for Solid State and Materials Research Dresden, P.O. Box 27 01 16, 01171 Dresden, Germany

1Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616, USA 2Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695-7907, USA 3Department of Materials Science and Engineering, University of North Texas, Denton, TX 76203, USA 4Department of Metals and Materials Engineering, University of British Columbia, Vancouver, BC, Canada V6T 1Z4