Nickel-carbon nanocomposites: Synthesis, structural changes and strengthening mechanisms

The present work investigates Ni-nanodiamond and Ni-graphite composites produced by mechanical synthesis and subsequent heat treatments. Processing of nickel-carbon nanocomposites by this powder metallurgy route poses specific challenges, as carbon phases are prone to carbide conversion and amorphization. The processing window for carbide prevention has been established through X-ray diffraction by a systematic variation of the milling parameters. Transmission electron microscopy confirmed the absence of carbide and showed homogeneous particle distributions, as well as intimate bonding between the metallic matrix and the carbon phases. Ring diffraction patterns of chemically extracted carbon phases demonstrated that milled nanodiamond preserved crystallinity, while an essentially amorphous nature could be inferred for milled graphite. Raman spectra confirmed that nanodiamond particles remained largely unaffected by mechanical synthesis, whereas the bands of milled graphite were significantly changed into the typical amorphous carbon fingerprint. The results on the annealed nanocomposites showed that milling with Ni accelerated graphitization of the carbon phases during heat treatments at 973 and 1073 K in both composites. At the finer scales, the nanocomposites exhibited a remarkable microhardness enhancement (∼70%) compared with pure nanostructured nickel. The Hall-Petch relation and the Orowan-Ashby equation are used to discuss strengthening mechanisms and the load transfer ability to the reinforcing particles.     

On the role of alloy composition and processing parameters in nanocluster formation and dispersion strengthening in nanostuctured ferritic alloys

An extensive experimental study characterizing the sequence of events that lead to the formation of a very high density of Y–Ti–O solute nanoclusters (NC) in mechanically alloyed, hot isostatically pressed ferritic stainless steels is reported. Yttria dissolves in the Fe–14Cr–3W(0.4Ti) powders during mechanical alloying. The dissolved Y and O, and when present Ti, subsequently precipitate during hot consolidation. The number densities and volume fractions of the NC decrease, and their radii increase, with increasing consolidation temperature. The NC form at 850 and 1000 °C in milled alloys containing Y, both with and without Ti additions. The presence of Ti refines the NC, and both Ti and high milling energy are necessary for the formation of NC at the highest consolidation temperatures of 1150 °C. However, the precise structure and composition of the NC are not well understood. Indeed, their character varies, depending on the alloy composition and processing variables.     

Effect of cold rolling on properties and microstructures of dispersion strengthened copper alloys

Mechanical properties and microstructures of unidirectionally and tandem rolled alumina dispersion strengthened copper（ADSC） alloys under different conditions were investigated by tensile test, optical microscopy（OM）, transmission electron microscopy（TEM） and scanning electron microscopy（SEM）. For unidirectionally rolled ADSC alloys, their strengths and elongations in the longitudinal direction are higher than those in the transverse direction under both cold rolling and annealing conditions. Once fracture appears in their longitudinal stress--strain curves, sudden reduction of overall stress level before complete fracture can be observed in the transverse tensile curves. The anisotropy of mechanical properties for the ADSC alloy can be greatly improved by tandem cold rolling. And no sudden reduction of overall stress level appears in the stress--strain curves for tandem rolled ADSC alloys. The differences of their microstructures and tensile fractures were analyzed. In order to compare the differences of tensile fracture mechanism in different directions, longitudinal and transverse fracture models for unidirectionally rolled ADSC alloys were also introduced.     

Comparison study on the annealing behaviors of dispersion strengthened copper alloys with different nanoparticles

The hardness measurement,optical microscopy （OM）,and transmission electron microscopy （TEM） microstructure observation on the annealing behaviors of Cu-Al2O3 （2.25 vol.% and 0.54 vol.% Al2O3） and Cu-0.52vol.%Nb alloys were carried out. The results show that with the increase of annealing temperature,the hardness of Cu-Al2O3 alloys decreases slowly. No change of the fiber structure formed by cold rolling in the Cu-2.25vol.%Al2O3 alloy is observed even after annealing at 900℃and the higher dislocation density can still be observed by TEM. Less combination of fiber formed by cold rolling and subgrains are observed in the Cu-0.54vol.%Al2O3 alloy annealed at 900℃. With the increase of annealing temperature,the hardness of the Cu-0.52vol.%Nb alloy exhibits a general decreasing trend,and its falling rate is higher than that of the Cu-Al2O3 alloys,indicating that its ability of resistance to softening at elevated temperature is weaker than that of the Cu-Al2O3 alloys. However,when annealed at a temperature of 300-400℃,probably owing to the precipitation strengthening of niobium,the hardness of the Cu-0.52vol.%Nb alloy arises slightly. The fibers formed by cold rolling be-come un-clear and un-straight and have less combination,and considerably more subgrains are observed by TEM.     

氧化物弥散强化MGH956合金板材的拉伸和持久性能

研究了具有粗、细两种不同晶粒组织状态的MGH956合金板材室温～1200℃拉伸及1100℃持久性能.采用扫描电镜和金相显微镜对拉伸和持久试样的断口形貌及纵剖面组织进行了检验,对比分析了两种板材不同温度下的强化因素、及变形和断裂模式.结果表明:正是由于强化因素、及变形和断裂模式上的不同,使得细晶板材的拉伸强度在低温高于粗晶板材,在高温则低于粗晶板材,以及细晶板材的持久强度大大低于粗晶板材;但两种板材从室温～ 1200℃的拉伸伸长率并无明显差异.     

Long-term high-velocity oxidation and hot corrosion testing of several NiCrAl and FeCrAl base oxide dispersion strengthened alloys

Several oxide dispersion strengthened (ODS) alloys have been tested for cyclic, long-term, high gas-velocity resistance to oxidation at 1100°C and hot corrosion at 900°C. Both nominally Ni-16Cr-4Al and Fe-20Cr-4.5Al ODS alloys were subjected up to 2500 cycles, where each cycle consisted of 1 hr in a hot, Mach 0.3 combusted gas stream followed by a 3-min quench in an ambient temperature, Mach 0.3 air blast. For comparison to existing technology, a coated superalloy was simultaneously tested. The ODS iron alloy exhibited clearly superior behavior, surviving 3800 oxidation and 2300 hot corrosion cycles essentially unscathed. While the ODS nickel alloys exhibited adequate oxidation resistance, the long-term hot corrosion resistance could be marginal, since the best life for such alloys under these conditions was only 1100 cycles. However, the hot corrosion resistance of the ODS Ni-base alloys is excellent in comparison to that of traditional superalloys.     

Microstructures, strengthening mechanisms and fracture behavior of Cu-Ag alloys processed by high-pressure torsion

Disks of a eutectic Cu-Ag alloy were processed by high-pressure torsion (HPT) up to 20 revolutions to reveal the microstructural evolution and mechanical properties. Both the Cu and Ag phases were thinned continuously with increasing numbers of revolutions. After 20 revolutions, the alternating Cu and Ag phases were significantly refined and became fibrous with dimensions as thin as 5 nm. The strain hardening behavior of the Cu-Ag alloy was characterized after different numbers of HPT revolutions, and a saturation microhardness was attained. It is shown that the tensile fracture mode changed from necking to fully brittle shearing with increasing numbers of revolutions, and some shear offsets with sizes of ∼5-20 μm were observed on the fracture surfaces. Based on the abnormal saturation microhardness value of the eutectic alloy, the strengthening mechanisms of various Cu-Ag alloys are discussed.     

Dispersion strengthened alloys: Properties and applications of light metals

This month, Journal of Metals presents the second part of a critical review of this field.     

On the superposition of flow stress contributions at finite temperatures and in the athermal limit

The functional form of the equation describing the superposition of contributions to the flow stress due to various strengthening mechanisms is analyzed. Considering that the superposition can be written as a sum to which each mechanism contributes through function f, that the dependence of the critical resolved shear stress on the density of obstacles to dislocation motion is given by function g, and that f and g have the same functional form for all strengthening mechanisms considered, it is shown that these two function are necessarily power functions, and their exponents are related. Furthermore, requiring that the superposition law is valid both at finite temperatures and at 0 K leads to an equivalent expression for the strain rate sensitivity and imposes restrictions on the way in which contributions of various mechanisms to the flow stress are evaluated at finite temperatures.     

内氧化-冷轧制备Al_2O_3弥散强化铜合金的组织与性能

以Cu-0.15%(质量分数)Al合金粉末为原料、Cu2O为氧化剂,采用内氧化-冷轧法制备Al2O3弥散强化铜合金,研究合金的组织与性能。结果表明:由内氧化-冷轧法制备的合金的σb520MPa、σ0.2/σb90%、电导率(IACS)也保持在80%以上;经950℃退火后,合金的σb400MPa;此合金具有高强、高导和优良的抗高温软化性能;冷轧态弥散强化铜合金的组织呈拉长变形的纤维状,试样断口分布有一定数量的韧窝;经950℃退火后,合金的纤维组织宽化,试样断口表面韧窝增多、变深,塑性提高。     

The mechanisms of microstructure formation in a nanostructured oxide dispersion strengthened FeAl alloy obtained by spark plasma sintering

A bulk dense nanostructured material, obtained by spark plasma sintering (SPS) of Y₂O₃ dispersion strengthened milled Fe–40Al powder, is characterized in detail using scanning (SEM) and transmission electron microscopies (TEM) in order to investigate the mechanisms of its microstructure formation. The sintered material displays a fairly heterogeneous microstructure that covers nano- and ultrafine together with large micrometric grains. The fine grains result from recovery and recrystallization, while the larger ones from grain growth or local melting. Under the present SPS conditions, large temperature differences in the range 570–670 °C, due to rapid heating–cooling and also to no holding stage applied, essentially account for such a structural heterogeneity. Controlling SPS of the milled powder thus provides a feasible processing route to get dense hetero-nanostructured material. In addition, complex oxide particles formed in the material are analyzed to be related to precipitation reaction and oxide evolution at different sintering temperatures.     

The mechanisms of crack growth in stress corrosion cracking of aluminium alloys

Modern understanding of possible mechanisms of crack growth in corrosion cracking of aluminium alloys immersed in various corrosion media such as aqueous solutions, saturated and undersaturated vapour, gaseous hydrogen, etc. is discussed in this review. The experimental data used as a basis to conclusions about the mechanisms of corrosion cracking in aluminium alloys are critically examined. A special emphasis is given to new methods to determining the effect of local anodic dissolution and hydrogen embrittlement in crack growth, i.e. the method of comparative tension and torsion tests deformations of types I and III, and the method based on evaluating ambiguous effect of cathodic polarization on crack growth.     

Mechanical model to evaluate the effect of the dispersion in nanocomposites

A key parameter in taking advantage of the exceptional mechanical and physical properties of carbon nanotubes (CNT) is getting a homogeneous dispersion of the nanofiller in the matrix. Micromechanical models are used to predict the properties of composites, but in most cases the calculated results are not comparable with those from experimentation. In the case of epoxy matrices, poorly dispersed CNT are obtained, as has been observed by scanning electron microscopy. The influence of the dispersion in nanocomposite stiffness is estimated developing a new micromechanical model. Several micromechanical models and experimental results are compared with this new model, showing that the latter has better agreement with mechanical tests. In addition, this mathematical formulation could be used with other reinforcements that are poorly dispersed, not only with CNT.     

Progress in the strengthening of aluminum alloys by heat treatment

This article examines progress recently made in the strengthening of aluminum alloys by heat treatment. The treatment maximizes strength while leaving the alloy with relatively high corrosion resistance.VILS. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 11, pp. 30–35, November, 1994.     

The multi-scale microstructure and strengthening mechanisms of Mo-12Si-8.5BxZr (at.%) alloys

Mo-12Si-8.5B alloys with different Zr contents (0 at.%, 1 at.%, 2 at.%, 3 at.%, 4 at.%) were manufactured via a mechanical alloying process followed by hot pressing sintering technology. The microstructure of Mo-12Si-8.5B alloy exhibited a continuous submicro- and micro-scale α-Mo matrix in which the sub-micron Mo₃Si/Mo₅SiB₂ particles were distributed. Addition of Zr to Mo-12Si-8.5B alloy promoted to form spherical nano-scale intermetallic Mo₂Zr and ZrO₂ particles, which were mainly located at the grain boundaries (GBs) as well as partially within the grains. The microstructure of Mo-12Si-8.5B-xZr alloys was remarkably refined by these Mo₂Zr/ZrO₂ nanoparticles. Additionally, results of mechanical properties indicated that the Zr addition improved the hardness, compression strength, yield strength and flexure strength of alloys. In particular, the Mo-12Si-8.5B-2Zr alloy exhibited extremely high compression strength (3.38 GPa), yield strength (3.17 GPa) and flexure strength (1.15 GPa). Quantitative analyses indicate that both fine-grained strengthening and Zr-rich particle strengthening mechanisms play a significant role in strengthening the Mo-Si-B-Zr alloys, the strengthening is dominantly governed by grain size reduction. Furthermore, Zr getters detrimental oxygen by synthesizing ZrO₂ distributed at grain/phase boundaries, which contributes to increasing the GBs cohesion. Fracture surfaces revealed that the fracture mode transformed from intergranular to transgranular fracture owing to Zr addition.     

Influence of ball-milling and annealing conditions on nanocluster characteristics in oxide dispersion strengthened steels

The characteristics of strengthening nanoclusters (NCs) have a major influence on the mechanical properties of oxide dispersion strengthened (ODS) steels. To determine how to control NC formation, ODS powders are synthesized in different ball-milling and annealing conditions, then characterized by electron probe micro-analysis and small angle neutron scattering. During ball-milling, reactants are dissolved into the metallic matrix until a Ti, Y and O solid solution is formed and then NC nucleation begins. Nucleation is greatly enhanced during the first minutes of annealing at 800 °C without any coarsening afterwards. The intensity and temperature of ball-milling influence this mechanism and thus the characteristics of the formed NC, whereas the nature of reactants, for a given composition, has no impact on NC size and volume fraction. Consequently, to promote the formation of fine and dense dispersion of NC, two main modifications to the usual process are proposed: (i) perform a long and/or intense ball-milling with a limited overheating, and (ii) anneal the as-milled powder at 800 °C before performing the thermo-mechanical treatment.     

The flow stress in polycrystalline films: Dimensional constraints and strengthening effects

An analytical model is presented in order to derive a general expression for the flow stress in polycrystalline films which encompasses and correlates dimensional constraints and strengthening effects. The model is based on the Thompson approach, which is extended to take into account both different grain aspect ratios and distinct strengthening contributions. It allows an accurate prediction of the growth textures in polycrystalline CdTe thick films when grain growth is driven by strain energy minimization. The model also matches the experimental data concerning the grain size and film thickness dependences of the yield stress in polycrystalline Cu thin films either deposited on a substrate or freestanding. Interestingly, the yield stress is found to be fitted by a modified Hall–Petch relation resulting in a d⁻ⁿ dependence in which the exponent n varies between ½ and 1 as a function of the grain size for a given thickness.     

Twinning stress in shape memory alloys: Theory and experiments

Utilizing first-principles atomistic simulations we present a twin nucleation model based on the Peierls–Nabarro formulation. We investigated twinning in several important shape memory alloys, starting with Ni₂FeGa (14M modulated monoclinic and L1₀ crystals) to illustrate the methodology, and predicted the twin stress in Ni₂MnGa, NiTi, Co₂NiGa, and Co₂NiAl martensites, all of which were in excellent agreement with the experimental results. Minimization of the total energy led to determination of the twinning stress accounting for the twinning energy landscape in the presence of interacting multiple twin dislocations and disregistry profiles at the dislocation core. The validity of the model was confirmed by determining the twinning stress from experiments on Ni₂FeGa (14M and L1₀), NiTi, and Ni₂MnGa and utilizing results from the literature for Co₂NiGa and Co₂NiAl martensites. This paper demonstrates that the predicted twinning stress can vary from 3.5 MPa in 10M Ni₂MnGa to 129 MPa for the B19′ NiTi case, consistent with the experimental results.