High strength-ductility of thin nanocrystalline palladium films with nanoscale twins: On-chip testing and grain aggregate model

The mechanical behaviour of thin nanocrystalline palladium films with an ∼30 nm in plane grain size has been characterized on chip under uniaxial tension. The films exhibit a large strain hardening capacity and a significant increase in the strength with decreasing thickness. Transmission electron microscopy has revealed the presence of a moderate density of growth nanotwins interacting with dislocations. A semi-analytical grain aggregate model is proposed to investigate the impact of different contributions to the flow behaviour, involving the effect of twins, of grain size and of the presence of a thin surface layer. This model provides guidelines to optimizing the strength/ductility ratio of the films.     

Structure–property optimization of ultrafine-grained dual-phase steels using a microstructure-based strain hardening model

Grain size reduction in single phase alloys is generally accompanied by a loss of ductility related to a decrease in the strain hardening capacity. The flow behaviour of fine-grained dual-phase steels produced by swaging was investigated in order to address the couplings between grain size reduction and incorporation of a second phase towards optimizing microstructures with different objectives. A physically based grain size dependent strain hardening model has been developed for the ferrite matrix, involving specific laws for the accumulation and saturation of dislocations along grain boundaries and for their net back stress contribution. The back stress increases with the dislocation density, reaches a maximum and finally decreases due to screening effects. The overall behaviour of the ferrite–martensite composite is then evaluated using a mean field homogenization method. After identification of the material parameters, a parametric study provides, for a given carbon content and grain size of the ferrite matrix, the optimum martensite volume fraction, leading either to the maximum strength ductility product or to the maximum strength under the constraint of a minimum ductility.     

Grain-size dependent accommodation due to intragranular distributions of dislocation loops

A grain-size dependent accommodation law for polycrystals is deduced from an inclusion/matrix problem (i.e., each grain is seen as embedded in a homogeneous equivalent medium) where plastic strain inside the inclusion is given as a discrete distribution of circular coaxial glide dislocation loops. The loops are assumed constrained at spherical grain boundaries. From thermodynamic considerations specific to a process of identical plastification in all the loops (considered as “super-dislocations”), an average back-stress over the grain is derived. In order to compute the very early stages of plastic deformation in a face-centred cubic polycrystal, this back-stress is incorporated into a diluted model in terms of concentration of plastic grains. Contrary to conventional mean-field approaches, a grain-size effect is obtained for the initial overall strain-hardening behaviour. This size effect results from an intrinsic contribution of intragranular slip heterogeneities on the kinematical hardening.     

Fabrication of Hadfield-Cored Multi-layer Steel Sheet by Roll-Bonding with 1.8-GPa-Strength-Grade Hot-Press-Forming Steel

An austenitic Hadfield steel was roll-bonded with a 1.8-GPa-strength-grade martensitic hot-press-forming (HPF) steel to fabricate a multi-layer steel (MLS) sheet. Near the Hadfield/HPF interface, the carburized and decarburized layers were formed by the carbon diffusion from the Hadfield (1.2%C) to HPF (0.35%C) layers, and could be regarded as kinds of very thin multi-layers of 35 μm in thickness. The tensile test and fractographic data indicated that the MLS sheet was fractured abruptly within the elastic range by the intergranular fracture occurred in the carburized layer. This was because C was mainly segregated at prior austenite grain boundaries in the carburized layer, which weakened grain boundaries to induce the intergranular fracture. In order to solve the intergranular facture problem, the MLS sheet was tempered at 200 °C. The stress–strain curve of the tempered MLS sheet lay between those of the HPF and Hadfield sheets, and a rule of mixtures was roughly satisfied. Tensile properties of the MLS sheet were dramatically improved after the tempering, and the intergranular fracture was erased completely. In particular, the yield strength up to 1073 MPa along with the high strain hardening and excellent ductility of 32.4% were outstanding because the yield strength over 1 GPa was hardly achieved in conventional austenitic steels.     

Bauschinger and size effects in thin-film plasticity

We present an experimental investigation of the effects of surface passivation, film thickness and grain size on the plastic behavior of freestanding Cu thin films. The stress–strain curves of the films are measured using the plane–strain bulge test. Films with a passivation layer on one or both surfaces have an offset yield stress that increases significantly with decreasing film thickness; the yield stress of unpassivated films, by contrast, is relatively independent of film thickness and increases mainly as a result of grain-size strengthening. The stress–strain curves of passivated films show an unusual Bauschinger effect on unloading. This effect is not observed for unpassivated films. Our experimental results suggest that passivation layers prevent dislocations from exiting the films and that they block slip bands at the film–passivation interface. The back stresses associated with these blocked slip bands increase the resistance to forward plastic flow on loading and cause reverse plastic flow on unloading. The effect of the back stresses increases with decreasing film thickness and leads to the observed strengthening of the passivated films. The constraint of a passivating layer on dislocation motion and hence on plastic flow cannot be described by classical plasticity theories, but can be modeled with some strain–gradient plasticity theories. We evaluate the suitability of the strain–gradient plasticity theory developed by Fleck and Hutchinson to describe our experimental results in a continuum framework. Comparison between experimental results and calculations yields very good agreement for the effect of film thickness, but the strain–gradient plasticity model fails to describe the Bauschinger effect observed in passivated films.     

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

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

变形态Mg-Nd合金的组织转变和拉伸性能特征

研究不同变形条件对Mg-2.2Nd-0.5Zn-0.5Zr合金室温拉伸性能和组织的影响.经过不同条件的热挤压变形后,该合金的强度和延性都有不同程度的增加,屈强比从0.58提高到0.87左右.固定变形温度时,强度随变形速率增大而降低,延性反之.固定变形速率时,升高变形温度则强度降低,延性增加.弥散于晶界的Mg9Nd化合物细化了晶粒.变形态Mg-Nd合金的高温超塑拉伸研究发现,375℃是该合金的最佳超塑变形温度,应变速率在1×10-2s-1时,延伸率达到329%;当变形速率提高到2×10-2s-1时,该合金的延伸率仍可达到213%.分析不同真应变下的组织发现,在变形初期发生动态再结晶,晶粒得到破碎而变得细小,随着变形程度的增加,晶粒长大程度较小.在变形后的断口形貌中发现,Mg-Nd合金的超塑变形机制为晶界滑移控制下的孔洞连接协调机制.     

Effects of tensile test parameters on the mechanical properties of a bimodal Al–Mg alloy

The properties of aluminum alloy (AA) 5083 are shown to be significantly improved by grain size reduction through cryomilling and the incorporation of unmilled Al particles into the material, creating a bimodal grain size distribution consisting of coarse grains in a nanocrystalline matrix. To provide insight into the mechanical behavior and ultimately facilitate engineering applications, the present study reports on the effects of coarse grain ratio, anisotropy, strain rate and specimen size on the elastic–plastic behavior of bimodal AA 5083 evaluated in uniaxial tension tests using a full-factorial experiment design. To determine the governing failure mechanisms under different testing conditions, the specimens’ failure surfaces were analyzed using optical and electron microscopy. The results of the tests were found to conform to Joshi’s plasticity model. Significant anisotropy effects were observed, in a drastic reduction in strength and ductility, when tension was applied perpendicular (transverse) to the direction of extrusion. These specimens also exhibited a smooth, flat fracture surface morphology with a significantly different surface texture than specimens tested in the axial direction. It was found that decreasing specimen thickness and strain rate served to increase both the strength and ductility of the material. The failure surface morphology was found to differ between specimens of different thicknesses.     

Architectured surface layer with a gradient nanotwinned structure in a Fe–Mn austenitic steel

A surface layer with a gradient decrease in twin density has been produced in a Fe–Mn austenitic steel, which corresponds to a gradient drop in hardness from 5.3 GPa in the top surface to 2.2 GPa in the coarse grained core. The dependence of hardness on the twin thickness was determined, showing a weaker strengthening effect of twin boundaries than that of conventional grain boundaries in this alloy. Superior strength–ductility synergy was observed in tensile tests of the gradient nanotwinned layer.     

Twinning-induced mechanical properties’ modification of CP-Ti by friction stir welding associated with simultaneous backward cooling

Commercial pure Ti (CP-Ti) plates with 2?mm in thickness were successfully joined by friction stir welding (FSW) associated with simultaneous backward cooling. The measured processing temperatures were strictly controlled below the α/β phase transformation point. The microstructural characterisation showed that the stir zone has an ultra-refined grain structure with abundant twin boundaries by decreasing processing temperature. The geometric dynamic recrystallisation and twinning-induced dynamic recrystallisation were the main mechanisms of the grain refinement process. The resulting tensile properties of the stir zone illustrate that both the strength and ductility enhanced due to the appearance of abundant twin boundaries. This work also provides an effective strategy to enhance the strength of the stir zone of FSW CP-Ti joint without ductility loss.     

Tailoring AZ91 laminates for combination of strength and ductility

Three types of laminates were designed by alternately stacking AZ91 extruded sheets in different states for extrusion to improve the mechanical properties. The tensile tests revealed that the combination of solid-solution-treated sheets with the aging-treated sheets achieved high tensile strength and ductility, i.e., ultimate tensile strength of ~386 MPa and elongation of ~19.8%, respectively. Electron backscatter diffraction (EBSD) and TEM results indicated that the aging-treated layers with more nano-sized precipitates and small grain size provided high strength and reasonable ductility, while the solid-solution-treated layers with low dislocation density facilitated strain hardening. Also, the strong interface bonding between the successive layers played an important role in the enhanced ductility.     

Superplasticity in electrodeposited nanocrystalline nickel

Electrodeposited nanocrystalline Ni films were processed with different levels of S, to evaluate the role of S on superplasticity. All the materials exhibited high strain rate superplasticity at a relatively low temperature of 777 K. Microstructural characterization revealed that the S was converted to a Ni₃S₂ phase which melts at 908 K; no S could be detected at grain boundaries. There was no consistent variation in ductility with S content. Superplasticity was associated with a strain rate sensitivity of ～0.8 and an inverse grain size exponent of ～1, both of which are unusual observations in superplastic flow of metals. Based on the detailed experiments and analysis, it is concluded that superplasticity in nano-Ni is related to an interface controlled diffusion creep process, and it is not related to the presence of S at grain boundaries or a liquid phase at grain boundaries.     

Cu/Ni多层膜对Ti811合金微动磨损和微动疲劳抗力的影响

在Ti811钛合金表面利用离子辅助磁控溅射沉积技术制备20～1200nm不同调制周期的Cu/Ni金属多层膜,分析多层膜的结构,测试膜基结合强度、膜层显微硬度和韧性,对比研究不同调制周期的Cu/Ni多层膜对钛合金基材常温下微动磨损性能和微动疲劳(FF)抗力的影响。结果表明:利用离子辅助磁控溅射技术可以获得致密度高、晶粒细化、膜基结合强度高的Cu/Ni多层膜,该类多层膜具有良好的减摩润滑作用,因而改善了Ti811钛合金常温下抗微动磨损和微动疲劳性能;Cu/Ni多层膜对钛合金FF抗力的改善程度随膜层调制周期呈现非单调变化趋势,调制周期为200nm的Cu/Ni多层膜对钛合金FF抗力的提高程度最大,原因归于该膜层具有良好的强韧和润滑综合性能。     

Size effects on the mechanical properties of thin polycrystalline metal films on substrates

An analysis of the effects of the thickness and grain size of polycrystalline thin films on substrates is presented with the objective of linking the film mismatch stress to the underlying characteristic size scales. The model is predicated on the notion that the relaxation of mismatch strain in the film is accommodated by the introduction of dislocation loops whose population, dimensions and interaction energies are controlled by the film thickness and microstructural dimensions. The model is capable of capturing the combined effects of these size scales by accounting for the interaction energies of the constrained dislocation structure, and provides quantitative predictions of the evolution of film stress during thermal excursions. The predictions of the analysis are compared with available experimental results for polycrystalline films of face-centered cubic materials on Si substrates. It is shown that the model correctly predicts the observed influence of film thickness and grain size on stress evolution during thermal excursions. Aspects of strain hardening in thin polycrystalline films with high dislocation densities are also discussed.     

Superior fatigue crack growth resistance, irreversibility, and fatigue crack growth–microstructure relationship of nanocrystalline alloys

While previous studies have reported that nanocrystalline materials exhibit poor resistance to fatigue crack growth (FCG), the electro-deposited nanocrystalline Ni–Co alloys tested in this paper show superior resistance to FCG. The high damage tolerance of our alloy is attributed to the following: alloying with Co, low internal stresses resulting in stability of the microstructure, and a combination of high strength and ductility. The high density of grain boundaries interact with the dislocations emitted from the crack tip, which impedes FCG, as predicted by the present model and measured experimentally by digital image correlation. Further, the addition of Co increases the strength of the material by refining the grain size, reducing the fraction of low angle grain boundaries, and reducing the stacking fault energy of the material, thereby increasing the prevalence of twinning. The microstructure is stabilized by minimizing the internal stress during a stress relief heat treatment following the electro-deposition process. As a result grain growth does not occur during deformation, leaving dislocation-mediated plasticity as the primary deformation mechanism. The low internal stresses and nanoscale twins preserve the ductility of the material, thereby reaching a balance between strength and ductility, which results in a superior resistance to FCG.     

Maragin behavior of an Fe-19.5Ni-5Mn alloy. III: Mechanical properties

A high maraging strength in Fe-Ni-Mn alloys can be achieved at the expense of a marked loss in ductility. Very fine precipitates are observed when peak strength is reached. At peak strength, the Fe-Ni-Mn alloys exhibit brittle failure, mainly along prior austenite grain boundaries, irrespective of the nickel content. The presence of a small amount of retained austenite prior to aging does not improve the ductility. Previous explanation of the embrittlement in Fe-Ni-Mn alloys was attributed to the segregation of Mn to prior austenite grain boundaries. However, this is not fully supported by the present studies. Auger electron spectroscopy reveals no decisive evidence of manganese segregation. Some degree of ductility in the aged martensite may be required in order to prevent brittle fracture. Dual aging recovers part of the ductility and improves the strength slightly. The effect of reversed austenite on ductility may vary, depending on its morphology. Matrix and recrystallized austenite are beneficial to both elongation and reduction of area, but lathlike austenite lowers the elongation, probably because of its lamellar morphology. The lamellar structure of the lath martensite is also detrimental to elongation.     

Thickness effect on shape memory behavior of Ti-50.0at.%Ni thin film

The thickness effect on the shape memory behavior of Ti-50.0at.%Ni thin films was investigated. It was found that the transformation strain and residual strain under a constant stress are very sensitive to the film thickness when the thickness is less than the average grain size, 5 μm. Cross-sectional observation showed that these strains are affected by two kinds of constraints from surrounding grains and surface oxide layers. With decreasing thickness, the former effect becomes weak, but the latter effect becomes strong. As a result, the transformation strain and residual strain show a maximum around a thickness of 1–2 μm. In addition, the transformation temperatures were also found to be affected by surface oxidation if the thickness is less than 1 μm.     

LOCAL CHEMISTRY AND THE COHESIVE STRENGTH OF GRAIN BOUNDARIES IN Ni_3Al

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The influence of microstructure on the ductility of iron aluminides

Considerable effort has been devoted over the last decade to the development of iron aluminides as materials for high temperature applications, where their good oxidation and corrosion resistance, combined with reasonable strength, may be utilised. Poor formability and ductility, however, particularly at room temperature, has hampered the exploitation of these materials. The present review examines the present state of understanding of the factors which influence the ductility. Recent research has made clear the important influence of testing environment, the role of Al content and minor additions of B, as well as the effect of quenched-in vacancies. The extent to which other factors, such as alloying additions and microstructural features, affect the ductility has not received the same attention, and is examined in the present study. Alloy strengthening, by almost any mechanism, is seen to lead to a dramatic loss of ductility. The only parameter allowing both strength increase and ductility improvement for a given set of Al/B/vacancy/environment conditions is the grain size. The best ductility for a given alloy, which should have as low an Al content as compatible with other requirements, is obtained by refining the grain size and by maintaining the alloy in the softest possible state. For the most part these conclusions are drawn from analysis of the behaviour of B2 ordered FeAl alloys, although similar trends seem also to apply to alloys of slightly lower Al content where DO3 ordering can occur. The observations drawn can be understood in terms of the mechanisms leading to the nucleation and propagation of brittle fracture, either as transgranular cleavage cracks or as grain boundary cracks. The possible role of additional factors, such as the texture, or grain and grain boundary distribution, surface layers producing protective stress effects, and strain homogenising or crack arresting dispersions, has not been sufficiently evaluated to determine whether any further improvements of ductility are possible.