Nanocrystalline gold with small size: inverse Hall–Petch between mixed regime and super-soft regime

ABSTRACT

Molecular dynamics simulations were used to study the atomic mechanisms of deformation of nanocrystalline gold with 2.65–18?nm in grain size to explore the inverse Hall–Petch effect. Based on the mechanical responses, particularly the flow stress and the elastic-to-plastic transition, one can delineate three regimes: mixed (10–18?nm, dislocation activities and grain boundary sliding), inverse Hall-Petch (5–10?nm, grain boundary sliding), and super-soft (below 5?nm). As the grain size decreases, more grain boundaries present in the nanocrystalline solids, which block dislocation activities and facilitate grain boundary sliding. The transition from dislocation activities to grain boundary sliding leads to strengthening-then-softening due to grain size reduction, shown by the flow stress. It was further found that, samples with large grain exhibit pronounced yield, with the stress overshoot decrease as the grain size decreases. Samples with grain sizes smaller than 5?nm exhibit elastic-perfect plastic deformation without any stress overshoot, leading to the super-soft regime. Our simulations show that, during deformation, smaller grains rotate more and grow in size, while larger grains rotate less and shrink in size.     

Grain Growth During Superplastic Deformation

Significant grain growth occurring during superplastic deformation is related to the micro-mechanism of superplastic flow. Observations performed on the deformed surface of superplastically deformed tensile and shear Pb-62%Sn samples and bi-axially formed AA7475 samples directly indicate that cooperative grain boundary sliding, i.e. sliding of grain groups, is accompanied by cooperative grain boundary migration that can result in an enhanced grain growth. Such a long range correlation in migration of sliding grain boundaries is related to movement of grain boundary dislocations having a step associated with its core. Observed correlation between grain size and strain measured in different regions of a superplastically formed Ti-alloy part and alignment of grain boundaries along shear surfaces support coupling of grain boundary sliding and migration. A model of grain growth considering climb of cellular dislocations, topological defects in a grain array, has been expanded to incorporate gliding and mixed cellular dislocations.     

A study on anomalous yield drop behaviour in modified beta titanium alloys

ABSTRACT

The yield drop phenomenon observed in the Ti–15V-3Al–3Sn-3Cr (Ti–15–3) beta-titanium alloy and its anomalous behaviour in the boron and carbon added Ti–15–3 alloys have been studied. While the base and the carbon containing alloys exhibit yield drop, the boron containing alloy with smaller grain size than base alloy does not appear to show this phenomenon. Tensile tests were interrupted at different stress levels followed by analyses of slip lines and sub-structural characteristics using scanning and transmission electron microscopes to understand this anomalous yield point phenomenon. Infrared thermal imaging technique was used to map the strain localisation and the spatiotemporal evolution of deformation along the gauge length of the specimens during the tensile tests. Deformation in these alloys initiates only in a few grains. Pile-up of dislocations in these grains subsequently triggers the formation of dislocations in other grains and their rapid multiplications. The spreading of deformation by the generation of dislocations from pile up dislocations in one grain to neighbouring un-deformed grains and their rapid multiplication to new regions influence the yield drop phenomenon and its characteristics. It is shown in this study that microscopic instability in the grain level is a necessary, but not the sufficient condition for the manifestation of macroscopic instability during tensile deformation in polycrystalline materials. The presence of boride particles at grain boundaries restricts the slip transfer across the grains as well as the spreading of deformation to new regions, which causes the suppression of yield drop in the boron containing alloy.     

纳米单晶与多晶铜薄膜力学行为的数值模拟研究

通过对不同温度下单晶薄膜的拉伸性能的分子动力学模拟，从微观角度揭示了温度效应对材料性能的影响. 结果表明温度效应对材料的变形机理影响很大.0K温度下由于缺乏热激活软化的影响, 粒子运动所受到的阻碍较大, 薄膜的强度较高, 塑性变形主要来自于粒子的短程滑移.温度升高，粒子的热运动加剧，屈服强度降低, 塑性变形将主要来自于大范围的位错长程扩展.多晶薄膜的模拟结果表明, 虽然其晶粒形状较为特殊, 但是它仍然遵循反Hall-Petch关系.在模拟过程中，侧向应力最大值比拉伸方向应力的最大值滞后出现.位错只会从晶界产生并向晶粒内部传播，晶粒间界滑移是多晶薄膜塑性变形的主要来源. 关键词： 纳米薄膜 变形机理 温度效应 分子动力学     

晶界对纳米多晶铝中冲击波阵面结构影响的分子动力学研究

用分子动力学方法研究了纳米多晶铝在冲击加载下的冲击波阵面结构及塑性变形机理.模拟研究结果表明:在弹性先驱波之后,是晶界间滑移和变形主导了前期的塑性变形机理;然后是不全位错在界面上成核和向晶粒内传播,然后在晶粒内形成堆垛层错、孪晶和全位错的过程主导了后期的塑性变形机理.冲击波阵面扫过之后留下的结构特征是堆垛层错和孪晶留在晶粒内,大部分全位错则湮灭于对面晶界.这个由两阶段塑性变形过程导致的时序性塑性波阵面结构是过去未见报道过的. 关键词： 晶界 塑性变形 冲击波阵面 分子动力学     

Nanostructures and deformation mechanisms in a cryogenically ball-milled Al-Mg alloy

An Al-7.6 at.% Mg alloy was ball milled in liquid N₂ for 8 h and its microstructures were investigated using transmission electron microscopy. Electron diffraction confirmed that the resulting powder is a supersaturated Al-Mg solid solution with an fcc structure. Three typical nanostructures with different grain-size ranges and shapes were observed and the deformation mechanisms in these structures were found to be different. High densities of dislocations were found in large crystallites, implying that dislocation slip is the dominant deformation mechanism. The dislocations rearranged to form small-angle subboundaries upon further deformation, resulting in the formation of medium-sized crystallites with diameters of 10-30 nm. In very small crystallites with dimensions less than 10 nm, twinning becomes an important deformation mechanism. The reasons for the different deformation mechanisms were discussed. Some defects, such as twin boundaries, and small- and large-angle grain boundaries were investigated in detail.     

Strength and plastic deformation of polycrystalline diamond composites

ABSTRACT

The use of nanopolycrystalline diamond has allowed a systematic study on deformation of polycrystalline diamond composites (PCDCs). Bulk PCDCs samples containing either Co or SiC as a binding agent were deformed under high pressure and temperature to strains up to 18% at strain rates ～10^?5?s^?1. All samples exhibit strong work hardening. The strength of PCDCs depends on the amount and type of binding agents and is consistently weaker than that of diamond single crystals. The weakening may be due to the binder materials, which play an important role in affecting grain boundary structures. In SiC-based PCDC, significant grain fragmentation occurs. Nearly all grain boundaries are wetted by SiC after large deformation, resulting in lower strength. In Co-based PCDC, the microstructure is dominated by dislocations, deformation twins, and separated grain boundaries. The density of deformation twins increases significantly with strain, with the twin domain width reaching as low as 10–20?nm at 14% strain.     

多晶银纳米线拉伸变形的分子动力学模拟研究

纳米尺度金属Ag以其独特的导电和导热性,广泛应用于微电子、光电子学、催化等领域,特别是在纳米微电极和纳米器件方面的应用.本文采用分子动力学方法模拟了不同晶粒尺寸下多晶银纳米线的拉伸变形行为,详细分析了晶粒尺寸对多晶银纳米线弹性模量、屈服强度、塑性变形机理的影响.发现当晶粒尺寸小于13.49 nm时,多晶Ag纳米线呈现软化现象,出现反Hall-Petch关系,此时的塑性变形机理主要以晶界滑移、晶粒转动为主,变形后期形成五重孪晶;当晶粒尺寸大于13.49 nm时,塑性变形以位错滑移为主,变形后期产生大量的孪晶组织.     

Study of the mechanical properties of alloy 36NKhTYu in relation to the mechanisms underlying the precipitation of the γ' and η phases

The nature of the dispersion (precipitation) hardening of the alloy 36NKhTYu is examined. It is shown that the increase observed in the resistance of the alloy to the motion of dislocations inside the grains during the continuous precipitation of the γ' phase is due to the presence of long-range order in the hardening particles. In addition to this, there is also a considerable increase in that part of the yield stress which is associated with the grain boundaries; this is because of the precipitation of carbides at grain boundaries and the corresponding sharp increase in slip localization. Overaging is characterized by a reduction in the resistance to the motion of the dislocations inside the grains, and a diminishing influence of the grain boundaries (ultimately to no influence at all). These results are in satisfactory agreement with calculations based on the Orowan theory, and also with the assumption that, as overaging progresses, the range of the dislocations is reduced from a distance equal to the grain size to a distance equal to that separating the particles ofγ′ phase, while localized slip is replaced by uniform slip. When the precipitation of theη phase reaches a well-developed stage, the yield stress is reduced. The result of the present investigation confirm our earlier conclusion as to the substantial hardening (strengthening) of the meterial which occurs as a result of the discontinuous precipitation of theγ′ phase.     

A thermally activated dislocation-based constitutive flow model of nanostructured FCC metals involving microstructural evolution

Abstract

Due to their interface and nanoscale effects associated with structural peculiarities of nanostructured, face-centered-cubic (FCC) ultrafine-grained/nanocrystalline (UFG/NC) metals, in particular nanotwinned (NT) metals exhibit unexpected deformation behaviours fundamentally different from their coarse-grained (CG) counterparts. These internal boundaries, including grain boundaries and twin boundaries in UFG/NC metals, strongly interact with dislocations as deformation barriers to enhance the strength and strain rate sensitivity (SRS) of materials on the one hand, and play critical roles in their microstructural evolution as dislocation sources/sinks to sustain plastic deformation on the other. In this work, building on the findings of twin softening and (de)twinning-mediated grain growth/refinement in stretched free-standing NT–Ni foils, a constitutive model based on the thermally activated depinning process of dislocations residing in boundaries has been proposed to predict the steady-state grain size and simulate the plastic flow of NT–Ni, by considering the blocking effects of nanotwins on the absorption of dislocations emitted from boundaries. It is uncovered that the stress ratio (η_stress) of effective-to-internal stress can be taken as a signature to estimate the stability of microstructures during plastic deformation. This model not only reproduces well the plastic flow of the stretched NT–Ni foils as well as reported NT–Cu and the steady-state grain size, but also sheds light on the size-dependent SRS and failure of FCC UFG/NC metals. This theoretical framework offers the opportunity to tune the microstructures in the polycrystalline materials to synthesise high performance engineering materials with high strength and great ductility.     

Effect of microstructure on fracture in age hardenable Al alloys

ABSTRACT

In the present study, the fracture behaviour of AA6016 alloy was investigated during bending deformation. Wrap-bend tests were conducted and the material was subjected to different bend angles to study crack propagation. The average grain size of the as-received material is approximately 45?μm. The aspect ratio of the grains was changed from 0.53 to 0.40 during bending. The presence of deformation bands was observed during bending in both tensile and compressive regions of the sample. No orientation correlation was observed between the deformation band and its corresponding parent grain. The Schmid factor inside the deformation bands was higher than that of the parent grain, which indicates that the deformation bands accommodate strain during bending. The crystallographic texture evolved significantly during bending deformation. The strength of cube texture component decreases with increasing bend angle and new texture components formed during bending. These new texture components favour either single slip or duplex slip. A mixture of intra-granular and inter-granular fracture occurs during bending. It is observed that inter-granular crack propagation is predominantly favoured along high-angle boundaries, and grain boundary de-cohesion occurs in regions where the misorientation angle is greater than 40°. The formation of deformation-induced coincidence lattice site (CSL) boundaries is also observed during bending and it is shown that the volume fraction of CSL boundaries of Σ3 type increases with increasing bend angle. The current study shows that the formation of deformation-induced CSL boundaries of Σ3 type in AA6016 alloy can improve its inherent resistance to crack propagation during bending.     

Dependence of grain boundary character distribution on the initial grain size of 304 austenitic stainless steel

Abstract

In order to study the dependence of the grain boundary character distributions (GBCD) on the grain size, annealing treatment was carried out on 304 austenitic stainless steel with different initial grain sizes. The evolution of the GBCD was analysed by electron backscatter diffraction. The experimental results showed that abnormal grain growth (AGG) occurred when grain size was small. With a smaller initial grain size, the number density of abnormally large grains and the fraction of low-Σ CSL boundaries increased but the size of abnormally large grains decreased and the random boundaries presented a continuous network. With a larger initial grain size, the fraction of low-Σ CSL boundaries also increased as well as the size of abnormally large grains but the number density of abnormally large grains decreased and the connectivity of random boundary network was disrupted by low-Σ CSL boundaries, especially Σ3ⁿ (n = 1, 2, 3) boundaries. However, with a very large initial grain size, normal grain growth (NGG) occurred, which had no effect on the fraction of low-Σ CSL boundaries and the connectivity of random boundary network.     

Grain boundary sliding during ambient-temperature creep in hexagonal close-packed metals

Even at ambient temperature or less, below their 0.2% proof stresses all hexagonal close-packed metals and alloys show creep behaviour because they have dislocation arrays lying on a single slip system with no tangled dislocation inside each grain. In this case, lattice dislocations move without obstacles and pile-up in front of a grain boundary. Then these dislocations must be accommodated at the grain boundary to continue creep deformation. Atomic force microscopy revealed the occurrence of grain boundary sliding (GBS) in the ambient-temperature creep region. Lattice rotation of 5° was observed near grain boundaries by electron backscatter diffraction pattern analyses. Because of an extra low apparent activation energy of 20 kJ/mol, conventional diffusion processes are not activated. To accommodate these piled-up dislocations without diffusion processes, lattice dislocations must be absorbed by grain boundaries through a slip-induced GBS mechanism.     

Influence of impurities on the fracture behaviour of tungsten

Ten tungsten materials with different impurity concentrations and different microstructures have been investigated by Auger electron spectroscopy and scanning electron microscopy with respect to their fracture behaviour. For almost all samples, both inter- and transgranular fracture are observed, and the proportion of each type varies. Due to the difference in their impurity content and grain boundary area, a large variation in the grain boundary impurities can be expected. By analysing the fracture surfaces the effect of grain boundary impurities, especially phosphorous and oxygen, on the fracture resistance of the boundaries was determined. The results indicate that for the analysed tungsten materials, grain boundary impurities do not have a significant influence on the fracture resistance of the boundaries. Other factors such as the size and shape of the grains, the amount of deformation and therefore the density of dislocations within the grains have a greater impact on the fracture behaviour of tungsten.     

Length scale effect on the deformation microstructures of grown-in twins in copper

Electrodeposited copper samples composed of columnar grains subdivided by alternating twin/matrix (T/M) lamellae have been cold rolled to 30–85% reduction in thickness. The thickness of the T/M lamellae varies over a wide range from a few nanometres to about 1?μm. The deformation microstructure has been characterized systematically. In thin T/M lamellae (below 50–100?nm) the deformation behaviours differ significantly from that of thick T/M lamellae, as the dislocation activity is concentrated at the T/M boundaries illustrated by the observations of stacking faults and Shockley partial dislocations. In thick T/M lamellae (100–1000?nm), the deformation microstructure is related to the grain orientation as also observed previously in deformed single crystals and polycrystals with a grain size at the micrometre scale. The experiment therefore suggests that the universal structural characteristics of deformation microstructure can be extended one order of magnitude from about 5?μm to the sub-micrometre scale (about 0.5?μm).     

Slip-accommodated superplastic flow in Zn-22 wt%Al

Experiments were conducted on the Zn-22 wt% Al eutectoid that contained nanometre-scale dispersion particles. These particles were introduced in the matrix of the alloy via powder metallurgy followed by cryomilling. Transmission electron microscopy observations made on specimens crept at a strain rate near the centre of the superplastic region (the intermediate-stress region or region II in the sigmoidal relationship between stress and strain rate) reveal clear evidence for lattice dislocation activities during superplastic flow. Such evidence is demonstrated in part by the presence of attractive particle-dislocation interactions that are only noted in some of the grains. It is suggested that each one of these grains serves as an obstacle for a group of grains sliding together as a unit. In addition, the configurations of the lattice dislocations in the interiors of the blocking grains are suggestive of viscous glide and single slip in the blocking grain. Combining the present findings with earlier observations reported for superplastic deformation leads to the conclusion that the generation and movement of lattice dislocations provide an accommodation process for grain-boundary sliding.     

Polycrystalline graphene: Atomic structure,energetics and transport properties

Recent experimental investigations show that large-area samples of graphene tend to be polycrystalline. Physical properties of such samples are strongly affected by the presence of intrinsic topological defects of polycrystalline materials—dislocations and grain boundaries. This article reviews recent progress in understanding dislocations and grain boundaries in graphene. First, a systematic approach towards constructing topological defects in graphene is introduced. Then, the review discusses the formation energies of these defects, stressing the dramatic stabilization of dislocations and small-angle grain boundaries in graphene due to the two-dimensional nature of this material. Finally, the electronic transport properties of polycrystalline graphene are considered, showing that topological defects may present novel opportunities towards engineering electronic devices based on graphene.     

Effect of atomic ordering on the role of grain boundaries in the plastic deformation of Ni<Subscript>3</Subscript>Fe alloy

The structure of dislocations and the defect structure of grain boundaries and their parameters in Ni₃Fe alloy with short-range order (SRO) and long–range order (LRO) at different stages of plastic deformation are studied by means of transmission diffraction electron microscopy using thin foils and replicas. It is found that atomic ordering reduces the Σ3 twins plasticizing effect, increases the density of grain boundary defects, slows their annihilation during deformation, and intensifies the microstrains at the triple junctions of grain boundaries.     

Synthesis and Characterization of a Polycrystalline Ionic Thin Film by Large-Scale Molecular-Dynamics Simulation

A simulation methodology for the synthesis of polycrystalline, ionic thin films is developed. The method involves the preparation of a polycrystalline substrate onto which a thin film is subsequently grown by crystallization from the melt. A detailed structural analysis of a textured sixteen-grain FeO film, with a grain size of approximately 4.7 nm, shows that the interiors of the grains are almost perfect single crystals with only a very few vacancies and no interstitials. The grains are delineated by 001 tilt grain boundaries; as expected, the low-angle grain boundaries in the film consist of arrays of dislocations, while the high-angle grain boundaries are relatively narrow and well ordered.     

Incipient plasticity and voids nucleation of nanocrystalline gold nanofilms using molecular dynamics simulation

In this study, the incipient plasticity and voids nucleation of nanocrystalline gold were investigated using a molecular dynamics simulation. The effects of mean grain size and temperature were evaluated in terms of the material's stress-strain diagram, Young's modulus, yield strength, common-neighbor analysis, slip vectors, and deformation behaviors. From the stress-strain diagram, at 300?K, the maximum stress value corresponding to a grain size of 3.2?nm was much lower and the stress curve was clearly different from those corresponding to other grain sizes. Young's modulus increased with increasing mean grain size. The inverse Hall–Petch relation was observed. The slip was the main deformation behavior at a mean grain size of 3.2?nm. Moreover, the internal stress was more pronounced with increasing temperature. At 700?K, the main deformation area range was concentrated in the lattice at the middle of the samples, resulting in an almost force–induced structural transformation phenomenon in the middle. Void damage occurred at the junction of three–grain boundaries during the tensile process. With decreasing mean grain size, the less internal differential slip was generated under the same temperature and strain conditions.