The effect of indenter angle on the microindentation hardness

Nix and Gao established an important relation between the microindentation hardness and indentation depth. It agrees well with the microindentation experiments for the Berkovich indenter. However, it is unclear whether it holds for other indenters with different indenter angles. We use the conventional theory of mechanism-based strain gradient plasticity established from the Taylor dislocation model to study the effect of indenter angles on the microindentation hardness. It is shown that the Nix–Gao relation between the microindentation hardness and indentation depth also holds for other indenter angles. The effect of friction is negligible for relatively flat indenters (e.g. Berkovich indenter), but may be significant for sharp indenters (e.g. cubic indenter).     

Influence of pre-existing dislocations on the pop-in phenomenon during nanoindentation in MgO

In order to study the influence of pre-existing dislocations on the pop-in phenomenon in magnesium oxide, an original experimental protocol, based on low-rate cleavage, has been developed to introduce a controlled dislocation density in MgO single crystals. Nanoindentation tests have been performed using a spherical indenter and the pop-in load has been measured as a function of the pre-existing dislocation density. The dislocation structures have been then characterized individually by chemical etching and atomic force microscopy observations. A double etching method has allowed distinguishing the pre-existing dislocation behaviour in the indenter stress field and the dislocation nucleated below the indenter during the pop-in. These experiments show that the pre-existing dislocations lower the pop-in load and promote the dislocation nucleation below the contact area. However, the analysis of the dislocation structures nucleated during the pop-in shows no direct relation between the pre-existing dislocations and those nucleated during the pop-in.     

Nanoindentation behavior of nanotwinned Cu: Influence of indenter angle on hardness,strain rate sensitivity and activation volume

The influence of strain on the mechanical properties and deformation kinetic parameters of nanotwinned (nt) copper is investigated by a series of nanoindentation experiments, which were performed by employing sharp indenters with five varying centerline-to-face angles (ψ). Comparison experiments were also conducted on (1 1 0) single crystalline Cu. Experimental results indicate that, unlike coarse-grained materials, nt-Cu is prone to plastic flow softening with large material pile-up around the indentation impression at high levels of strains. Localized detwinning becomes more significant with decreasing ψ, concomitant with reduced strain-rate sensitivity (m) and enhanced activation volume (V^*). The m of nt-Cu is found to depend sensitively on ψ with a variation of more than a factor of 3, whereas V^* exhibits a much less sensitive trend. This paper discusses the validation of the experimental techniques and the implications of various deformation kinetic parameters on the underlying deformation mechanisms of nt-Cu.     

Electronic transport in Si and Ge polymers

Time-resolved transport studies which have now been carried out on a wide variety of alkyl- and aryl-substituted Si and Ge backbone polymers demonstrate the ability of these unipolar systems to transmit holes through the specimen bulk with negligible loss of these positive carriers to deep traps, making these materials attractive candidates for electronic device applications. Transport has been studied by measuring drift mobilities as a function of field, temperature and composition, using the small-signal time-of-flight technique. Transport occurs among chain backbone derived states, yet exhibits behavior essentially identical to systems in which transiting carriers are known to undergo thermally activated tunneling transitions among discrete molecular sites. On this basis it is suggested that disorder causes the chain backbone to be suborganized into domain-like units of varying conjugation length. The sensitivity of transport to large-scale thermodynamic processes, sidegroup substituent and effect of specific dopants is demonstrated.     

Sn合金化对Gd5Si2Ge2磁致冷材料结构和性能的影响

以商业纯Gd为原料,采用非自耗电弧炉氩气保护下熔炼了Gd5Si2Ge2-xSnx(x=0.2,0.5,1)和Gd5Si2-yGe2Sny(y=0.1,0.2,0.5)系列合金,研究Sn合金化对Gd5Si2Ge2晶体结构和磁热性能的影响.粉末XRD结果表明Sn代Ge样品具有正交Gd5Si4型结构;Sn少量代Si(y=0.1,0.2)的样品具有单斜Gd5Si2Ge2型结构;Gd5Si1.5Ge2Sn0.5则为单斜和正交的混合结构.用超导量子磁强计(SQUID)测定了样品的M-T和不同温度的M-H曲线,结果表明Gd5Si2Ge2-xSnx(x=0.2,0.5,1)不具有巨磁热效应;Gd5Si1 9Ge2Sn01合金的最大磁熵变达15.3 J/kg·K(0 T～5 T),具有巨磁热效应.     

Time-resolved studies of electronic transport in Si and Ge backbone polymers

Time-resolved transport studies in alkyl- and aryl-substituted Si and Ge polymers, together with xerographic discharge measurements, clearly demonstrate the ability of a wide variety of these polymers to transmit holes, photogenerated (or photoinjected) at one surface under an applied field, through the film bulk to the opposite surface with negligible loss of these carriers to deep traps. The average carrier velocity per unit field (i.e., the drift mobility μ) is measured by the small-signal current-mode time-of-flight technique (TOF). Two key results are discussed in detail: (1) the mechanism of electronic transport in the glassy state, which is established to be hopping among energetically inequivalent sites on the polymer main chain (rather than band motion); and (2) elucidation of the mechanism by which drift mobility in a given polymer can be modified by chemical doping.     

Mechanical properties of intermetallic phases in multi-component Al–Si alloys using nanoindentation

The presence of additional elements in a multi-component Al–Si alloy system allows many complex intermetallic phases to form. The mechanical properties of different intermetallic phases have been investigated using nanoindentation. In particular, the hardness and modulus of a number of phases have been established for a range of alloy compositions. The results show that both hardness and reduced modulus increase as the Ni ratio of the Al–Cu–Ni phases increases. The elastic modulus can be correlated with the formation temperature of the intermetallic phases. The intermetallic phases with a high heat of formation have a strong binding between atoms and therefore, their elastic modulus is also higher.     

Effects of indenter geometry and material properties on the correction factor of Sneddon’s relationship for nanoindentation of elastic and elastic–plastic materials

Finite element simulations of indentations on elastic and elastic–plastic materials have been performed to systematically study the effects of indenter geometry and mechanical properties, namely Poisson’s ratio and the E/σ_y ratio, on the correction factor for Sneddon’s equation used for analysis of nanoindentation data. Two indenter geometries, namely conical and Berkovich indenters, have been considered. It has been shown that the first-order correction for conical indentation on elastic materials previously developed by Hay et al. [Hay JC, Bolshakov A, Pharr GM. J Mater Res 1999;14:2296–305] can be applied only to conical indentation of elastic deformation-dominated materials but not to Berkovich indentation. A new general relationship for the estimation of the correction factor for Berkovich indentation is proposed based on the finite element simulation results. This relationship gives a better estimation of correction factor for Berkovich indentations on both elastic and elastic–plastic materials.     

Discrete and continuous deformation during nanoindentation of thin films

This paper describes nanoindentation experiments on thin films of polycrystalline Al of known texture and different thicknesses, and of single crystal Al of different crystallographic orientations. Both single-crystalline and polycrystalline films, 400–1000 nm in thickness, are found to exhibit multiple bursts of indenter penetration displacement, h, at approximately constant indentation loads, P. Recent results from the nanoindentation studies of Suresh et al. (Suresh, S., Nieh T.-G. and Choi, B.W., Scripta mater., 1999, 41, 951) along with new microscopy observations of thin films of polycrystalline Cu on Si substrates are also examined in an attempt to extract some general trends on the discrete and continuous deformation processes. The onset of the first displacement burst, which is essentially independent of film thickness, appears to occur when the computed maximum shear stress at the indenter tip approaches the theoretical shear strength of the metal films for all the cases examined. It is reasoned that these displacement bursts are triggered by the nucleation of dislocations in the thin films. A simple model to estimate the size of the prismatic dislocation loops is presented along with observations of deformation using transmission electron microscopy and atomic force microscopy. It is demonstrated that the response of the nanoindented film is composed of purely elastic behavior with intermittent microplasticity. The overall plastic response of the metal films, as determined from nanoindentation, is shown to scale with film thickness, in qualitative agreement with the trends seen in wafer curvature or X-ray diffraction measurements.     

大直径棒材二辊矫直安装角对工艺参数的影响

二辊矫直机在辊形曲线确定后,通常通过调整矫直辊的安装角度来实现不同直径棒材的矫直。针对矫直辊的安装角度调整原则问题,运用大型非线性有限元软件ABAQUS建立二辊矫直的弹塑性有限元模型,模拟了大直径棒材二辊矫直过程,比较分析了在不同矫直辊安装角度的情况下,棒材矫后的直线偏差、残余应力、等效塑性应变以及矫直过程中的矫直力,综合得出了矫直辊安装角度的调整原则。根据获得的调整原则进行了实际实验验证,实验结果与有限元分析结论相吻合。     

Deformation of anodic aluminum oxide nano-honeycombs during nanoindentation

Anodic aluminum oxide with a nano-honeycomb structure is subjected to nanoindentation along the axial direction of the honeycomb. The load–displacement behavior is discontinuous with periodic strain excursions. Top-view and cross-sectional microscopic examination reveals a localized mode of deformation with very clear-cut elastoplastic boundaries. A crack system that is self-similar with respect to the indent size is also found, and this is thought to correspond to the discontinuous load–displacement behavior. A simple column model is proposed to explain certain features of the deformation microstructure.     

Possibility of high-pressure transformation during nanoindentation of SiC

High-pressure transformation to the rocksalt structure has been proposed as a mechanism that underlies the ductile wear observed during nanomachining of SiC. However, in contrast to other brittle materials (e.g. Si), no such transformation has been directly observed either during machining or during nanoindentation of SiC. Here, we performed large-scale molecular dynamics simulations of nanoindentation with spherical indenters of various sizes and surface roughness to determine whether SiC can undergo a nanoindentation-induced transformation from the zincblende to the rocksalt structure. The calculations of possible states of stresses under the indenter have been combined with a thermodynamic analysis to estimate the effects of dislocation density, shear stresses and temperature on the phase transformation pressure in SiC. Our analysis shows that the high-pressure transformation is highly unlikely under the conditions of nanomachining. We conclude that the primary response of SiC to nanoindentation is dislocation nucleation and propagation in the low-pressure (zincblende) phase.     

Correlation between dislocation density and nanomechanical response during nanoindentation

The crucial role of dislocations in the nanomechanical response of high-purity aluminum was studied. The dislocation density in cold-worked aluminum is characterized by means of electron channeling contrast and post-image processing. Further in situ heat treatment inside the chamber of a scanning electron microscope was performed to reduce the dislocation density through controlled heat treatment while continuously observing the structure evolution. The effect of dislocation density on both the pure elastic regime before pop-in as well as elastoplastic deformation after the pop-in were examined. Increasing the dislocation density and tip radius, i.e. the region with maximum shear stress below the tip, resulted in a reduction in the pop-in probability. Since the oxide film does not change with dislocation density, it is therefore clear that pop-ins in aluminum are due to the onset of plasticity by homogeneous dislocation nucleation and not oxide film breakdown. Hertzian contact and the indentation size effect based on geometrically necessary dislocations are used to model the load-displacement curves of nanoindentation and to predict the behavior of the material as a function of the statistically stored and geometrically necessary dislocation density.     

增强调制掺杂Si80Ge20基热电材料功率因子的研究

采用成熟工艺制备了N型、P型调制掺杂型Si80Ge20基固溶体合金及等化学计量比的均匀掺杂型Si80Ge20基固溶体合金,重点研究了两类固溶体合金的热电性能。结果表明：温度为773 K时,N型系列、P型系列,调制掺杂型固溶体合金较均匀掺杂型的功率因子分别提高了13.6%和49.2%,热电优值ZT分别提高了7.9%和12.9%     

On the determination of coating toughness during nanoindentation

Nanoindentation tests have been widely used to evaluate fracture toughness of brittle materials. For the thin coatings, energy based models have been shown to be effective. In this study, a new energy based method is proposed based on the analysis of unloading curve at the start and end points of the crack induced pop-in. The semi-analytical generalized expressions have been presented to determine the fracture toughness of coatings under both load and displacement control. This provides valuable theoretical guideline to determine fracture toughness from the energy point of view.     

Plastic deformation in nanostructured bulk glass composites during nanoindentation

Nanoindentation experiments of a Ti₄₅Zr₁₆Be₂₀Cu₁₀Ni₉ bulk metallic glass and partially vitrified nano-composite metallic glass matrix have been performed under a constant maximum load of 10 mN and constant loading rate of 0.08 mN s^?1 with the aim of comparative study of their micro-plastic deformation behavior. Remarkable difference in deformation behavior was found in load–displacement curves of nanoindentation and pile-up morphologies around the indents. The difference in shear banding behavior has been attributed to the presence of nanosized icosahedral particles in amorphous matrix.     

Incipient plasticity and deformation mechanisms in single-crystal Mg during spherical nanoindentation

Incipient plasticity in Mg single crystals was investigated using the pop-ins generated during spherical nanoindentation on (0 0 0 1), (1 0 ?1 2) and (1 0 ?1 0) surfaces. Representative deformed regions extracted from underneath indents by means of focused ion beam machining were examined by transmission electron microscopy (TEM) to identify the deformation mechanisms. Anisotropic elastic Hertzian contact theory was used to calculate indentation Schmid factors and the relevant resolved shear stresses at pop-in from the load–displacement curves. The pop-in statistics in conjunction with the TEM analysis showed that the most likely deformation mechanism responsible for pop-in is slip via 〈a〉 dislocations even in the case of indentation along the c-axis.     

Yb2Ge,Eu2Ge and Eu2Si: new PbCl2-type compounds

The intermetallic compounds Yb₂Ge, Eu₂Ge and Eu₂Si were synthesized from the elements by HF melting in tantalum crucibles. The three phases crystallize in the PbCl₂ structure type, as shown by Rietveld refinement of powder pattern intensity data. The structure of Yb₂Ge was confirmed by a single crystal study.     

TEM study on Si0.65Ge0.35/p-Si HIP infrared detector

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