Tension-Compression Asymmetry in Mechanical Behavior and Crystal Defect of Thin Ni/Ni3Al (001) Nanowires

利用分子动力学方法研究超薄Ni/Ni3Al(001)纳米线力学性能和晶体缺陷的拉压不对称,对应力-应变曲线和晶体曲线在不同的温度下进行比较。模拟表明在拉伸载荷作用下不全位错的施密特因子大于压缩载荷下的施密特因子,在10 K温度下的流变应力行为异常,同时超薄Ni/Ni3Al(001)纳米线在不同温度下都表现为抵抗压缩载荷能力比拉伸载荷强。结果显示超薄Ni/Ni3Al(001)纳米线具有显著的拉伸不对称特性。此外,堆积层错提高原子移动,不全位错从堆垛层错处发射。在促进位错发射过程中原子移动扮演着重要的角色;而且在拉-压载荷下不同晶体缺陷主要是点位错和层错,层错主要发生在4个{111}方向。研究拉压不对称与温度之间的关系,可以更准确和全面的理解超薄Ni/Ni3Al(001)纳米线的力学性能。     

Effect of temperature on mechanical behavior of AZ31 magnesium alloy

Strain rate sensitivity and tension/compression asymmetry of AZ31 magnesium alloy at different temperatures and strainrates were investigated.Both of mechanical behaviors are temperature dependent.Strain rate sensitivity increases with increasingtemperature.Thermally activated slip is the source of strain rate sensitivity.At the temperature below or near 373 K,strain ratesensitivity is very little.Tension/compression asymmetry in yielding decreases with increasing temperature.Twinning is the reasonof tension/compression asymmetry.At the temperature above or near 573 K,the material shows little tension/compressionasymmetry of the flow stress.     

Mechanical response of nanoporous gold

We report the results of computational tensile and compressive tests for model bi-continuous nanoporous gold structures using atomistic simulations with empirical many-body potentials and molecular dynamics. The results are compared with the predictions of scaling laws for coarser-scale foams and with available experimental data. We find a surprising substantial tension/compression asymmetry in yield due to the surface stress that sets the filament under compression, providing a bias favoring yielding in compression. We provide a model for our results based on a ligament strength value close to the theoretical strength of Au, and the surface stress. The model predicts a significant tension/compression asymmetry for ligament sizes below ～10 nm and pore collapse for ligament sizes below 1 nm. We also observe an unexpected compaction tendency under tension characterized by a decrease in the total volume of the sample of 15% for samples deformed under tension by 30%. The mechanism of the compaction involves a decrease in the average pore size and pore collapse resulting from plasticity within the ligaments.     

Mechanical and Corrosion Behavior of a Biomedical Mg-6Zn-0.5Zr Alloy Containing a Large Number of Twins

The strong texture of Mg alloys can lead to strong tension–compression yield asymmetry and corrosion anisotropy, and this will consequently affect the effectiveness of hard tissue implants. A biomedical Mg–6Zn–0.5Zr alloy containing a large number of {10$\overline{1}$2} primary twins and {10$\overline{1}$2}–{10$\overline{1}$2} secondary twins is successfully prepared by cross compression. The dual twin structure not only removes the tension–compression yield asymmetry completely, but effectively reduces the corrosion anisotropy without compromise of corrosion resistance. The difference between the largest corrosion rate and smallest one is ~ 1.2 times compared to ~ 1.6 times of the original materials. It is found that the reduced corrosion anisotropy is related to re-distribution of crystallographic orientations by twins.     

Structural performance of metallic sandwich beams with hollow truss cores

The article focuses on the structural performance of sandwich beams with hollow truss lattice cores made from a ductile stainless steel. The trusses are arranged in an orthogonal (cross-ply) configuration, in either ±45° (diamond) or 0°/90° (square) orientations with respect to the face sheets. The responses in shear, tension and compression, as well as simply supported and fully clamped bending, are measured for specimens with both core orientations. While the two cores perform equally well in compression, the diamond orientation exhibits higher shear strength but lower stretch resistance. For bend-dominated loadings of the sandwich beams, the core in the diamond orientation is preferred because of its higher shear strength. For stretch-dominated loadings encountered in large-displacement, fully clamped bending, the square orientation is superior. Models of core and beam yielding are used to rationalize these observations. Optimizations are then performed to identify strong lightweight designs and to enable performance comparisons with other sandwich structures.     

Two-phase intermetallic alloy Ni3(Al, Si):microstructure and mechanical properties

Yield stress in compression (0.2% flow stress) from ambient temperature up to 800 °C has been studied on Ni₃(Al, Si) alloy with the atomic composition Ni₇₈Al₁₁Si₁₁. When annealed at 1000 °C, the alloy has a pure L1₂ (γ′) ordered structure. After subsequent annealing at 750 °C, the disordered solid solution of Al and Si in Ni (face centred cubic, γ) precipitates in fine coherent particles. Calorimetry helps to describe the various phase transformations necessary to obtain the last microstucture. Solute addition of Si, which replaces Al atoms, increases the 0.2% flow stress of Ni₃Al in the fully γ′ microstructure. The γ precipitation shifts the peak stress towards higher temperatures and stresses.     

Role of yttrium content in twinning behavior of extruded Mg—Y sheets under tension/compression

The influence of Y content on the grain-scale twinning behavior in extruded Mg—xY (x=0.5, 1, 5, wt.%) sheets under uniaxial tension and compression along the extruded direction was statistically investigated. An automatic twin variant analysis was employed, based on large data sets obtained by electron backscatter diffraction (EBSD), including 2691 grains with 977 twins. The {10

2} tension twinning (TTW) dominance and prevailing anomalous twinning behavior (Schmid factor (m) <0) under both tension and compression were found. The anomalous twinning behavior was more pronounced as Y content increased under tensile loading, indicating a promoted stochasticity of twin variant selection for more concentrated Mg—Y alloys. However, the trend for the Y-content dependent anomalous twinning behavior was opposite in compression. The fractions of the anomalous TTWs were found to be well correlated with the maximum Schmid factor (m_max) values of basal slip and prismatic slip in the corresponding parent grains for compression and tension, respectively, indicating that twinning and dislocation slip might be closely related in the present Mg—Y alloys.     

EBSD studies of the stress-induced B2–B19′ martensitic transformation in NiTi tubes under uniaxial tension and compression

In situ electron backscattering diffraction (EBSD) investigations were conducted on polycrystalline NiTi tube specimens during tensile and compressive deformation. The long-range cooperative and catalytic martensitic transformation under tension induces the transformation to proceed in the form of helical Lüders band. Propagation of the band is closely related to the spatial distribution of the orientations of individual grains. In uniaxial compression, the larger variation in Schmid factors, and consequently the larger variation in the critical transformation stresses among grains, leads to a homogeneous martensitic transformation, and therefore the absence of the Lüders band. To interpret the observed tension–compression asymmetry, a crystallographic model of the critical transformation stress and transformation strain for polycrystalline NiTi under tension and compression is proposed. The model defines three crystallographic regions: tension-favorable, compression-favorable and neutral zones. The orientation population in which tensile strains are larger than compressive strains is much higher than that of orientations with higher compressive strains. For resolved shear stress, orientation populations favoring tension and compression do not show any great difference.     

Tension–compression asymmetry of the stress–strain response in aged single crystal and polycrystalline NiTi

The purpose of this work is to thoroughly understand tension–compression asymmetry in precipitated NiTi using unique experimental results and micro-mechanical modeling. For the first time, tensile and compressive stress–strain behaviors were established on aged single crystals ([100], [110], and [111] orientations) and polycrystalline NiTi. The single crystal and polycrystalline Ti–50.8 at.% Ni materials were given both peak aged and over aged heat treatments. The drawn polycrystalline NiTi has a strong texture of the 〈111〉{110} type, thus it deformed in a manner consistent with the [111] single crystals. In contrast to the phenomenological theory of martensitic transformations (analogous to Schmid's law), the critical resolved shear stress required to trigger the transformation, τ_crss, in the peak-aged single crystals was dependent on both the stress direction and crystallographic orientation. Using micro-mechanical modeling, the deviation from Schmid's law was attributed to the unique orientation relationship that exists between the Ti₃Ni₄ precipitates (their coherent stress fields) and the 24 martensite correspondence variant pairs. The over-aged single crystals generally obeyed Schmid's law within experimental error, consistent with the proposed micro-mechanical model. Qualitatively, the tension–compression asymmetry and orientation dependence of the recoverable strain level, ε₀, was consistent with the phenomenological theory for martensitic transformations. However, the peak- and over-aged single crystals generally both demonstrated smaller ε₀ magnitudes than predicted. The differences for both crystals were attributed to the inhibition of martensite detwinning coupled with several unique microstructural effects.     

TA2拉压力学性能不对称性及应变速率敏感性

本文研究了不同应变速率下工业纯钛TA2室温拉伸和压缩力学行为,结果发现TA2具有明显的拉压屈服不对称性和拉压应变硬化不对称性;随着应变速率的增加,拉压不对称性有明显增加。采用电子背散射衍射（EBSD）技术分析了拉伸和压缩时TA2微观变形机制及微观组织演化规律,结果表明,TA2压缩时以孪晶变形机制为主,拉伸时以位错滑移为主;TA2微观变形行为的载荷敏感性导致了宏观力学性能的不对称性。     

Deformation behavior of Ni3Al single crystals during nanoindentation

The deformation behavior of (1 1 1), (1 1 0) and (0 0 1) oriented Ni₃Al single crystals was investigated by means of nanoindentation. Upon loading with the blunt and sharp indenter tips, the crystal deforms elastically in the initial stage followed by plastic deformation of which the onset is characterized by an obvious displacement burst (or pop-in) in the loading curve. This pop-in corresponds to homogeneous nucleation of dislocation loops under the indenter. When a sharp indenter tip was used, a major pop-in can be identified at a large load (7 mN for (1 1 1) crystal) in the plastic regime on loading. The major pop-in may be correlated with the special dislocation structure of the K–W locks in the Ni₃Al crystals that are formed during loading. The pop-in behavior during plastic deformation of Ni₃Al crystals is found to be closely related to the crystal orientation, pre-existing dislocation density in the sample surface, loading rate, and holding (at a constant sub-critical load) time as well. Pop-ins were also observed at critical loads in unloading processes.     

镍基单晶高温合金[111]取向拉压不对称分析

试验表明,[111]取向镍基单晶高温合金宏观滑移迹线符合六面体滑移特征,而微观位错滑移机制为八面体滑移。针对上述宏、微观现象,提出了一种"之"字形交滑移模型,使宏观六面体滑移迹线与微观位错的八面体滑移在该模型中得到了很好的统一。同时,在上述"之"字形交滑移变形机制的基础上,对[111]取向的拉压不对称特性进行了分析,提出一种位错分解方式,对[111]取向产生拉压不对称特性的微观机制进行了表征。最终,基于上述两种机理,建立了[111]取向镍基单晶高温合金的拉压不对称预测模型。该模型不仅能够对[111]取向镍基单晶高温合金屈服强度进行预测,而且可以表征屈服强度的拉压不对称特性,即压缩屈服强度大于拉伸屈服强度以及拉压不对称随温度升高愈加严重的现象。     

Anomalies of the plastic yield stress in the intermetallic compound Fe–30 at.% Al

The plastic properties of Fe–30 at.% Al were investigated in compression, cyclic tension/compression and shear tests between room temperature and 870 K. At elevated temperatures in all tests a positive temperature dependence of the yield stress was observed. At room temperature an asymmetry of the flow stress with respect to the deformation direction (tension or compression) was found, which disappeared for temperatures T⩾570 K. The results are discussed on the basis of the present state of knowledge about the decoupling of D0₃-superdislocations at elevated temperatures and the deformation behaviour of b.c.c. metals at low temperatures.     

Molecular Dynamics Simulations of the Orientation Effect on the Initial Plastic Deformation of Magnesium Single Crystals

Molecular dynamics simulation is employed to study the tension and compression deformation behaviors of magnesium single crystals with different orientations.The angle between the loading axis and the basal a direction ranges from 0° to 90°.The simulation results show that the initial defects usually nucleate at free surfaces,but the initial plastic deformation and the subsequent microstructural evolutions are various due to different loading directions.The tension simulations exhibit the deformation mechanisms of twinning,slip,crystallographic reorientation and basal/prismatic transformation.The twinning,crystallographic reorientation and basal/prismatic transformation can only appear in the crystal model loaded along or near the a-axis or c-axis.For the compression simulations,the basal,prismatic and pyramidal slips are responsible for the initial plasticity,and no twinning is observed.Moreover,the plastic deformation models affect the yield strengths for the samples with different orientations.The maximum yield stresses for the samples loaded along the c-axis or a-axis are much higher than those loaded in other directions.     

Directional solidification and microstructures of near-eutectic Cr–Cr3Si alloys

Near-eutectic Cr–Cr₃Si alloys were directionally solidified in a high-temperature optical floating zone furnace. At the eutectic composition, uniform and well-aligned lamellar structures were obtained over a fairly wide range of solidification conditions, but not at very low or very high growth rates where degenerate and cellular structures, respectively, were obtained. The lamellar spacing was found to increase with decreasing solidification rate, in agreement with the Jackson–Hunt theory. In addition, for a fixed growth rate, the lamellar spacing was found to increase with increasing rotation rate. The growth directions in the lamellar eutectic are found to be 1 0 0 for the Cr₃Si phase and 1 1 1 for the Cr solid-solution phase. Eutectic microstructures (rod or lamellar) could also be produced at off-eutectic compositions, but only for a limited range of growth conditions.     

Sigmoidal creep of Ni3(Al, Ta)

Incremental creep tests have been used to explore the time-dependent plastic behavior of single-slip oriented Ni₃(Al, Ta) at low temperatures in the anomalous flow regime. For selected incremental creep experiments at 20 and 100 °C, it was discovered that Ni₃Al exhibited sigmoidal creep, where there is a significant time delay before the plastic strain rate accelerates to a maximum value during a creep experiment. Several of the factors that affect the sigmoidal creep response have been identified. The origin of sigmoidal creep is accounted for using a simple model of work hardening in Ni₃Al, where the acceleration of the creep rate is a direct result of the annihilation of the existing dislocation substructure.     

时效消除AZ80镁合金的屈服不对称性(英文)

AZ80镁合金在380°C挤压之后,分别在170°C和310°C下进行不同时间的时效,以比较析出相的形貌和结构对镁合金屈服不对称性的影响。对沿挤压方向切取的样品在室温下进行单轴拉伸及压缩实验。结果发现,时效样品的拉压屈服不对称显然比未时效挤压样品的弱很多,这是因为时效样品中的孪生发生远少于挤压样品中的。此现象可以通过时效样品中孪生的临界剪切力增量比基面滑移的增量大,而其取向因子却减小使得其发生更困难来证实。镁合金中的拉压屈服不对称性可以通过增加析出相分数来减弱甚至消除。实验结果显示,当合金内析出相分数达到饱和后,拉压不对称性消失。     

Experimental and finite-element analysis of the anisotropic response of high-purity α-titanium in bending

In this paper, we present results of four-point bending tests performed on beams of high-purity α-titanium material. These tests have been performed at room temperature for different beam configurations and loading orientations with respect to the orthotropy axes of the material. Digital image correlation was used to determine local strains in the deformed beams. Experimental results compare very well with the predictions of finite-element simulations obtained using the elastic/plastic model developed by Nixon et al. (2010) [12]. Specifically, we compare local deformations and the cross-sections of each beam for all loading configurations. We show that the model predicts with great accuracy the tension–compression asymmetry and the evolving anisotropy of the material. The experimentally observed upward shift of the neutral axis, as well as the rigidity of the response along the hard to deform c-axes are very well described by the proposed model.     

Tin whiskers studied by synchrotron radiation scanning X-ray micro-diffraction

A large number of Sn whiskers have been found on the Pb-free solder finish on leadframes used in consumer electronic products. Some of the whiskers on eutectic SnCu finishes are long enough to short the neighboring legs of the leadframe. Tin whisker growth is known to be a stress relief phenomenon. We have performed synchrotron radiation X-ray micro-diffraction analysis to measure the local stress level, the orientation of the grains in the finish around a whisker, and the growth direction of whiskers. The compressive stress in the solder finish is quite low, less than 10 MPa; nevertheless, there exists a stress gradient around the root of a whisker. From the orientation map and pole figure, we found that the growth direction of whiskers is [0 0 1] and there exists a preferred orientation of [3 2 1] grains on the solder finish. In one of the whisker analyzed, we found that the normal orientation of the grain just below the whisker is different; it is [2 1 0].     

高应变速率下AZ31镁合金的各向异性及拉压不对称性

采用分离式霍普金森拉杆及压杆装置,研究挤压态AZ31镁合金高速变形下的各向异性及拉压不对称性,并从微观变形机制的角度探讨具有强烈初始基面织构的挤压态镁合金各向异性及拉压不对称性产生的原因。结果表明:在高速变形条件下,依据加载方向及应力状态挤压态AZ31镁合金的拉伸行为表现出很强的各向异性,但压缩行为的各向异性不明显;在挤压方向表现出很强的拉压不对称性,而在垂直于挤压方向的拉压不对称性很低。挤压态AZ31镁合金宏观上的各向异性及拉压不对称性是由于不同的微观变形机制所引起的。沿挤压方向拉伸的主要变形机制为柱面滑移,沿垂直于挤压方向拉伸及压缩的主要变形机制为锥面滑移;沿挤压方向压缩时初始变形机制为拉伸孪晶,当变形量为0.08(8%)左右时由于孪晶消耗殆尽,变形变而以滑移的方式进行。