Capturing slip band formation in Ni3Al nanocubes during compression

The intermittent plasticity, and stochastic size effect, observed experimentally during the compression of fibbed Ni₃Al nanocubes are modelled by implementing gradient plasticity within a cellular automaton simulation. The simulation results are in agreement with the experimentally observed stress–displacement curves and can capture both their serrated and stochastic response. Analysing the simulation data allows for additional information to be obtained regarding the deformation mechanisms and microstructural changes during compression, such as the thickness of the individual localised slip zones that formed, as well as the total volume percentage that deformed plastically. Both of these length scales are shown to follow a size-dependent behaviour, suggesting that although the overall deformation may appear to be stochastic, the evolution of deformation still depends on the cube size.     

运用应变梯度塑性理论模拟颗粒增强铝合金强度及延伸率的尺寸效应

运用基于细观机制的应变梯度塑性理论模拟了不同晶粒尺度、不同第二相颗粒直径及体积分数的铝合金应力应变曲线.结果表明,在相同条件下,随着第二相颗粒直径的减小,或随着第二相体积分数的增加,合金的强度明显增强.相反,随着第二相颗粒体积分数的增加,或随着第二相颗粒直径的减小,合金的均匀延伸率均有所下低.同时对不同晶粒尺寸的铝合金应力应变相应的分析表明,第二相颗粒分布的不均匀性对其力学性能也有一定的影响.     

Zirconium-Induced Softening in Hyperstoichiometric Ni3Al

The room temperature compressive properties and microhardness of Ni3AI alloys doped with Zr were studied. For the hypostoichiometric Ni3AI alloys, the compressive strength and microhardness increased with an increase in Zr content, while softening behavior induced by doping with a certain amount of Zr was observed in hyperstoichiometric Ni3AI alloy. Possible mechanisms for the softening effect were suggested.     

长程有序金属间化合物Fe_3Al合金的研究现状与发展

近年来,金属间化合物 Fe_3Al 合金由于具有高强度、耐腐蚀、低密度和低成本等一系列优点而受到世界各国材料科学工作者的普遍重视,开展了大量的研究工作,获得了许多基本数据,取得了很多有规律的结果。本文对国外在 Fe_3Al 方面发表的大量文献和作者近两年开展的工作进行综合评述,主要内容包括相结构和晶体缺陷;物理和力学性能;合金脆性以及组织结构和力学性能等方面。     

Dynamic observation of dislocation-free deformation process in Al, Cu, and Ni thin foils

Dislocation-free plastic deformation, which occurs under extraordinarily high internal stress comparable to ideal strength of metals, was discovered in thin foil portion produced by ductile fracture of fcc Au by dynamic observation of the deformation process [1, 2, 3, 4 and 5]. In the present study, the deformation process of thin foil portion in other fcc metals (Al, Cu, Ni) was examined in the same manner. In all these fcc metals, production of vacancy-type point defect clusters was confirmed during deformation without dislocations. Also, the dislocation-free deformation was found to progress under extraordinarily high internal stress levels corresponding to 14% elastic deformation in Ni, 12% in Cu, and 4% in Al. Especially in Al, as temperature decreased, the number density of stacking fault tetrahedra produced during deformation increased, along with increasing of the detected elastic deformation. These results indicate that internal stress level is a key factor in generalizing the new theory regarding dislocation-free plastic deformation.     

金属间化合物Ni3Al薄膜的制备及性能研究

采用直流磁控共溅射法,在衬底温度为450℃的SiO2基体上制备了厚度为500nm的Ni3Al薄膜,X射线衍射(XRD)和透射电子显微镜(TEM)等测试表明,薄膜为(111)取向的L12型晶体结构金属间化合物。采用纳米压痕方法测试了薄膜的力学性能,其硬度为8.00GPa,弹性模量为200GPa。为克服亚微米级薄膜氧化增重难以测量的困难,采用四探针测试金属薄膜电阻的方法,间接给出了薄膜的腐蚀性能和高温氧化程度。结果表明Ni3Al金属间化合物薄膜的氧化速率为2.28×10-13g2/(cm4.s),薄膜具有良好的高温抗氧化性能。     

SiC颗粒尺寸对Al2O3-SiC纳米复合陶瓷的影响

采用非均相沉积法制备含有不同粒径ＳｉＣ颗粒的Ａｌ２Ｏ３－ＳｉＣ复合粉体,粉体呈Ａｌ２Ｏ３包裹ＳｉＣ的开貌,经热压烧结获得致密烧结体,通过ＳＥＭ观察,Ａｌ２Ｏ３基体晶粒尺寸随着加入ＳｉＣ颗粒粒径的减小而减小。但减小的趋势比Ｚｅｎｅｒ模型预测的弱,力学性能随着加入ＳｉＣ颗粒粒径的减小而得到改善,这主要同ＳｉＣ颗粒对基体的弱化作用减弱及基体粒径变小有关。     

Hf在Ni和Ni3Al中的扩散行为

用含Ｈｆ扣间层合金钎接或暂存液相连接单晶高温合金时，为了使Ｈｆ尽快地由焊缝区向本合金扩散，研究了Ｈｆ在基体相Ｎｉ（γ）和Ｎｉ３Ａｌ（γ）中的扩散行为，并算出扩散系娄和扩散激活性，找到了扩散九与温度，浓度的关系，为选择合适的焊接条件提供依据。     

Superhard Nanobuttons: Constraining Crystal Plasticity and Dealing with Extrinsic Effects at the Nanoscale

The compressive plastic strength of nanosized single‐crystal metallic pillars is known to depend on their diameter D. Herein, the role of pillar height h is analyzed instead, and the suppression of the generalized crystal plasticity below a critical value h_CR is observed. Novel in situ compression tests on regular pillars as well as nanobuttons, that is, pillars with h < h_CR, show that the latter are much harder, withstanding stresses >2 GPa. A statistical model that holds for both pillars and buttons is formulated. Owing to their superhard nature, the nanobuttons examined here underline with unprecedented resolution the extrinsic effects—often overlooked—that naturally arise during testing when the Saint‐Venant assumption ceases to be accurate. The bias related to such effects is identified in the test data and removed when possible. Finally, continuous hardening is observed to occur under increasing stress level, in analogy to reports on nanoparticles. From a metrological standpoint the results expose some difficulties in nanoscale testing related to current methodology and technology. The implications of the analysis of extrinsic effects go beyond nanobuttons and extend to nano‐/microelectromechanical system design and nanomechanics in general.     

Al2O3 Particle Size Effect on Reinforced Copper Alloys: An Experimental Study

Abstract: Bonding between dissimilar metallic (copper) and non‐metallic particles (Al₂O₃) was successfully achieved. The mechanical properties of the copper composite nano‐structure with different Al₂O₃ particle sizes ranging from 100 nm to 20 μm were investigated. The alloys were prepared by using electro‐less precipitation and particle metallurgy technique at three different sintered temperatures, 900, 950 and 1000 °C. The chemical composition of composites were 85 atomic % Cu and 15 atomic % Al₂O₃, of which commercially available Al₂O₃ particles with a purity of 99.5% were used. The microstructure, hardness and compressive stress–strain behaviours were determined. A pin‐on‐disc system was used to study wear. Electrical resistivity was also measured.     

18Ni(250)钢伸长率的尺寸效应

实际生产中检测18Ni(250)钢的伸长率并不高,当钢板厚度为1.4 mm时.其伸长率在4%左右,似乎其塑性很差,接近于脆性材料。通过对不同厚度的18Ni(250)钢板进行拉伸试验,研究了该钢伸长率较低的原因。结果表明:该钢伸长率有尺寸效应,即伸长率随着拉伸试样截面尺寸的减小而减小;该钢塑性很好,其伸长率反映的塑性是一个假象。     

TiN-Al2O3纳米复合材料的力学性能和导电性能

以纳米TiN和α-Al2O3粉体为原料,采用球磨混合法制备了纳米TiN-Al2O3复合粉体,通过热压烧结得到致密烧结体.研究了纳米TiN颗粒对Al2O3材料力学性能和导电性能的影响,实验结果表明:在Al2O3基体中加入15vol％TiN纳米颗粒时,Al2O3材料的弯曲强度和断裂韧性分别从370MPa和3.4MPa·m1/2提高到690MPa和5.1MPa·m1/2,随着TiN添加量的增加,复合材料的电阻率逐渐降低,在25vol％TiN时达到最低值(6.5×10-3Ω·cm).     

Ni3Al合金热/机械疲劳裂纹扩展行为研究

对ＩＣ１０定向凝固高温合金进行了４５０￣９９０℃热／机械疲劳裂纹扩展行为的试验研究。分别研究了相位角、保持时间、温度、频率对ＩＣ１０合金裂纹扩展行为的影响,并建立了热／机械疲劳裂纹扩展速率预测模型。研究发现：温度的升高和频率的降低均会加速裂纹扩展;同相位热／机械疲劳裂纹扩展速率大于反相位热／机械疲劳裂纹扩展速率,而且它们两者介于最大温度和最小温度的等温度裂纹扩展速率之间;载荷保持加速裂纹扩展。     

核壳结构纳米Ni／Al复合粉末的制备

在含氟离子的水溶液中，采用Al粉直接置换还原Ni盐的方法，实现了纳米Ni在Al粉表面上的快速化学沉积，制备出核壳结构的Ni／Al复合粉末。探讨了反应的过程，利用SEM、XRD、BET等测试手段对复合粉末进行了微观测试和表征。结果表明：Al粉表面连续、均匀包覆了由晶粒大小约20．6nm的Ni纳米颗粒组成的壳层。     

残余热应力对Al2O3/Ni金属陶瓷断裂行为和力学性能的影响

运用残余热应力理论定性地解释也残余热应力对Al2O3/Ni金属陶瓷断裂行为和力学性能的影响，Ni颗粒位于Al2O3晶内或Ni含量低时，在Al2O3-Al2O3晶界产生张应力，易发生沿晶断裂；而其位于Al2O3晶界或Ni含量高时，在Al2O3-Al2O3晶界产生压应力，易产生穿晶断裂。     

Finite element analysis of geometrically necessary dislocations in crystal plasticity

We present a finite element method for the analysis of ductile crystals whose energy depends on the density of geometrically necessary dislocations (GNDs). We specifically focus on models in which the energy of the GNDs is assumed to be proportional to the total variation of the slip strains. In particular, the GND energy is homogeneous of degree one in the slip strains. Such models indeed arise from rigorous multiscale analysis as the macroscopic limit of discrete dislocation models or from phenomenological considerations such as a line‐tension approximation for the dislocation self‐energy. The incorporation of internal variable gradients into the free energy of the system renders the constitutive model non‐local. We show that an equivalent free‐energy functional, which does not depend on internal variable gradients, can be obtained by exploiting the variational definition of the total variation. The reformulation of the free energy comes at the expense of auxiliary variational problems, which can be efficiently solved using finite element approximations. The addition of surface terms in the formulation of the free energy results in additional boundary conditions for the internal variables. The proposed framework is verified by way of numerical convergence tests, and simulations of three‐dimensional problems are presented to showcase its applicability. A performance analysis shows that the proposed framework solves strain‐gradient plasticity problems in computing times of the order of local plasticity simulations, making it a promising tool for non‐local crystal plasticity three‐dimensional large‐scale simulations. Copyright © 2012 John Wiley & Sons, Ltd.     

镍离子注入对Al2O3陶瓷机械性能的改善

离子注入工艺是一种新兴的束流表面强化技术,离子注入陶瓷材料可改变陶瓷材料的表面硬度、表面断裂韧度、抗弯强度等机械性能.本文在不同工艺条件下,观察Al-2O-3陶瓷表面注入镍离子后机械性能的变化,发现最佳注入离子剂量5×1016ioncm-2时机械性能有很大改善.结合应力分析,发现陶瓷表面压应力对机械性能的改善有很大贡献.     

23Co13Ni11Cr3Mo超高强钢的高温变形行为

在Glreeble 1500热模拟机上进行大变形等温压缩实验,研究了23Co13Ni11Cr3Mo钢的高温变形行为.结果表明:流变应力随着变形温度的降低和应变速率的升高而增大,在高温、低应变速率下动态再结晶软化效应显著,在温度为1000℃、应变速率为0.001 s-1时流变应力下降29.6％.23Co13Ni11Cr3...