Effect of coherency strain on the deformation of In x Ga1− x As superlattices under nanoindentation and bending

It has been shown elsewhere that the room temperature yield pressure of In _x Ga_{1? x} As superlattices measured by nanoindentation, decreases from a high value as the volume averaged strain modulation is increased, while at 500°C under uniaxial compression or tension the yield stress increases from a low value with increasing strain modulation. We have used cross-sectional transmission electron microscopy to examine the deformation mechanisms in these two loading regimes. At room temperature both twinning and dislocation flow was found with the proportion of twinning decreasing with increasing strain modulation. The coherency strain of the superlattice is retained in a twin but partially relaxed by dislocation flow. The strain energy released by the loss of coherency assists dislocation flow and weakens the superlattice. Twins are only nucleated when a critical elastic shear of about 7° is achieved at the surface. The plastic zone dimensions under the indent are finite at the yield point, with a width and depth of approximately 1.3?µm and 1.1?µm respectively. Under uniaxial compression and tension at 500°C the superlattices deform by dislocation flow along {111} planes. The most highly strained samples also partially relax through the formation of misfit dislocations.     

Yield phenomenon in oriented polyethylene terephthalate

The object of this investigation was to determine the origin of deformation bands in one-way oriented polyethylene terephthalate (PET) and their relation to the upper yield point. The tensile yield point was measured at room temperature as a function of the angle (θ) between the original draw direction and the tensile axis. The effect of strain rate was observed. The growth and geometry of deformation bands were studied. The results showed that the formation of a deformation band in PET cannot be interpreted as the strength-limiting yield mechanism for all θ and that the viscoelastic behavior plays the primary part in determining the yield point of this semicrystalline polymer.     

Role of the entangled amorphous network in tensile deformation of semicrystalline polymers

Being composed of crystalline lamellae and entangled amorphous polymeric chains in between, semicrystalline polymers always show a complicated deformation behavior under tensile deformation. In recent years, the process of tensile deformation was found to exhibit several regimes: intralamellar slipping of crystalline blocks occurs at small deformation whereas a stress-induced crystalline block disaggregation-recrystallization process occurs at a strain larger than the yield strain. The strain at this transition point is related to the interplay between the amorphous entanglement density and the stability of crystal blocks. We report experimental evidence from true stress-strain experiments that support this argument. It is emphasized that tie molecules, which connect adjacent lamellae, are of lesser importance with respect to the deformational behavior.     

船载光电转台设计及特定环境下可行性分析

结合船载光电转台的技术要求,通过pro/E软件设计舰载光电转台的机械结构。根据转台的表面材料,运用ABAQUS软件对其在海洋的等价静力风载、极端温度场和海浪振动3种特殊环境下进行表面变形分析,校验结构的应力和应变,确保满足船载光电转台的设计精度要求;得到在等价静力风载作用下,光电转台的最大应力点大小为255 MPa,小于钢的屈服强度355 MPa;在温差作用下最大应力点分别为174.8 MPa、247.1 MPa,均小于铝合金的屈服强度274 MPa;在海浪振动作用下,1~5阶振型的最大形变位置均不在转轴附近。     

红外测温法研究Al-Mg合金中的Portevin-Le Chatelier效应

试件塑性变形过程伴随着机械能向热能的转化.利用红外测温法,通过分析红外热像仪采集的温度场图像,系统研究了Al-Mg合金中的Portevin-Le Chatelier （PLC）效应.在不同应变率下,实验得到了三类锯齿形应力-应变曲线,分析了相应情况下试件温度变化曲线的异同及其原因,探讨了三种类型PLC变形带的空间传播特性.研究发现,试件表面的温升随着应变率的增加而增加;PLC带的倾角转向发生在试件的两端或者带外的温度最高处. 关键词： Portevin-Le Chatelier效应 红外测温 Al-Mg合金     

Low-stabilty states of aluminum crystal lattice and acoustic emission under high-temperature straining

Using aluminum as an example it is experimentally established that strain accumulation under low-stability condition at the temperatures higher than 0.5 melting temperature exhibits a non-monotonic behavior and represents sharp deformation changes within a wide range of scale levels (including macroscale). The effects of this stepwise deformation are accompanied by high-amplitude acoustic emission signals. An analysis of the anomalous metal behavior and acoustic emission shows that the activation volume of an elementary deformation event under this low-stability state is exponentially increased with the deformation temperature, indicating an increased scale of cooperative atomic displacements and manifestation of low stability of the crystal lattice of this atomic configuration. The macroscopic character of this stepwise deformation provides evidence on a correlation between elementary deformation events within a volume larger than one strain band. It is assumed that the amplitude of an acoustic signal can serve a criterion of this correlation.     

Local heating under deformation in the range of liquid-helium temperatures and its measurement with strain gages

The jumplike deformation of stainless steel specimens in liquid helium was shown to be accompanied by local heating with an increase in the temperature to ～80 K at each serration. Measurements were performed with wire strain gages glued on the specimen and used in the range of small deformations to determine the elastic modulus and yield point. In the range of deformations ε ～ 0.06–0.08, the strain gages were unloaded spontaneously because of the destruction of the glue layer and responded only to variations in the temperature of the specimen at a serration.     

Point defect generation,nano-void formation and growth. I. Validation

The volume fraction of point defects generated as a function of plastic shear strain squared, γ², was derived from crystal plasticity concepts. The evolution was determined from the stress–strain values using a new constitutive relation which replicates the measured behavior with at least two fitted loci. Assuming that nano-voids form by clustering of vacancies, the nano-void diameter was found to be proportional to their spacing and shear strain with the constant being characteristic of point defect production during deformation. The predicted amount of point defect generated was validated using the previously determined resistivity of [100] copper single crystals deformed at 4.2?K and annealed at 296?K. Similar analysis of super-pure polycrystalline copper data affirmed that the dynamic annihilation parameter extrinsically incorporated in the new derivation is larger due to formation of slip clusters. Moreover, the temperature dependence of the mean slip-distance to inter-forest spacing ratio at Stage II to III transition indicates that the thermally activated drag of vacancy-creating jogs occurs above 150?K. For polycrystalline aluminum deformed at 296?K, it was concluded that the nuclei of the nano-voids were not part of the evolving dislocation array but were embedded in the grown-in microstructure. This hypothesis is pursued in the accompanying paper, Part II, and its prediction results in a criterion for ductile failure.     

考虑底充胶固化过程的InSb面阵探测器结构分析模型

液氮冲击中In Sb面阵探测器的易碎裂特性制约着探测器的成品率,建立适用于面阵探测器全工艺流程的结构模型是分析、优化探测器结构的有效手段.本文提出了用底充胶体积收缩率来描述底充胶在恒温固化中的体积收缩现象,同时忽略固化中底充胶弹性模量的变化来建立底充胶固化模型,给出了底充胶在恒温固化中生成的热应力/应变上限值.借鉴前期提出的等效建模思路,结合底充胶固化后的自然冷却过程和随后的液氮冲击实验,建立了适用于In Sb面阵探测器全工艺流程的结构分析模型.探测器历经底充胶固化、自然冷却至室温后的模拟结果与室温下拍摄的探测器形变分布照片高度符合.随后模拟液氮冲击实验,得到面阵探测器中累积的热应力/应变随温度的演变规律,热应力/应变值极值出现的温度区间与液氮冲击实验结果相符合.这表明所建模型适用于预测不同工艺阶段中面阵探测器的形变分布及演变规律.     

基于缠结微结构的非晶态玻璃态高分子材料屈服行为的分子动力学研究

非晶态玻璃态高分子材料作为结构材料在工程领域应用广泛,其机械力学性能特别是屈服变形行为受到热处理、加载应变率和环境温度的影响.采用分子动力学模拟方法研究非晶态玻璃态高分子材料不同工况下的单轴拉伸变形,基于分子链缠结微结构的概念,阐明了非晶态玻璃态高分子材料屈服和应变软化过程的内在变形机制.结果表明,拓扑缠结具有较为稳定的空间结构,难以发生解缠,决定了非晶态高分子材料屈服后的软化平台.由相邻分子链的局部链段相互作用形成的次级缠结在一定外界条件下可发生破坏或重新生成,次级缠结微结构及其演化是非晶态高分子材料发生屈服及软化的内在物理原因.     

Thermophysical characterization of the deformation behavior of polyethylene/kaolin composites

The deformation behavior at room temperature of injection-molded and blow-molded samples of high-density polyethylene (HDPE) differing in orientation (with respect to the melt flow direction) and in filler content (untreated and surface-treated kaolin, respectively) were characterized by the stretching calorimetry technique. Samples with longitudinal and transversal orientations were examined. Strain-softening and strain-hardening phenomena were observed, respectively, in the range of inelastic strains above the yield point. The depression of yield stresses for the filled composites compared to those for the corresponding neat polymers is associated with the onset of debonding of the matrix polymer from the filler surface. The analysis of the energy balance of the debonding process suggests that the fraction of a debonded interface is smaller in samples containing a coupling agent and larger filler particles.     

EXPLORATION OF OPTIMUM SHAPE IN SYMMETRY FISSION

In the paper,large deformation of symmetry fission has been described by a biquadratic surface revolution instead of several quadratic equations.Based on the standard liquid drop model,variation calculation of deformation energy have been performed,a better shape with a gruop of coefficient (bi) than Cassinian ovaloid obtained.The shape with (bi) has lower deformation energy than that of ovaloid,at same separation distances between C.M. of half bodies.The so-called shape optimum Ⅱ with (bi) can generally be used in mass range 200—300. The fission barrier for opt.Ⅱ is lower and thiner than that of the ovaloid,because after the saddle point the potential descends more quickly.The saddle point is a little bit pressed for the shape opt.Ⅱ.There are significant differences of deformation energy between the shape opt.Ⅱ and ovaloid,when neck in middle appears.The neck cross-section of the opt.Ⅱ is larger than that of ovaloid before the scission point be reached,and equal or thiner near the point.     

CRITICAL STRAINS DETERMINING THE YIELD BEHAVIOR OF s-PP*

Video-controlled tensile deformation experiments giving true stressstrain curves were carried out on samples of s-PP. Dividing the total tensile deformation into elastic and plastic parts shows three critical strains (A, B, C) for which the differential compliance and the recovery properties change. It was found that all critical strains remain constant on varying the crystallite thickness or the testing temperature. Point A marks the end of the linearelastic range. The yield point, as given by the maximum on the engineering stress-strain curve for a necking sample, essentially corresponds to the second critical point, although the yield point is shifted relative to B to higher strains and varies with temperature. It is associated with the collective onset of inter- and intralamellar slip processes. At the third critical strain (C), the lamellae become destroyed, and fibrils are formed. The texture changes accompanying the drawing determined by a simultaneous measurement of wide-angle X-ray scattering (WAXS) patterns indicate a common activity of intra- and interlamellar slip processes setting in at A and a dominance of intralamellar block slips at moderate deformations above B. The Young's modulus and the yield stress show a large decrease with temperature, whereas there are only minor changes in the crystallinity. We understand it as resulting from a change of coupling, both between the crystalline lamellae and of the blocks within the lamellae.

     

Investigation on Tensile Deformation Behavior of Semi-Crystalline Polymers

The strain rate, temperature, and microstructure-dependent, tensile-yielding behavior of three semi-crystalline polymers, namely high-density polyethylene (HDPE), polyamide 6 (PA6) and low-density polyethylene (LDPE), was investigated. It is found that, depending on the strain rate and temperature, the three polymers exhibit markedly different tensile deformation behavior, especially the shape of the stress-strain curves. LDPE exhibits a uniform extension and shows no obvious geometrically unstable effect, such as necking, during the overall tensile process. HDPE and PA6, on the other hand, show clear necking and cold-drawing phenomena during the uniaxial tensile process. When considering the effect of strain temperature on necking, significant differences between HDPE and PA6 emerge. For both, the heterogeneous necking disappears and homogeneous deformation occurs with increasing temperature. For HDPE, the homogeneous deformation takes place in the vicinity of the melting temperature, while for PA6, it takes place close to the glass transition temperature instead. The conventional yield point, corresponding to the force maximum in stress-strain curves, becomes less defined as the testing temperature is increased. It is applicable, to some extent, to combine the Brereton analysis and Considère construction to predict such a point quantitatively. However, this combination can only be suitable for homogeneously deformed material. In addition, it is found that the special, double yielding behavior will take place under certain deformation conditions for all three semi-crystalline polymers. With respect to judging the appearance of the double yielding of polymers, it seems that it can be estimated qualitatively by plotting the compression residual strain-applied strain curves of the samples.     

小直径光纤光栅的研制及传感交叉敏感研究

研究了小直径光纤光栅的研制以及传感中的温度应变交叉敏感问题.首先根据耦合模理论,分析了小直径光纤Bragg光栅光谱特性,确定了包覆层为80 μm的单模光纤加工成中心波长为1 528 nm的Bragg光栅的栅长及周期,并研究了小直径光纤光栅与解设备之间的连接方式.其次利用等强度梁的变形特点,结合矩阵法,提出基于等强度悬臂梁双Bragg光纤光栅矩阵算法,对小直径光纤Bragg光栅的交叉敏感问题进行研究.温度和应变的实验辨别误差分别为5%和6%.实验结果表明,该方法可以分离温度和应变对光纤Bragg光栅传感的影响.采用该方法去除交叉影响,K矩阵始终存在逆矩阵,因此对所采用的光栅无特殊要求,从而扩大了光纤光栅选用范围,并将温度和应变识别出来.     

Effects of twin and stacking faults on the deformation behaviors of Al nanowires under tension loading

The effects of twin spacing and temperature on the deformation behavior of nanotwinned Al under tensile loading are investigated using a molecular dynamic(MD) simulation method.The result shows that the yield strength of nanotwinned Al decreases with the increase of twin spacing,which is related to the repulsive force between twin boundary and the dislocation.The result also shows that there is no strain-hardening at the yield point.On the contrary,the stress is raised by strain hardening in the plastic stage.In addition,we also investigate the effects of stacking fault thickness and temperature on the yield strength of the Al nanowire.The simulation results indicate that the stacking fault may strengthen the Al nanowire when the thickness of the stacking fault is below a critical value.     

Frustum pyramid shape of indium bump to lengthen cycling life of InSb infrared detector

The thermal deformation appearing in indium antimonide infrared focal plane arrays (InSb IRFPAs) subjected to thermal shock tests, will easily incur the fracture of InSb chip, this phenomenon restricts the final yield of InSb IRFPAs. In light of the proposed equivalent method, the three dimensional structural modeling of InSb IRFPAs is developed, and the simulated strain distributions are consistent with the buckling pattern, the shallow groove and the local flatness, appearing on the top surface of InSb IRFPAs at the corresponding regions. After comparing the deformation profiles at different regions, we deduce that the top surface flatness of InSb IRFPAs will be improved with frustum pyramid indium bump arrays, and this deduction is verified by the subsequent simulation results. That is, when the top surface area of indium bump is smaller than its bottom surface area, in this paper, the diameter of indium bump bottom surface is set with 24 μm, the simulated Z-components of strain is uniformly covering the whole top surface of InSb IRFPAs, and the deformation amplitude is decreased slowly with the decreasing top surface area of indium frustum pyramid arrays. These findings are beneficial to further improve the flatness of InSb IRFPAs, correspondingly, to lengthen its temperature cycling life.     

Effect of temperature on the anisotropy of AZ31 magnesium alloy rolling sheet under high strain rate deformation

In order to investigate the effect of temperature on the anisotropic behaviour of AZ31 magnesium alloy rolling sheet under high strain rate deformation, the Split Hopkinson Pressure Bar was used to analyse the dynamic mechanical properties of AZ31 magnesium alloy rolling sheet in three directions, rolling direction(RD), transverse direction (TD) and normal direction (ND). The texture of the rolling sheet was characterised by X-ray analysis and the microstructure prior and after high strain rate deformation was observed by optical microscope (OM). The results demonstrated that AZ31magnesium alloy rolling sheet has strong initial {0?0?0?2} texture, which resulted at the obvious anisotropy in high strain rate deformation at 20 °C. The anisotropy reflected in stress–strain curve, yield stress, peak stress and microstructure. The anisotropy became much weaker when the deformation temperature increased up to 250 °C. Continuing to increase the deformation temperature to 350 °C the anisotropy of AZ31 rolling sheet essentially disappeared. The decreasing tendency of anisotropy with increasing temperature was due to the fact that when the deformation temperature increased, the critical resolved shear stress (CRSS) for pyramidal 〈c + a〉 slip, which was the predominant slip mechanism for ND, decreased close to that of twinning, which was the predominant deformation mechanism for RD and TD. The deformation mechanism at different directions and temperatures and the Schmid factor (SF) at different directions were discussed in the present paper.     

Interactions between high-temperature deformation and crystallization in zirconium-based bulk metallic glasses

High-temperature deformation of a ZrTiCuNiBe bulk metallic glass (BMG) is investigated by compression tests in the supercooled liquid region. When the temperature is decreased or strain rate increased, the amorphous alloy exhibits the usual Newtonian/non-Newtonian transition behaviour. Using specific heat treatments, partially crystallized alloys are produced, the associated microstructures characterized and the volume fractions of the crystal measured. The interaction between high-temperature deformation and crystallization is investigated by appropriate mechanical testing. According to these measurements, partial crystallization is responsible for a significant increase in flow stress and the promotion of non-Newtonian behaviour. Deformation does not significantly change the volume fraction, composition or size of the crystal. The flow-stress increase with crystallization is analyzed under different hypotheses. We conclude that the flow-stress increase cannot be interpreted through a compositional change in the residual amorphous matrix, either by reinforcement due to hard crystallites or by connections between crystals. It appears that the effect is due to the nanometric size of the crystals alone.