Cyclic saturation behavior of tungsten monofilament-reinforced monocrystalline copper matrix composites

Studies on saturation behavior produced by cyclic deformation have been conducted on tungsten monofilament-reinforced monocrystalline copper composites. The effect of the fiber on strain localization has been investigated using interferometry. For a given applied strain amplitude, local strain and volume fraction of the persistent slip bands (PSBs) in the composite appeared no different from those observed in monolithic copper single crystals. However, the distribution of the PSBs was observed to be more uniform, and the total number of PSBs is substantially higher than that in monolithic crystals. The PSBs appeared mostly in the form of micro-PSBs or macro-PSBs with very limited width. Instead of expanding existing PSBs, new PSBs were more likely to nucleate at new locations during cyclic deformation. The volume fraction and width of the PSBs were observed to increase during saturation, which indicates that some of the PSBs become aged and new PSBs form in order to continue to carry the plastic strain. A rule of mixtures model was established to link the cyclic stress–strain response of the monocrystalline composites to the behavior of monolithic single crystals and fibers. The results calculated from the model show very good agreement with the experimental data.     

Effect of long-range order on the cyclic deformation behaviour of Ni3Fe single crystals

Effect of ordering on cyclic deformation in disordered and ordered Ni₃Fe single crystals was investigated focusing on stress–strain response and deformation substructure. The cyclic hardening depended strongly on the long range order. The maximum stress in the disordered crystals increased gradually with increasing number of cycles and then reached a saturation, while ordered ones exhibited cyclic softening after an initial strong cyclic hardening. The cyclic hardening at an early stage of fatigue in ordered crystals may be due to APB tubes and debris which were produced by the intersection between primary and secondary slips. Coarse slip bands were observed in fatigued ordered Ni₃Fe single crystals. In the bands, three-dimensional dislocation structure was formed accompanied by a decrease in the degree of order, which was responsible for the cyclic softening.     

化学镀铜短碳纤维-铜-石墨复合材料的性能研究

本文采用化学镀铜法对短碳纤维表面进行镀铜,并用电镀法对长碳纤维表面进行连续镀铜后,再切割成镀铜短碳纤维.随后用粉末冶金法制备了含有这两种镀铜短碳纤维的碳纤维-铜-石墨复合材料和不含碳纤维的铜-石墨复合材料,对它们的物理和力学性能进行了测试,并在滑动速度为15 m/s、载荷为4.9N的干摩擦条件下进行了30 h磨损试验,结果表明:化学镀铜短碳纤维-铜-石墨复合材料的导电性、硬度、抗弯强度和耐磨性优于电镀铜短碳纤维-铜-石墨复合材料和不含碳纤维的铜-石墨复合材料.     

纤维种类对C/C复合材料摩擦磨损性能的影响

目的为了降低C/C复合材料制造成本,扩展C/C复合材料应用领域,选用低成本预氧丝纤维取代碳纤维,制备出C/C复合材料,并研究纤维种类对C/C复合材料摩擦磨损性能的影响。方法以两种纤维为原材料,采用CVI工艺制备出C/C复合材料,用MM-2000摩擦试验机进行摩擦磨损试验,采用扫描电镜对摩擦面进行形貌分析。结果随着载荷的增大,预氧丝基C/C复合材料在与金属摩擦时摩擦因数保持在0.22左右,平均磨损量为0.82 mg/min,而碳纤维基C/C复合材料与金属配副相对摩擦因数较小(0.15~0.20),平均磨损量为1.17 mg/min。三种碳与碳配副中,预氧丝基C/C复合材料同预氧丝基C/C复合材料配副之间的摩擦因数随载荷波动的范围为0.28~0.33,较稳定,平均磨损量为1.76mg/min。碳纤维基复合材料与碳纤维复合材料配副时,随着载荷的增大,摩擦因数变化范围较大(0.15~0.33),平均磨损量为2.35 mg/min。预氧丝基复合材料与碳纤维基复合材料之间相互配副,其磨损最大,平均磨损量为2.95 mg/min。结论 C/C复合材料的摩擦磨损性能与纤维种类有很大关系,采用预氧丝纤维制备出的C/C复合材料,无论与金属相互摩擦,还是与自身材料摩擦,均易形成较为稳定的润滑膜。随着载荷的增加,摩擦因数变化较小,磨损量和摩擦功也最低,表现出比碳纤维基C/C复合材料更优异的摩擦性能。     

Compressive strength of continuous fiber reinforced aluminum matrix composites

Aluminum matrix composites can exhibit exceptionally high compressive strength (ε 4 GPa). The main failure mechanism has been identified to be plastic kinking, although an upper bound seems to be attained when the fibers reach their compressive strength. The experimental results are consistent with a plastic kinking model in strain hardening composites. The important parameters controlling the compressive strength are the composite shear modulus, the shear yield strength, the strain hardening rate and the maximum initial fiber misorientation. Interfaces can also play an important role; composites reinforced with weakly bonded fibers have a reduced compressive strength due to their low shear strength in the fiber direction. The results can be used to design composite systems with high compressive strengths; in practice this is done by selecting matrices with high yield strengths and fast hardening rates.     

Fatigue mechanism in Ni3Fe single crystals with L12 superlattice structure

Ni₃Fe single crystals with the L1₂ structure were cyclically deformed with various loading axes at different plastic strain amplitudes. The crystals demonstrated initial strong cyclic hardening followed by rapid cyclic softening at any orientation tested. There existed a typical plateau region in the cyclic stress–strain curves of the crystals oriented for single slip, while the saturation shear stress (τ_s) monotonically increased with increasing plastic shear strain amplitude (γ_pl) for double-slip oriented crystals. Persistent slip bands (PSBs) with cell-like dislocation structure developed in fatigued Ni₃Fe single crystals. The relationship between the volume fraction of PSBs and γ_pl suggests that Winter’s two-phase model can be applied to explain the cyclic stress–strain response in Ni₃Fe single crystals. Activation of the secondary slip is closely related to the cyclic hardening/softening behaviour and the formation of PSB in Ni₃Fe single crystals.     

铜双晶体中晶界及组元晶体的疲劳寿命

本文对具有单,双滑移取向组元晶体的近生趣晶界铜双晶体进行拉－拉循环变形,得出铜双晶体中晶界及组元晶体的Ｓ－Ｎ曲线,比较了铜双晶体晶界及组元晶体疲劳寿命的差别,发现在相同的应力幅下晶界的疲劳寿命明显低于组元晶体的疲劳寿命。     

Al2O3,TiB2粒子增强铝基复合材料的动态压缩性能和高温蠕变性能

对ＴｉＯ２－Ａｌ－Ｂ和ＴｉＯ２－Ａｌ－Ｂ２Ｏ３体系制备的两种Ａｌ２Ｏ３和ＴｉＢ２原位粒子增强铝基复合材料进行了动态压缩试验和高温拉伸蠕变试验。动态压缩试验表明,随着应变速率的提高,复合材料的强度和初始加工硬化率明显增加。然而,复合材料中含有的条状Ａｌ３Ｔｉ对复合材料的动态机械响应基本没有影响。透射电镜观察表明,在高应变速率下两种复合材料强度和初始加工硬化率的明显提高可由复合材料基体中位错密度的显著     

Cyclic deformation behaviour of γ′-Ni3(Al,Ti) single crystal containing disordered γ precipitates

Effect of disordered γ precipitates on the cyclic deformation behaviour of γ′-Ni₃(Al,Ti)-based single crystals with L1₂ structure was investigated. The γ precipitates with fine spherical and coarse plate-like shapes were formed in the γ′-based single crystals by annealing at 1073 K for 3 h (peak-aged) and 100 h (over-aged), respectively. The over-aged crystal shows stronger cyclic hardening than does the peak-aged crystal, although the yield stress of the peak-aged crystal is higher than that of the over-aged one. In these two crystals the precipitates were sheared by superlattice dislocations dissociated into superpartials. The separation distance between superpartials increased in the precipitates because there was no APB in the disordered phase and the cyclic hardening was enhanced in the aged crystal. The size of the precipitates in the over-aged crystal was reduced by shear deformation resulting in acceleration of the cyclic hardening. Dissolution of γ phase into γ′ matrix did not occur during cyclic loading and cyclic softening did not appear.     

INFLUENCE OF FIBER COATING ON TENSILE BEHAVIOR OF UNIDIRECTIONAL C/Mg COMPOSITES

1.Intr0ductionAm0ngtheseveralmethodsavailablef0rcombiningamagnesiummatrixwithacar-bonfiberreinforcement,l0wpressureinfiltrati0nprocesseshavetheadvantages0frelativesimplicityandpotentialf0rl0w-costpr0ductionofnet-0rnear-net-shapec0mpositecom-ponents.Unfortunately,thereexistcomplexproblemsatfiber/matrixinterfacesuchasinterfacialwetting,illterfacialreacti0nandinterfacialb0nding,allofwhichneedtobesolvedsimultaneously.Afunctionalgradientcoating0nfibersurface,C/SiC/SiO2,wasthuspresentedinourprevio…     

纯度对ECAP制备超细晶铜疲劳性能的影响

采用应力比R=–1的对称加载疲劳试验,研究了ECAP制备的超细晶高纯铜(HPCu)、低纯铜(LPCu)的疲劳行为,分析了循环应力-应变响应、疲劳寿命和疲劳前后晶粒取向分布,讨论了纯度与超细晶材料疲劳稳定性的关系。结果表明:在任何应力幅下,获得的超细晶低纯铜的寿命都大于ECAP变形前的粗晶铜;在相同应力幅下,循环周次提高1.6~2.0倍。而超细晶高纯铜的疲劳曲线,表现出不同的特性,在高应力幅下,超细晶高纯铜具有较高的疲劳寿命,但在低应力幅下,超细晶高纯铜循环周次下降,疲劳寿命低。在应力控制条件下,随应力幅的降低,超细晶纯铜的循环应力-应变响应从循环软化逐渐过渡为循环硬化。杂质的存在能有效阻止疲劳过程中晶粒的转动和位错的运动,降低其回复软化,减小相邻晶粒间取向差变化,使超细晶低纯铜与超细晶高纯铜相比有较大的循环硬化指数n和循环硬化系数K,具有较好的疲劳稳定性。     

真空压力浸渗制备CF/Al复合材料热收缩与残余应力数值模拟与实验研究

针对连续碳纤维增强铝基复合材料(CF/Al复合材料),采用细观力学数值模拟与热性能试验结合的方法,研究了真空压力浸渗制备过程中的热收缩行为和热残余应力分布。结果表明,复合材料的横向热收缩应变量远大于轴向热收缩应变量,且具有横观各向同性,纤维随机分布的单胞有限元模型能够准确地预测复合材料轴向与横向热收缩行为曲线;复合材料制备完成后纤维和基体合金分别处于压应力和拉应力状态,基体和纤维的横向残余应力均小于其轴向残余应力,且均表现出横观各向同性;基体合金在轴向残余拉应力作用下会出现不同程度的损伤现象,特别是纤维间距较小部位过高的残余应力会引发界面的局部失效,从而不利于发挥复合材料承载性能,减少纤维局部偏聚是进一步改善提高复合材料力学性能的重要技术手段。     

CYCLC DEFORMATION OF Nb SINGLE CRYSTALS

The Nb single crystals of both[321]and[110]orientations have been cyclicly deformed intension-compression at constant strain rate 8×10~(-4)s~(-1)over a range of plastic strainamplitudes between 10~(-3)and 10~(-4).The cyclic hardening,the changes in shape of crystals andthe asymmetry of stress have been studied.The hardening curve can be divided into threestages,i.e.first,rapid-hardening and saturated stage.In the first stage of cyclic hardening curve dominant features of dislocation configura-tions are high density networks and debris loops.In the rapid-hardening stage the main fea-ture is the formation of dislocation bundles.In the saturated a well defined bundle structurefully develops and between them it is filled with only screw dislocations and the imposed strainis accommodated mainly by the motion of screw dislocations travelling to and from betweenthe bundles.Three-dimension cell,two-dimension cell or bundle structures are summarized asthe saturated structures of bcc metals.     

Modeling of the consolidation of continuous- fiber metal matrix composites via foil- fiber- foil techniques

The consolidation of metal-matrix composites (MMC) via hot isostatic pressing (HIP) of foil-fiber-foil layups has been investigated using finite element method (FEM) metal flow analysis. For this purpose, the deformation pattern for various fiber arrangements was determined using representative unit cells to describe extremes in behavior. For a given fiber architecture, the consolidation time was found to be heavily dependent on the ratio of the HIP pressure to average flow stress and the friction conditions at the matrix-fiber interface. The specific influence of material properties such as the rate sensitivity of the flow stress appears to enter only as a second order effect. The FEM solutions were used to construct HIP diagrams delineating temperature-time-pressure combinations for full composite consolidation. Laboratory trials on subscale foil-fiber coupons were used to validate the FEM predictions.     

CuCrZr铜合金高温阶梯疲劳与循环塑性行为研究

对CuCrZr铜合金在300和400℃开展了阶梯疲劳试验研究,从循环应变幅、平均应变、平均应变率和能量耗散率等方面研究其循环塑性行为。研究发现,CuCrZr铜合金的循环软硬化特征以及棘轮效应受到温度与循环应力的共同作用,温度越高,越易发生循环软化现象,同时棘轮效应也更加显著。基于对CuCrZr铜合金高温拉伸断裂能与高温阶梯疲劳总耗散能的对比发现,两者均与温度相关,因此将高温拉伸断裂能作为温度补偿参数,提出了一种基于能量法的线性损伤疲劳寿命预测模型,对CuCrZr铜合金高温阶梯疲劳寿命进行预测。最后,基于断口观察分析了CuCrZr铜合金与温度相关的失效机制：在较低的温度下容易发生疲劳裂纹失效,而随着温度的升高,更易发生棘轮应变累积的韧性失效。     

循环变形铜单晶体的滞后回线形状变化与驻留滑移带的萌生

本文系统测量并总结了不同类型双滑移取向铜单体循环变形中滞后回线性形状参数ＶＨ随循环周次Ｎ的变化关系,结果表明,与单滑移晶体不同晶体取向双滑移晶体的滞后回线形状变化趋势有显著的影响,ＶＨ与驻留滑移带（ＰＳＢ）的形成有无明业对应关系与该取向单晶体的循环应力－应变（ＣＳＳ）曲线有无平台区或准平台区出现密切相关,ＶＨ随Ｎ的变化关系反映了晶体在循环变形中的微观组织结构的不同变化,通过ＶＨ确定了不同类型双滑移     

Self-Diagnosis of GFRP composites containing carbon powder and fiber as electrically conductive phases

The electrical characteristics of glass fiber reinforced plastic (GFRP) composites have been investigated in order to incorporate a self-diagnosis function suitable for monitoring the health of structural materials. The electrical conductivity was introduced by adding carbon powder (CP) or carbon fibers (CF) into GFRP rods and sheets. The self-diagnosis ability of the composites was evaluated by measuring the change in electrical resistance as a function of stress (or strain) in tensile tests. The resistance of CFGFRP changed only slightly at small strain levels and increased nonlinearly with the applied stress due to cutting of the fibers at higher levels. CPGFRP showed high sensitivity to stress and the resistance changed linearly over a wide strain range. During cyclic loading tests, a residual resistance was also observed in CPGFRP composites after unloading. The residual resistance increased with maximum applied strain, showing that it can be used as an indicator of previously applied strain or stress. It is concluded that the CPGFRP composite is a promising material for simple diagnosis of dynamic damage and cumulative strain.     

短纤维增强金属基复合材料拉伸变形的屈服行为的理论研究

运用空间轴对称弹塑性有限元方法的混合律模型,推导出应力应变分配系数的解析表达式。并由此提出了一种新的定义复合材料比例极限和屈服强度的方法,进而研究了材料参数（纤维长径比、纤维体积分数、纤维根间距和基体应变硬化指数）对短纤维金属基复合材料拉伸变形行为的影响。     

Fracture behavior of monofilament-reinforced multicrystalline copper composites

Studies on monotonic tensile fracture and fatigue fracture have been performed on tungsten monofilament-reinforced multicrystalline copper composites and the failure modes are reported. Monotonic tensile fracture is not very sensitive to the initial fiber breaks and grain boundaries in tungsten monofilament-reinforced multicrystalline copper composites. Multiple breaks of the fiber and multiple necking of the matrix near the fiber segment breaks, commonly observed in tensile tests of composites, are reported here. However, the fatigue fracture is very sensitive to a fiber break, and to microstructural features such as grain boundaries and dislocation structures. Once the fiber breaks into two segments, the composite will quickly fail. The fatigue fracture mechanism of the composites depends on applied plastic strain amplitudes. Most cracks initiate at grain boundaries in the matrix at low and intermediate plastic strain amplitudes and the fiber greatly improves the fatigue behavior. At a high plastic strain amplitude, fatigue cracks initiate at the fiber, and the fiber seems ineffective for improving the fatigue life.     

短纤维增强金属基复合材料拉伸应力场的有限元数值分析

运用空间轴对称弹塑性有限元方法,研究了短纤维增强金属基复合材料拉伸应力场分布。研究表明,基体和纤维的应力分布及基体塑性行为具有明显的不均匀性,材料参数（纤维长径比,纤维体积分数,纤维根间距和基体应变硬化指数）以不同方式通过影响应力传递基体约束变形和基体应变硬化进而影响应力场分布。