Order-strengthening in Cu−Al alloys

Cu−Al alloys can be significantly strengthened after deformation by a suitable low temperature thermal treatment. Possible mechanisms for this strength increase have been investigated using a number of techniques including differential thermal analysis, electron microscopy and diffraction. It has been found that strength increases could be associated with the development of order in the alloys, and the magnitude of the increases have been found to be dependent on the amount of prior deformation, the amount of order and thee/a ratio. An ordered phase based on an fcc lattice has been identified at higher aluminum concentrations. It is suggested that this phase is Cu₃Al. It is postulated that strain fields associated with the formation of ordered regions which nucleate on stacking faults are responsible for the observed strengthening.     

Flow localization in OFHC Cu and 99.99% Al

The work hardening and rate sensitivity coefficients determined in an earlier investigation are used to integrate numerically the evolutionary constitutive relation proposed by Kocks et al. under conditions where the acceleration remains constant during testing. The numerical predictions are compared with flow curves obtained from constant acceleration tests and the agreement found is excellent. These experiments were performed with the aid of a diametral transducer attached at the minimum cross-section of the tensile samples which monitored and controlled the rate of reduction of the sample diameter. The material coefficients are also employed for the numerical integration of the first and second order differential equations describing flow localization. Tensile tests were carried out on Cu and Al samples at room temperature and on Al specimens at 473 K in which the flow localization process was followed by photographic means. The strain measurements obtained are reproduced more closely when the combined “first” and “second order” material coefficients rather than only the ‘first order’ coefficients are used. This is largely because of the nonnegligible value of B_σ, the rate sensitivity of the work hardening rate. In the case of the Al, the onset of flow localization is retarded by 0.15–0.2 strain beyond the UTS because of the magnitude of B_σ.     

The Effects of Fatigue on the Atomic Structure with Cyclic Loading in Zr50Cu40Al10 and Zr60Cu30Al10 Glasses

The potential damage effect from fatigue on Zr bulk metallic glass alloys of Zr₅₀Cu₄₀Al₁₀ at the eutectic point and Zr₆₀Cu₃₀Al₁₀ away from the eutectic point (in atomic percent) is examined via the local atomic structure, which was obtained from the pair density function analysis of the synchrotron X-ray radiation and neutron data. Samples cut from the same rods were subjected to 10⁴, 10⁵, and 10⁶ compression cycles ex situ, and the evidence for fatigue damage was investigated by comparing alloys before and after cyclic loading. Bond orientation was observed particularly in Zr₅₀Cu₄₀Al₁₀, suggesting that fatigue damage occurs even in the elastic range, below the yield point, and during cyclic loading. The initiation of fatigue changes is observed first within small localized atomic regions.     

Cu—Al合金内氧化工艺的研究

对Cu-Al合金的内氧化工艺及其动力学进行了实验研究。结果表明:Cu-Al合金内氧化主要发生在初期较短的一段时间内(本实验条件下10min左右),内氧化动力学曲线具有抛物线特征。提高内氧化温度可大大缩短完成内氧化所用的时间,实际内氧化宜采用较高的温度。与Al的固溶强化相比,Al_2O_3粒子的弥散强化不仅大大提高了电导率,而且显著提高了材料的硬度。实验条件下以1223 K×0.5h内氧化工艺制备的Cu-Al_2O_3复合材料性能最佳,其中电导率可达49m/Ω·mm~2(84.5％IACS),硬度可达HRF98。     

细晶Al2O3/Cu复合材料的研究

在粉末冶金工艺的基础上，研究了一套新工艺，制取了纳米级Al2O3/Cu复合材料，观察并测试其性能。结果表明：制取的纳米级Al2O3/Cu复合材料，Al2O3为50－70nm，组织中晶粒细小，电导率大于80％IACS，软化温度超过660℃，室温硬度达130HV，综合性能优良。     

The columnar-to-equiaxed transition in Al 3 Pct Cu

A columnar-to-equiaxed transition is observed in Al 3 pct Cu solidified directionally from a chill face. The transition occurs when the temperature gradient in the melt ahead of the columnar dendrites decreases to 0.6 ‡C/cm at dendrite growth rates of about 5 x 10^-3 cm/s. Increasing the nuclei density by adding 171 ppm of TiB₂ to the melt produces a fine-grained structure without columnar growth. Adding 100 ppm TiB₂ has no effect on the cast structure or columnar-to-equiaxed transition. The results are considered in relation to the model for the columnar-to-equiaxed transition proposed by Hunt.^[2]     

Interfacial Reaction during Friction Stir Welding of Al and Cu

Commercially pure copper was joined to a 1050 aluminum alloy by friction stir welding. A specific configuration where the tool pin was fully located in the aluminum plate was chosen. In such a situation, there is no mechanical mixing between the two materials, but frictional heating gives rise to a significant thermally activated interdiffusion at the copper/aluminum interface. This gives rise to the formation of defect-free joints where the bonding is achieved by a very thin intermetallic layer at the Cu/Al interface. Nanoscaled grains within this bonding layer were characterized using transmission electron microscopy (TEM). Two phases were identified, namely, Al₂Cu and Al₄Cu₉ phases. The nucleation and growth of these two phases are discussed and compared to the standard reactive interdiffusion reactions between Cu and Al.     

Dislocation microstructure and internal-stress measurements by convergent-beam electron diffraction on creep-deformed Cu and Al

Creep experiments were conducted on aluminum single crystals and copper polycrystals deformed within the five-power-law regime. The dislocation structure of copper, which has not been extensively characterized in the past, consists of less-well-defined subgrain walls of relatively low misorientation, typically between 0.1 and 0.3 deg, with a Frank network of dislocations within the subgrains. The aluminum, as expected, consisted of well-defined subgrain boundaries with a typical misorientation between 1.0 and 2.0 deg. The subgrains were probed from one boundary to another in copper and aluminum using convergent-beam electron diffraction (CBED). This allowed a determination of any changes in the lattice parameter, which would indicate the presence of any internal stresses. Earlier investigations by others suggested that internal stresses may be high in the vicinity of the “hard” subgrain boundaries in both loaded and unloaded specimens, based on a variety of techniques including X-ray diffraction (XRD), stress-dip tests, as well as some preliminary CBED. It was determined in this work that the lattice parameter was unchanged at the equilibrium or stress-free value within the interior of the subgrains and along (within a one-beam diameter) the subgrain boundaries. This article is based on a presentation made in the workshop entitled “Mechanisms of Elevated Temperature Plasticity and Fracture,” which was held June 27–29, 2001, in San Diego, CA, concurrent with the 2001 Joint Applied Mechanics and Materials Summer Conference. The workshop was sponsored by Basic Energy Sciences of the United States Department of Energy.     

Viscosities of aluminum-rich Al−Cu liquid alloys

Viscosity data for Al−Cu liquid alloys in the ranges of 0≤C _L≤33.1 wt pct Cu and 1173≤T ≤973 K are reviewed. It was found that Andrade's equation can be used to represent the variation of viscosity with temperature for a given composition, but that each of the two parameters in Andrade's equation shows no systematic variation with composition of the liquid-alloys. Consequently, arithmetic averages of the parameters were used and assumed to apply to all compositions in the range 0≤C _L ≤33.1 wt pct Cu. Such a procedure implies that the viscosity happens to vary with composition solely because the specific volume varies with composition. In order to establish the predictability of extrapolating such simple behavior, a more complex model was considered. The latter model was recently presented by Kucharski and relates viscosity to the structure and thermodynamics of liquid alloys. Viscosities obtained by interpolating Andrade's equation and Kucharski's model compare closely; furthermore, values obtained by extrapolations to lower temperatures also compare favorably. Finally the simpler model was used to calculate the viscosity of the interdendritic liquid during solidification.     

铜含量对注射成形Cu/Al_2O_3复合材料性能的影响

采用注射成形方法制备了Cu/Al2O3复合材料,研究了铜的含量对复合材料性能的影响,结果表明:铜含量为10%时,复合材料具有最佳综合性能。通过扫描电镜观察了复合材料的断口形貌,分析了其断裂机制,表明Cu/Al2O3复合材料断裂时存在沿晶断裂、穿晶断裂以及铜颗粒的拔出三种方式,且三者比例依次降低。     

Investigation on the Electron Beam Welding of Al/Cu Composite Plates

The Al/Cu composite plates composed of 2.5 mm thick Al base plate and 0.5 mm thick Cu cladding plate were joined by electron beam welding (EBW). The butt joints of Al/Cu composite plates were obtained successfully in Modes I (Cu cladding plate was placed upon the Al base plate, welding speed of 1400 mm/min) and II (Al base plate was placed upon the Cu cladding plate, welding speed of 1300 mm/min), respectively. The results showed that microstructures under two modes were similar, but there existed some obvious differences in fracture behavior of the joints and damage behavior of Cu cladding plate. For two butt joints, the (Al2Cu + α-Al) eutectic structure was distributed in continuous networks around the α-Al grains in the weld zone. In addition, the interface between Cu cladding plate and weld zone was composed of Al2Cu intermetallic compound and (Al2Cu + α-Al) eutectic structure. The destruction width of Cu cladding plate was greater in Mode I than that in Mode II. Furthermore, the average loads of the EBW joints were 4.8 kN and 4.5 kN in Modes I and II, respectively.     

Studies on Electro Mechanical Aspects in Ultrasonically Welded Al/Cu Joints

Ultrasonic welding has been widely used to bond dissimilar conductive wires, battery cell terminals in relay applications. In this paper, dissimilar metals, Al/Cu were joined using ultrasonic welding for conductive applications. Welding trials were carried out by varying three control parameters: (1) vibrational amplitude (40, 60, and 80 µm), (2) clamping pressure (1, 1.2, and 1.4 bar), (3) weld time (0.1, 0.2, and 0.3 s). Experimental trails were designed based on L₉ Taguchi method. Interpretation of tensile strength and microhardness results revealed that the satisfactory weldments were obtained for higher welding energies when compared to low welding energies. From the microstructural analysis, the bond formation of metals and failure modes were studied. SEM and XRD images revealed the four major intermetallic compounds at the interface of joint; AlCu, Al₂Cu, Al₃Cu₄, and Al₄Cu₉ with resistivity values of 11.415, 8.027, 10.612, and 14.243 Ω-cm respectively. The resistivity values of intermetallic compounds observed in the joint was almost 5–6 times higher than the Al.     

Self-diffusion of copper in Cu−Al solid solutions

Self-diffusion coefficients of copper in Cu?Al solid solutions in the concentration interval 0 to 19 at. pct Al and in the temperature range 800° to 1040°C have been determined by the residual activity method using the isotope Cu⁶⁴. The values of the self-diffusion coefficients in the concentration interval 0 to 14.5 at. pct Al satisfy the Arrhenius relation and their temperature dependence can be expressed by the following equations $$\eqalign{ & D_{Cu}^{Cu} = \left( {0.43_{ - 0.11}^{ + 0.15} } \right) exp \left( { - {{48,500 \pm 700} \over {RT}}} \right) cm^2 /\sec \cr & D_{Cu - 2.80 at. pct Al}^{Cu} = \left( {0.46_{ - 0.16}^{ + 0.23} } \right) exp \left( { - {{48,000 \pm 900} \over {RT}}} \right) cm^2 /\sec \cr & D_{Cu - 5.50 at. pct Al}^{Cu} = \left( {0.30_{ - 0.07}^{ + 0.09} } \right) exp \left( { - {{47,000 \pm 600} \over {RT}}} \right) cm^2 /\sec \cr & D_{Cu - 8.83 at. pct Al}^{Cu} = \left( {0.46_{ - 0.09}^{ + 0.11} } \right) exp \left( { - {{47,100 \pm 500} \over {RT}}} \right) cm^2 /\sec \cr & D_{Cu - 11.7 at. pct Al}^{Cu} = \left( {0.61_{ - 0.13}^{ + 0.17} } \right) exp \left( { - {{47,200 \pm 600} \over {RT}}} \right) cm^2 /\sec \cr & D_{Cu - 14.5 at. pct Al}^{Cu} = \left( {4.2_{ - 1.5}^{ + 2.2} } \right) exp \left( { - {{51,110 \pm 1000} \over {RT}}} \right) cm^2 /\sec \cr} $$ An analysis of the results leads to the conclusion that, in the concentration interval 0 to 11.7 at. pct Al, the frequency factor and activation enthalpy concentration dependences can be described by the following equations whereD _0Cu ^Cu and ΔH _Cu ^Cu are diffusion characteristics for self-diffusion in pure copper,X _Al is the atomic percent of aluminum, andK andB are experimental constants.     

Fatigue crack propagation in copper and Cu−Al single crystals

The effect of crystallographic orientation, temperature, and stacking fault energy on the rate of fatigue crack propagation was studied in polycrystalline copper, copper single crystals, and Cu−Al single crystals at room temperature and at liquid nitrogen temperature. A stress intensity factor was used to normalize the crack propagation data. It was found that dislocation cross slip plays a critical role on the rate of fatigue crack propagation. Existing mathematical crack propagation formulae could not explain the data on single crystals. A new fatigue crack propagation model to explain the observed results is proposed. H. ISHII, formerly Research Assistant at Materials Science Department, Northwestern University, Evanston, Ill. This paper is based on a thesis submitted by H. ISHII in partial fulfillment of the requirements of the degree of Doctor of Philosophy at Northwestern University.