Fatigue in CuZnAl single crystals

Fatigue properties associated with the β-18R martensitic transformation are analyzed in CuZnAl single crystals. The fatigue induced changes are studied by optical, scanning and electron microscopy. It is found that in the interior of the crystals dislocation defect arrays are created which act as local obstacles for the martensitic transformation and lead to the formation of extrusions and holes at the surface, which with increasing numbers of cycles join to form continuous cracks of about 1 μm width. Crack propagation seems to be little affected by composition or orientation.     

Direct observation of coarsening in AlCu alloys

Observations have been made on coarsening of θ-CuAl₂ precipitates at the surface of AlCu alloys. One specific area was examined over extended periods so that dissolution and growth of individual precipitates could be followed. It was found that nearly all precipitates shrank on ageing, even those larger than the average particle size. Only the largest 8% increased in dimension. A simple theory was used which gave good agreement with the observed dissolution kinetics of all precipitates except the very largest. This theory assumes a solute flux from the precipitate to a larger precipitate located a significant distance away. The theory gave a mean value for interfacial energy of 0.81 J/m². Particles were observed to spheroidise during ageing, and by the time their dimension had decreased 50%, they were completely spherical. Particle coalescence took place in a few cases but was generally of minor importance in the overall coarsening process.     

Hydride formation in an AlLiCu alloy

A hydride phase, LiAlH₄, has been identified in Al-2.0%Li-2.2%Cu alloy electrochemically charged with hydrogen. This is a hydride having the composition of LiAlH₄. The orientation relationship of the hydride and the matrix has been determined and rationalized with the O lattice theory. The thermodynamic stability of the hydride is discussed and possible formation mechanisms explained. The hydride forms from the grain boundary phase, AlLi, as the maximum amount of available Li is already present in the AlLi (δ) phase.     

高韧性Al—Cu合金

美国专利US 7494552本专利发明了一种AA2000系列的Al-Cu合金,其特点是韧性很高,并改进了强度。该合金的化学成分为(w_B,%):4.5～5.5Cu,0.5～1.6Mg,Mn≤0.80,Zr≤0.18,Cr≤0.18,Si≤0.15,Fe≤0.15,     

Microstructure Evolution and Grain Growth Competition in Directionally Solidified Ternary Al–Ag2Al–Al2Cu Eutectic

Metallurgical and Materials Transactions A - Eutectics serve as an interesting material class for fundamental research on solidification as well as holding promise for new, high-tech applications....     

The pre-bainitic transformation in CuZnAlMn alloy

The behaviour of the pre-bainitic transformation in the CuZnAlMn alloy was investigated by using internal friction (Q⁻¹) measurements and TEM. The results show that there always exists an internal friction peak associated with the segregation of solute atoms before the formation of orthorhombic 9R bainite and that the 9R bainite nucleates martensitically in depleted regions of solute atoms in the B₂ phase. The transformation processes mentioned above were also confirmed in isothermal internal friction and TEM experiments.     

Production of Al–Cu and Al–Cu–Si Alloys by PM Methods

Abstract

The possibilities of the production of aluminium-base copper and/or silicon alloys by conventional powder compaction and sintering methods have been studied. The effects of various lubricants, pressing, and sintering conditions on the behaviour of Al–Cu and Al–Cu–Si alloys were evaluated systematically. The role of copper and silicon additions during compaction and sintering and their advantages or disadvantages are discussed. All alloys underwent large dimensional changes (sudden swelling followed by rapid contraction) during sintering at temperatures greater than Al–Cu eutectic temperature and it is suggested that a process of particle rearrangement is largely responsible for this behaviour. The mechanical properties of the alloys were highly dependent on the sintering temperature. PM/0215     

The G.P. zones in AlCu alloys—II

Measurements on an absolute scale of the diffuse X-ray scattering by an aged single crystal of Al-1.94 at.% Cu have been made at a synchrotron source. Aging times were chosen to correspond to a mixture of G.P.1 and G.P.2 states, and to G.P.2 itself. The results indicate that the transition is in reality a coarsening reaction. There is not a unique structure for G.P.2 different than that for G.P.1. The G.P.1 state is a mixture of almost pure Cu single layer and multilayer platelets with their broad faces parallel to {100} planes of the Al-rich matrix. With increased aging time, these regions thicken and the number of single layer regions decreases. The so-called extra reflections in the G.P.2 state are actually thickness fringes. The strains around a multilayer zone are similar for all aging treatments, oscillate with distance from a zone, and are ~10% near the zone boundary, where the A1 planes collapse toward the zone.     

Numerical modelling of dendritic solidification in aluminium-rich AlCuMg alloys

By modification of a numerical iterative computer simulation worked out for binary alloys, the solidification behaviour of ternary alloys can be quantitatively predicted. The program finds the cooling curves, the amount of binary and ternary eutectic, the concentration distribution of the solute elements in the dendrite arms and the spacing of the dendrite arms during and after complete solidification. Comparison of the numerical predictions with experimental results obtained for five alloys from the aluminium rich corner of the AlCuMg system at four cooling rates shows very good agreement.     

The G.P. zones in AlCu alloys—I

Absolute measurements of the diffuse X-ray scattering from an Al-1.94 at.% Cu single crystal aged 1 hr at 353 K into the G.P.1 state have been made at a synchrotron source. It is confirmed that this state consists of a mixture of single and multilayer zones on {100} planes of the matrix, and essentially 100% copper. The multilayer zones have roughened or incomplete surfaces. Due to the high intensity and resolution of such an X-ray source, the interatomic displacements are obtained to a high precision for the first time. These displacements indicate that the interplanar strains parallel and normal to the zone faces oscillate in sign. For the single layer zones, the nearest Al plane collapses toward the zone by ~ 10%, whereas the next plane moves away by a similar amount. These displacements vanish near the fourth or fifth At plane. The strains near the multilayer zones are similar.     

Age-Induced Precipitating and Strengthening Behaviors in a Cu–Ni–Al Alloy

Microstructural response and variations in strength and electrical conductivity of a Cu−20 at. pct Ni–6.7 at. pct Al alloy during isothermal aging at temperatures from 723 K to 1023 K (450 °C to 750 °C) were investigated to discuss the age-induced precipitation behavior and strengthening mechanism. At all aging temperatures, fine spherical γ′-Ni₃Al particles were found to nucleate coherently with parent Cu grains by continuous precipitation and then grew gradually by Ostwald ripening. Domains with a high density of twins developed at grain boundaries during aging below 873 K (600 °C) followed by cellular components composed of fiber-shaped γ′-Ni₃Al and Cu solid solution phases at the domain boundaries later. Both the domains and cellular components were suppressed at aging above 923 K (650 °C). The age-induced strengthening principally resulted from fine dispersion of γ′-Ni₃Al coherent particles in the grains. The precipitation strengthening by the fine γ′-Ni₃Al coherent particles exhibited a maximum at an aging temperature of 873 K (600 °C), resulting in excellent mechanical properties such as a high hardness of 340 ± 7 HV and an ultimate tensile strength of 980 ± 14 MPa, which are comparable to those of other commercial age-hardened Cu–Be, Cu–Ni–Si, and Cu–Ti alloys.

     

Investigation of thermoelastic martensitic transformation in a CuZnAl alloy

Thermoelastic martensitic transformation in a Cu-29%Zn-3%Al alloy was investigated experimentally and theoretically. The phase structures and morphological changes occurring during transformation were studied using optical microscopy and high voltage TEM (1000–1200 kV), both equipped with combination heating and cooling stages. A “single crystal pure shear” experiment was designed to measure the relationship between M_s and shear stress, and from these data changes of enthalpy and entropy of the transformation were calculated through the Clausius-Clapeyron equation. The changes of enthalpy and entropy were also obtained by using calorimetric measurement. Both results were in good agreement. The kinetic behavior of the transformation in polycrystalline alloy as a function of applied stress was followed using electric resistance measurement. The slope of the transformation rate was constant over the range of 20–70% transformed, which corresponds physically to transformation occurring by plate growth in the unpartitioned parent phase or equivalently, the interphase boundary “moving freely.” A phenomenological theory was suggested to describe the constant slope portion of the transformation. Here, phase boundary motion is related to thermal hysteresis and quantitatively described the effect of applied stress on the transformation behavior.     

Hysteresis loops in stress induced β-18R martensite transformation in CuZnAl

A detailed study of the characteristics of the hysteresis loops in stress induced transformations was carried out using a high resolution apparatus. Samples with two different thermal treatments, air cooled and water quenched from 1070 K, were tested. The contribution of the nucleation processes to the hysteretic behaviour was evaluated. It was found that the intrinsic pseudoelasticity, due to the interaction of the martensite with the dislocations, plays a crucial role in the phenomenology of the transformation. This interaction prevents the growth of a unique plate and the transformation proceeds by successive nucleation of new plates. The friction associated to the movement of a single interface was also measured. From the experiments the relevant parameters which determine the hysteretic behaviour were identified. The hysteresis loops were computer simulated on the base of these parameters. The results showed a good agreement with the experimental observations.     

Precipitation and dissolution kinetics in AlLiCuMg alloy 8090

Resistivity and differential scanning calorimetry (DSC) techniques are used to study the kinetics of precipitation and dissolution of GPB zones and metastable phases (δ′ and S′) in AlLiCuMg alloy 8090. Three stages of the precipitation sequence during aging have been analyzed. A new analytical method is developed for the DSC technique, which requires only one heating rate to obtain the kinetic parameters, and the results show good agreement with the more conventional method of varying heating rate. The low-temperature endothermic peak in the DSC thermogram is interpreted as the dissolution of Li-bearing zones, which is supported by the hardness results. The activation energy, Q, and the growth parameter, n, determined by resistivity and DSC techniques are in good agreement with previously published data.