Materials Science Forum Vols. 519-521

Abstract: X-ray tomography allows the microstruture of aluminum alloys to be imaged non destructively in three dimensions (3D). This paper shows different examples of the use of this technique for the quantification of damage in model and industrial Al based materials. The model materials are used to setup the technique. The spherical shape of their inclusions makes it easy to compare the measurements with the prediction of standard model for damage. The industrial materials are characterized during in situ tensile but also ex situ bulging and plane strain tension tests. The respective contribution of initiation and growth of damage is measured separately and discussed. The 3D data are also used to quantify the anisotropy of the effect of damage.

Abstract: A micromechanical model has been developed in order to capture the influence of a second population of voids on the coalescence of large primary voids. FE unit cell simulations have been performed by introducing the primary voids explicitly in a finite element mesh and by using a Gurson type model for the surrounding matrix in order to reproduce the influence of the second population. These simulations have guided the development of a closed-form void coalescence model. The new coalescence condition accounts for the softening introduced by the second population by integrating the Gurson model based on an approximate solution for the stress and strain field near the surface of the primary voids. The evolution of the primary voids is modelled using an advanced Gurson model involving an evolution law for the void shape. The model is applied to the prediction of the fracture strain of 6xxx aluminium alloys measured on smooth and notched round bars. The model successfully captures, without any parameter adjustment, the variation of the ductility as a function of the stress triaxiality for various shapes of the primary particles and various volume fraction of second population.

Abstract: Supersaturated Al-4.8Zn-1.2Mg-0.14Zr and Al-5.7Zn-1.9Mg-0.35Cu (wt.%) alloys were processed by Equal-Channel Angular Pressing (ECAP) at 200°C. The crystallite size distribution and the characteristic parameters of the dislocation structure of both Al matrix and precipitates were determined by X-ray diffraction line profile analysis, which has been complemented by transmission electron microscopy (TEM) observations. The results show that severe plastic deformation promotes the precipitation process and consequently has a strong influence on the strength of these alloys.

Abstract: In the present study the influence of strain rate and temperature on the behaviour of two commercial aluminium alloys, 6063-T6 and 7030-T6, was investigated. Both alloys are high strength precipitation hardened alloys that are expected to have low strain rate and temperature sensitivity. Tensile tests were performed at room temperature at strain rates ranging from 10-4 to 102 s-1, and at -40°C and +60°C at strain rates of 10-4 and 10-1 s-1, due to equipment limitations. Both alloys showed low but positive strain rate sensitivity at all temperatures. Also the temperature sensitivity was low, showing negative values in all cases. The dependence of the flow stress on temperature was more pronounced than the strain rate dependence. The area reduction at fracture was higher in 6063 than 7030, although the uniform elongation was larger in 7030. 6063 showed almost no strain rate dependence of the ductility and a limited reduction with increased temperature. 7030 showed markedly increasing area reduction with increasing temperature and decreasing values with increasing strain rate. The energy absorption was higher in 7030 by a factor of approximately three.

Abstract: Recently the method for obtaining ultra-fine grained metallic materials has developed using severe plastic deformation (SPD), such as equal channel angular pressing (ECAP), accumulative roll bonding (ARB), torsion straining, and warm multiple deformation (WMD) etc. In order to enhance thermal stability of ultra-fine grained aluminum alloys manufactured by SPD process, the addition of Sc and Zr elements has been considered to devise fine Al3Sc, Al3Zr and Al3(Scx Zr1-x) precipitates for inhibiting the grain growth. In this study, the microstructure evolution has been investigated in Al-Mg alloys with and without Sc and Zr addition during the warm multiple deformation process. In addition Al-Mg alloys were compressed at a strain rate of 10-1 sec-1 by two different routes, that is, route A and route B. Route A is to rotate the specimen throughout 90o around the vertical axis of loading direction at every pass. Route B is to rotate the specimen throughout 90o around the parallel axis of loading direction and then rotate it again as route A. The specimen deformed by route B had finer grain size and more uniform distribution of grains than those deformed by route A. When the warm multiple deformation process repeated up to 8 passes at 673 K, the specimen consisted of ultra-fine grained structure with the average grain size less than 3 μm. The superplastic behavior can also be observed at the high strain rate and low temperature regime.

Abstract: The tensile strength, proof strength, hardness and electrical conductivity of Al alloy 7010 under different temper and ageing conditions were investigated with the aim to correlate strength with hardness and electrical conductivity so that the strength of the alloy can be determined nondestructively. Following the solutionising treatment, continuous age hardening was performed on a series of test coupons, taken from a large plate, to produce a wide range of precipitation hardening conditions, which gave rise to progressive variations of strength, hardness and conductivity. The relationship between strength and hardness was found to be reasonably linear, whereas the relationship between hardness and strength with electrical conductivity was non-linear. The ageing conditions and therefore the mechanical properties of the components can be predicted more accurately by the simultaneous combination of hardness and conductivity values.

Abstract: The influence of the iron content from 0.1 to 1.0 mass% on the mechanical properties of AA6016-T4 sheets was investigated. The amount of the Al-Fe-Si second phase particles increased with the iron content, thus the solute silicon atoms decreased. Increasing of the Al-Fe-Si particles lowers the bendability, while decreasing of the solute silicon atoms lowers the paint bake response (PBR) and improves the bendability. The bendability of the samples then became the worst at the 0.5 mass% iron content, while it at 0.8 and 1.0 mass% became better or the same as that at 0.5 mass%. The increasing of the silicon content in the 1.0 mass% iron content alloy improved the PBR and lowered the bendability.

Abstract: Gradient-dependent plasticity where a characteristic length is involved to consider the microstructural effect (interactions and interplaying among microstructures due to the heterogeneous texture) and the measured nonlinear shear stress-shear strain curves for different loading strain rates are used to calculate the distribution of local temperature rise in adiabatic shear band (ASB) for aluminum-lithium alloy specimen of thin-walled tube in dynamic torsion test. ASB is assumed to initiate just at peak shear stress in the specimen. The temperature rise in ASB is decomposed into the uniform temperature rise in strain-hardening stage and the nonuniform temperature rise in strain-softening stage. The former depends on the measured nonlinear shear stress-shear strain curve prior to the peak, the density, the work to heat conversion factor and the heat capacity. The latter is related to the softening branch of the measured nonlinear shear stress-shear strain curve, the internal length parameter and the physical parameters. For binary Al-Li alloy, the predicted maximum temperatures in ASB are 413K at strain rate of 2000s-1 and 433K at strain rate of 2600s-1. These peak temperatures are lower than the recrystallization and phase transformation temperatures. Higher loading strain rate results in higher pre-peak and post-peak temperature rises, steeper profile of local temperature and higher peak local temperature in ASB. These predictions qualitatively agree with the previously analytical solution for ductile metal exhibiting linear strain-softening behavior beyond the peak shear stress based on gradient-dependent plasticity.

Abstract: The influence of Sc addition on the high temperature compressive strength of a commercial alloy 7010 (hereafter termed base alloy) has been examined. The base alloy, and the base alloy with 0.23 wt% Sc were cast, homogenized and subjected to compression tests at temperatures ranging from 300 to 450oC and strain rates of 10-3, 10-2, 10-1 and 1 sec-1. It is shown that Sc addition to the base alloy increases the compressive flow stress under these deformation conditions. The increase in peak flow stress is nearly 3-6 times the peak flow stress of the base alloy at temperatures 300-350oC over the strain rate range investigated. Whilst, at temperatures ³ 400oC, the flow stresses decrease significantly irrespective of the strain rate used. Transmission electron microscopy (TEM) revealed that a combination of (1) increased nucleation frequency of dispersoids, (2) evolution of smaller subgrain size, and (3) refinement of alloy phases in the Al-Zn-Mg-Cu system contribute to superior strengthening in the alloy containing Sc. Whilst, it is primarily a combination of coarsening and instability of the alloy phases in the Al-Zn-Mg-Cu system that dramatically reduces the flow stresses in both the alloys at temperatures ³ 400oC.

Abstract: The effects of strain path reversal on the macroscopic orientation of microbands in AA5052 have been studied using high resolution electron backscatter diffraction. Deformation was carried using two equal steps of forward/forward or forward/reverse torsion at a temperature of 300°C and strain rate of 1s-1 to a total equivalent tensile strain of 0.5. In both cases microbands were found in the majority of grains examined with many having more than one set. The microbands appear to cluster at specific angles to the macroscopic deformation. For the forward/forward condition microbands clustered around -20° and +45° to the maximum principle stress direction and at ± 30-35° to the principal strain direction. For the forward/reverse condition significantly more spread in microband angle was observed though peaks were visible at ±35° with respect to principal stress direction and at -40° and +30° with respect to the principal strain direction of the reverse torsion. This suggests the microbands formed in the forward deformation have or are dissolving and any new microbands formed are related to the deformation conditions of the final strain path.