The effect of density and feature size on mechanical properties of isostructural metaffic foams produced by additive manufacturing

Simple models describing the relationship between basic mechanical properties and the relative density of various types of porous metals (such as foams, sponges and lattice structures) are well established. Carefully evaluating these relationships experimentally is challenging, however, because of the stochastic structure of foams and the fact that it is difficult to systematically isolate density changes from variations in other factors, such as pore size and pore distribution. Here a new method for producing systematic sets of stochastic foams is employed based on electron beam melting (EBM) additive manufacturing (AM). To create idealised structures, structural blueprints were reverse-engineered by inverting X-ray computed tomographs of a randomly packed bed of glass beads. This three-dimensional structure was then modified by computer to create five foams of different relative density ρr, but otherwise consistent structure. Yield strength and Young’s modulus have been evaluated in compression tests and compared to existing models for foams. A power of 3 rather than a squared dependence of stiffness on relative density is found, which agrees with a recent model derived for replicated foams. A similar power of 3 relation was found for yield strength. Further analysis of the strength of nominally fully dense rods of different diameters built by EBM AM suggest that surface defects mean that the minimum size of features that can be created by EBM with similar strengths to machined samples is ∼1 mm

Influence of Grain Boundary Character on Creep Void Formation in Alloy 617

Alloy 617, a high temperature creep-resistant, nickel-based alloy, is being considered for the primary heat exchanger for the Next Generation Nuclear Plant (NGNP) which will operate at temperatures exceeding 760oC. Orientation imaging microscopy (OIM) is used to characterize the grain boundaries in the vicinity of creep voids that develop during high temperature creep tests (800-1000oC at creep stresses ranging from 20-85 MPa) terminated at creep strains ranging from 5-40%. Observations using optical microscopy indicate creep rate does not significantly influence the creep void fraction at a given creep strain. Preliminary analysis of the OIM data indicates voids tend to form on grain boundaries parallel, perpendicular or 45o to the tensile axis, while few voids are found at intermediate inclinations to the tensile axis. Random grain boundaries intersect most voids while CSL-related grain boundaries did not appear to be consistently associated with void development

Cast aluminium single crystals cross the threshold from bulk to size-dependent stochastic plasticity

Metals are known to exhibit mechanical behaviour at the nanoscale different to bulk samples. This transition typically initiates at the micrometre scale, yet existing techniques to produce micrometre-sized samples often introduce artefacts that can influence deformation mechanisms. Here, we demonstrate the casting of micrometre-scale aluminium single-crystal wires by infiltration of a salt mould. Samples have millimetre lengths, smooth surfaces, a range of crystallographic orientations, and a diameter D as small as 6 μm. The wires deform in bursts, at a stress that increases with decreasing D. Bursts greater than 200 nm account for roughly 50% of wire deformation and have exponentially distributed intensities. Dislocation dynamics simulations show that single-arm sources that produce large displacement bursts halted by stochastic cross-slip and lock formation explain microcast wire behaviour. This microcasting technique may be extended to several other metals or alloys and offers the possibility of exploring mechanical behaviour spanning the micrometre scale

Creep of aluminium-magnesium open cell foam

Aluminium-5 wt.% magnesium open cell foam produced by replication and tested in tension at 300, 350 or 450 degrees C creeps at rates between 10(-3) and 10(-8) s(-1). The behaviour of the foam matches that of the alloy from which it is made: three-power law creep with the same activation energy as for Al-Mg alloy creeping by viscous dislocation glide in the high stress regime and five-power law creep in the low stress regime. The model of Mueller et al. [Mueller R, Soubielle S, Goodall R, Diologent F. Mortensen A. Scripta Mater 2007;57:33], itself a simplified adaptation of previous variational estimates, predicts well the measured foam creep rates, in terms of both absolute value and dependence on temperature and applied stress. Agreement with the model of Andrews et al. [Andrews EW, Gibson LJ, Ashby MF. Acta Mater 1999;47:2853] is somewhat inferior, but nonetheless satisfactory. A strong dependence of creep rate on relative density is found; this feature is also captured by the variational estimate. (c) 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Surface oxide in replicated microcellular aluminium and its influence on the plasticity size effect

The oxide film present along the interface between metal and pores in microcellular aluminium produced by replication depends on the leaching medium that is used to dissolve the NaCl preform: leaching in water produces a thick and irregular multilayered hydroxide layer, whereas leaching in a chromate conversion solution produces a smooth layer of oxide, approximate to 10 nm thick. The pore-size-dependent flow stress of replicated microcellular aluminium exhibits a marked dependence on the leaching process used to produce the foam when the pore size is below roughly 100 mu m. This dependence on the leaching medium is a result of the influence exerted by the outer surface oxide layer on dislocational glide in micron-sized metal struts making the foam. (C) 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved