Activation volume in microcellular aluminium: Size effects in thermally activated plastic flow

Diologent, Frédéric; Goodall, Russell; Mortensen, Alicja

doi:10.1016/j.actamat.2011.07.021

Cited by 20 publications

(9 citation statements)

References 62 publications

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“…Turning to the Al2Mg microcellular foams, one finds broadly similar stress-strain curves, with the difference that a close-up on the stress-strain curves reveals the presence of many serrations, Fig. 1b, similar to serrations that were observed in earlier work on replicated Al5Mg foams [6]. Such saw-tooth curves are a typical signature of the PLC effect.…”

Section: Tab 1 Main Characteristics Of Al and Al2mg Foam Samples Inmentioning

confidence: 48%

Occurrence of the Portevin Le-Châtelier effect in open-cell microcellular Al-2wt% Mg

et al. 2017

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Microcellular open-cell metal foams with pores of 75 or 400 µm in diameter and made of pure Al or Al-2 wt% Mg are tested in compression. The aluminum foam exhibits a typical smooth plastic flow and a positive strain rate sensitivity, whereas plastic instabilities and a negative strain rate sensitivity appear in the stress-strain curves of the Al-2 wt% Mg foams, indicating the presence of the Portevin Le-Châtelier (PLC) effect. The effect is further confirmed by acoustic emission analysis. Taken together, the data indicate that the PLC effect is caused in this material by the sudden collective deformation of many struts across the material.

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Section: Tab 1 Main Characteristics Of Al and Al2mg Foam Samples Inmentioning

confidence: 48%

Occurrence of the Portevin Le-Châtelier effect in open-cell microcellular Al-2wt% Mg

et al. 2017

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“…This difference in flow stress, by a factor near two, is likely another example of the frequently reported "knock-down" factor between theory and data for the plastic flow stress of microcellular materials [2,6,46,47].…”

Section: Scaling Of the Flow Stressmentioning

confidence: 99%

“…This collapsed curve is, according to the model, the effective (von Mises) back-calculated insitu stress-strain curve of the metal making the foam -with the caveat that it is likely scaled down by a fixed "knock-down" factor on the order of two to three, which has often been found in confronting data with theory but remains so far essentially unexplained [2,6,46,47].…”

Section: Scaling Of the Flow Stressmentioning

confidence: 99%

Influence of microstructural heterogeneity on the scaling between flow stress and relative density in microcellular Al–4.5 %Cu

2014

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We explore the influence of the metal microstructure on the compressive flow stress of replicated microcellular 400 µm pore size Al-4.5wt%Cu solidified at two different solidification cooling rates, in as-cast and T6 conditions. It is found that the yield strength roughly doubles with age-hardening but does not depend on the solidification cooling rate.Internal damage accumulation, measured by monitoring the rate of stiffness loss with strain, is similar across the four microstructures explored and equals that measured in similar microcellular pure aluminium. In-situ flow curves of the metal within the open-pore microcellular material are back-calculated using the Variational Estimate of Ponte Castañeda and Suquet. Consistent results are obtained with heat-treated microcellular Al-4.5wt%Cu, and are also obtained with separate data for pure Al; however, for as-cast microcellular Al-4.5wt%Cu the back-calculated in-situ metal flow stress decreases, for both solidification rates, with decreasing relative density of the foam. We attribute this effect to an interplay between microstructural and mesostructural features of the microcellular material: variations in the latter with the former held constant can alter the scaling between flow stress and relative density within microcellular alloys.

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“…Rather, it displays a finite rate of work-hardening, which reflects dual influences of hardening in the metal making the foam, coupled with an absence of the collapse bands that are observed when most other metal foams are compressed [11,16,[19][20][21]. Curves from samples of this work are shown in Fig.…”

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confidence: 99%

“…The flow stress of the resulting microcellular high-purity aluminium increases, all else constant, as the pore size falls below roughly 100 µm [13][14][15][16]. The oxide layer covering the pores exerts a strong influence on its flow stress [17], and thermally activated slip in this material is altered by the free surfaces within it [16]. Here, we show that the plasticity size effect in these materials is dislocational in nature and that it has the same origin as in metal matrix composites.…”

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confidence: 99%

The plasticity size effect in replicated microcellular aluminium

2013

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Microcellular aluminium can be produced by replication with pores across a wide range of sizes but otherwise identical structures. Compressive tests reveal a plasticity size effect, with samples showing higher strengths and higher rates of work hardening for smaller pore diameters. This size effect is shown to be dislocational, its main origin being dislocation emission during the composite stage of foam processing. Keywords: Foams; Compression Test; Dislocation Theory; Size EffectThe plastic flow stress of metals often becomes size-dependant when this falls below roughly 10 µm: beyond this point, the finer the microstructure or size of a deforming metal, the higher its flow stress. The effect has been documented in many configurations, ranging from wire torsion [1], FIBmilled micropillar compression [2,3], nanoindentation [4], dispersion and grain boundary hardening [5,6], to the flow stress of metal matrix composites [7,8], metal microparticles [9] or thin films [6,10].Plasticity size effects are also manifest in porous metals: when pores of a microcellular metal decrease below a certain size (all else constant) its flow stress starts increasing [11]. Microcellular aluminium is one of the most convenient materials for the study of this effect. The aluminium replication process, described in more detail in Ref. [12], consists of preparing a packed preform of NaCl particles, which is infiltrated with aluminium that is then solidified before leaching the salt. This leaves a network of interconnected aluminium struts, which delineate pores, or controlled size, shape and volume fraction. The flow stress of the resulting microcellular high-purity aluminium increases, all else constant, as the pore size falls below roughly 100 µm [13][14][15][16]. The oxide layer covering the pores exerts a strong influence on its flow stress [17], and thermally activated slip in this material is altered by the free surfaces within it [16]. Here, we show that the plasticity size effect in these materials is dislocational in nature and that it has the same origin as in metal matrix composites.Foams were made with 99.99% pure aluminium using NaCl particles crushed and sieved to produce size ranges with three different mean particle diameters, 400 µm, 75 µm or 26 µm. The particles were packed and cold isostatically pressed to produce preforms. These were then infiltrated at 710ºC under different argon gas pressures, chosen to drive uniform infiltration of the metal into the preform (0.4 MPa for 400 µm, and 8 MPa for the other two sizes), and cooled slowly in the furnace before being machined. The salt was then removed by dissolution in ordinary tap water, significant corrosion being avoided by frequently changing the water and using the minimum amount of immersion time required. Note that, since a corrosion inhibitor was here not used during

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Activation volume in microcellular aluminium: Size effects in thermally activated plastic flow

Cited by 20 publications

References 62 publications

Occurrence of the Portevin Le-Châtelier effect in open-cell microcellular Al-2wt% Mg

Occurrence of the Portevin Le-Châtelier effect in open-cell microcellular Al-2wt% Mg

Influence of microstructural heterogeneity on the scaling between flow stress and relative density in microcellular Al–4.5 %Cu

The plasticity size effect in replicated microcellular aluminium

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