The plasticity size effect in replicated microcellular aluminium

Abstract

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

Reviewing recently developed technologies to direct cell activity through the control of pore size: From the macro‐ to the nanoscale

“…The polymer is subsequently hardened, and the salt is removed via dissolution in a solvent such as water or alcohol (Lee, Kim, Chong, Hong, & Lee, ). This technique has been adopted for many years as it allows to achieve a precise control of the pore size and pore morphology (Goodall, Despois, & Mortensen, ; Jia et al, ; Osorio‐Hernández et al, ). In this study, the authors fabricated a porous polyetheretherketone scaffold (PEEK‐SP) using sodium chloride with different sizes (200–312, 312–425, and 425–508 μm).…”

Design and Fabrication of Highly Porous Replicated Aluminum Foam Using Double-Granular Space Holder

“…Goodall et al revealed the plasticity size effect. The authors reported that higher strength and rates of hardening are obtained when the structure contains smaller pores [ 9 ]. One of the most promising methods of porosity formation is an infiltration process using NaCl as a space holder.…”

Amino Acids Aided Sintering for the Formation of Highly Porous FeAl Intermetallic Alloys

“…After formation of the massive, porous element, the NaCl is leached out and porosity is revealed. Following this approach, metallic foams made of Al [ 15 , 16 , 17 ], Cu [ 18 ], FeAl [ 19 ], Mg [ 20 , 21 ], Ti [ 22 ], Ti 5 Si 3 [ 23 ], TiAl [ 24 ], and TiAl 3 [ 25 ] have been obtained. Nevertheless, using sodium chloride as the space holder may bring some risks since, when the leaching out process is too long, potentially galvanic cells may be formed and corrosion may occur.…”

“…The major motivations of the use of the amino acids as gas releasing agents are the volume of the in situ produced gases ( Table 1 ) and the formation of gaseous nitrogen, diluting remaining gaseous products including carbon dioxide, and thus decreasing the chemical interaction between the formed sinter and produced gas. Moreover, amino acids as well as other organic compounds [ 32 , 33 , 34 ] work as mentioned gas releasing agent, but first they act as space holders in the same way as e.g., NaCl [ 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 ].…”