Effect of strain rate on the yielding mechanism of amorphous metal foam

Abstract

Stochastic amorphous Pd 43 Ni 10 Cu 27 P 20 foams were tested in quasistatic and dynamic loading. The strength/porosity relations show distinct slopes for the two loading conditions, suggesting a strain-rate-induced change in the foam yielding mechanism. The strength/porosity correlation of the dynamic test data along with microscopy assessments support that dynamic foam yielding is dominated by plasticity rather than elastic buckling, the mechanism previously identified to control quasistatic yielding. The strain-rate-induced shift in the foam yielding mechanism is attributed to the rate of loading approaching the rate of sound wave propagation across intracellular membranes, thereby suppressing elastic buckling and promoting plastic yielding. © 2010 American Institute of Physics. ͓doi:10.1063/1.3279132͔Recent progress in the processing of metallic glasses has led to the development of open-and closed-cell amorphous metal foams fabricated from various alloys via a variety of methods.1-6 Because of the unique mechanical properties of amorphous metals ͑e.g., high strength and elasticity, broadly varying toughness, and lack of ductility͒, 7 amorphous metal foams inherit a set of mechanical properties not previously seen in porous solids of any kind. Specifically, cellular structures consisting of struts thinner than the process zone size of the amorphous metal were found to be heavily deformable, as catastrophic failure due to global brittle fracture is avoided. 8,9 Highly stochastic cellular structures consisting of struts with broadly varying thicknesses and aspect ratios were found to yield by percolation of elastic buckling, a consequence of the high elastic limit of the amorphous metal.10 Elastic yielding gives rise to a steep strength/ porosity relation, resulting in very high strengths at high relative densities, 11 however, as the limit of cooperative buckling is approached at low relative densities, the attainable foam strengths are rather low. Interestingly, by matching the structural scales controlling these two yielding mechanisms ͑brittle fracture and buckling percolation͒, i.e., by attaining cellular structures consisting of thin struts with uniform slenderness ratios, foams of low relative densities ͑Ͻ10%͒ yield plastically.12 Consequently, such foams are able to inherit the high plastic yield strength of the amorphous metal at very high porosities, and emerge among the strongest foams of any kind. The strain-rate sensitivity of these porous solids has not yet been investigated. Monolithic ͑pore-free͒ amorphous metals are known to be strain-rate insensitive. 13,14 In this letter we demonstrate that unlike the parent solid, the yielding mechanism of a stochastic amorphous-metal cellular structure is sensitive to the rate of the applied strain. Hence, a drastic change in strain rate results in a shift in the overall strength/porosity relation.Amorphous Pd 43 Ni 10 Cu 27 P 20 foam specimens with highly stochastic closed-cell porosity were utilized. The foams were produced by thermoplastically expanding...

Metallic Glass Structures for Mechanical-Energy-Dissipation Purpose: A Review

“…Fortunately, the MG foams can have both larger sample dimensions and better room-temperature plasticity. As compared with the solid MG samples, MG foams can exhibit greatly-enhanced nominal plasticity, where wide plastic-flow-plateau stages can be achieved for achieving a higher plastic energy absorption capacity [37][38][39][40][41][42][48][49][50][51][52]. Moreover, the glass transition nature endows MGs with a relatively-lower processing temperature as compared with conventional metals [14].…”