Yielding of Metallic Glass Foam by Percolation of an Elastic Buckling Instability

Abstract

Recent advances in the processing of closed- [1][2][3][4][5] and opencell [6][7][8][9] metallic glass foam have focused increasing attention on the mechanical behavior of this emerging structural material. This interest evolved after recognizing that metallic glasses impregnated with pores can be effectively alleviated from their inherent brittle nature and yet retain a considerable fraction of their outstanding strength.[10] The mechanical behavior of open-and closed-cell metallic glass foams has recently been assessed by means of compressive experiments performed using standard mechanical testing procedures. Open-cell Zr 57 Nb 5 Cu 15.4 Ni 12.6 Al 10 foams investigated in the porosity range of 72-86 % are found to exhibit impressive compressive plasticity extending to nearly 80 % strains, however, they are found to yield at relatively low stresses ranging between 5 and 34 MPa. [7][8][9] In contrast, closed-cell Pd 43.5 Ni 7.5 Cu 30 P 20 and Pd 35 Pt 15 Cu 30 P 20 foams investigated in the porosity range of 36-64 % were found to exhibit substantially higher strengths, ranging between 250 and 600 MPa, but a more modest plasticity ranging between 20 and 30 % strains. [4,5] Plasticity in these closed-cell foams is shown to propagate by recurring non-catastrophic crushing events associated with the formation of collapse bands through the foam structure. More interestingly, ahead of a crushing event these foams are found to exhibit an unusual yielding response characterized by an extended stress plateau. This nonlinear foam yielding response bears resemblance to the yielding of a monolithic metallic glass near zero temperature. [11][12][13] This resemblance hints at a possible similarity between floppybonded networks (such as foams) and zero-temperature glasses, as originally proposed by Alexander. [14] In this communication we seek to investigate the underlying mechanisms governing the nonlinear yielding of these closed-cell foams by examining the intercellular interactions within a bulk-foam specimen by using real-time X-ray microtomography.For the loading experiment, we utilized a 70 % porosity closed-cell amorphous Pd 43 Ni 10 Cu 27 P 20 foam. In situ X-ray microtomography during loading was performed at beam line ID15A of the ESRF. [15,16] A cycle involving compressive loading towards a non-catastrophic collapse event followed by unloading was implemented. 18 radiographic images of the cellular structure were recorded at constant strain. The stress-strain diagram is presented in Figure 1. The stress spikes appearing in the loading curve are a consequence of ra- COMMUNICATION

Anelastic to Plastic Transition in Metallic Glass-Forming Liquids

“…He further argued that the elastic matrix confinement of an isolated transformed STZ would lead to reversible elastic energy storage in the STZ-matrix system, implying that transformed STZ's have a memory of their original untransformed state. Interestingly, Argon's concept was recently studied in a deforming metallic glass foam, where buckled membranes and their accommodating stress fields were observed to behave like elastically confined STZ's [5].…”

“…He further argued that the elastic matrix confinement of an isolated transformed STZ would lead to reversible elastic energy storage in the STZ-matrix system, implying that transformed STZ's have a memory of their original untransformed state. Interestingly, Argon's concept was recently studied in a deforming metallic glass foam, where buckled membranes and their accommodating stress fields were observed to behave like elastically confined STZ's [5].As recognized by Johari and Goldstein [6], the underlying relaxation mechanisms of liquids and glasses are governed by two kinetic processes: a fast process, termed the process, viewed as a locally initiated and reversible process, and a slow process, termed the process, viewed as a large scale irreversible rearrangement of the material. From a potential energy landscape perspective, Debenedetti and Stillinger [7] have identified the transitions as stochastically activated hopping events across ''subbasins'' confined within the inherent ''megabasin'' (intrabasin hopping) and the transitions as irreversible hopping events extending across different landscape megabasins (interbasin hopping).…”

Oxidation Behavior of a Pd43Cu27Ni10P20 Bulk Metallic Glass and Foam in Dry Air

“…5 Cu 30 Ni 7.5 P 20 and Pd 43 Cu 27 Ni 10 P 20 foams exhibited strengths in the range of 100 to 600 MPa, elastic moduli in the range of 6 to 32 MPa, and plastic deformation up to 20 to 30 pct strain. [6][7][8] Nevertheless, the use of Pd4-BMG and its foam in practical applications at higher temperatures requires knowledge of its degradation behavior during oxidation. Thus, it is of interest to investigate the oxidation behavior of the Pd4-BMG and its foamy material in this study.…”

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

“…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.…”

“…The postyielding behavior of these foams under quasistatic loading has been studied extensively elsewhere. 10,11 A typical stress strain response of a 0.4 relative density foam deformed under a strain rate of 10 −4 s −1 is shown in Fig. 3.…”