Stochastic Metallic-Glass Cellular Structures Exhibiting Benchmark Strength

Abstract

By identifying the key characteristic ''structural scales'' that dictate the resistance of a porous metallic glass against buckling and fracture, stochastic highly porous metallic-glass structures are designed capable of yielding plastically and inheriting the high plastic yield strength of the amorphous metal. The strengths attainable by the present foams appear to equal or exceed those by highly engineered metal foams such as Ti-6Al-4V or ferrous-metal foams at comparable levels of porosity, placing the present metallic-glass foams among the strongest foams known to date. DOI: 10.1103/PhysRevLett.101.145702 PACS numbers: 64.70.pe, 61.43.Gt, 62.20.mm, 62.20.mq The fundamentals governing the strength capabilities of foam materials have been well studied over the past three decades [1]. Depending on the mechanism accommodating foam failure, the failure stress will be determined by the relevant structural property of the parent solid. Specifically, the strength of a plastically yielding foam will be determined by the solid yield strength, the strength of a brittle foam by the solid fracture stress, and the strength of an elastically buckling foam by the solid modulus. Since fracture and buckling stresses of solids are generally lower than plastic yield strengths, brittle or elastically buckling foams tend to be weaker than plastically yielding foams. Owing to remarkably high plastic yield strengths, amorphous metals are thought to be attractive parent materials for ultrastrong foams [2]. Considerable advances in the synthesis, characterization, and testing of metallic-glass foam have been reported to date [3][4][5][6][7][8][9][10][11][12][13][14]. Low-to moderate-porosity foams are found to exhibit strengths that are roughly consistent with the high plastic yield strength of the monolithic amorphous metal [7][8][9][10][11][12]; however, higher-porosity foams ( > 80%) fail at relatively low stresses that cannot be correlated to the strength of the parent solid [13,14]. In this Letter, we demonstrate that, by matching the key characteristic ''structural scales'' that dictate the resistance of a metallic-glass cellular structure against buckling and fracture, the metallic-glass foam can inherit the plastic yield strength of the amorphous metal up to porosities as high as 92%. The achievable foam strengths reported herein appear to equal or exceed those by strong highly engineered metal foams, placing the present metallic-glass foams among the strongest foams of any kind.Amorphous metals exhibit superb plastic yield strengths; however, upon unconfined loading they fail plastically by shear localization attaining very limited global plasticity terminated by brittle fracture. Unlike ceramics, though, most amorphous metals are capable of undergoing plastic yielding prior to failing catastrophically by fracture, an ability attributed to a toughness adequate to suppress fracture until plasticity is initiated. Fracture toughness values for amorphous metals typically range between 10 and 100 MPa m 1=2 , going from relati...

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