Owing to a lack of microstructure, glassy materials are inherently strong but brittle, and often demonstrate extreme sensitivity to flaws. Accordingly, their macroscopic failure is often not initiated by plastic yielding, and almost always terminated by brittle fracture. Unlike conventional brittle glasses, metallic glasses are generally capable of limited plastic yielding by shear-band sliding in the presence of a flaw, and thus exhibit toughness-strength relationships that lie between those of brittle ceramics and marginally tough metals. Here, a bulk glassy palladium alloy is introduced, demonstrating an unusual capacity for shielding an opening crack accommodated by an extensive shear-band sliding process, which promotes a fracture toughness comparable to those of the toughest materials known. This result demonstrates that the combination of toughness and strength (that is, damage tolerance) accessible to amorphous materials extends beyond the benchmark ranges established by the toughest and strongest materials known, thereby pushing the envelope of damage tolerance accessible to a structural metal.
The development of metal alloys that form glasses at modest cooling rates has stimulated broad scientific and technological interest. However, intervening crystallization of the liquid in even the most robust bulk metallic glass-formers is orders of magnitude faster than in many common polymers and silicate glass-forming liquids. Crystallization limits experimental studies of the undercooled liquid and hampers efforts to plastically process metallic glasses. We have developed a method to rapidly and uniformly heat a metallic glass at rates of 10(6) kelvin per second to temperatures spanning the undercooled liquid region. Liquid properties are subsequently measured on millisecond time scales at previously inaccessible temperatures under near-adiabatic conditions. Rapid thermoplastic forming of the undercooled liquid into complex net shapes is implemented under rheological conditions typically used in molding of plastics. By operating in the millisecond regime, we are able to "beat" the intervening crystallization and successfully process even marginal glass-forming alloys with very limited stability against crystallization that are not processable by conventional heating.
An alloy development strategy coupled with toughness assessments and ultrasonic measurements is implemented to design a series of iron-based glass-forming alloys that demonstrate improved glass-forming ability and toughness. The combination of good glass-forming ability and high toughness demonstrated by the present alloys is uncommon in Fe-based systems, and is attributed to the ability of these compositions to form stable glass configurations associated with low activation barriers for shear flow, which tend to promote plastic flow and give rise to a toughness higher than other known Fe-based bulk-glass-forming systems. © 2009 American Institute of Physics. ͓DOI: 10.1063/1.3184792͔The remarkably high strength, modulus, and hardness of iron-based glasses, combined with their low cost, prompted an effort over the past five years to design amorphous steel suitable for structural applications. The development effort yielded glasses with critical rod diameters as large as 12 mm 1,2 and strengths in excess of 4 GPa. 3 These low-cost ultrastrong materials however exhibit fracture toughness values as low as 3 MPa m 1/2 , 4 well below acceptable toughness limits for structural materials. The low toughness has been linked to their elastic constants, specifically their high shear modulus, 5 which for some compositions exceeds 80 GPa. 3 Recent efforts to toughen these alloys by altering their composition yielded glasses with lower shear moduli ͑below 70 GPa͒, which exhibit improved notch toughness ͑as high as 50 MPa m 1/2 ͒ but compromised glass-forming ability ͑criti-cal rod diameters less than 3 mm͒. 5,6 In this study, we implement an alloy development strategy coupled with toughness assessment and ultrasonic measurements to design glassy steel alloys with particularly low shear moduli ͑below 60 GPa͒ that demonstrate high toughness ͑notch toughness in excess of 50 MPa m 1/2 ͒ yet adequate glass-forming ability ͑critical rod diameters as large as 6 mm͒.The link between the high shear modulus and the low toughness of Fe-based glasses rests on the argument that a high shear modulus implies a high resistance to relax stress by shear flow. In turn, this promotes cavitation and early fracture and thus limits toughness. Using a Frenkel-like analysis to study cooperative shearing, Johnson and Samwer 7 arrived at a quantitative expression for the activation energy for shear flow, that is, the energy barrier to initiate plastic flow. Specifically, a relationship was proposed between the shear-flow barrier W and the shear modulus G for a frozen-in atomic configuration at the glass transition temperature T g , given by W͑T g ͒ ϰ G͑T g ͒v m ͑T g ͒, 7 where v m is the molar volume, which usually varies little within an alloy family. Aside from their high G, the brittle behavior of these glasses can also be predicted by their high T g , which for some compositions exceeds 600°C. 1,2 The glass transition temperature is also a measure of W͑T g ͒, since the requirement for the liquid viscosity at T g ͑10 12 Pa s͒ gives W͑T g ͒Ϸ37RT g ....
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...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.