Propagation and Deflection of Shear Bands in Metallic Glass under Circumferential Constraint

Abstract

For monolithic bulk metallic glasses, deformation and fracture are always associated with the initiation and propagation of localized shear bands, finally leading to failure in a shear mode. [1,2] Thus, it is of considerable scientific interest to investigate the formation and propagation of shear bands in metallic glass experimentally and theoretically. [3][4][5][6][7] Generally, due to the long-range disorder and macroscopically isotropic nature of amorphous alloys, [8] shear bands often initiate along some shear plane, which is convenient to investigate the yielding or fracture criterion based on the observations of the angles between shear bands and stress axis. [9,10] For metallic glass composites, the primary shear bands are restrained due to the strong impeding effect of second phase particles or fibers, leading to the deflection or formation of secondary shear bands. [11][12][13][14][15] The same phenomenon also occurs in metallic glassy specimens with a small aspect ratio under compressive loading. [16,17] In addition, under bending or indentation, due to the complexity of the stress distribution, shear bands are not always planar but often proceed along some curved plane. [18][19][20][21] Especially, it is interesting to find that profuse shear bands appear on the surface of metallic glassy samples subjected to small punch test, forming completely symmetrical patterns with dense shear bands along both circumferential and radial directions, even for the highly brittle BMGs composites. [22] The above findings illustrate that the formation and propagation of shear bands in metallic glass have a close relationship with the stress state and the circumferential limitation/ constraint. This gives rise to an interesting question: how the stress condition and the circumferential limitation affect the initiation and propagation of shear bands? However, until now there are few reports with the explicit intent to investigate the propagation and deflection of shear bands by using a circumferential limiting boundary. In this letter, we introduce an unsymmetrical circumferential limiting boundary to a Zr-based metallic glass sample during the small punch test, in order to further reveal the propagation and deflection of shear bands. Finally, we propose a new strategy to enhance the plasticity by strengthening the surface of metallic glasses without any expenditure of the strength. ExperimentalA Zr 52.5 Cu 17.9 Al 10 Ni 14.6 Ti 5 master alloy was prepared by arc melting elemental Zr, Cu, Al, Ni, and Ti with a purity of 99.99 % or better in a Ti-gettered argon atmosphere. The ingots were re-melted for several times, followed by casting into a copper molds with a cavity of 3 × 100 mm. Analyzed by X-ray diffraction (XRD) using a Rigaku diffractometer with Cu-radiation as a source, the final sample shows only broad diffraction maxima without any peaks of crystalline phases, revealing the typical amorphous structure, in accord with the reference. [23] In the current study, the plastic deformation behaviors were investigated...

Improved plasticity and fracture toughness in metallic glasses via surface crystallization

“…Subsequently, a new strategy was proposed to improve the plasticity of BMGs, i.e. strengthening the surface of metallic glasses [20].…”