Plasticity in small-sized metallic systems: Intrinsic versus extrinsic size effect

At ambient temperature the plastic flow shown by metallic glasses is localized into shear bands1,2. This localization and the liquid-like features seen on fracture surfaces are consistent with shear softening in the bands. The extent to which this softening is a result of local heating has remained controversial, with estimates of the local temperature rise ranging from less than 0.1 kelvin to a few thousand kelvin3,4,5,6,7,8,9,10,11. Here we present a new experimental method based on a fusible coating, which shows that the temperature rise, over a few nanoseconds, can be as high as a few thousand kelvin; nevertheless, the temperature rise does not seem to control the shear-band thickness. It is important to understand the mechanisms of shear banding and associated softening because these are the principal factors limiting structural applications of bulk metallic glasses, which have some attractive mechanical properties such as high yield strength12,13.

Temperature rise at shear bands in metallic glasses

Compression, tension and high-velocity plate impact experiments were performed on a typical tough Zr41.2Ti13.8Cu10Ni12.5Be22.5 (Vit 1) bulk metallic glass (BMG) over a wide range of strain rates from similar to 10(-4) to 10(6) s(-1). Surprisingly, fine dimples and periodic corrugations on a nanoscale were also observed on dynamic mode I fracture surfaces of this tough Vit 1. Taking a broad overview of the fracture patterning of specimens, we proposed a criterion to assess whether the fracture of BMGs is essentially brittle or plastic. If the curvature radius of the crack tip is greater than the critical wavelength of meniscus instability [F. Spaepen, Acta Metall. 23 615 (1975); A.S. Argon and M. Salama, Mater. Sci. Eng. 23 219 (1976)], microscale vein patterns and nanoscale dimples appear on crack surfaces. However, in the opposite case, the local quasi-cleavage/separation through local atomic clusters with local softening in the background ahead of the crack tip dominates, producing nanoscale periodic corrugations. At the atomic cluster level, energy dissipation in fracture of BMGs is, therefore, determined by two competing elementary processes, viz. conventional shear transformation zones (STZs) and envisioned tension transformation zones (TTZs) ahead of the crack tip. Finally, the mechanism for the formation of nanoscale periodic corrugation is quantitatively discussed by applying the present energy dissipation mechanism.

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Energy dissipation in fracture of bulk metallic glasses via inherent competition between local softening and quasi-cleavage

https://pure.rug.nl/ws/files/6718469/2008ActaMaterMatthews.pdf

An electron microscopy appraisal of tensile fracture in metallic glasses

Shear transformation zone volume determining ductile–brittle transition of bulk metallic glasses

Fractographic studies of ductile shear failure under the uniaxial compression for rod–like samples of the Zr41.2Ti13.8Ni10Cu12.5Be22.5 and Cu50Zr35Ti8Hf5Ni2 bulk amorphous alloys at temperatures 300 and 77 K are presented. The mechanisms of shear deformation and failure appeared to have characteristics in common with other amorphous alloys prepared in the form of thin ribbons. However, there were a number of new fractographic features observed due to the bulk character of the samples and to the large supercooled liquid region of these alloys.

VV Molokanov

Papers

New features of the low temperature ductile shear failure observed in bulk amorphous alloys

Metastable, mechanically alloyed and nanocrystalline materials, pts 1 and 2

Low Temperature Ductile Shear Failure of Zr 41.2 Ti 13.8 Ni 10 Cu 12.5 Be 22.5 and Cu 50 Zr 35 Ti 8 Hf 5 Ni 2 Bulk Amorphous Alloys

Experimental observation of the ``vein'' pattern on the surface of a viscous shear fracture in the compression of an amorphous alloy

On Causes of High Local Plasticity of a Bulk Cu50Zr35Ti8Hf5Ni2 Amorphous Alloy during Low Temperature Shear Failure under Compression