Shear-banding is a ubiquitous plastic-deformation mode in materials. In metallic glasses, shear bands are particularly important as they play the decisive role in controlling plasticity and failure at room temperature. While there have been several reviews on the general mechanical properties of metallic glasses, a pressing need remains for an overview focused exclusively on shear bands, which have received tremendous attention in the past several years. This article attempts to provide a comprehensive and up-to-date review on the rapid progress achieved very recently on this subject. We describe the shear bands from the inside out, and treat key materials-science issues of general interest, including the initiation of shear localization starting from shear transformations, the temperature and velocity reached in the propagating or sliding band, the structural evolution inside the shear-band material, and the parameters that strongly influence shear-banding. Several new discoveries and concepts, such as stick-slip cold shear-banding and strength/plasticity enhancement at sub-micrometer sample sizes, will also be highlighted. The understanding built-up from these accounts will be used to explain the successful control of shear bands achieved so far in the laboratory. The review also identifies a number of key remaining questions to be answered, and presents an outlook for the field.

Shear bands in metallic glasses

Amorphous, nanoquasicrystalline and nanocrystalline alloys in Al-based systems

This chapter aims to review our recent research results on new bulk amorphous alloys. The main topics are the following: (1) the finding of new amorphous alloys with high glass-forming ability in a number of alloy systems; (2) the mechanism for achieving high glass-forming ability; (3) the fundamental properties of the new amorphous alloys; (4) successful examples of producing bulk amorphous alloys by different techniques of water quenching, metallic mold casting, arc melting and unidirectional zone melting, etc.; (5) the high tensile strength, low Young’s modulus, and high impact fracture energy of nonferrous metal-based bulk amorphous alloys; (6) the soft magnetic properties of Fe- and Co-based bulk amorphous alloys; (7) hard magnetic properties of Nd- and Pr-based bulk amorphous alloys; (8) the viscous flow and microformability of bulk amorphous alloys in a supercooled liquid region, and (9) future aspects of applications. These new results enable eliminating of the limitation of sample shape which has prevented the development of amorphous alloys as engineering materials. They are expected to give rise to a new era of amorphous alloys.

Bulk Amorphous Alloys

Amorphous alloys, and the partially or fully crystallised materials derived from them, can have properties attractive for a diverse range of applications. In some cases, their wear resistance can be of primary concern, in others, it is an important secondary property. The distinctive mechanical properties of amorphous alloys make their wear resistance of fundamental interest also. This review focuses on the influence of a variety of factors in wear testing, on the mechanisms of wear, on the characteristics of different categories of amorphous alloy, and on the effects of partial or complete crystallisation. It is shown that amorphous alloys can have very good resistance to sliding and abrasive wear. The wear resistance of related quasicrystalline phases is also considered.

Wear resistance of amorphous alloys and related materials

The electronic interaction of SnO2 with Ag and Pd particles dispersed on its surface was examined by means of X-ray photoelectron spectroscopy (XPS). The binding energies (BE) of Sn3d and O1s of Ag(1.5 wt%)-SnO2 and Pd(3.0 wt%)-SnO2, which were lower by 0.5–0.7 eV than those of pure SnO2 in the as-prepared state, shifted reversibly by reduction and oxidation treatments. These shifts in BE are shown to reflect the shifts of Fermi energy of SnO2 which are interacting electronically with the metal additives. The electronic interaction depended on the metal loading, being strongest at 1.5 wt% and 3.0 wt% loadings of Ag and Pb, respectively. The implication is that the electronic interaction is of primary importance to the inflammable gas detection by Ag-SnO2 sensors.

Electronic Interaction between Metal Additives and Tin Dioxide in Tin Dioxide-Based Gas Sensors : Surfaces, Interfaces and Films

Ultrahigh tensile strengths of al88y2ni9m1(m = mn or fe) amorphous alloys containing finely dispersed fcc-al particles

Amorphous Al 88 Y 2 Ni 10-x M x (M=Mn, Fe or Co) alloys containing nanoscale fcc-Al particles form in the composition ranges 0 to 2 at% Mn and 0 to 5%Fe or Co and exhibit tensile fracture strength (σ f ) and hardness (H v ) higher than those of amorphous single phase alloys with the same compositions, without detriment to good bending ductility. The particle size of the fcc-Al phase increases in the range of 2 to 30 nm with a decrease of cooling rate and decreases with an increase of M content

Ultrahigh mechanical strengths of Al88Y2Ni10-xMx (M = Mn, Fe or Co) amorphous alloys containing nanoscale fcc-Al particles

An Al 88 Ni 9 Ce 2 Fe 1 alloy with ultrahigh tensile fracture strength exceeding 950 MPa in the temperature range from room temperature to 573 K was found to be obtained by rapid solidification in the structural state where the nanoscale fcc-Al particles without internal defects disperse homogeneously in the amorphous matrix. The ultimate tensile strength is as high as 1560 MPa at room temperature and 970 MPa at 573 K

Elevated-temperature strength of an Al88Ni9Ce2Fe1 amorphous alloy containing nanoscale fcc-Al particles

An Al 88 Ni 10 Ce 2 amorphous alloy with the two-stage crystallization process of amorphous→amorphous plus fcc-Al→Al plus compounds exhibited a large elongation reaching 45% in the temperature range of 450 to 465 K. The temperature range agrees with that where the primary Al particles with a size of about 12 nm precipitate homogeneously at an interparticle spacing of about 6 nm in the amorphous matrix. It is therefore concluded that the appearance of the significant elongation is due to a crystallization-induced elongation phenomenon

A Large Tensile Elongation Induced by Crystallization in an Amorphous Al 88 Ni 10 Ce 2 Alloy

Amorphous Al88(Y1-xCex)2Ni9Fe1 alloys containing nanoscale fcc-Al particles have been found to exhibit tensile fracture strength (σf) and hardness (HV) higher than those of amorphous single phase alloys with the same compositions, without detriment to good bending ductility. The particle size of the fcc-Al phase increases in the range of 3 to 30nm with a decrease in cooling rate. The HV and Young's modulus (E) increase monotonously with increasing volume fraction of the fcc phase (Vf), while the σf shows a maximum in the Vf range of 5 to 26% (e.g., 1560MPa at 25% Vf for Al88Ce2Ni9Fe1).The increase in σf in the Vf range below 26% is presumably due to an enhancement of the resistance to shear deformation caused by the nanoscale fcc particles which have hardness higher than that for the amorphous phase with the same compositions. The highest σf for the Al88Ce2Ni9Fe1 alloy is much higher than that (1320MPa) for the Al88Y2Ni9Fe1 alloy. It is presumably a reason for this difference that the bonding force between Al and Ce atoms is stronger than that between Al and Y.

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Yeong-Hwan Kim

Papers

Ultrahigh tensile strengths of al88y2ni9m1(m = mn or fe) amorphous alloys containing finely dispersed fcc-al particles

Ultrahigh mechanical strengths of Al88Y2Ni10-xMx (M = Mn, Fe or Co) amorphous alloys containing nanoscale fcc-Al particles

Elevated-temperature strength of an Al88Ni9Ce2Fe1 amorphous alloy containing nanoscale fcc-Al particles

A Large Tensile Elongation Induced by Crystallization in an Amorphous Al 88 Ni 10 Ce 2 Alloy

Mechanical properties of Al88(Y1-xCex)2Ni9Fe1(x = 0, 0.5, 1) amorphous alloys containing nanoscale fcc-Al particles