Plastic deformation in metallic glasses

A theory is presented for the plastic deformation of metallic glasses below their glass transition temperature. The theory is based on two modes of thermally activated shear transformations initiated around free volume regions under an applied shear stress. The regions are typically conceived to be about 5 atom diameters across. At high temperatures (0.6 T_g ≲ T ≲ T_g) the transformation is a diffuse rearrangement producing a relatively small local shear strain in a roughly spherical region. At low temperatures (0 < T ≲ 0.6 T_g) the transformation is in a narrow disk shaped region and resembles closely the nucleation of a dislocation loop. The theory is in good accord with experimental observations.Based on the theory, possible levels of flow dilatation have been computed from which rates of shear localization can be obtained. At low temperatures, very rapid shear localization is predicted which is in good accord with the observations reported in the literature and with recent cinematographic observations.     

Properties of bulk metallic glasses

A relatively small number of revolutionary materials have been discovered in the field of physical metallurgy and metal physics in the last several decades, and bulk metallic glasses are among them. Their strength and hardness are considerably higher and their modulus of normal elasticity is considerably lower than that of crystalline alloys, which leads to large stored elastic strain energy. These materials also have very good corrosion resistance. In this article, we present the properties of bulk metallic glasses, in particular, thermal, mechanical, magnetic, and electrical properties, corrosion resistance, as well as the application fields of these alloys.     

An investigation of contact deformation,fracture, and fatigue behavior in bulk metallic glasses

This article presents the results of a study of the contact-induced deformation, fracture/resistance-curve behavior, and fatigue-crack-growth behavior of two bulk metallic glasses (BMGs), one in the fully amorphous condition and the other containing a dispersion of microscale crystallites. Hertzian contact indentation experiments were performed on both materials under monotonic loading conditions. The contact-induced deformation characteristics observed in the Hertzian experiments are then compared with the stress distribution determined from finite-element analyses. In addition, the cracking patterns associated with resistance-curve behavior in single-edge notched bend (SENB) specimens are incorporated into a fracture-mechanics framework for the estimation of toughening due to microcrack shielding. The predicted steady-state toughness values are shown to be comparable to the measured values obtained from resistance-curve experiments. Subsequently, fatigue-crack-growth rare data obtained at stress ratios of 0.1 and 0.5 are presented and analyzed using a crack-tip opening displacement (CTOD) model. The implications of the results are then discussed.     

On the kinetics of structural relaxation in metallic glasses

A model is proposed to describe the simultaneous changes in topological (TSRO) and chemical (CSRO) short range order during structural relaxation of metallic glasses. Changes in free volume and order parameter during isothermal and isochronal annealing were calculated. Corresponding changes in internal energy and Curie temperature are—at least qualitatively—in good agreement with experimental observations. The amount of free volume and short range order frozen in during the quench from the liquid state were also calculated and found to be of the correct order of magnitude.     

Bulk metallic glasses: Fabrication,structure, and structural changes under heating

Bulk metallic glasses (BMGs) of Pd–Cu–Si and Pd–Ni–P systems were formed from a melt in the 1970s–1980s. However, in view of the extremely high cost of the main component (palladium), they have been outside the realm of special interest of scientists and engineers for a long time. Relatively recently, BMGs in the form of macroscopic-sized ingots have been fabricated in alloys based on industrial metals (iron, copper, magnesium, and titanium), which opened up wide possibilities for their application. BMGs possess high strength, hardness, wear resistance, elastic deformation, and corrosion resistance. In this study, a review of publications is presented and main scientific achievements in this field are described. It is noted that main scientific problems, which are not solved completely, are describing the BMG structure as well as vitrification and plastic deformation, while the technical problem attracting the attention of scientists in many countries is to increase the plasticity and impact fracture toughness of these materials.