K. Hajlaoui

New ternary Fe-based bulk metallic glass with high boron content

To satisfy thermodynamic and kinetic requirements, Fe-based alloys capable of forming bulk metallic glasses often contain five or more elements. Usually, such compositions are of the type transition-metals/metalloids, with metalloid content around 20 atomic%. Starting from known Fe-based compositions used to make melt-spun glassy ribbons, purifying the master alloy by fluxing with B2O3 and using copper mould casting, a ternary Fe66Nb4B30 bulk metallic glass was obtained. To our knowledge this is the first Fe-based fully amorphous bulk metallic glass with just three atomic constituents. The alloy is ferromagnetic with Curie temperature T c = 646 K, glass transition temperature T g = 845 K, crystallization temperature T x = 876 K, liquidus temperature T liq = 1451 K and having a mechanical strength of 4 GPa.

A Synchrotron Radiation Study of Plastic Zones Generated by Compressive Deformation in Zr-Based Metallic Glasses

X-ray diffraction using synchrotron radiation has been used to follow diffraction line profile changes after compressive deformations generated through brinell indentations of Zr60Ni10Cu20Al10 and Zr65Ni10Cu7.5Al7.5Pd10 metallic glass forming systems at different loads below the glass transition temperature Tg. In going from str ained to unstrained zones of the same specimen, we have found that both the integral breadth and t e first diffraction maximum intensity were significantly affected. A correlation between larger relative integral breadth and free volume concentration distribution has been observed in zones of v rlapping shear bands around impressions. The surface uplift of the pile-up around i ndents has been assessed using a profilometer. It is shown that inhomogeneous deforma tion results in an inhomogeneous distribution of free volume concentration. Comparison between the experimental relative variation of free volume concentration and that predicted based on the well defined strain localization model is developed.

Synchrotron Radiation and Instrumented Indentation Studies of Compressive Plastic Deformation in Zr-Based Bulk Metallic Glasses

X-ray diffraction using synchrotron radiation and instrumented indentation experiments has been carried out to trac k free volume changes and local reodering following plastic straining by indentation below the glass transition temperature Tg in the Zr60Ni10Cu20Al10, Zr 65 Ni 10 Cu 15 Al 10 and Zr 65 Ni 10 Cu 7.5 Al 7.5 Pd10 metallic glass forming systems. A quantitative diffraction line profiles modifications are observed during scanning strained and unstrained zones of the same specimen. We have found that the first diffractogram maximum intensity in transmission and the average inter-atomic spacing position are significantly affected in the overlapping shear bands around impressions. The surface uplift of the pile-up around indents has been assessed using a profilometer. It is shown that the inhomogeneous deformation results in an inhomogeneous distribution of free volume concentration. Substantial plastic deformation is determined form hysteresis loops recorded during nanoindentation experiments. While the ratio of the dissipated energy to total indentation work is quite similar for Zr-based metallic glasses, continuous stiffness measurements showed an appreciable difference in the evolution of the elastic modulus and hardness as function of penetration depth.

On the Electromechanical Engraving of Bulk Metallic Glasses: Modelling and Predictive Response

The intrinsic materials properties of bulk metallic glasses (BMGs) such as Newtonian viscous-flow and high electric resistivity have been successfully used to achieve electromechanical engraving on BMG. The technique explores joule heating by an electrical current flowing through a sharp W-tip electrode brought into contact with a sheet of the BMG. The application of a small force allows penetration of the electrode at low applied stresses once Joule heating leads to temperatures T >Tg of the glass transition temperature of the BMG. A simple energetic model of the electromechanical engraving process is developed to identify key parameters that govern load-temperature-penetration depth behavior. Writing the conservation equilibrium energy of the system “heating section/neighbourhood”, and applying the first thermodynamic principle lead to the establishment of general thermal equilibrium equation of the heating section/neighbourhood system. By using the well known quadrupoles formalism and adopting simplifying hypotheses, an analytical expression describing thermophysical behavior during the contact is proposed. The coupling of thermal and mechanical behavior leads to the load-temperature-penetration depth evolution. Example of resolution of predictive expressions for electromechanical engraving on BMG penetration responses as function of time for given loading charges and applied current is given. Introduction Bulk metallic glasses with elastic strain range of about 2% can store mechanical energy have been proposed for a range of industrial application as springs, machinery structural materials, optical precision materials, sporting goods, electrode materials, writing tool materials, cell phone castings etc. Technologies for engraving, grooving and writing on BMG surfaces are important for their future use. Very recently, Yavari et al [1,2] have experimentally demonstrated the ability of electromechanical process for engraving exclusively on BMGs. The previous experiments were performed with tungsten tip of arbitrary dimensions pushed on the stage of an optical microscope. In this work we present the first modelling of microengraving process on BMG. Analytic expressions describing thermomechanical behaviour during contact between the fine micro-tip of W and BMG surface are developed. Resolution of these expressions gives predictive response of electromechanical engraving on BMG. It shows that, in the first stage, the temperature increases under the w-tip is logarithmic then reaches a local steady-state value. Taking account of conductive dissipation of heat Q, the modelling can predict the time at which BMG becames super-plastic and therefore the onset of w-tip penetration slightly before super-plasticity. Simultaneously, this model predicts the quantitative decrease of applied stress and specific heating power with penetration due to significant increase of contact cross-section S. Coupling the mechanical and thermal behavior Journal of Metastable and Nanocrystalline Materials Online: 2004-07-07 ISSN: 2297-6620, Vols. 20-21, pp 89-94 doi:10.4028/www.scientific.net/JMNM.20-21.89