An Analysis of Thermophysical and Mechanical Properties of Glass-Forming Alloys
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1048-Z02-08
AN ANALYSIS OF THERMOPHYSICAL AND MECHANICAL PROPERTIES OF GLASS-FORMING ALLOYS Livio Battezzati1, Daniele Baldissin1, Marcello Baricco1, Tanya Aycan Baser1, Donato Firrao2, Paolo Matteis2, and Giovanni M. Mortarino2 1 Centro NIS e Dipartimento di Chimica IFM, Università di Torino, Via P. Giuria 7, Torino, 10125, Italy 2 Dipartimento di Scienza dei Materiali ed Ingegneria Chimica, Politecnico di Torino, Corso Duca degli Abruzzi 24, Torino, 10129, Italy ABSTRACT Metallic glass-forming liquids and glasses are ranked using thermophysical properties (glass transition, excess entropy, fragility indexes). The relationships between mechanical properties and some of the above quantities are discussed to get insight into the mechanism of shear band propagation during mechanical failure when, following up a shear offset event, a local temperature rise occurs. The mechanism is supported by evaluating the energy content of the shear band, as well as finite element modelling of temperature profiles around it.
INTRODUCTION Thermophysical properties of metallic glass formers show definite trends in the liquid state. The temperature dependence of the excess specific heat and entropy, as well as the rise in viscosity, on undercooling are well documented [1, 2]. These quantities describe the melt in terms of the concept of strength or fragility. Since the glass should retain features of the atomic bonding and short range order existing in the liquid, it should be possible to find correspondence between thermophysical and mechanical properties at least in a given family of materials. It has been claimed that the Poisson ratio, a quantity which provides information on the toughness of the glass [3], scales with the liquid fragility [4]. However, this cannot be demonstrated for metallic glasses and is questioned also for other classes of glasses [5, 6]. On the other hand, the glass transition, Tg, and heat content of the glass up to Tg do relate to mechanical properties [7]. This topic is reviewed with the help of a database produced by collecting data on mechanical properties appeared in the literature so far. Suggestions are derived for the energy content of shear bands and the mechanism of failure of the material also by making use of a finite element simulation of the heat production and flow in and around a shear band.
INDEXES OF FRAGILITY AND ELASTIC PROPERTIES Thermodynamic indexes, such as the slope of ∆Sg /∆S (entropy frozen in at Tg to the entropy difference between liquid and crystal) versus Tg /T which has the same shape as the viscosity plot, fail in providing a scaling of alloys with respect to their fragility because of uncertainties in thermodynamic data and in the reference state and inadequate approximation of
the liquid configurational entropy [2]. Kinetic indexes of fragility are most reliable since they correlate well with each other for all glasses: especially the slope of the logarithm of viscosity at Tg, m, and the ratio of Tg to the temperature span of the glass transition range, ∆Tg [5]. Envisa
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