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CRITICAL COOLING RATE AND THERMAL STABILITY IN Zr-Ti-Cu-Ni-Be BULK METALLIC GLASSES Theodore A. Waniuk, Jan Schroers, and William L. Johnson Keck Laboratory of Engineering Materials, California Institute of Technology, Pasadena, CA 91125 ABSTRACT The crystallization behavior of a series of alloys in the Zr-Ti-Cu-Ni-Be system is studied. Upon cooling from the molten state with different rates, alloys with compositions ranging along a tie line from (Zr75Ti25)55(Ni45Cu55)22.5Be22.5 (Vit1) to (Zr85Ti15)55(Ni57Cu43)17.5Be27.5 (Vit4) show a continuous increase in the critical cooling rate to suppress crystallization. In contrast, thermal analysis of the same alloys shows that the undercooled liquid region, the temperature difference between the glass transition temperature and the crystallization temperature, is largest for compositions midway between the two endpoints, revealing that glass forming ability does not correlate with thermal stability. The relationship between the change in glass forming ability and thermal stability is discussed with reference to a chemical decomposition process. INTRODUCTION Alloy systems with critical cooling rates for glass formation below 100 K/s, i.e. with good glass forming ability (GFA), are a relatively new development. Because of their resistance to crystallization, alloys such as Zr41.2Ti13.8Cu12.5Ni10Be22.5 (Vit1) [1] and Pd40Cu30Ni10P20 (PCNP) [2] can be examined in the deeply undercooled liquid region on accessible laboratory time scales. As shown by Turnbull [3], GFA (represented by the critical cooling rate) scales with the reduced glass transition temperature, Trg, defined as the glass transition temperature, Tg, divided by the liquidus temperature, Tl. This correlation has been confirmed in many experiments (see, for example, Ref. 4). Thermal stability in metallic glasses is usually quantified by measuring the temperature difference, ∆T, between the glass transition and the first crystallization event upon heating at a constant rate. For some systems, it has been demonstrated that larger values of ∆T tend to be associated with lower values of critical cooling rate, Rc [5,6]. As a result, the thermal stability has served as an indicator of GFA in these alloys. In recent years, the crystallization of Vit1 has been extensively examined [7-10]. Several studies of this alloy have revealed a tendency to undergo chemical decomposition in the undercooled liquid [7-9], which has a direct influence on the subsequent nucleation and growth of crystalline phases. Since the decomposition occurs at a temperature close to Tg, the isothermal crystallization behavior of Vit1 for low undercooling is quite different from its behavior when deeply undercooled [10]. In addition, a study by Schroers et al. [11] involving constant heating and cooling experiments has shown that Vit1 crystallizes in a different manner upon heating from the amorphous solid than upon cooling from the melt. In this study, a cooling rate of approximately 1 K/s was required to bypass crystallization during cooling, whereas