Structural Relaxation and Crystallization in a Pd 40 Cu 30 Ni 10 P 20 Bulk Metallic Glass
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Structural Relaxation and Crystallization in a Pd40Cu30Ni10P20 Bulk Metallic Glass R. Raghavan1, U. Ramamurty1, J. Basu1, S. Ranganathan1 and N. Nishiyama2 1 2
Department of Metallurgy, Indian Institute of Science, Bangalore-560 012, INDIA Institute for Materials Research, Tohoku University, Sendai, JAPAN
ABSTRACT The stability of a Pd40Cu30Ni10P20 bulk metallic glass (BMG) against structural relaxation is investigated by isothermal and isochronal annealing heat treatments below and above its glass transition temperature, Tg, for varying periods. Differential scanning calorimetry (DSC) of the annealed samples shows an excess endotherm at Tg, irrespective of the annealing temperature. This recovery peak evolves exponentially with annealing time and is due to the destruction of anneal-induced compositional short range ordering. The alloy exhibits a high resistance to crystallization on annealing below Tg and complex Pd- and Ni-phosphides evolve on annealing above Tg. INTRODUCTION The rapid quenching techniques (~105 – 106 K/s) used to form metallic glasses lead to the retention of significant amounts of free volume. The quenched-in free volume can lead to the degradation of functional properties of metallic glasses, such as coercivity and susceptibility. Annealing the glass below its Tg structurally relaxes the glass and alters its properties. Structural relaxation is understood as a process of atomic rearrangement aided by free-volume annihilation or redistribution [1]. The driving force for structural relaxation is the availability of other minima in the potential energy hyper-surface [2]. Relaxation results in localized ordering [compositional short-range ordering (CSRO) or topological short-range order (TSRO) ([3]] and stabilization of the glass. BMGs, which are processed with relatively low cooling rates and hence retain negligible free volume, can be expected to exhibit little or no structural relaxation [4]. This study examines this possibility critically with the aid of experiments on a Pd-based glass, a BMG with one of the lowest critical cooling rates. MATERIALS AND EXPERIMENTAL PROCEDURES The alloy of the nominal composition Pd40Cu30Ni10P20 was induction melted and cast in a water-cooled copper mould in the form of a 10 mm diameter rod. Slices of the alloy were sectioned using a slow speed diamond saw, vacuum-sealed and heat-treated at different temperatures below Tg (0.84xTg = 483 K, 0.91xTg = 523 K and 0.95xTg = 543 K) for 1, 12 and 24 h and above Tg at 623 K (1.09xTg) for 0.5, 1 and 2 h. The structural and thermal characterization of the as cast and annealed alloys were carried out by X-ray diffractometry (XRD), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC).
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RESULTS The DSC thermograms of the samples annealed at 523 K are shown in Fig. 1. An enthalpy recovery peak at Tg for 12 and 24 h is noteworthy. Thermal cycling between ~530 and 620 K of the 24 h annealed alloy (Fig. 2) shows that the endotherm observed is not only reproducible but also mostl
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