Structural and Phase Transformations During Crystallization of Pd 43 Ni 10 Cu 27 P 20 Metallic Glass
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Structural and phase transformations during crystallization of Pd43 Ni10 Cu27 P20 metallic glass Evgenia Pekarskaya, Jan Schroers and William L. Johnson
Keck Laboratory of Engineering Materials California Institute of Technology Pasadena, CA, 91125, USA ABSTRACT
Crystallization of the Pd43 Ni10 Cu27 P20 amorphous alloy during isothermal annealing in the undercooled temperature region is studied by electron microscopy and di erential scanning calorimetry (DSC). It is established that di erent crystallization processes take place above and below the nose temperature of the time-temperature-transformation (TTT) diagram. Detailed analysis of the microstructural evolution at the early stages of the crystallization is performed. In addition, the stable phases in the Pd-Ni-Cu-P system are identi ed. INTRODUCTION
The exceptional stability of the undercooled liquid with respect to crystallization observed in Pd-Ni-Cu-P metallic glasses has attracted a lot of attention during the last decade (e.g. [1,2]). However, the reasons for such excellent glass forming ability and crystallization mechanisms of these alloys have not been fully understood. Microstructural studies have been scarce. Some work has been done on the Pd40 Ni10 Cu30 P20 system, where heterogeneous nucleation due to \quenched -in" nuclei was observed in a wide temperature interval [3]. The Pd43 Ni10 Cu27 P20 alloy, which exhibits superior glass forming ability and shows some di erences in the crystallization kinetics compared to the Pd40 Ni10Cu30 P20 alloy, [2,4] has been studied much less. A detailed analysis of the microstructural evolution in the Pd43 Ni10 Cu27 P20 metallic glass during isothermal annealing is performed in the present work. Initiation of the crystallization process, types of phases formed, their morphology and chemical content are analyzed. EXPERIMENTAL TECHNIQUES
The amorphous samples were prepared by induction melting of the constituents ( uxed in B2 O3 ) in a quartz tube for 20 min at 1200 K followed by water quenching. Composition of the specimens was veri ed by WDX analysis in the electron probe microanalyzer. Annealing experiments were performed as follows: the specimens were cooled down from above the liquidus temperature to the annealing temperatures, 675 K and 720 K. Annealing duration was 240 s (sample A), 300 s (sample B), 600 s (sample C) and 800 s (sample D) at 720K, and 200 s (sample E), 220 s (sample F), and 300 s (sample G) at 675 K (Fig. 1). The choice of annealing times was done on the basis of the DSC analysis with the aim of detecting various stages of crystallization. For scanning electron microscopy (SEM), the specimens were etched in the solution of hydrochloric and nitric acids, 5:1 volume ratio. Transmission electron microscopy (TEM) samples were prepared by ion beam thinning. A Philips EM430 TEM operating at 300kV, Camscan Series II SEM and JEOL JXA-733 Electron Probe Microanalyzer were used for the microstructural analysis. L12.7.1
A B
C
D
720 700 680
E
720 K
Exothermic
Temperature,
K
740
F G
675 K
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