Probing Phase Evolution Behavior during Nanocrystallization of Metallic Glass Using Positron Annihilation Spectroscopy
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INTRODUCTION
POSITRON annihilation spectroscopy (PAS) has long been recognized as a very sensitive probe of the electronic structure of solids.[1,2] The positron lifetime reflects the electron density at the annihilation site and can be utilized to study open volume defects up to a concentration of a few parts per million.[3–8] The positron lifetime gives information about the free volume, concentration and size of positron trapping sites in amorphous and crystalline structures.[3] The Doppler broadening measurement gives the momentum distribution of the annihilating electrons.[1,2] The low-momentum part of the spectrum (511-keV gamma line) arises mainly from annihilation with the valence electrons; the core electrons contribute to the high-momentum part of the spectrum. Thus, the chemical surrounding of the annihilation site or the elemental specificity is obtained from the shape/magnitude of the high-momentum core component, which carries the signature of the element. However, a very small fraction of positrons annihilate with core electrons, due to the repulsion of the positron by the positively charged nucleus. The amplitude of the high-momentum or core electron component is, therefore, low; it is buried in the background in the conventional Doppler spectrum. The coincidence Doppler broadened (CDB) technique, using two high-purity A.P. SRIVASTAVA, Scientific Officer (D), D. SRIVASTAVA, Scientific Officer (H), and G.K. DEY, Head, are with the Materials Science Division, Bhabha Atomic Research Center, Mumbai 400 094, India. Contact e-mail: [email protected] K. SUDARSHAN, Scientific Officer (E), and P.K. PUJARI, Scientific Officer (H), are with the Radiochemistry Division, Bhabha Atomic Research Center, Mumbai 400 094, India. Manuscript submitted February 21, 2008. Article published online April 24, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
germanium (HPGe) detectors (Eurisys Mesures, St Quentin Yvelines Cedex, France), a relatively new development,[9] is capable of eliminating the background to a great extent, e.g., a peak-to-background ratio of ~106 can be obtained, as compared to a few hundreds in the conventional Doppler technique. This permits the unambiguous extraction of the shape and magnitude of the high-momentum part of the Doppler spectrum. Following the first lifetime measurement on Pd77.5Cu6Si16.5 metallic glass by Chen et al.,[4] a large number of positron annihilation studies on metallic glasses have been reported.[3–8] The broad observations in all these studies can be summarized as follows. In general, a single lifetime component is observed in metallic glasses that is interpreted in terms of the saturation trapping of the positron into a large number of cavities (vacancy-like defects, Bernal holes, microvoids, quasidislocations, Egami defects, crystalline embryos, etc.) of different sizes on the atomic scale, representing an irregular array of potential wells with different binding energies. The irreversible decrease in the mean lifetime upon annealing before the crystallization temperature is a well-k
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