Structural study of amorphous Pd41Ni41Si18 alloy associated with structural relaxation by anomalous X-ray scattering

Abstract

Since the amorphous state is not thermodynamically stable, it is known that this state easily transfers to a more stable state when annealed at temperatures sufficiently low that crystallization does not occur. This transfer process has now been interpreted in terms of structural relaxation. The structural relaxation is frequently described by two categories of a topological short-range ordering (TSRO) process and a chemical short-range ordering (CSRO) process [1]. The TSRO process involves irreversible structural change via highly collective atomic rearrangements with the annihilation of density fluctuation and stress relief in the as-quenched specimen [2, 3]. On the other hand, the anneal-induced reversible relaxation behaviour is also well documented in some amorphous alloys [4] and it is considered to be the atomic rearrangement between specific constituent elements. Such reversible relaxation behaviour in amorphous alloys is mainly attributed to the CSRO process [5]. Regarding the structural change due to lowtemperature annealing, much reliable information has been obtained using X-ray and neutron diffraction [6, 7]. These results clearly indicate that a highly disordered frozen-in structure, which is metastable in amorphous alloys prepared by rapid quenching from the melt, relaxes structurally owing to low-temperature annealing and the atoms occupy more stable positions in the disordered atomic arrangement. Thus, we now have sufficiently reliable information about the TSRO process [2, 3]. However, these structural studies have still had relatively little impact on the CSRO process due to low temperature annealing, because the available structural data are very limited for the individual chemical constituents in amorphous alloys. The use of anomalous X-ray scattering may bring about a significant breakthrough in this subject by obtaining the environmental structure around a specific element in amorphous alloys. The main purpose of this work is to present the results on the structural change of amorphous Pd41Ni41Si18 alloy due to low-temperature annealing by applying anomalous X-ray scattering (AXS) at the Ni K absorption edge, coupled with ordinary Xray diffraction data. Amorphous ribbons were prepared by the meltspinning method with a single copper roller in a purified Ar atmosphere. The ribbon shape was about 30 μm in thickness and 2 mm in width. The quality of samples was tested by X-ray diffraction and differential scanning calorimetry (DSC). The samples were annealed at 603 K for 330 s to reduce partially the excess free volume. It may be worth mentioning that the crystallization temperature was estimated to be 668 K, defined as a departure from the base line of the DSC thermogram taken with a heating rate of 0.33 K sy1 and the present annealing condition was determined from the temperature– time–transformation diagram determined from electrical resistance measurements. Ordinary X-ray scattering profiles of the 2θ range between 5 and 1308 were obtained by Mo Kα radiation with a pyrolitic grade monochromator in the diffraction beam path. AXS experiments were carried out with the synchrotron radiation source at Photon Factory of the National Laboratory for High Energy Physics in Japan. The X-ray energies of 8.032 or 8.307 keV, which are 300 and 25 eV below the Ni K absorption edge, were selected by the double Si(1 1 1) monochromator. The energy resolution of the monochromator is about 7 eV [8]. The 2θ range was varied from 10 to 1408. The experimental details including data processing have been reported previously [9, 10]. It must be stressed that special effort has been paid to reduce the statistical fluctuation in the AXS measurements by accumulating at least 1:5 3 105 counts so as to maintain a statistical error of less than 0.3%. Fig. 1 shows the interference functions, Qi(Q)as (solid curve) and Qi(Q)an (dotted curve), obtained from ordinary X-ray scattering for as-quenched and annealed samples together with the difference, Q i(Q) ˆ Qi(Q)an y Qi(Q)as. Fig. 2a represents the reduced radial density function, G(r), which is given by Fourier transformation of Qi(Q), for both asquenched and annealed samples. In the case of the