Local order and nanostructure induced by microalloying in Al–Y–Fe amorphous alloys
Introduction
For over fifty years microalloying has played a major role in the development of a high-performance crystalline materials; a prominent example is the Ni3Al alloys [1], [2]. The formation, crystallization and physical properties of glasses can also be profoundly influenced by microalloying with particular elements. Examples include the trace additions of Fe, Nb, Ta, Y, B, Si and C in Zr-based bulk metallic glasses [3], Y in Cu-based glasses [4], and Co in Ce-based alloys [5]. A recently proposed model for metallic glasses emphasizes the importance of packing atoms into clusters and provides a way to understand their short- and medium-range order [6]. This is supported by previous experimental studies. For example, many glasses are believed to contain strong icosahedral short-range order (ISRO), which is incompatible with crystalline periodicity. If microadditions enhance the ISRO, crystal nucleation will become more difficult and glass formation will be enhanced. This speculation is supported by a tendency for some Zr-based glasses to crystallize to the icosahedral phase when small amounts of noble elements such as Ag, Au, Pd and Pt [7] and the transition metals Nb, Ta, V [8] are added. Ag, Au, Pd, and Pt have strong negative heats of mixing with Zr, but interact only weakly with Ni or Cu, suggesting that strongly bound icosahedral clusters may form around the transition metals, giving rise to strong ISRO in the liquid/glass.
Recently, it was reported that the addition of as little as 0.5 at.% of Ti or V dramatically improves glass formation in Al88Y7Fe5 alloy and radically alters the crystallization route [9], [10]. Based on X-ray and TEM diffraction studies, the alloys prepared without the transition metal microadditions appear to be amorphous, containing no evidence for crystal diffraction peaks. However, isothermal differential scanning calorimetry (DSC) studies show a monotonic decrease in the rate of heat evolved with annealing time, reminiscent of grain coarsening. The possibility that the microadditions were scavenging oxygen was ruled out experimentally, leading to the speculation that they were altering the local structure of the glass. ISRO might be enhanced, but this is not the only possibility. If the small additions were to establish any local order that was incompatible with that of the nucleating ordered phases, the nucleation barrier would be raised and glass formation would be enhanced.
The Al–rare earth–transition metal (Al–RE–TM) glasses with a high Al content, discovered by He et al. [11] and Tsai et al. [12], are particularly intriguing. Their significant improvement in strength over conventional Al-based crystal alloys and their high ductility makes these metallic glasses of interest for aerospace applications [13]. The reasons for glass formation in Al-based alloys are unclear. They differ from traditional metallic glasses and from most of the recently discovered bulk metallic glasses, not forming near deep eutectics in the phase diagram for example. It is, therefore, of both fundamental and practical interest to understand the formation and mechanism of microalloying in these glasses.
Here we explore the structural role of microadditions on glass formation in Al–Y–Fe alloys. Using EXAFS we have investigated the local atomic structures in alloys prepared with and without small additions of Ti (0.5 and 2%) or V (0.5 and 0.65%). The short-range order around the major constituents, Al, Y and Fe, and also around the Ti and V atoms was probed. The Y and Fe environments are compared to previous results obtained by X-ray diffraction and EXAFS in Al88Y7Fe5 by Saksl et al. [14]. These measurements demonstrate for the first time, to our knowledge, that a well-defined Al nanostructure exists around the V and Ti atoms, the microalloying elements, that is different from that around the Al, Y and Fe atoms, the major alloy constituents, even at the lowest microalloying concentrations. This local structure, while not ISRO, is significantly different from that of the primary crystallizing phase, α-Al, demonstrating a structural basis for the improved glass formation with microalloying in the Al–Y–Fe alloys.
The paper is organized as follows. The sample preparation and the EXAFS experiments are described in Section 2. The EXAFS analysis and the results for the Al, Y and Fe edges are presented in Section 3. The local structure around the V and Ti atoms is discussed in Section 4 for the different samples. A summary and conclusion are presented in Section 5.
Section snippets
Experimental methods
Samples of Al88Y7Fe5, Al88−xY7Fe5Tix (with x = 0.5, 2), Al87.5Y7Fe5V0.5 and Al85.35Y8Fe6V0.65 were prepared and characterized at Washington University [15]. Alloy ingots of the desired composition were first produced by arc-melting mixtures of the pure elements on a water-cooled copper hearth in a high purity argon atmosphere. A Ti-getter located close to the samples was melted prior to arc-melting to further remove oxygen from the chamber. To ensure a homogeneous composition, samples were
EXAFS analysis
Standard procedures of normalization and background removal were followed to determine the EXAFS oscillations, χ, versus the energy, E, of the photoelectron from the absorption coefficient, ln I0/I , in transmission mode or If/I0 in fluorescence mode using a program package [17]. The data were then converted to momentum space (k-space) using , where Eo is the threshold energy origin and k is the photoelectron wave vector. The normalized EXAFS signals, kχ(k), are compared in Fig. 2
Local structure around vanadium and titanium atoms
Calculations for the Ti and V edges were made using multiple scattering contributions. The sum in Eq. (1) is taken over the different paths, taking into account their degeneracy. The neighboring atoms were taken to be only Al. Several types of local structures were considered, simple cubic, body centered cubic (bcc), face centered cubic, (fcc), and hexagonal clusters. The structure of crystalline V is bcc with eight atoms at 2.6189 Å, six at 3.0240 Å and 12 at 4.2766 Å [21]. Aluminium has a fcc
Conclusions
The effect of the addition of small amount of Ti and V on the atomic structure of Al–Y–Fe metallic glasses was examined from extended X-ray absorption fine structure (EXAFS) measurements at the Al, Y, Fe and Ti or V K absorption edges.
The addition of 0.5% Ti does not change the local atomic structure around the Fe atoms, and changes it only slightly around the Y atoms. However, the local structure appears more ordered around the Al atoms.
There exists a well-defined atomic structure around the V
Acknowledgments
It is a pleasure to thank A.M. Flank, for the Al experiment, J. Moscovici and F. Villain for their help during the EXAFS experiments at DCI and all the staff of the BM30B beam line for the experiments in ESRF. We also thank A. Mukhopadhyay for preparing the Al–Y–Fe–V samples and A. Michalowicz for the EXAFS programs. K.F. Kelton gratefully acknowledges support from the United States Air Force Office of Scientific Research, contract AFOSR FA 9550-05-1-0110 and the National Science Foundation
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