Mutual solubility of Si and Ge in the Al-rich amorphous and crystalline phases

Abstract

Rapidly solidified alloys of the Al-Si-TM and Al-GeTM (TM-transition metals) systems obtained by melt spinning technique were found to have wide compositional ranges for the formation of the amorphous single phase [1–5]. Moreover, such compositional ranges in the Al-Ge-TM ternary alloys are relatively close to that observed in the Al-Si-TM alloys [2, 3]. Mechanical and electrical properties of these alloys change greatly with composition [3]. Quaternary Al-Si-Fe-TM (TM= 3d transition metals) system alloys were studied recently [6, 7]. One of the widest composition ranges in the direction of Si concentration axis from 16 to 45 at % Si was obtained for Al86−x Six Fe10Cr4 alloys. The microstructure of rapidly solidified Al-Si-TM [8] and Si-Al-TM [9] alloys studied recently was found to change from homogeneously amorphous to mixed structure consisting of amorphous phase and nanocrystalline Fd3m Ge-Si solid solution by the Ge addition. However, as it will be shown in the present paper, at sufficiently low total Si and Ge concentration both these elements are dissolvable in the Al-rich amorphous phase. Ingots of Al-Si-Ge-Fe-Cr alloys were prepared by arc melting the mixture of Al (99.99%), Si (99.9999%), Ge (99.9999%), Fe (99.9%) and Cr (99.9%) in an argon atmosphere. From these alloys, ribbon samples of about 0.015–0.02 mm in thickness and 0.9 mm in width were prepared by rapid solidification of the melt on a single copper roller at the wheel surface velocity of 41.9 m s−1. The structure of ribbon samples was examined by X-ray diffraction with monochromatic CuKα radiation. Transmission electron microscopy (TEM) was carried out using a JEM 2010 microscope operating at 200 kV. Samples for TEM were polished electrolytically in a solution of 10 vol % perchloric acid and 90 vol % methanol at 208–213 K. Crystallization temperature and heat of crystallization were examined by differential scanning calorimetry (DSC) at a heating rate of 0.67 K s−1. Contrary to rapidly solidified Si-based and Ge-based alloys in which replacing of Si by Ge and Ge by Si, respectively, causes precipitation of nanocrystalline Ge particles, Al61Si25−x Gex Fe10Cr4 alloys form an amorphous phase at different x values from 0 to 25 as shown in Fig. 1. It should be mentioned, that the mixed amorphous+ nanocrystalline Ge structure also forms in Albased alloys by replacement of Al by Ge and increment of the total Si+Ge content [8]. Some characteristic features of Al61Si25−x Gex Fe10Cr4 alloys should be described. The split of the first halo peak slightly seen in the alloys containing 0–15% Ge becomes significant at higher Ge content and the left shoulder of the first halo peak marked by an arrow in Fig. 1 appears. This effect is commonly observed in Al-Ge-TM alloys and results from phase separation of the amorphous phase into Aland Ge-rich phases over a short range [2, 3]. The brightand dark-field TEM images of the Al61Si5Ge20Fe10Cr4 alloy shown in Fig. 2 exhibit modulated contrast that was observed in Al-Ge-TM alloys [2, 3] whereas the structure of the Al-Si-Fe-Cr amorphous alloys studied recently [7] is homogeneous. Although crystallization temperature very slightly changes with increase in Ge content the shape of the DSC curves shown in Fig. 3 varies greatly. Phase composition in the fully crystallized state also changes significantly in alloys containing more than 15 at % Ge. Two heat effects seen in the DSC curve of the Al61Si25Fe10Cr4 alloy (see Fig. 3) are related to the precipitation of the Fd3m Si and a quaternary (Fe,Cr)4Si4Al13 phase with a cubic structure of a= 1.260 nm [10]. The addition of 5 at % Ge causes the appearance of the small left shoulder of the second exothermic peak. During the crystallization process Ge dissolves in Si and the quaternary (Fe,Cr)4Si4Al13 phase replacing Si atoms and forming a substitutional solid solution. Lattice parameters of Si and (Fe,Cr)4Si4Al13 phase increase and the diffraction peaks relevant to these phases shift to lower 2θ angle because the atomic radius of Ge (0.123 nm) is higher than that of Si (0.117 nm) as well as the corresponding lattice parameter of pure crystalline Ge is higher than that of Si. Some unidentified