A comparison between the structures of amorphous and liquid Ag–Cu and Cu–Mg alloys

Abstract

Amorphous films of AgCu and CuMg2, approximately 3000 A in thickness, were prepared by co-evaporation of Ag and Cu, and Cu and Mg, respectively, onto 25 #m thick Be sheets, held at liquid nitrogen temperature. Mo K~ X-rays were used as a radiation probe to determine the structure of the films, at room temperature, and of the liquid alloys of Cu with 50 at.~ Ag and with 0 and 67 at.~o Mg at 50°C above the liquidus temperature. With the transmission technique, the interference functions (or structure factors) I(K) were determined in the range of K=4rt sin 0/2 between 0.8 A -1 and 12.5 A-1, and then Fourier transformed to obtain the radial distribution functions (RDF). The I(K) and RDF of the amorphous AgCu and CuMg2 films were compared with those of the liquid Ag-Cu and Cu-Mg alloys, respectively. It was found that the structures of the amorphous and liquid Ag-Cu alloys were similar with a more well-defined short-range order occurring in the solid alloys, whose I(K) exhibited the well-known shoulder on the second peak. The I(K) and RDF of the amorphous CuMg2 and the liquid Cu-Mg alloys cannot be explained by a common structure, although I(K) showed a small premaximum below the first main peak in both the amorphous and liquid alloys, a feature observed in many liquid Mg alloys.

Tetragonal phase in the Mg–Ni–Sn system

“…In many works, the strong inter-atomic interaction and the formation of associations in the liquid state were related to the high GFA [5,7,[11][12][13][14][15]. Audebert et al [5] observed in the amorphous alloys of Mg-Cu-RE, an amorphous pre-peak in the X ray diffractogram at Q = 1.50 Å −1 (Q: scattering vector), similar to that observed for the Mg 2 Cu amorphous alloy [16]. They proposed that the strong interaction between Mg and Cu atoms produces associations like Mg 2 Cu in the liquid, which increases the viscosity and the GFA of this alloy.…”

Effect of Al addition to Rapidly Solidified Mg-Cu-Rare Earth Alloys

“…This could be a consequence of an intensification of the chemical heterogeneity of the amorphous structure or the formation of small fraction of crystalline order at the nanoscale. In addition, the diffractograms of the Y and Al-0 showed a weak and broad pre-peak around 2θ~20.3 o , which reveals a strong chemical short and medium range order, as it was observed in the literature 22,28,29 . The X-ray diffractograms of the Al-5 and Al-10 alloys do not show a clear pre-peak, which is in agreement with the criterion of the micro-alloying to reduce the fraction of the strong hetero-atomic clusters in the amorphous phase.…”

Modeling of Thermodynamic Properties and Phase Equilibria for the Cu-Mg Binary System

“…[15] Thus, the Gibbs free energy of the liquid phase is given by The interaction energies ð L liq i;j Þ are described here as constants, while the Gibbs free energies of formation ðDG liq Cu m Mg n ) are described as functions of temperature, using the form given in Eq 1. Where sufficient heat capacity data are available, the parameters a)e are used.…”

“…First, in each of these treatments, the liquid phase is modeled as a regular solution, a model that cannot describe the chemical ordering observed in this system. For example, X-ray diffraction experiments reported by Lukens and Wagner [15] indicate the existence of chemical short range order in the liquid phase with Cu 2 Mg and CuMg 2 stoichiometries. This discrepancy is clearly observed in the inability of these models to accurately describe the heat capacity of the undercooled liquid, as shown in Fig.…”