Thermodynamic, structural and surface properties of liquid Cd–Zn alloys

Abstract

The mixing behaviour of cadmium–zinc binary liquid alloys have been examined by computing thermodynamic properties, such as free energy of mixing (G_M), heat of mixing (H_M), entropy of mixing (S_M), activity (a_i) and microscopic functions, such as concentration fluctuations in long wavelength limit (Scc(0)), Warren–Cowley short range order parameter (α₁) and ratio of diffusion coefficients of the alloys at 800 K using statistical model in the framework of quasi-chemical approximation. Surface properties have also been analysed using four different approaches. The theoretical study reveals that the ordering energy parameters are temperature dependent and all the observed thermodynamic parameters are in reasonable agreement with the observed values. The Cd–Zn liquid alloys at 800 K represent a segregating system with segregation of Cd-atoms at the surface. The values of surface tension obtained from different approaches have been found to be comparable with each other.

Introduction

In metallurgical science, metal alloys are generally beneficial over metals for their increased mechanical strength, heat resistance, chemical resistance, decreased production costs, and a wider range of colour not found naturally. Cadmium and cadmium based alloys have frequently been the subject of both theoretical and experimental research studies [1], [2], [3], [4], [5], [6]. Cadmium–zinc is one of numerous metal alloys, which has potential applications in bearing assemblies, ballasts, casting, step soldering, radiation shielding, etc. Smaller cadmium content cadmium–zinc alloys exhibit lower melting point, improved thermal and electrical conductivities and increased mechanical properties such as hardness, wear resistance, tensile and fatigue strength [7]. Cadmium–zinc alloys are used as special solders at medium temperature which provide excellent corrosion resistant joints on most metals. Devices with such superior strength joints can work in high vibration and high stress applications in electronics, lighting and electrical products [7]. In addition cadmium is toxic and its use in electroplating as in cyanide baths poses problems of environmental concern. Cadmium–zinc alloys can be used in dry electroplating and can be a safer potential substitute for cadmium.

With such prospective applications and benefits, it is useful to study the mixing properties of cadmium–zinc alloys in metallurgical science. These studies are, indeed, important because a good knowledge of their mixing properties in the liquid state is necessary for understanding process of design and preparation of desired materials. Transport properties such as diffusivities of metals in the liquid state are required for many metallurgical processes and heterogeneous chemical reactions. Similarly, the knowledge of surface properties is required for understanding of the surface related phenomena such as wetting characteristics of solders, corrosion and kinetics of phase transformation [8]. The role of these properties at initial melt is important in the formation of solid alloys. The alloying behaviour of binary alloys can be studied theoretically by computing thermodynamic, structural, transport and surface properties of alloys in the liquid state. For a long time many researchers [5], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32] have been working with several models to explain the mixing properties of binary liquid alloys.

In the present work, we have studied the Cd–Zn alloys in molten state at 800 K. The concentration fluctuations in long wavelength limit, Scc(0) for Cd–Zn liquid alloys computed directly from observed activity data [9] indicates that self associations of atoms are energetically favoured. With the aim of understanding from theoretical point of view the mixing behaviour of the alloy in bulk and at the surface we have computed thermodynamic, structural and transport properties in the framework of quasi-chemical approximation [10], and surface properties as function of bulk concentration from the formulation of Prasad et al. [12]. In order to see the reliability of the data on surface properties obtained from Prasad et al., we have also employed other approaches, namely the approaches reported by Novakovic [13], and March and Alonso [15].

Basic theoretical formalism is presented in Section 2. Section 3 deals with results and discussion and conclusions are outlined in Section 4.