Compound formation in Sn-based liquid alloys

The phenomenon of compound formation in Sn-based liquid alloys (MgSn and CuSn) has been discussed through the study of concentration-dependent thermodynamic and microscopic functions such as activity, excess free energy of mixing G_M^XS, concentration fluctuations in the long-wavelength limit Scc(0) and chemical short-range order (CSRO) parameters. The expressions for these functions have been obtained by using statistical mechanical theory based on compound formation. All the above-mentioned properties of mixing of MgSn and CuSn liquid alloys have been found to be asymmetric around equiatomic compositions. The larger negative values of G_M^XS and maximum deviation of Scc(0) from ideal values (Scc(0,id) at C_Mg = 0.66 in MgSn and at C_Cu = 0.75 in CuSn suggest that Mg₂Sn and Cu₃Sn compounds are formed in MgSn and CuSn systems, respectively. Our study of CSRO reveals that maximum order exists at compound-forming concentrations, i.e. C_Mg = 0.66 in MgSn and C_Cu = 0.75 in CuSn. It is also observed that the tendency for compound formation in MgSn is greater than that of the CuSn system, although the interactions in both systems are not very strong.