The high-temperature phase equilibria of the Ni-Sn-Zn system: Isothermal sections

In this work three complete isothermal sections of the Ni-Sn-Zn system at 700, 800 and 900 degrees C are presented. They were constructed based on experimental investigation of more than 60 alloy samples. Powder XRD, single crystal XRD, EPMA, and DTA measurements on selected samples were carried out. Two new ternary compounds, designated as tau 2 (Ni(5)Sn(4)Zn) and tau 3 (Ni(7)Sn(9)Zn(5)), were identified and their homogeneity ranges and crystal structures could be described. Whereas tau 3 is only present at 700 degrees C, the tau 2-phase was found at both 700 and 800 degrees C. No truly ternary compound could be found in the isothermal section at 900 degrees C. A seemingly ternary compound at 20 at% Sn in the Ni-rich part of Ni-Sn-Zn was found at 800 and 900 degrees C. Our XRD results, however, indicate that this phase is a ternary solid solution of Ni(3)Sn-HT from constituent binary Ni-Sn. It is stabilized to lower temperatures by additions of Zn. These new experimental results will provide valuable information to the thermodynamic description of alloy systems relevant for high-temperature lead-free soldering.