Phase Equilibria in the Ni-Si-B System

Abstract

A thermodynamic study has been carried out on the Ni-Si-B ternary system, which is an important system in view of the development of Ni-base filler metals. A regular solution approximation based on the sublattice model was adopted to describe the Gibbs energy for the individual phases in the binary and ternary systems. Thermodynamic parameters for each phase have been evaluated using the available experimental information on phase boundaries and other related thermodynamic properties. Thermal analysis experiments have also been conducted on several ternary alloys to re-examine the available ternary experimental data on phase boundaries. The set of evaluated parameters in this study enables reproducible calculations of the liquidus and solidus temperatures and vertical section diagrams satisfactorily. Ni-base brazing filler metals are widely used in the fields of aircraft, various engines, and nuclear engineering due to their high strength at high temperatures, corrosion resistance and oxidation resistance. At present, the study on the development of Ni-base filler metals is done by trial and error. The melting point has to be measured for many alloy specimens in spite of only a few candidate alloys out of them are used for brazing processes. Therefore, the prediction of liquidus and solidus temperatures and phase equilibria is significantly useful in the design of candidate alloy compo- sition. The CALPHAD (CALculation of PHAse Diagrams) approach 1) of phase equilibria calculation using thermody- namic descriptions from databases provides a powerful tool for obtaining such information. In the present study, a thermodynamic analysis of the phase equilibria in the Ni-Si-B system, which is an important system of Ni-base brazing filler metals, has been carried out. And also thermal analysis experiments have been conducted on several ternary alloys in order to re-examine the available experimental data on phase boundaries. 2. Experimental Procedures The phase boundaries of the ternary system were deter- mined by differential scanning calorimetry (DSC). The starting materials are powders of Ni(99.9%), Si(99.9%), and B(99%). The alloys were prepared by arc melting of cold-pressed pellets in an atmosphere of argon. A titanium button was melted to getter the residual oxygen in the chamber, prior to melting the actual charges. The arc-melted alloys were re-melted in vacuum by induction heating in order to ensure homogeneity. The as-cast alloys were encapsulated in quartz tubes under vacuum and then annealed at 850 � C for 18 days before water quenching. No chemical analysis for the alloys was conducted, since the weight losses in preparing alloys were generally less than 1%. The prepared alloy compositions are shown in Table 1. Thermal analysis (DSC) was carried out using a Rigaku ThermoPlus DSC8270 (Rigaku Corp., Tokyo, Japan). A cylindrical specimen was heated and cooled in Al2O3 crucible at a rate of 5 � C/min under a purified argon-flow atmosphere with � -Al2O3 as the standard materials. However, in order to avoid experimental error caused by supercooling, the peak temperature values during heating were adopted in a thermodynamic analysis. The peak temperatures in heating processes are shown in Table 1. In addition, in order to confirm the eutectic temperature of the Si-B system, two alloys of the Si-B binary system were Table 1 Experimental results for phase boundaries of the Si-B binary and the Ni-Si-B ternary alloys determined by DSC. Alloy composition (mol%) Peak temperature ( � C)