Phase Formation Modeling of an Alloy 706 Casting Using Computational Thermodynamics
Computational thermodynamics to perform alloy and process modeling for Ni-based superalloys is a new technology which has become available to industry. This paper presents an application of the technology to a superalloy of current interest. The objective of this paper is to show how results obtained from the computational program Thermo-Calc ‘1 using a recently developed thermodynamic database for Ni-based superalloy9 can be applied to help understand solidification in INCONEL’ alloy 706. The ability of the computational method to predict both stable and metastable phase formation is explored. INCONEL@ alloy 706 was chosen because of its importance in the manufacture of large land-based gas turbine disks. In addition, the alloy’s complex structure tested the viability of the computational method. The phases examined were: Laves, MC carbide, and the y matrix. Quantitative microstructure evaluation and X-ray dispersive analysis were performed on an as-cast, vacuum induction melted (VIM) ingot. Differential Thermal Analysis (DTA) was performed on selected samples of the alloy. The experimental data was compared to the data generated via the computational approach. TM Thermo-Calc, trade name of the Division of Computational Thermodynamics, Royal Institute of Technology, Stockholm, Sweden. 8 INCONEL is a trademark of the INCO group of companies.
Phase diagram calculations for high-temperature structural materials
Materials in use at high temperatures can reach states which are close to equilibrium and knowledge of the stable phase structure at fabrication and working temperatures can therefore be very important. The field of high-temperature structural materials provided one of the areas where thermodynamic phase diagram calculations were first used some two to three decades ago. However, although some general features of phase equilibria were predicted reasonably well, the early attempts were not able to provide sufficient accuracy for more general, practical use. Recent work has now shown that it is possible to make very accurate predictions for phase equilibria in a number of high-temperature structural materials and it is possible to demonstrate that predictions for phase equilibria in ‘real’ multicomponent alloys provide results close to those which are measured experimentally. This paper will present typical results which can now be obtained for γ-TiAl-based intermetallics and Ni-based superalloys and some specific examples of usage will be shown. A further advantage of the CALPHAD (calculation of phase diagrams) route is that other properties can be predicted using input data from the calculations, for example non-equilibrium solidification phenomena, time-temperature-transformation diagrams, antiphase domain boundary and stacking fault energies. This extension of the CALPHAD method will be briefly discussed.
Phase Calculation and its Use in Alloy Design Program for Nickel-Base Superalloys
A series of phase calculation equations for Y and Y' phases in nickelbase superalloys was established by regression analysis on the EPMA data obtained from 30 experimental or commercial alloys together with data from a Ni-Al binary phase diagram. By using these equations, an accurate calculation of 'Y/Y' equilibrium, at 9OO"C, was made possible for various compositions of alloys, from Ni-Al binary to Ni-Co-Cr-Mo-W-Al-Ti-Nb-Ta-Hf-Re multi-component alloys. These phase calculation equations were substituted for those having been used in our alloy design program to extend the applicable composition range and to improve the accuracy of phase and property calculations of the program. Furthermore, a calculation equation for creep rupture life of SC alloys was established and adapted to the program. Using this new version of alloy design, the phase and property were successfully calculated for various types of 'Y' precipitation hardening nickel-base superalloys. Superalloys 1988 Edited by S: Reich&n, D.N. Duhl, G. Maurer, S. Antolovich and C. Lund The Mehlurgical Society, 1988