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.
Heat treatment of 706 alloy for optimum 1200 °F stress-rupture properties
Evaluation of a commercial heat treatment for 706 alloy indicated that it resulted in relatively low 1200° F stress rupture ductility. It was determined that this was caused by a solution treatment which dissolved all of the age-hardening phases in the alloy and caused a coarse grain size and supersaturated matrix condition. Based upon extensive fine structure study of the 706 alloy as well as previous experience with 718 alloy and other Fe−Ni-base superalloys, a heat treatment is developed which effectively optimizes the 1200°F stress-rupture properties of the alloy. The key to best properties was found to be the precipitation of globular to plate-like Ni3Cb/Ni3Ti at the grain boundaries in conjunction with maintaining a fine as-forged grain structure.
Phase diagram modeling for titanium alloys with light element impurities
It is well known that low levels of light element impurities, such as oxygen and nitrogen, can significantly modify phase equilibria in conventional titanium alloys. However, although the role of nitrogen and oxygen as alpha stabilizers is well established, little quantitative work exists in ternary and higher order systems. Moreover the effect of such elements on equilibria with phases other than cx-and P-Ti is often unknown. The problems in measuring light elements at the microscopic level adds to the difficulty of establishing partitioning, and old axioms are often transferred to new alloys with little evidence that they are applicable. As part of a current program on titanium alloys, phase diagram modeling has been performed for a variety of titanium alloys with additions of oxygen and nitrogen. This paper presents results for the system Ti-Al-V-O-(N,C) with particular reference to the O additions to the commercially important alloy Ti-6A1-4V.