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.
THE APPLICATION OF CALPHAD CALCULATIONS TO NI-BASED SUPERALLOYS
In recent years thermodynamic modelling via the CALPHAD method has been extensively applied to industrial alloys of many types. Although pertaining to equilibrium conditions, use has shown that valuable information can be gained for a variety of practical applications. A paper presented at the last Seven Springs meeting gave some theoretical background to the CALPHAD method and described the development of the methodology to Ni-based superalloys. The main purpose of that paper was to provide validation of results against an extensive experimental literature which, at the time, concentrated on γ/γ' equilibria and liquid/solid equilibria. The present paper will present an extension of the validation process to take into account η formation and provide a review and examples of the practical application of the CALPHAD method to industrial alloys. It will expand on some of the topics briefly raised in the previous paper and demonstrate that the CALPHAD route is readily extendable to conditions that depart from equilibrium. It will also be shown that it can be used to provide fundamental input for calculations of physical and mechanical properties.
The age hardening effect in Ti-6 Al-4 V due to ω and α precipitation in the β grains
The structure at room temperature of a quenched TA6V titanium alloy has been investigated. This structure isα +α′ orα +β according to the treatment temperature; it is always metastable. During ageing theβ grains decomposed by the reactionβ →β +ω +α →β +α; this decomposition was accompanied by a large increase of the 0.2% yield stress. No structural modification was observed inα. Theα andβ phase of TA6V were separately investigated in the form of single-phase alloys. The hardness ofα was insensitive to ageing, whileβ was considerably hardened byω andα; we deduced that the strengthening of the minor phaseβ during ageing is mainly responsible for the hardening of TA6V.