Effect of Nickel Content on Delta Solvus Temperature and Mechanical Properties of Alloy 718

Abstract

Knowledge and control of the delta phase (Ni3Nb) solvus temperature is critical to optimize processing and heat treatment of alloy 718 for best mechanical properties. This is the case in aerospace applications where delta phase is used to produce fine grain structures during forging, and in oil-field applications where the delta phase is dissolved during solution treatment. Delta phase solvus is primarily controlled by niobium content. The effect of nickel content on delta phase solvus is not well understood as thermodynamic models and previous experiments have produced conflicting results. The purpose of this study was to confirm the effect of nickel content on delta solvus temperature using experimental heats and to document mechanical properties vs. nickel content. In addition, solvus results were compared with modeling predictions and an improved database was developed to more accurately predict delta phase solvus temperature in alloy 718. Introduction Delta is an orthorhombic phase that is incoherent with the face-centered cubic matrix. Delta phase size, morphology, and distribution dictate whether the phase is beneficial or detrimental to alloy performance. A uniformly-dispersed globular precipitate will facilitate grain refinement while a lathlike or needle morphology will adversely affect stress rupture and fracture toughness properties [1,2]. For aerospace applications, knowledge of the delta solvus temperature is essential for identifying forging and heating temperature practices that incorporate a dispersed delta phase to obtain the desired uniform fine grain structures. For oil-field applications where delta phase is undesirable for fracture toughness, knowledge and control of the solvus temperature is important for determination of super-solvus solution treatment temperatures to fully dissolve delta phase while limiting grain growth. It is well known that the delta phase solvus is primarily controlled by niobium content [3]; however, niobium content in alloy 718 is typically tightly controlled by industry specifications, either directly or indirectly by the mechanical properties required (niobium controls strength). The effect of nickel content on delta phase solvus is not well understood and thermodynamic models and previous experiments have produced conflicting results. Work by E. Raymond [4] indicates that delta solvus of 718 billet increases with nickel content in the 52-54% range. Other researchers [5] found no significant effect of nickel, cobalt, and iron levels on delta solvus temperature. Modeling simulations using the databases from Thermotech and CompuTherm