Polycrystalline c-c¢ superalloys with varying grain sizes and unimodal, bimodal, or trimodal distributions of precipitates have been studied. To assess the contributions of specific features of the microstructure to the overall strength of the material, a model that considers solid-solution strengthening, Hall-Petch effects, precipitate shearing in the strong and weak pair-coupled modes, and dislocation bowing between precipitates has been developed and assessed. Crossslip-induced hardening of the Ni 3 Al phase and precipitate size distributions in multimodal microstructures are also considered. New experimental observations on the contribution of precipitate shearing to the peak in flow stress at elevated temperatures are presented. Various alloys having comparable yield strengths were investigated and were found to derive their strength from different combinations of microconstituents (mechanisms). In all variants of the microstructure, there is a strong effect of antiphase boundary (APB) energy on strength. Materials subjected to heat treatments below the c¢ solvus temperature benefit from a strong Hall-Petch contribution, while supersolvus heat-treated materials gain the majority of their strength from their resistance to precipitate shearing.
The effect of varying amounts of Laves phase on the mechanical properties of wrought and cast + HIP Inconel718 is discussed. When present as a continuous or semicontinuous grain boundary network in wrought Inconel718, Laves phase dramatically reduces room temperature tensile ductility and ultimate tensile strength, with room temperature impact and fracture toughness properties and elevated temperature ductility also reduced. Laves may also act as a preferred crack initiation and propagation site, resulting in reduced low cycle fatigue (LCF) p blty d ca a i i an accelerated fatigue crack growth rates. Laves present as large globular aggregates in cast+HIP Inconel 718 significantly reduces room temperature tensile and elevated temperature stress rupture properties. In addition, the phase acts as a preferred crack initiation and propagation site, resulting in significant reductions in smooth and notch LCF capability and an accelerated fatigue crack growth rate. Methods for controlling Laves phase in wrought and cast +HIP Inconel 718 are discussed.
The introduction of Alloy 71 8 at P&W in the early 1960's represented a significant advance in gas turbine engine technology, enabling the manufacture of engines with lower cost, lighter weight and simplified construction. This paper traces the applications and evolution of this unique material over the last four decades at P&W, along with some of the reasons for its introduction. From its initial use in 1963 for the diffuser case of the 558 engine for the SR-71 Blackbird, the alloy is now the most widely used of all nickel alloys at P&W. Applications include disks, cases, shafts, blades, stators, seals, supports, tubes and fasteners. Some of the key studies conducted at P&W to increase our understanding of the alloy and to improve its properties, uniformity and quality are also described. Additional challenges remain, however, if we are to exploit this alloy system even further into the 21St century. Most notable among these include a need for improved understanding and process models to allow us to tailor properties to specific applications, and the development of a higher temperature derivative. An alloy is needed which would retain all of the property, cost and fabricability advantages of 718, but with greater resistance to overaging to allow a 50-100F (28-56C) increase in use temperature.
An experimental program was conducted in order to determine the effects of microstructure on the high temperature constitutive behavior or powder-processed IN100. The relevant application is the high pressure turbine disk in a gas turbine engine. Eight different microstructures were prepared, with 260 o C yield strengths ranging from 945 to 1165 MPa.For each microstructure, tensile tests and cyclic constitutive tests were conducted at 260 o C and 650 o C. This paper reports on the tensile test results. The tensile curves were fit to a modified Ramberg-Osgood constitutive model, which contains four material constants. It was found that two of the constants were sensitive to microstructure, while two others were not. The constant K 1 , which is analogous to the strength coefficient in the traditional power-law hardening expression, varied in exactly the same way as the yield strength. The variations in yield strength and K 1 were consistent with the Huther-Reppich strength model, with the secondary γ' size being the dominant microstructural term. There was also a smaller effect of tertiary γ' and grain size. The hardening exponent n, which is basically the inverse of the hardening exponent in the traditional power-law hardening expression, was sensitive to secondary γ' size and also to the presence of primary γ', but not to grain size.
A study was conducted on the influence of grain boundary elements (GBE) on several P/M nickel base superalloys. Evaluations included microstructural response to chemistry and processing, tensile, creep, rupture, and fatigue crack growth testing (FCG). Boron was found to be the most influential element in the study, with increased boron leading to increased grain boundary triple point void formation and tendency for Thermally Induced Porosity (TIP). Boron reduced creep life but increased both rupture life and rupture ductility and slightly reduced hold time crack growth rates. Carbon also influenced microstructure by slightly refining grain size, and slightly reduced creep and hold time crack growth rates. Hafnium had slight impacts on yield strength and hold time crack growth rates. Magnesium overall had negligible effects. Tantalum had several significant effects, improving creep life and reducing crack growth rates. Overall, the results confirm that careful control and optimization of grain boundary chemistries and grain boundary microstructure are necessary to achieve maximum performance of P/M superalloys.
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