80 Aerospace Materials Handbook Superalloy density is influenced by alloying additions:aluminum,titanium,and chromium reduce density,whereas tungsten,rhenium,and tantalum increase it.The corrosion resistance of superal- loys depends primarily on the alloying elements added,particularly chromium and aluminum,and the environment experienced [5]. The mechanical properties of nickel-based superalloys are determined by the chemical composi- tion and the processing conditions that control the state of microstructure.The microstructure of a typical superalloy consists of different phases.The major phases that may be present in most nickel- based superalloys are as follows [2,7-9]: Gamma matrix (Y).The continuous matrix is a fcc nickel-based nonmagnetic phase that usu- ally contains a high percentage of solid-solution elements such as cobalt,iron,chromium, molybdenum,and tungsten.All nickel-based alloys contain this phase as the matrix. Gamma prime (y).This is the primary strengthening phase in nickel-based superalloys. Aluminum and titanium are added in amounts required to precipitate fcc y'[Ni(Al,Ti)]. which precipitates coherently with the austenitic gamma matrix.Other elements,particu- larly niobium,tantalum,and chromium,also enter y.This phase is required for high-tem- perature strength and creep resistance.Gamma prime phase has an ordered Ll2 structure, which coherently precipitates in the austenitic gamma phase.The close match in matrix/ precipitate lattice parameter combined with chemical compatibility allows the y'to pre- cipitate homogeneously throughout the matrix and have long-time stability.It is interesting that the flow stress of y'increases with increasing temperature up to about 650C.Other intermetallics behave in a similar way;the flow stress increases with temperature.This unique characteristic provides the basic ground for Ni-based superalloys. Gamma double prime(Y").It is a body-centered tetragonal (bct)phase which is the primary strengthening phase in alloys containing niobium or niobium and tantalum.In this phase, nickel and niobium combine in the presence of iron to form bct NiaNb,which is coherent with the gamma matrix,while including large mismatch strains of the order of 2.9%.This phase provides very high strength at low to intermediate temperatures,but is unstable at temperatures above about 650C.This precipitate is found in nickel-iron alloys. Carbides.Carbon is added in an amount of about 0.02-0.2 wt%;combining with reactive elements,such as titanium,tantalum,hafnium,and niobium to form metal carbides(MC). During heat treatment and service,these MC carbides tend to decompose and generate other carbides,such as M23C6 and/or M6C,which tend to form at grain boundaries. Carbides in nominally solid-solution alloys may form after extended service exposures. These common carbides all have an fec crystal structure.It is believed that carbides are beneficial by increasing rupture strength at high temperature in superalloys with grain boundaries,though results vary on whether carbides are detrimental or advantageous to superalloy properties. Topologically close-packed (TCP)-type phases.These are generally undesirable,brittle phases that can form during heat treatment or service.The cell structure of these phases have close-packed atoms in layers separated by relatively large interatomic distances. TCPs are usually platelike or needle-like phases such as o,u,and Laves that may form for some compositions and under certain conditions.These cause lowered rupture strength and ductility.The likelihood of their presence increases as the solute segregation of the ingot increases. The development of viable superalloys has been achieved by a combination of compositional modifications that control aspects of yly'relationship,the use of more conventional alloying approaches to solid solution strengthening and corrosion resistance,and the introduction of a range of novel processing techniques such as directional modification,single crystal technology,powder processing,mechanical alloying,and so on [9].80 Aerospace Materials Handbook Superalloy density is influenced by alloying additions: aluminum, titanium, and chromium reduce density, whereas tungsten, rhenium, and tantalum increase it. The corrosion resistance of superal￾loys depends primarily on the alloying elements added, particularly chromium and aluminum, and the environment experienced [5]. The mechanical properties of nickel-based superalloys are determined by the chemical composi￾tion and the processing conditions that control the state of microstructure. The microstructure of a typical superalloy consists of different phases. The major phases that may be present in most nickel￾based superalloys are as follows [2,7–9]: Gamma matrix (γ). The continuous matrix is a fcc nickel-based nonmagnetic phase that usu￾ally contains a high percentage of solid-solution elements such as cobalt, iron, chromium, molybdenum, and tungsten. All nickel-based alloys contain this phase as the matrix. Gamma prime (γ ′). This is the primary strengthening phase in nickel-based superalloys. Aluminum and titanium are added in amounts required to precipitate fcc γ ′ [Ni3(Al,Ti)], which precipitates coherently with the austenitic gamma matrix. Other elements, particu￾larly niobium, tantalum, and chromium, also enter γ ′. This phase is required for high-tem￾perature strength and creep resistance. Gamma prime phase has an ordered L12 structure, which coherently precipitates in the austenitic gamma phase. The close match in matrix/ precipitate lattice parameter combined with chemical compatibility allows the γ ′ to pre￾cipitate homogeneously throughout the matrix and have long-time stability. It is interesting that the flow stress of γ ′ increases with increasing temperature up to about 650°C. Other intermetallics behave in a similar way; the flow stress increases with temperature. This unique characteristic provides the basic ground for Ni-based superalloys. Gamma double prime (γ″). It is a body-centered tetragonal (bct) phase which is the primary strengthening phase in alloys containing niobium or niobium and tantalum. In this phase, nickel and niobium combine in the presence of iron to form bct Ni3Nb, which is coherent with the gamma matrix, while including large mismatch strains of the order of 2.9%. This phase provides very high strength at low to intermediate temperatures, but is unstable at temperatures above about 650°C. This precipitate is found in nickel–iron alloys. Carbides. Carbon is added in an amount of about 0.02–0.2 wt%; combining with reactive elements, such as titanium, tantalum, hafnium, and niobium to form metal carbides (MC). During heat treatment and service, these MC carbides tend to decompose and generate other carbides, such as M23C6 and/or M6C, which tend to form at grain boundaries. Carbides in nominally solid-solution alloys may form after extended service exposures. These common carbides all have an fcc crystal structure. It is believed that carbides are beneficial by increasing rupture strength at high temperature in superalloys with grain boundaries, though results vary on whether carbides are detrimental or advantageous to superalloy properties. Topologically close-packed (TCP)-type phases. These are generally undesirable, brittle phases that can form during heat treatment or service. The cell structure of these phases have close-packed atoms in layers separated by relatively large interatomic distances. TCPs are usually platelike or needle-like phases such as σ, μ, and Laves that may form for some compositions and under certain conditions. These cause lowered rupture strength and ductility. The likelihood of their presence increases as the solute segregation of the ingot increases. The development of viable superalloys has been achieved by a combination of compositional modifications that control aspects of γ/γ ′ relationship, the use of more conventional alloying approaches to solid solution strengthening and corrosion resistance, and the introduction of a range of novel processing techniques such as directional modification, single crystal technology, powder processing, mechanical alloying, and so on [9]