The precipitation of the γ’ (Ll2) and γ" (DO22) phases has been studied in four alloys Fe-Ni-Cr-Ti-Al-Nb containing a higher Ti + Al/Nb ratio than that of the INCONEL 718 alloy. For these alloys, the precipitation microstructure varies rapidly with aging temperature and composition. Bct γ"particles have always been found to precipitate on γ’ phase. Moreover, by aging three alloys above a critical temperature, a “compact ntorphology” has been observed: cube-shaped γ’ particles coated on their six faces with a shell of γ" precipitate. This microstructure has proved to be very stable on prolonged aging. A thermal stability better than that encountered in nominal INCONEL 718 alloy can thus be achieved. The influence of composition and aging temperature on the conditions that bring about this “compact morphology” has been investigated. A minimal Ti + Al/Nb ratio between 0.9 and 1 has been determined, allowing the “compact morphology” to be obtained.

Morphology of y’ and y” precipitates and thermal stability of inconel 718 type alloys

The precipitation of the equilibrium δ-Ni3Nb phase has been studied in two niobium bearing nickel base superalloys—INCONEL 718 and INCONEL* 625—both of which are hardenable by the precipitation of the metastableγ″-Ni3Nb phase. The morphology and the distribution of precipitates have been examined and the crystallographic orientation relationship between the austenite and theδ phases has been determined. The nucleation of theδ phase at stacking faults within pre-existing δ" precipitates has been discussed.

Precipitation of the δ-Ni 3 Nb phase in two nickel base superalloys

The constituents of nickel-base superalloys have been classified into solid solution formers, precipitate formers, carbide formers and surface stabilizers. The characteristics of solutes which would make them most suitable in each category have been specified and appropriate alloying elements have been identified. Nickel-base superalloys are hardened primarily by the precipitation of Ni3X type compounds. The occurrence and crystallography of precipitation of various kinds of Ni3X type precipitates have been considered. The role of substitution by alloying elements on mismatch and stability of phases has been discussed. The free electron model and the Engel-Brewer model have been applied for evaluating the stabilities of precipitates, and the role of the alloying elements in determining the stabilities of external and internal surfaces such as grain boundaries have been briefly outlined.

The role of alloying elements in the design of nickel-base superalloys

Effect of heat treatment on the microstructural evolution of a nickel-based superalloy additive-manufactured by laser powder bed fusion.

Nickel based superalloys are critical to the safe operation of many energy conversion systems operating at high temperatures. Time dependent intergranular cracking of these alloys, under both sustained and cyclic loads, is dominated by environmental interactions at the crack tip. This review is concerned mainly with the interaction of oxygen in alloys used for combustion turbine discs, although interactions with other more aggressive species are considered. The phenomenology of this cracking is shown to be consistent with the same mechanism as that associated with oxygen embrittlement resulting from pre-exposure of uncracked material, and also with environmentally induced reduction in creep rupture life. Gas phase embrittlement (GPE), resulting from intergranular oxygen penetration, is shown to be responsible for all four streams of experimental observations. Three distinct processes of intergranular embrittlement involving oxidation reactions have been confirmed experimentally. One of these, the oxidation of intergranular sulphides, results in elemental sulphur embrittlement and subsequent local decohesion under stress. The other two, oxidation of carbon or carbides to form carbon dioxide gas bubbles and oxidation of strong oxide formers to form intergranular internal oxides, result in a reduction of the local ability to accommodate stress concentrations associated with sliding grain boundaries in an intermediate temperature range. This in turn leads to a temperature dependent minimum in ductility and maximum in crack propagation rate. Attempts to reduce the sensitivity to time dependent cracking based on chemistry (chromium level or trace element addition), microstructure control (using thermal–mechanical treatment or controlled cooling), or reversal of environmental embrittlement, are all considered. The conclusions form a basis for the development of life prediction methods for energy materials operating in diverse environments.

Gas Phase Embrittlement and Time Dependent Cracking of Nickel Based Superalloys

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.

Heat treatment of 706 alloy for optimum 1200 °F stress-rupture properties

Alloy 706 is a precipitation-strengthened Fe−Ni-base superalloy which has been reported to have improved machining characteristics over other similar alloys. The present work was conducted to document the phase relationships of the alloy in comparison with other Fe−Ni-base alloys. The precipitated phases identified in the alloy include γ′, Ni3Cb, Ni3Ti, M23C6, and MC. A small amount of Laves phase was also detected. The effects of some composition variations on solvus relationships for γ′ and Ni3Cb/Ni3Ti are documented. Means to take advantage of the structural response of the alloy to develop desired properties are suggested. It is shown that on the basis of chemistry and microstructural response, 706 alloy can be considered to be intermediate to the well-known alloys, 718 and 901.

The microstructure of 706, a new Fe−Ni-base superalloy

A sulfidation-resistant alloy having high stress-rupture strength at about 1350°-1500°F, containing about 0.02-0.08% carbon, 21-26% chromium, 52-58% nickel, 1-3.5% molybdenum, 1.75-3.25% titanium, 0.75- 2.25% aluminum, 0.50-2.00% columbium, up to 0.02% boron and the balance iron.

Sulfidation resistant nickel-iron base alloy

A PROCESS FOR MAKING NICKEL BASE PRECIPITATION HARDENABLE ALLOYS, AND ALLOYS MADE THEREBY, WHICH INCLUDES TREATING THE METAL WHILE IN A MOLTEN STATE UNDER VACUUM OR AN INERT GAS AT LOW PRESSURE WITH A REACTIVE ELEMENT SELECTED FROM THE GROUP CONSISTING OF MAGNESIUM, CALCIUM, NEODYMIUM, AND THE RARE EARTH ELEMENTS HAVING A MELTING POINT GREATER THAN NEODYMIUM.

Process for making nickel base precipitation hardenable alloys

Precipitation hardenable or hardened nickel-cobalt-iron base alloy containing nickel, cobalt, iron, columbium, titanium, aluminum and boron as the only essential elements, characterized by a mean coefficient of linear expansion of about 3×10 - 6 /° F to 6×10 - 6 /° F from room temperature to its Curie temperature of at least about 600° F to 1000° F or higher. The alloy contains about 35-45% nickel, 13-18% cobalt, 2.5-7.0% columbium, 1.0-4.0% titanium, 0.1-2% aluminum, and a small but essential amount of boron.

Donald R. Muzyka

Papers

Heat treatment of 706 alloy for optimum 1200 °F stress-rupture properties

The microstructure of 706, a new Fe−Ni-base superalloy

Sulfidation resistant nickel-iron base alloy

Process for making nickel base precipitation hardenable alloys

Controlled expansion alloy