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

Nucleation in solids: The induction and steady state effects

This article provides an overview of structural changes that occur during the hot working of superalloys and provides insight into the use of precipitated particles and other thermomechanical factors to achieve desired structures. Examples will focus primarily on alloys 718 and 720, which are iron-nickel and nickel-based alloys, respectively. The availability of a second phase to control grain size is a characteristic of some iron-nickel-and nickel-based superalloys that is not usually available to cobalt-based superalloys; processing with and without the use of a precipitated phase that influences microstructures will be illustrated by the use of these examples.

The structural evolution of superalloy ingots during hot working

Transformation of DO24 η-Ni3Ti phase to face-centered cubic austenite during isothermal aging of an Fe–Ni–Ti alloy

The advantages of using select thermomechanical processing for improving mechanical properties of γ′ strengthened nickel-base superalloys in the range room temperature to 1400°F are demonstrated in the present study. Based upon analysis of the deformation and precipitation behavior of this class of alloys, several processing conditions have been evaluated for the alloy Udimet 700. The working operation and resulting dislocation substructure are shown to be of critical importance. In the preferred treatment the alloy is warm worked in the two-phase γ-γ′ condition at a temperature below that of rapid recrystallization such that a polygonal substructure is formed on the scale of the γ′ interparticle spa cing. A thermomechanical treatment consisting of 1) solution heat treatment, 2) precipitation aging at 1950°F to form γ′, 3) warm working to 1.50∈ (92 pct RA) at 1950°F, plus 4) final aging at 1550° and 1400°F was found to be beneficial for Udimet 700. The resulting property improvements included large increases in ultimate and yield strength, creep resistance, and low and high cycle fatigue resistance. These were achieved with minimal loss in ductility. The mechanical properties characterized were intended to be broad in scope and no exhaustive attempt was made to optimize processing in order to achieve maximum improvement. However, the experimental results confirm the assessment that nickel-base superalloys can be effectively strengthened through controlled thermomechanical processing.

Thermomechanical strengthening of a γ′ precipitation-hardened nickel-base alloy

Heterogeneous precipitation of metastable γ′-ni3ti in a nickelbase alloy

A thermo-mechanical treatment process is disclosed. The process comprises a first heating step comprising heating a nickel-base alloy workpiece to a temperature greater than a M 23 C 6 carbide solvus temperature of the nickel-base alloy; a first working step comprising working the heated nickel-base alloy workpiece to a reduction in area of 20% to 70%, wherein the nickel-base alloy workpiece is at a temperature greater than a M 23 C 6 carbide solvus temperature when the first working step begins; a second heating step comprising heating the worked nickel-base alloy workpiece to a temperature in a range of 927°C (1700°F) to less than the M 23 C 6 carbide solvus temperature of the nickel-base alloy, wherein the as-worked nickel-base alloy workpiece is maintained at elevated temperature and not permitted to cool to ambient temperature between completion of the first working step and beginning of the second heating step; and a second working step comprising working the heated nickel-base alloy workpiece to a second reduction in area of 20% to 70%, wherein the nickel-base alloy workpiece is at a temperature greater than 927°C (1700°F) and less than the M 23 C 6 carbide solvus temperature of the nickel-base alloy when the second working step begins.

W. A. Owczarski

Papers

Heterogeneous precipitation of metastable γ′-ni3ti in a nickelbase alloy

Thermomechanical processing of nickel-base alloys

The relationship of microstructure to workability in a high strength nickel-base superalloy