Discontinuous cellular precipitation in a high-refractory nickel-base superalloy

Abstract

A discontinuous precipitation reaction has been investigated in a high-refractory content nickel-base alloy. The reaction transforms the two-phase ␥-␥ ' parent microstructure into a three-phase cellular structure with a ␥' matrix containing Re-rich P-phase and agglomerated ␥ lamellae. The reaction has been studied in polycrystalline material and in bicrystals with varying degrees of boundary misorientation at temperatures in the range of T/T m ϭ 0.78 to 0.85. The early stages of the reaction are characterized by heterogeneous nucleation of P-phase precipitates and migration of the grain boundary. At low-angle, near-tilt boundaries misoriented by less than 10 deg, nucleation of P-phase particles was observed, but the cellular reaction did not occur, due to limited boundary mobility and diffusivity. The high degree of supersaturation of Re and W in the initial ␥-␥' alloy appears to be the primary driving force for the reaction. Small amounts of creep deformation did not significantly influence the extent of the transformation. The diffusivity of Re associated with the moving boundary was calculated to be 5 ϫ 10 Ϫ8 cm 2 s Ϫ1 at 1093 ЊC, which is approximately four orders of magnitude greater than the bulk lattice diffusivity of tungsten.

Formation of an Intermediate Layer Between Grains in Nickel-Based Superalloy Turbine Blades

“…This result is important for the understanding of the formation mechanism of this boundary layer because it is known that the development of the discontinuous precipitation reaction, which forms similar microstructures to the layer observed in this study, can only occur along boundaries with mis-orientation greater than 10 deg. [22][23][24] From the observation, it is clear that there is a distinct boundary layer between a matrix and surface defect grains.…”

“…[39][40][41] To reduce the energy of the grain boundary, the DP reaction can easily occur from the following reaction [22][23][24]42] :…”

Discontinuous Precipitation in Ni-Base Superalloys During Solution Heat Treatment

“…[2] It has been observed in the substrate beneath coatings [3][4][5] or on grain boundaries. [6][7][8] In all cases, there is the requirement of a super-saturated matrix, which provides the driving force for growth. [2] In multi-component Ni-base superalloys, DP morphology constitutes a polycrystalline colony comprising of c¢ matrix with/without c-lamellae and needle-like tetragonal close-packed (TCP) phases segregated in Cr, W, and Re, which constitute the precipitate.…”

“…[2] In multi-component Ni-base superalloys, DP morphology constitutes a polycrystalline colony comprising of c¢ matrix with/without c-lamellae and needle-like tetragonal close-packed (TCP) phases segregated in Cr, W, and Re, which constitute the precipitate. [3][4][5][6][7][8] The advancement of the DP zone occurs by partitioning of Cr, W, and Re to the growing tips of the precipitates along the grain boundary.…”

“…[6][7][8] (ii) Beneath an aluminized or Pt-aluminized surface coating-the reaction can be written as c + Al fi c¢ + TCP and in this case the DP zone is also referred to as the ''secondary reaction zone'' or SRZ. [3][4][5] It should be pointed out that DP is also referred to as ''Type II'' re-crystallization.…”