Secondary Reaction Zones in Coated 4th Generation Ni-Based Blade Alloys

Abstract

The morphologies and growth kinetics of the Secondary Reaction Zone (SRZ) formed at the interface between the Pt-Aluminised coating and 4 th generation Ni-base superalloy substrates have been investigated. Three alloys with systematically varying Ru content in the range of 2-5wt% were studied. Each alloy was tested using four different surface finishes prior to Pt coating. At the lowest Ru content sporadic formation of SRZ was observed, whilst in the higher Ru-containing alloys a continuous SRZ was formed. These alloys also showed more rapid SRZ growth, regardless of the original surface finish. The precipitation of TCPs in the substrate inhibited the growth of the SRZ towards the end of the exposure reversing the advantage of early nucleation. The volume fraction of the SRZ formation followed that in the substrate, reducing with Ru content. EBSD analysis revealed that the higher Ru-containing alloys nucleate many more grains of the SRZ but the number of grains adjusts with growth towards a constant grain size. On balance Ru leads to a deeper penetration depth into the substrate.

Discontinuous Precipitation in Ni-Base Superalloys During Solution Heat Treatment

“…[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. This occurs when re-crystallization originates at the surface and more importantly occurs below the solvus temperature.…”

“…[1] Discontinuous precipitation (DP) refers to an abrupt change in composition and orientation between parent phase and product phases. [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. [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.…”

Effect of Thermal History on Microstructural Changes in Aluminized Nickel-Based Single-Crystal Superalloy

“…The SRZ is defined as the region between the interdiffusion zone (IDZ) and the substrate that characteristically shows a high density of fine topologically close-packed (TCP) phase precipitates and a boundary at a high angle relative to the substrate. 8) In other words, the SRZ can be characterized as the region of the polycrystallized substrate that contains TCP phase precipitates. The formation of the SRZ destroys the coherent £ and £A two-phase structure, degrading the mechanical properties and the creep resistance of the substrate.…”

Effect of Surface Treatment and Crystal Orientation on Microstructural Changes in Aluminized Ni-Based Single-Crystal Superalloy

“…The SRZ is defined as the region between the inter-diffusion zone (IDZ) and the substrate, characteristically showing a high density of fine topological close-packed phase (TCP) precipitates, and a high angle boundary to the substrate. 4) In other words, the SRZ is a region consisting of the re-crystallized substrate with TCP precipitates. Since the SRZ formation can destroy the and 0 coherent two-phase microstructure, which accordingly leads to a deterioration of mechanical properties of the substrate, understanding of formation kinetics of SRZ is important.…”