Modeling of microstructural evolution and lifetime prediction of MCrAlY coatings on nickel based superalloys during high temperature oxidation

Abstract

MCrAlY coatings are deposited onto superalloys to provide oxidation and corrosion protection at high temperature by the formation of a thermally grown oxide scale. In this project, the oxidation behaviour of a HVOF CoNiCrAlYSi coating on IN792 was studied for both isothermal oxidation (900, 1000 and 1100 °C) and thermal cycling (100-1100 °C). The microstructural evolution of the CoNiCrAlYSi coatings after oxidation was investigated. It was found that the Al-rich β phase is gradually consumed due to two effects: surface oxidation and coating-substrate interdiffusion. Some voids and oxides along the coating-substrate interface, or inside the coating, were considered to play a role in blocking the diffusion of alloying elements. Based on the microstructural observation, an oxidation-diffusion model, considering both surface oxidation and coating-substrate interdiffusion, was developed by using Matlab and DICTRA software to predict the useful life of MCrAlY coatings.A new concept of diffusion blocking is also introduced into the model to give more accurate description on the microstructural evolution. The results from modeling and the experimentally established composition profiles showed good agreement.

A Continuous β-NiAl Layer Forming at the Interface of a MCrAlY and CMSX-4

“…A developed oxidation-diffusion model [4,7,8] was adopted to study the influence of heating at 1100 °C on systems combining the substrates and coatings given in Table 1. The modelling scheme is as follows (Fig.…”

“…In our previous studies [4,7,8], the "rule of mixture" (the 5th homogenization module in the DICTRA software, shorten as Ho5) has been adopted to calculate the elemental diffusivities in the multi-phase material systems. The Ho5 module is an upper diffusion bound for which the diffusion of elements in the high-mobility phase has a dominant role.…”

“…The upper diffusion bound tends to be more applicable in cases where low-mobility phase(s) has a low volume faction in the microstructure. One successful application of the Ho5 module can be found in [4]. The diffusivity of an element ( ) in the Ho5 module was derived as below:…”

“…3a). The secondary β phase (β formed in the substrate) was usually not predicted by the model [4,7], which could be due to the limitation of the diffusion database used.…”

“…Under continued thermal exposure, continued oxidation at the coating surface consumes Al in the coating and thus the β-phase. In addition, the coating also loses Al into the substrates due to the interdiffusion of alloying elements through coating-substrate interface (the substrate is considered to be an Al sink), leading to the development of an inner β-depletion zone of the coating near the interface [4,5]. If the diffusion of Al from the coating into the substrate can be hindered or slowed, more Al will be available for maintaining the protective TGO and therefore the coating life can be prolonged.…”