In-situ study of microscale fracture of diffusion aluminide bond coats: Effect of platinum
- PDF / 499,815 Bytes
- 11 Pages / 584.957 x 782.986 pts Page_size
- 63 Downloads / 177 Views
Sanjit Bhowmick and S.A. Syed Asif Hysitron Inc., Minneapolis, Minnesota 55344, USA
Vikram Jayaram Department of Materials Engineering, Indian Institute of Science, Bangalore, Karnataka 560012, India (Received 6 April 2015; accepted 2 September 2015)
The influence of Pt layer thickness on the fracture behavior of PtNiAl bond coats was studied in situ using clamped micro-beam bend tests inside a scanning electron microscope (SEM). Clamped beam bending is a fairly well established micro-scale fracture test geometry that has been previously used in determination of fracture toughness of Si and PtNiAl bond coats. The increasing amount of Pt in the bond coat matrix was accompanied by several other microstructural changes such as an increase in the volume fraction of a-Cr precipitate particles in the coating as well as a marginal decrease in the grain size of the matrix. In addition, Pt alters the defect chemistry of the B2-NiAl structure, directly affecting its elastic properties. A strong correlation was found between the fracture toughness and the initial Pt layer thickness associated with the bond coat. As the Pt layer thickness was increased from 0 to 5 lm, resulting in increasing Pt concentration from 0 to 14.2 at.% in the B2-NiAl matrix and changing a-Cr precipitate fraction, the initiation fracture toughness (KIC) was seen to rise from 6.4 to 8.5 MPam1/2. R-curve behavior was observed in these coatings, with KIC doubling for a crack propagation length of 2.5 lm. The reasons for the toughening are analyzed to be a combination of material’s microstructure (crack kinking and bridging due to the precipitates) as well as size effects, as the crack approaches closer to the free surface in a micro-scale sample.
I. INTRODUCTION
Diffusion aluminide bond coats are an example of compositionally and microstructurally graded coatings with significant variation in mechanical properties through depth.1 Although having a B2-(Pt,Ni)Al matrix throughout, the process of pack aluminizing during deposition of these coatings on Ni superalloy substrates and thermo-mechanical cycles during service leads to extensive inter-diffusion of elements between the substrate and the coating. This results in a gradient in chemistry, microstructure, and hence properties.2 These substrate elements either get into the solid solution (e.g., Co, Ti) or precipitate out as complex intermetallics (e.g., constituting W, Ta, Cr) with varying morphologies and distributions through the coating depth. Pt is deliberately introduced since it is known to improve oxidation resistance of bond coats that form a part of thermal Contributing Editor: Yang-T. Cheng a) Address all correspondence to this author. e-mail: [email protected] b) Present address: Max Planck Institute for Iron Research, Max Planck Strasse-1, Duesseldorf 40237, Germany. DOI: 10.1557/jmr.2015.285 J. Mater. Res., Vol. 30, No. 21, Nov 13, 2015
barrier coatings on gas turbines and aero-engine blades,3 and this is expected to alter the mechanical properties further. According to the distinct micros
Data Loading...
