Microstructure refinement of NiCoCrAIY alloys by laser surface melting
- PDF / 380,101 Bytes
- 9 Pages / 576 x 792 pts Page_size
- 93 Downloads / 223 Views
I. INTRODUCTION
MULTIPHASE MCrAlY (M Ni,Co) alloys, consisting of an Al-rich b phase (NiAl) and an Al-poor g phase (Ni solid solution), are used as overlay coatings or as bond coatings in high-temperature coating systems to protect the components in gas turbine engines against high-temperature oxidation and corrosion.[1–4] The protection offered by the MCrAlY coating relies on the capability of the alloy to establish and maintain a continuous a-Al2O3 layer on its surface during service.[1–4] Theoretical analysis of the high-temperature oxidation of multiphase alloys indicates that the formation of such a continuous alumina layer on a MCrAlY coating is promoted if the b precipitates within the alloy are small and homogeneously distributed.[5] Also, the adherence of the a-Al2O3 to the MCrAlY alloy is improved if Y is finely and homogeneously distributed in the alloy near its surface.[6,7] However, in cast MCrAlY alloys, the b precipitates are generally very coarse and the Y is normally distributed heterogeneously as large NiY-rich precipitates along the g/b phase and g/g grain boundaries.[7] To refine the microstructure of a cast NiCoCrAlY alloy, laser surface melting (LSM) can be employed. The rapid melting of the alloy surface by high-energy density laser irradiation and the subsequent rapid solidification of the melted alloy surface after laser irradiation produces a zone below the alloy surface with a fine-grained and homogeneous microstructure.[8–10] The maximum depth of the melt pool and the degree of microstructure refinement achieved by LSM is to a large extent determined by the laser proC. KWAKERNAAK, Research Assistant, and W.G. SLOOF, Associate Professor, are with the Department of Materials Science and Engineering, Delft University of Technology, 2628 AL Delft, The Netherlands. Contact e-mail: [email protected] T.J. NIJDAM, formerly Postdoctoral Student, Department of Materials Science and Engineering, Delft University of Technology, is a Postdoctoral Researcher with The Netherlands Institute for Metals Research, Mekelewgz, 2628 CD Delft, The Netherlands. Manuscript submitted June 13, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
cessing conditions, e.g., the laser power density, the laser irradiation times, and the initial reflectivity of the alloy surface prior to LSM.[8] In this work, the relations between the laser processing conditions and the alloy microstructure are established for the LSM of a NiCoCrAlY coating alloy. To this end, the surface of a cast Ni-20Co-19Cr-24Al-0.2Y (at. pct) alloy was melted with 75 MW/m2 laser pulses for five different irradiation times and two different alloy surface conditions (i.e., different initial reflectivity). After LSM, the microstructure, chemical composition, and phase constitution of the original and laser-melted alloy were investigated with scanning electron microscopy (SEM), electron probe X-ray microanalysis (EPMA), and X-ray diffractometry (XRD). The SEM backscattered electron (BSE) images were used to determine the maximum depth of the melt po
Data Loading...
