Characterization of microstructure in laser-surface-alloyed layers of aluminum on nickel
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9/27/03
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Characterization of Microstructure in Laser-Surface-Alloyed Layers of Aluminum on Nickel S. BYSAKH, S.K. MITRA, G. PHANIKUMAR, J. MAZUMDER, P. DUTTA, and K. CHATTOPADHYAY In order to obtain basic understanding of microstructure evolution in laser-surface-alloyed layers, aluminum was surface alloyed on a pure nickel substrate using a CO2 laser. By varying the laser scanning speed, the composition of the surface layers can be systematically varied. The Ni content in the layer increases with increase in scanning speed. Detailed cross-sectional transmission electron microscopic study reveals complexities in solidification behavior with increased nickel content. It is shown that ordered B2 phase forms over a wide range of composition with subsequent precipitation of Ni2Al, an ordered phase in the B2 matrix, during solid-state cooling. For nickel-rich alloys associated with higher laser scan speed, the fcc phase is invariably the first phase to grow from the liquid with solute trapping. The phase reorders in the solid state to yield Ni3Al. The phase competes with AlNi, which forms massively from the liquid. The AlNi transforms martensitically to a 3R structure during cooling in solid state. The results can be rationalized in terms of a metastable phase diagram proposed earlier. However, the results are at variance with earlier studies of laser processing of nickel-rich alloys.
I. INTRODUCTION
LASER surface alloying is a novel technique of applying desired protective coatings on engineering components using the intense power of a laser. It is capable of producing a thin surface layer on a substrate alloyed with the desired element. The details of laser surface modification are reviewed elsewhere.[1] Through proper control and optimization of the various experimental parameters such as laser power, spot size, laser scan speed, and feed rate of the alloying powder, an alloyed layer of the desired composition and improved surface properties can be achieved on to the workpiece.[2,3,4] Laser surface alloying is increasingly becoming an important industrial tool for surface modification to impart additional functionality in actual use. In order to achieve this goal, one needs to employ rational alloy design criteria to obtain the required chemistry and the microstructure within the alloyed layer. The understanding of the microstructure evolution in the alloyed surfaces as a function of processing parameters and chemistry is, therefore, crucial. Nickel aluminides are well known for exhibiting interesting properties such as high strength and corrosion /oxidation resistance at elevated temperatures. Since laser surface alloying is capable of producing fine as well as metastable S. BYSAKH, Visiting Scientist, is with the Nanocharacterization Research Group, Nanomaterials Laboratory (NML), National Institute for Materials Science (NIMS), Ibaraki–305003, Japan. S.K. MITRA, Programmer Analyst, Syntel(I) Limited, Chennai–600 015, India. G. PHANIKUMAR, Visiting Scientist, is with the Institut fu
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