Laser surface alloying of nickel by molybdenum and aluminium— microstructural studies

Abstract

It is now well established that a considerable improvement in the mechanical and chemical properties of the near surface regions of materials may be achieved by the use of high energy laser beams. By manipulating the laser power density and the time of interaction of the laser beam with an appropriately coated work piece, it is possible to achieve a surface chemistry that would have an improved resistance to wear, fatigue and corrosion failures. The change in chemistry at the surface is attained through the process of melting and mixing of the coating and a thin layer of the substrate. Solidification of this molten region at the surface results, due to an interplay of various forces, in the development of very complex microstructures. To analyse these in a piece of nickel that had been coated with a mixture of molybdenum and aluminium powders and then treated with a continuous wave CO₂ laser, extensive transmission electron microscopy was done on a thin foil obtained from near the bottom region of the recast pool. The foil was found to have a cellular microstructure comprising cells of the Ni₃Al (γ') ordered phase. In the intercellular regions, two dispersed phases, namely, martensitic Ll₀ phase and Ni₂Al₃, and a contiguous phase, identified as a close derivative of the equilibrium δ -NiMo phase were found. A gradient in the degree of order could be observed within the γ' cells. In the central portion of the cells, rapid solidification resulted, by the process of sequential ordering, in the development of a partially ordered alloy with very fine domains. In the peripheral regions of the cells, the alloy solidified by direct ordering into larger domains. This paper discusses some of these results.