Microstructure of laser-induced surface-alloyed α-Fe

Substrates of pure alpha-Fe were laser alloyed with Ni2B powder, which was injected into the molten bath created by the irradiation of the substrate with a CW-CO2 laser. A 50 % overlap between successive laser passes allowed a larger amount of Ni2B to react with the alpha-Fe melt. Solidification of the Fe-Ni-B melt produced an off-eutectic microstructure of gamma-<(Fe)under bar>Ni and orthorhombic (Fe,Ni)(3)B in the similar to 650 mu m thick surface-alloyed layer. Its matrix consists of the gamma-<(Fe)under bar>Ni phase, which contains about 60 at.% Fe and 40 at.% Ni, the relatively large amount of Ni stabilizing the fee gamma-phase. Between the gamma-<(Fe)under bar>Ni dendrites, which are up to similar to 80 mu m long, a fine lamellar microstructure of gamma-<(Fe)under bar>Ni, and (Fe,Ni)(3)B solidifies. The gamma-<(Fe)under bar>Ni and (Fe,Ni)(3)B lamellae are about 200 nm wide and extend over several mu m. Since only a small amount of the injected Ni2B particles are discernible by optical microscopy and X-ray diffraction studies, it is assumed that the nearly complete dissolution of the Ni2B particles in the melt results in the off-eutectic gamma-<(Fe)under bar>Ni and (Fe,Ni)(3)B microstructure, which in turn causes the improved surface hardness of similar to 680 Hv0.1 compared with similar to 160 Hv0.1 of the alpha-Fe substrate.