Microstructure of iron substrates borided with Ni2B particles by laser-induced surface-alloying

Armco iron substrates were laser-induced surface-alloyed using Ni2B particl es. The injected particles dissolve in the melt pool created by irradiation of the substrate surface with a CW-CO2 laser. Solidification of the Fe-Ni- B melt causes the formation of boride phases imparting hardness values of s imilar to 600 Hv to the surface layer. The evolving microstructure of surfa ce alloyed-layers prepared by single laser tracks and overlapping laser sca ns was analyzed for varying laser powers and laser scan velocities. The stu dies revealed that either fee gamma-(Fe,Ni) or bcc alpha-Fe(Ni) dendrites f orm the matrix of the alloyed layers, depending on the amount of Ni2B disso lved in the melt. In the first case a (Fe,Ni)(3)B-gamma-(Fe,Ni) eutectic so lidifies in the space between the gamma-(Fe,Ni) dendrites, while in the lat ter case alpha-Fe(Ni) and the metastable Fe3B phase solidify eutectically i n the interdendritic regions. The thickness and microstructure of the Ni2B laser-alloyed coatings determine their proneness to cracking.