STUDY OF MICROSTRUCTURES IN γ / γ '-- α Mo DIRECTIONAL EUTECTOID ALLOY INDUCED BY LASER RAPID MELTING SOLIDIFICATION

The principle purpose of surface modification by laser rapid melting solidification is to improve the surface properties of materals through the change of their surface microstructures. Theγ/γ'-αMo directional eutectoid alloy consisting of a chemical composition(atomic fraction) of 15.41%A1,2.56%V,3.25%Cr,8.41%Mo and balance Ni was chosen in the present study.The master alloy was melted in a vacuum induction furnace,and then the directional eutectoid specimens were prepared in a vacuum solidified furnace.Two ways,i.e.pulse and scan,were used for the surface laser treatment.A finer directional eutectoid structure grows again in the melting zone when the laser pulse is parallel to the eutectoid growth direction.However,when the laser pulse is perpendicular to the eutectoid growth direction,the microstructures of melting zone are composed of much denser cell-like crystalline and dendrites.The dendritic arm in dendrite structure after laser melting consists of theγ-solid solution and the fineγ'-participites,and the microstructures in the interdendrites are composed of theγ-solid solution containingγ' and the metastable phase precipitates which are enriched by Mo. These metastable phases are the ordered[αMo_((ordered))],Ni_3Mo and ordered[Ni_3Mo_((ordered))].