Microstructure refinement of NiCoCrAlY alloys by laser surface melting

The surface of a cast gamma + beta Ni-20Co-19Cr-24Al-0.2Y (at. pct) alloy is melted with 75 MW/m2 laser pulses for five different laser irradiation times and two different alloy surface conditions (different initial alloy reflectivity). Detailed analysis of the chemical composition, microstructure, and phase constitution of the laser-melted and original alloy reveals that after laser surface melting (1) the average size of the beta dendrites is much smaller, (2) the Y is distributed more homogeneously as smaller NiY-rich precipitates along gamma/beta phase and gamma/gamma grain boundaries, (3) the alloy contains a higher volume fraction of the beta phase, (4) the beta phase contains less Al and more Cr and has a lower lattice parameter, and (5) the gamma phase contains more Al and less Cr and has a higher lattice parameter. Based on both experimental results and model calculations, it is shown that the maximum depth of the melt pool increases with increasing laser irradiation time and decreasing initial alloy reflectivity. The arm spacing of the secondary beta dendrites, which also increases with increasing laser irradiation time, is related to the average alloy cooling rate according to the power of -1/3, in agreement with theories on solidification.