Effect of microstructure evolution on machinability of cast Ni 51−x Ti 49 Co x shape memory alloys

The effects of Co addition and heat treatment on the microstructure and machinability of cast Ni 51−x Ti 49 Co x shape memory alloys were investigated using conventional drilling. The alloys exhibited a monoclinic B19’ martensite matrix with Ti 2 Ni and Ni 4 Ti 3 precipitates identified by X-ray diffraction (XRD). Increasing Co content from 0.5 to 1.5 at.% reduced hardness from ∼440 to 280 Hv, followed by a slight increase to 295 Hv at 4 at.% Co, with the same trend observed after heat-treatment. The 0.5 and 4 at.% Co alloys developed pronounced subsurface deformation with indistinct grain boundaries. In contrast, the 1.5 at.% alloy exhibited the lowest tool wear and subsurface deformation, indicating that machinability depends on the combined effects of matrix strength, precipitate state and adhesive tool-workpiece interaction.