Monotectic four-phase reaction and self-lubricating performance of directionally solidified Ni-Nb-Cu ternary immiscible alloys

The monotectic four-phase equilibrium reaction ${\mathrm{L}}^{\prime }\to {\mathrm{L}}^{\prime \prime }+{\mathrm{S}}_1+{\mathrm{S}}_2$ of ternary immiscible alloys provides a new window for the one-step design of novel in-situ composites. In this work, a differential scanning calorimetry (DSC) study was performed for the Ni-Nb-Cu alloys to find the ternary monotectic composition point Ni_48.5Nb₃₉Cu_12.5 and determine the four-phase reaction ${\mathrm{L}}^{\prime }\to {\mathrm{L}}^{\prime \prime }({\mathrm{Cu}}^{\mathrm{L}})+{\mathrm{Ni}}_3\mathrm{Nb}+{\mathrm{Ni}}_6{\mathrm{Nb}}_7$ at 1134℃. Subsequently, continuous directional solidification experiments were carried out for the monotectic-point alloy. The results indicate that the microstructure of the rod sample is of in-situ composite characteristic with Cu-rich soft particles embedded in eutectic-like hard Ni₃Nb and Ni₆Nb₇ compounds. The average size of the Cu-rich particles and the interlamellar spacing of the Ni₃Nb compound decrease with the increase of the withdrawal rate. The formation mechanism of the composite structure at the solidifying front was discussed in detail. The interaction between the eutectic-like microstructure with the Cu-rich droplets depends on the solute diffusion, interfacial wetting, and droplet migration. The Cu-rich droplets prefer to wet the Ni₃Nb compound, and interfacial bonding properties among the Ni₃Nb, Ni₆Nb₇ and Cu-rich phases were characterized. The measurements of hardness and wear characteristics indicate that the Vickers hardness and wear resistance of the samples significantly increase with the increase of the withdrawal rate. The combination of the well-dispersed soft Cu-rich particles as solid lubricant and the hard Ni₃Nb and Ni₆Nb₇ compounds as strong matrix causes that the Ni-Nb-Cu monotectic alloy exhibits high self-lubricating and wear-resistance performances.