Grain boundary wetting and prewetting in Ni-doped Mo

Grain boundary (GB) wetting and prewetting in Ni-doped Mo are systematically evaluated via characterizing well-quenched specimens and thermodynamic modeling. In contrast to prior reports, the δ-NiMo phase does not wet Mo GBs in the solid state. In the solid-liquid two-phase region, the Ni-rich liquid apparently wets Mo GBs completely. Furthermore, high-resolution transmission electron microscopy demonstrates that nanometer-thick quasiliquid films persist at GBs into the single-phase region where the bulk liquid phase is no longer stable. This is interpreted as a case of GB prewetting. An analytical thermodynamic model is developed and validated; this model can be applied to other alloys.