Influences and Mechanisms of the Alloying Elements Ta and Ag on the Mechanical and Tribological Properties of Nickel-Base Alloys

Nickel-based composites with addition of Ta and Ag were prepared by high-energy ball milling and vacuum hot-pressing (HP) sintering methods. The tribological and mechanical properties of the composites in a wide temperature range were investigated. The phase composition, wear mechanism and fracture mechanism were analyzed by scanning electron microscopy and X-ray diffraction. During the HP sintering process, Ta reacted with C in the graphite die and formed a TaC ceramic phase which dispersed in the matrix. The addition of Ta and Ag reduced the friction and wear of the material, and the NiCrMoAl-Ta-Ag composite achieved continuous lubrication from room temperature to 800 ℃. Ag provided lubrication at room temperature, and discrete lubricant film was formed by abrasive debris and Ag at intermediate temperature, while at 800 ℃, a continuous lubricant film containing oxide, silver molybdate and Ag was formed on the worn surface. The addition of Ta significantly enhanced the mechanical properties of the material. The NiCrMoAl-Ta alloy had excellent mechanical properties in wide temperature range (room temperature ~ 1 000 ℃), which was attributed to the dispersion strengthening effect of the in-situ generated TaC and Al₂O₃ ceramic phase. The fracture mechanism of the materials varied from micro-void coalescence fractures to the fracture combining micro-void coalescence and oxidative fractures with increasing temperature.