Experimental and First Principles Study of the Ni-Ti-W System

摘要

Nickel based superalloys are structural materials with a chemical composition and structure which has been developed to enable good high temperature performances leading to a wider range of applications. Their unique properties are due to their microstructure characterized by the coexistence of L1_2-ordered intermetallic precipitates like Ni_3Al or Ni_3Ti - γ' phase - in a face-centered cubic nickel based solid solution matrix, (Ni) - y phase. Solid solution strengthening at high temperatures can also be provided by the addition of refractory alloying elements, like tungsten, W. Therefore, the mechanical properties behaviour of the alloys is very strongly related to their composition and microstructures. The purpose of this work is to study the effect of composition and microstructures in a series of Ni-rich prototype alloys, Ni_(100-2x)-Ti_x-W_x (in which x is in at.%), in order to understand and ultimately optimize the performance of these materials. The adopted strategy was to combine experimental studies using Neutron Diffraction, Electron Probe Micro Analysis - EPMA, Differential Scanning Calorimeter - DSC and micro-hardness measurements, with first principles calculations for structure optimization and Gibbs energies at different temperatures, for each phase, leading to thermodynamic assessment.