Effects of Tungsten Addition and Isothermal Annealing on Microstructural Evolution and Hardening Behavior of Two-Phase Ni3Al-Ni3V Intermetallic Alloys

The effects of tungsten (W) addition and isothermal annealing on the microstructure and related hardening behavior were investigated for the dual two-phase Ni3Al-Ni3V intermetallic alloys. The microstructures in the homogenized alloys depended on the W substitution manner for Ni, Al or V. Distinctively fine and coherent dual two-phase microstructures were developed for the homogenized alloy in which W was substituted for Al and resulted in the largest hardness in the investigated alloys. Disk-shaped and fine Ni4W particles were precipitated in the channel region of the isothermally annealed alloy in which W was substituted for Ni. Hardness of the alloys increased with increasing annealing time and made a peak, followed by decrease, irrespective of the W substitution manner. The initial increase of hardness at early annealing time was attributed to the maturity of the eutectoid microstructure prematurely developed in the channel region before annealing. The simultaneous addition with W and Nb stabilized the eutectoid microstructure in the channel region and suppressed softening at a prolonged annealing time. Hardening mechanisms for the homogenized and annealed alloys were discussed in terms of the microstructural and phase stability and compared with the alloys with the simultaneous addition with W and Nb.