High Temperature Compression And Oxidation Properties Of Ni₃Al And Its Composites Fabricated By Spark Plasma Sintering

In this thesis, a practical avenue was pointed out to fabricate Ni₃Al and 20vol.%(TiBB2 +TiC)/Ni₃Al composites by mechanical alloying of elemental powders and subsequent spark plasma sintering (SPS). The mechanical alloying process and microstructure of Ni₃Al and its composites after sintered were investigated by scanning electronic microscope (SEM), transmission electronic microscope (TEM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD) and so on. The effects of sintering temperature on the properties of composites were analyzed. The hot comporession behavious and high temperature oxidation properties of composites were also investigated.SEM, DSC and XRD observation of powder mixture indicated that during the mechanical alloying process, Ni(Al) solid solution were formed in Ni-Al powder mixture after milling for 30h, and each elements distributed homogeneously in Ni-Al-Ti-B-C powder mixture.The SPS process of Ni₃Al matrix composites could be divided into initial, intermediate and final stages. The spark plasmas were mainly generated in the initial and intermediate stages. Plastic flow and diffusion creep of the powders were controlling mechanisms in the final stage of SPS. TiB₂, TiC and Ni₃Al were the major phase in composites as sintered. Ni₃Al and its composites have high relative density and fine microstructure. Ceramic particles distributed homogeneously in Ni₃Al matrix. Interface between particles and matrix is clean. Ceramic particles and effectively restrict dislocation and interface mobility. As a result, the hardness and strength are improved.The investigation of the hot compressive deformation of composites showed that the"R"effect is not obviously in Ni₃Al and its composites due to their fine microstructure. Compressive strength of Ni₃Al and its composites decrease with increasing deformation temperature. The strength of Ni₃Al is highly improved at low temperature. The flow stress difference between the composites sintered at different temperature decreases with increasing deformation temperature. Ni₃Al has good anti-oxidation ability due to the fine microstructure: (1) the fine grain and high density of grain boundaries provide large number of nucleation points for forming oxides; (2) the promotion of selective oxidation of Al produces an external layer of Al₂O₃; (3) the adhesion between oxide scales and base metal is improved due to nano- crystallization.TiO₂ is the major phase in the oxide scales formed on (TiB₂+TiC)/Ni₃Al composites. The continuity of Al₂O₃ was broken because of the formation of TiO₂. The formation of CO2 and B2O3 made the scales loose. All these decrease the anti-oxidation properties of the composites. The isothermal oxidation analysis shows that the oxidation of the composites is controlled by the formation of TiO₂. The sintering temperature greatly influences the anti-oxidation properties of (TiB₂+TiC)/Ni₃Al composites. (TiB₂+TiC)/Ni₃Al composites sintered at 900oC has low anti-oxidation property due to the very low relative density, which results in the speedup of the inner and outer oxidation of the composite. Anti-oxidation ability of (TiB₂+TiC)/Ni₃Al composites decreases when the sintering temperature increases (except for 900oC).A low temperature stable oxide TiO₂(B) was formed during the composites oxidation due to the fine grain size and lower surface energy of the TiO₂(B) phase.