Pulse Current Sintering And Formability Of NiAl Alloy Powder

The NiAl intermetallic is considered as a promising candidate for elevatedtemperature structural materials and aeronautic materials due to its low density, highmelting point, good thermal conductivity and excellent oxidation and frictionresistance. However, its application is restricted by its poor ambient-temperatureductility and high-temperature strength. Many methods have been carried out in orderto overcome the drawbacks. Among various methods, preparing fine-grained materialis a feasible and effective approach. Nevertheless, using traditional methods, such astraditional casting, NiAl bulk material with fine and homogeneous micro-structurecannot be easily prepared. Currently, mechanical alloying is a viable method to preparefine-grained material. Furthermore, Pulse Current Auxiliary Sintering (PCAS), for itsadvantages, such as quicker heating rate, lower sintering temperature, shorter sinteringtime, higher material density, lower cost etc., has been applied to fabricate NiAl bulkmaterials with refined micro-structure.In this research, the nano-crystalline NiAl powder was synthesized by mechanicalalloying, and dense NiAl-Al₂O₃ composite bulk materials with submicron-grain weresubsequently consolidated by Pulse Current Auxiliary Sintering (PCAS). Ni,Alelement powders'transformation into NiAl powder during the process of mechanicalalloying was investigated and the microstructure of bulk materials were characterizedwith the help of Optical Microscopy, X-ray techniques, SEM,TEM, etc. Moreover,mechanical properties of NiAl bulk material have also been researched by density test,hardness test, fraction and oxidation resistance experiment, compression test, fracturetoughness test, etc. In the thesis, the influences of sintering temperature on bulkmaterials'microstructures and mechanical properties were investigated. At the end ofthe research work, a model of submicron-grained NiAl-Al₂O₃ hot extrusion mold wasfabricated by sinter-forging technology of short process in the PCAS apparatus.The results indicated that nanocrystalline NiAl powders with average size of 8μmwere synthesized after 22 h of milling. Little Al₂O₃ particles have been observed. Theresults of comparing the microstructure and mechanical properties of the bulkmaterials fabricated at 1200℃and 1300℃revealed that 1200℃was a morereasonable sintering temperature. After being sintered at 1200℃, Al₂O₃ particleshomogeneously distributed in NiAl matrix with a farraginous intragranularintergranularstructure. The average size of NiAl and Al₂O₃ were 200nm and 50nm,respectively. Due to the effect of grain refinement and dispersion-strengthening, thecompressive yield strength and compressive plasticity at room temperature and elevated temperature were obviously improved. The result of fracture toughness testindicated that the fracture toughness increased to 8.2 MPa·m1/2 and the morphology offracture surface exhibits a typical brittle fracture mode with atransgranular-intergranular mixture mode. The result of friction and oxidationresistance test showed that NiAl-Al₂O₃ bulk material performed well at elevatedtemperature because of its excellent oxidation resistance and friction resistance. Lastly,a model of hot extrusion mold with fine NiAl-Al₂O₃ grains, high surface and fillingquality was obtained by sinter-forging technology of short process. Compared with theNiAl-Al₂O₃ composite bulk materials, its average grain was slightly larger.Sinter-forging technology of short process has been dedicated as a promising potentialuse in the field of fabricating elevated temperature structural and aeronauticfabrications.