Study On Nanocrystalline Nickel-Aluminum And Its Composites Prepared By Mechanical Alloying

Mechanical alloying is a novel technology for preparing alloys and their compounds through diffusion of atoms, solid reaction and phase transition at room temperature. At present mechanical alloying is extensively used to synthesis microcrystalline, amorphous and nanocrystalline materials. Mechanical alloying shows a promise for adjustable composition, extended solid solubility of alloy, occurrance of the metastable and intermetallic phase during processing.Using element powders as raw materials, the NiAl intermetallic, oversaturating solid solution NiAl and microalloying NiAl were synthesized by mechanical alloying. Raney Ni was prepared through heat treatment and remobilized leaching using alkali liquor. The bulk NiAl-TiC in-situ composite was prepared using element powder as raw materials by high energy ball milling combining spark plasma sintering. Microstructure and mechanical properties of these products by mechanical alloying were studied by XRD, SEM, EDS and testing. The synthesizing mechanisms of Ni-Al and its overstaturating solid solution were also discussed.It was shown that, the nanocrystallite NiAl intermetallics, oversaturating solid solution NiAl and microalloying NiAl could be obtained by mechanical alloying using element powder as raw materials. After heat treatment, the average grain size of products by mechanical alloying was increased and the initial microstructure of powder by mechanical alloying was disappeared. Powder particles become uniform and spheroidize. There was appearance of melting and merging between the boundaries of powder particles. Solid phase transition appeared and new phases were formed from NiAl alloy with severely oversaturating solid solution during heat treatment, but the phase of NiAl alloy with less oversaturating solid solution were hardly change during heat treatment. The synthesizing mechanism of nanocrystalline NiAl intermetallic was combustion synthesis induced by mechanical alloying. The synthesizing mechanism of nanocrystalline NiAl oversaturating solid solution was that the NiAl intermetallic phase was synthesized at first, following the oversaturating solid solution of NiAl formed through diffusion of Al into by combustion reaction.The Raney Ni and its alloying catalyst were prepared by leaching nanocrystalline Ni-Al alloys. Monatomic Raney Ni was easily obtained by leaching Ni20Al80 system powders by mechanical alloying. And Raney Ni with NiAl phases from Ni30Al70 system, Raney Ni with NiAl and Ni3Al phases from Ni50Al50 system similarly. Compared with Ni-Al alloys prepared by casting, Al in Ni-Al alloy prepared by mechanical alloying were more easily leached, so the phase component restriction of leaching technology on Ni-Al alloy produced by conventional method was broken. The Raney Ni produced by mechanical alloying had the advantage of tiny grain size, uniform organization, adjustable composition et al. Its technology and image of microstructure were also superior to commercial Raney Ni. Raney Ni alloy catalyst containing active ingredients could be prepared in producing preparatory alloy of Raney Ni if some Ti, Fe, Cr et al was added into it. The component of catalyst was uniform and its microstructure was fine.The bulk NiAl-TiC in-situ composite could be prepared by thermal mechanical alloying which was combination of high energy ball milling and plasma sintering. The grain size of the in-situ composite was fine and its microstructure was uniform. Compared with those prepared by other preparing processes, the NiAl-TiC in-situ composite prepared by thermal mechanical alloying showed higher hardness and better high temperature yield stress.