INVESTIGATION OF NiAL ALLOY AND ITS COMPOSITES BY MECHANICAL ALLOYING

Reactive mechanism, structural evolution and final products of NiAl and NiAl-based composites fabricated by mechanical alloying have been studied by XRD, SEM, TEM and DSC in this paper. Hot pressing (HP) and hot isocratic pressing (HIP) has been used to consolidate resultant powders to near full density bulk compacts with fine structure, and then, the microstructure, mechanical properties and deformation behaviors of these materials have been investigated in detail on the basis of the microstructural analysis.Nanocrystalline NiAl has been quickly synthesized by mechanical alloying with elemental powders of Ni and Al. The formation mechanism is attributed to explosive reaction and the length of incubation period is affected seriously by intensity of milling energy inputted. After compacted by hot pressing at high temperature, the dense of the bulk material reached 95% that of theoretical density with the grain size of 200-300nm. Both compressive and tensile testing has been carried out on the MA NiAl. The compressive yield stress at room temperature is much higher that that of cast NiAl, and compressive strain to failure reaches 6%, which is about 2.1 times that of as-cast NiAl. The yield strength at room is independent on strain rate, which indicated deformation behavior of the material is controlled by dislocation movement. Based on analysis and calculation, grain refining is main reason in improving of toughness of the material at room temperature while it also was affected by the deviation from stoichiometric resulting from oxidation and a little Al2O3. However, the tensile strength of MA NiAl was low and almost no elongation below 973K. The examination of microstructure and fracture surface illustrated existence of mini-cavity decreased the effect of strengthening. With the temperature increasing, the difference between tensile and compressive strength is reduced gradually. At high temperature, the yield stress was dependent on strain rate, which reflected the fact that deformation of the materials was controlled by diffusion mechanism.The effects of the alloying elements Co, Cr added to substitute for the Al element on the alloying process, reaction mechanism and products of mechanicalrvNiAlalloying were studied systematically for the first time. The results show that the addition of element (>10at.%) lowered the heat release of the formation, which made explosive reaction vanished and was replaced by diffusion reaction. Addition of Co affected the structure of end products significantly: Addition 15at.% resulted in the formation of the "amorphous-like" NiAl(Co) phase. When Co content reached or more than 20at.%, the end milling product has transform to supersaturated Ni-based solid solution. Compared with Co, Cr has a lower effect on the structure of end product, but the ordering degree of β-NiAl(Cr) is decreased with the increasing of Cr and total disordering when Cr addition more than 20at.%.Fine structure bulk composites of NiAl(Cr) with different contents of Cr(5, 15, 25at.%) have successfully synthesized by the means of HP and HIP. The microstructure and mechanical properties of the bulk composites have also investigated. The results showed that the microstructure has become balanced and coarsened significantly. The improvement of strength and toughness were due to the combination of both the even separation of y' -Ni3Al and a-Cr and fine grains. The yield strength of 1094-1318 MPa at room temperature was due to fine structure, sediment strengthening and solid strengthening, which was about 1.19 to 1.58 times that of as cast NiAl(Cr). Furthermore, the bulk composites of MA NiAl(Cr) had excellent compression ductility with the maximum compression rate of 30% while strength at high temperature was low and needed to be improved.NiAl/HfC and NiAl/HfB2 nanocomposites have been synthesized by mechanical alloying of elemental mixed powders for first time. The formation mechanism is attributed to double explosive reaction. Incubation time for Ni-Al-Hf-C system was between that of Ni50...