The purpose of this study is to perform a fundamental study on the design and the development of an advanced high temperature intermetallic alloy by investigating the relationships between the microstructure and the mechanical properties of two phase $NiAl/Ni_{3}Al$ alloys. The $NiAl/Ni_{3}Al$ two phase alloys have been studied to get advantages of both NiAl and Ni3Al intermetallic compound, but they lack room temperature ductility and high temperature strength and so these have been their main drawback to their practical applications.
In the previous study, the β + γ' alloy containing Ti and C showed a more refined mesh microstructure than the lamellar microstructure. This study revealed that the titanium among alloying elements helped to refine the microstructure. The $M_{s}$ temperature of NiAl lowered rapidly with an increasing titanium content. This effect therefore contributed to the formation of the fine microsturucture. In the case of a low titanium content (below 2.2 at.%), the $M_{s}$ temperature was above ∼255℃, and when the martensite was heated, the exothermic β' → $Ni_{5}Al_{3}$ transformation appeared first. Because the martensite twins could be inherited during the β' → $Ni_{5}Al_{3}$ transformation, the lamellar β + γ' structure similar to martensite plates was finally formed. However, in the case of a high titanium content, the $M_{s}$ temperature sufficiently lowered, so the endothermic β'+ β transformation could occur before the β' + $Ni_{5}Al_{3}$ transformation. The endothermic transformation extinguished the martensite plates, therefore allowing the mesh microstructure to be independently formed from the martensite plates.
The microstructural evolution of β - $NiAl/γ'- $Ni_{3}Al$ two-phase $(Ni_{66}Al_{34})_{100-χ} X_{χ}(X=Ti, Si, Nb)$ alloys during various heat treatment (quenching and aging) were systematically studied. It was found that the microstructural features of the two-phase $NiAl/Ni_{3}Al$ alloys could be divided into three types : lamellar, mesh and Widmanst tten microstructures. The reason why those three types of microstructure were formed could be explained by the change of austenite start temperature ($A_{s}$ temperature) depending on the ternary elements. In case of $A_{s} > 250℃, the lamellar microstructure was formed by following phase transformation: Martensite → Ni_{5}Al_{3 → β + γ'. In this case, the NiAl martensite was quickly transformed into the $Ni_{5}Al_{3}$ phase at 250℃ by a re-ordering reaction. In case of 20˚ < A_{s} < 250℃, two types of mesh microstructure were formed depending on the ternary element. When Ti and Nb were added as a ternary element, the β → $Ni_{5}Al_{3}$ transformation occurred very quickly, but not in the case of Si addition. Hence, the final mesh microstructures showed somewhat different microstructural features. On the other hand, in case of $A_{s}$ < 20℃, the direct β → β + γ' transformation occurred because the martensitic transformation could not occur due to its very low $M_{s}$ temperature, resulting in the Widmanst tten type microstructure.
In the my studies about the two-phase $NiAl/Ni_{3}Al$ alloys, the various microstrucutures of polycrystalline $NiAl/Ni_{3}Al$ alloys could be obtained by the $M_{s}$ temperature control of NiAl-martensite including titanium, and the mechanical properties were considerably dependent on their microstructures. In this study, the grain boundaries were tried to align by directional solidification to suppress the intergranular fracture at room temperature or grain boundary sliding at high temperature. To effectively maintain the grain boundary alignment in the course of the heat-treatment of a DS ingot, suppressing recrystallization during solutionizing treatment, which is the preceding process of martensitic transformation of a DS alloy, is very important. Based on the orientation relationship between parent β - NiAl (the B2 structure) and product β' - martensite $(the L1_{0} structure)$, the optimum heat-treatment conditions for DS ingot were determined through XRD observation. Consequently, the columnar grained $NiAl/ Ni_{3}Al$ alloys could be achieved for the only Ti-added ternary ingot contrary to the recrystallized binary ingot. And, the columnar grained alloys including Ti could show higher yield strength at high temperature than the usual polycrystalline alloys.
The Ni-rich β - NiAl alloys are considered as potential materials for high temperature shape memory alloys because of their higher thermoelastic martensite transformation temperature. But the transformation to $Ni_{5}Al_{3}$ phase occurring at 450∼550℃ during heating of NiAl martensite interrupts the reverse martensitic transformation, and causes the shape memory effect in NiAl martensite to disappear. In the present study, the phase transformation process in binary Ni-(33∼37at.%)Al martensite was investigated by DTA, and it was found that the blocker of reversible martensitic transformation was not the β → $Ni_{5}Al_{3}$ transformation but the M → Ni_{5}Al_{3}$ transformation occurring at 250∼300℃. Therefore, the transformation temperature of M → $Ni_{5}Al_{3}$ determined the critical temperature for operation of shape memory effects. In addition, for verifying the critical temperature, the phase transformation process was investigated for various ternary Ni-33Al-X alloys (X = Cu, Co, Fe, Mn, Cr, Ti, Si, Nb). As a result, Ti, Si and Nb were very effective to lower the $A_{s}$ temperature of Ni-33Al-X alloys, facilitating the shape memory effect in Ni-33Al-X alloys. Especially, Si and Nb additions were beneficial for increment of the transformation temperature of M → Ni_{5}Al_{3}$, resulting in higher operating temperatures of NiAl-based shape memory alloys.
Since the intermetallic compound $Ni_{3}Al$ has attractive high-temperature properties, $Ni_{3}Al$ thin foils could be used as lightweight, high-temperature structural materials. T. Hirano et al. previously found that the directional solidification by the floating-zone (FZ) technique was very effective in improving the ductility of $Ni_{3}Al$ at room termperature without any alloying elements. The FZ method makes it possible to fabricate the thin foils with smooth and crack-free surface by cold rolling up to 96% without any intermediate annealing steps. And through annealing of the cold-rolled foils at temperatures over 1273K, the recrystallized foils could possess some room-temperature ductility (3.0 to 14.6%) in air and be bent plastically around TD more than 100℃ without cracks. These results demonstrate that it may be possible to utilize $Ni_{3}Al$ thin foils as lightweight, high-temperature structural materials, e.g. honeycomb structures. For the practical application of $Ni_{3}Al$ foils, the other properties such as oxidation resistance, wear resistance and friction resistance should be considered. The β - NiAl, which is an important intermetallic compound in Ni-Al systems together with γ'- $Ni_{3}Al$, have been used for coating materials of Ni-base superalloys because of its excellent oxidation resistance. In this study, the NiAl coating was intended to deposit on the $Ni_{3}Al$ foils. For the purpose, we used a RF magnetron sputtering. But binary Ni-50Al coating could not form α - $Al_{2}O_{3}$ oxide, and moreover, the rapid Al diffusion between coating and substrate was occurred. To improve oxidation resistance and to retard Al diffusion from coating layer, the coating composition of Ni-47Al-3Ti-0.1Y was selected, and it could show superior oxidation resistance due to stable α - $Al_{2}O_{3}$ formation.
