The purpose of this work is to perform a fundamental study on the design and development of an advanced high temperature intermetallic alloy by investigating the relationships between the microstructure and the mechanical properties of the two phase (NiAl+$Ni_3Al$) alloys. Although polycrystalline NiAl alloys exhibited an excellent high temperature properties, the extreme brittleness at room temperature has limited their applications as high temperature structural materials. Therefore, the improvement of mechanical properties of polycrystalline NiAl alloy by introducing a second phase reinforcement such as $Ni_3Al$ was attempted in this work.
The additions of B and Zr to the (NiAl+$Ni_3Al$) alloys were made for the grain and interface strengthening and its effects on the modification of the prior NiAl-β phase interfaces were studied. The microstructural variations of the matrix by changing the conditions for the solution and aging treatment were also studied. Elevated temperature deformation techniques such as hot extrusion, hot compression were utilized in order to induce the grain size refinement. To illuminate the relationships between the mechanical properties and the microstructural variation, the compression tests at both room and elevated temperatures were made. In addition, the effects of the addition of Boron to the (NiAl+$Ni_3Al$) alloys on the oxidation resistance and the wear resistance at both room and elevated temperature were evaluated.
Microstructure and compressive fracture mode in the vacuum induction melted nickel aluminides, Ni-32.9Al and Ni-32.6Al-1.2B(at%), were investigated in order to understand the effect of boron addition. It was found that the addition of boron causes formation of a thin layer along the grain boundary and island-type precipitates inside grains. Compressive tests were made from room temperature up to 1300℃ for both alloys and the results indicated that additional boron gave rise to a significant improvement in the room temperature fracture strength and strain. The thin layer may contribute to improve the resistance of grain boundary cracking during the compressive test. Contrary to the beneficial effect at room temperature, the strength improvement by additional boron was not realized at high temperatures between 1000 and 1300℃. Moreover, lower plastic strain at the maximum stress was obtained from the boron-doped alloys tested below 1200℃, but higher plastic strain was seen above 1200℃, which might be the result of the additional boron promoting dynamic recrystallization above 1200℃.
The addition of zirconium caused a formation of a thin layer along grain boundary for the Ni-31.7Al(at%) with the two phases of NiAl and $Ni_3Al$ as in the case of the boron addition. The thin layer was composed of two phases which were divided into outside and center region phases. These phases were identified with $Ni_3Al$($L1_2$ phase) in outside region and a zirconium segregated compound ($Ni_{14}Al_3Zr_3$, ordered fcc of 6.9Å lattice parameter) in the center region. Compressive fracture strength and strain at room temperature was increased with zirconium addition owing to the enhanced grain boundary cracking resistance by the thin layer. At high temperature, the addition of zirconium promoted the grain refinement by dynamic recrystallization and the zirconium-doped alloy showed the highest strength and plastic strain.
To analyze the mechanism for the formation of the $Ni_3Al$ films on the prior NiAl-β interfaces, direct quenching during a directional solidification of the alloy was made and the microstructures of the alloy were examined in the wide temperature range from liquid state to room temperature. The quenched solid/liquid interface showed that solidification ended up with γ phase at the β interdendritic region in the boron added alloy, while the γ' phase formed at the interdendritic region in the binary alloy. It was thought that the γ' phase formed around the β dendritic phase grew between γ and β phases by a diffusion couple manner in the boron added alloy. It was also interesting to find out that boron addition changed the phase field and solidification behavior.
The effect of boron addition to the (NiAl+$Ni_3Al$) alloys on the wear and oxidation resistance of the nickel aluminides was investigated. Wear tests of various heat-treated specimens at room temperature and at 500℃ were performed under no lubricant condition in air by using a ball-on-disk type tribotester. Isothermal oxidation tests were made at 1100℃ in air flowing at the rate of 70cc/min. and at 1000℃ in air by using TGA. Experimental results from wear tests showed that boron-added nickel aluminide had remarkably improved wear resistance at both temperature conditions. Especially, the minimum weight loss and the minimum wear volume were measured in solutionized and aged boron-added specimen from the wear test at 500℃. On the contrary, the addition of boron to the nickel aluminides has affected the oxidation resistance of the aluminides adversely. Ni-32Al-0.5B aluminide showed poorer oxidation resistance than Ni-32Al aluminide during the isothermal oxidation test. Although both types of aluminides revealed a parabolic oxidation behavior and the formation of the $Al_2O_3$ on the surface, the differences in the oxidation resistance between the aluminides with and without boron addition seemed to be attributed to the microstructural difference between the aluminides. An accelerated oxidation along the thin layer of $Ni_3Al$ along the grain boundary observed in the microstructure of the Ni-32Al-0.5B aluminide could be attributed to the poor oxidation resistance, compared with the Ni-32Al showing no grain boundary $Ni_3Al$ phase.
