Study of Ni3Al- and MoSi2-based Materials for High Temperature Structural Applications

Licentiate thesis, 2007

In general, the potential of current conventional high-temperature materials has been exploited to a large extent. The industries are therefore looking forward to development of alternative materials for their industrials application, which would be lighter, stronger and having improved higher temperature potential compared to the presently available alloys. Among the various high temperature materials for the future, intermetallics-base alloys are prime candidates for alternative materials. In this thesis work, Ni3Al- and MoSi2-based materials were specially investigated for their industrial. On Ni3Al-based materials Attractive engineering properties of Ni3Al alloys is their increasing yield strength with increasing temperature up to about 650-750 ºC. This type of strength behaviour suggested that the Ni3Al-based intermetallic alloys may have good wear properties in the peak-strength temperature range. Consequently, investigations of their sliding friction and wear were initiated. In fact, a number of laboratory studies have indicated that Ni3Al-based alloys have significant potential in wear-critical applications, especially in cavitations erosion and in sliding wear at the temperature range between 400 ºC and 650 ºC. In this work, an existed Ni3Al-based alloy 77.7Ni-9.8Al-11.5Fe-0.5Mn-0.5Ti-0.1B in weight percentages and its composite reinforced by 6 vol. % Cr3C2-particles were studied and related to their microstructures. Hot isostatic pressing and casting were applied for preparing the materials. Wear rate of the test materials under different loads and their scuffing resistance were measured by Pin-on-Disk method. SEM, EDS, XRD, and EBSD techniques were used to observe the microstructures and identify the phase constitutions of the tested materials. The tribological performance related to the analytical results revealed that the single phase Ni3Al-based alloy showed a better wear-resistance compared to duplex microstructure. And the investigation also indicated that the hard Cr-carbide played a positive role on tribological intermetallics reducing the wear-rate. On MoSi2-based materials MoSi2 is presently considered as one of the most promising due to its high melting point (2030ºC) and excellent elevated-temperature oxidation resistance. However, like most of other intermetallics, the major problem, which has impeded their use, as high temperature materials, is their poor ductility and toughness during brittle-to-ductile transition temperature, BDTT, and low strength level above at temperature. In other words, their low temperature strength is limited by brittle fracture while the high temperature strength is governed by plastic flow. From the earlier investigations it was clear that the addition of alloying elements could not make the monolithic MoSi2 compound having the right combination of ambient temperature toughness and high temperature strengthen. For the most demand depends on the development of engineered composite materials. In this work, the attention was mostly focused on ZrO2-particles reinforced MoSi2-matrix composites. The working results indicated (1) a PLS process could be a practical and economical method for producing MoSi2-ZrO2 composites; (2) ZrO2-particles of less than 1 μm usually generated the better sintered density, RT-hardness and toughness than of the composites compared with the bigger particles; (3) the composites containing 15 – 25 vol.% unstable zironia (USZ) showed a better toughening effect, compared to the composites having less or more particles; and (4) A deteriorated oxidation resistance of MoSi2-ZrO2 composite compared to its monolithic counterpart is due to the formation of the porous oxide layer of ZrSiO4+SiO2 mixture and a retarded Si diffusion by the inner oxidation and reaction between SiO2 and ZrSiO4. Therefore, an alloying addition for further forming a protective oxide layer is necessary on developing this type of composites. Keywords: Intermetallics, Ni3Al, MoSi2, High temperature materials, Wear-resistant materials, Composites, Toughening.