Structural Silicides - Processing, Microstructure and Toughening
Molybdenum disilicide is a promising candidate material for high temperature structural uses. However, its room-temperature toughness is rather low and has to be improved. The objective of this thesis is to improve the fracture toughness of MoSi2 by alloying and composite processes.
The feasibility of toughening of MoSi2 by crystal structure modification was studied. MoSi2-based alloys containing the transition metals Cr, Fe, Co, Ni, V, Ti and Nb were prepared by arc melting and characterised by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS). It is found that the alloying elements Cr, V, Ti and Nb exhibit a limited solubility in the C11b structure of MoSi2, while Fe, Co, Ni show no detectable solubility. Alloying with the above elements leads to precipitation of second phases. In a Mo-Cr-Si alloy, for instance, the precipitated phase is the (Cr,Mo)Si2 with a C40 structure, and it has a certain orientation relationship with the C11b matrix. In contrast, the solubility of some alloying elements in Mo5Si3 is relatively high, and this offers an opportunity for crystalline modification. In fact, it was found that the Fe substitution for Mo in Mo5Si3 leads to a D8m-to-D88 structure change. This study shows that the toughening of MoSi2 can not be simply achieved by crystal structure modification, while Mo5Si3 has a potential of structure modification.
Toughening of MoSi2 by composite processing was more successful. MoSi2-ZrO2 composites were produced by a powder metallurgy (PM) process, using powders of mechanical alloyed MoSi2, unstabilised zirconia (UZ) and ytteria partially stabilised zirconia (YPSZ). Zirconia phases present in UZ- and YPSZ-composites were identified to be m-ZrO2 and t-ZrO2, respectively. MoSi2-SiC in-situ composite was prepared by hot pressing of MoSi2 and C powders. The SiC phase is formed through solid reaction of MoSi2 and C during fabrication. The fracture toughness of MoSi2-ZrO2(UZ) and MoSi2-SiC composites was found to be 2.4 and 1.8 times higher than that of pure MoSi2, respectively. Mechanisms responsible for the toughening have been suggested and discussed.
The fracture behaviour of MoSi2 base materials was also investigated by SEM, electron spectroscopy for chemical analysis (ESCA), and Auger electron spectroscopy (AES). The dominating fracture mode is transgranular for pure MoSi2 and MoSi2+0.2%B, a mixed mode of transgranular and intergranular for YPSZ-composite, and intergranular for UZ-composite. AES and ESCA analyses show that the grain boundaries of these materials are covered with a SiO2 film. In pure MoSi2, the thickness of the SiO2 film is estimated to be 10-20 nm. The influence of the SiO2 phase on the fracture behaviour of the materials was discussed.
As a part of this work, reaction synthesis of MoSi2 and other refractory disilicides was studied. Nearly full-density disilicides MoSi2, NbSi2 and TaSi2 were produced by reactive isostatic hot pressing (HIPing) from the compacts of elemental powders. Main phases present in the HIPed materials were identified to be disilicides, but small amount of 5:3 silicides were also detected by EDS. The fracture toughness and fracture modes of the HIPed materials were also determined. This study shows that the reaction HIPing is an effective process to produce high-density refractory metal disilicides directly from pure components.