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Deformation Response of Mo (Si,B) Solid Solution and Multiphase Mo-Si-B Alloys

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Abstract:
The increasing demand for materials that can perform at high temperatures beyond the capabilities of Ni-base superalloys has generated interest in refractory metal-based systems (Mo-based and Nb-based). Within the family of Mo alloys, three-phase alloys at the Mo-rich end of the Mo-Si-B system containing Mo5SiB2 (T2) and Mo3Si intermetallic phases in a Mo (Si,B) solid solution matrix have drawn greater attention. Previous studies on multiphase alloys have confirmed the central role of the Mo-rich solid solution phase in affecting creep resistance and low temperature toughness in these alloys. The main focus of the thesis is to understand the deformation and fracture response of a Mo (Si, B) solid solution phase, in isolation, in order to tailor and enhance the requisite properties of the multiphase alloys. Uniaxial compression and tension tests were conducted on a solid solution alloy containing a very low fraction (< 5 vol.%) of the T2 phase over a strain rate regime spanning 10-4 - 10-7 s-1 in vacuum; the compression tests were performed between 1000ºC and 1200ºC whereas the temperature range for tension tests was between 400ºC and 1200ºC. The manifestations of dynamic strain aging effect were observed in two different temperature and strain rate regimes in the solid solution alloy. Uniaxial tests were conducted on commercial purity Mo for comparison and to understand the role of B and Si on DSA in the solid solution alloy. Constant load tensile tests were performed on the solid solution alloy, a two-phase alloy containing ~35 vol.% of the T2 phase, and a three-phase alloy with ~50 vol.% of T2 + Mo3Si phases between 1000ºC and 1300ºC at different loads to evaluate the creep behavior of these alloys. The resulting fracture surfaces and the microstructure of the tested specimens were characterized using various techniques. The results from this study confirmed that Si in solid solution significantly enhances both the yield strength and the creep resistance of these materials. The extent of Si dissolved in the solid solution phase varies in these three alloys and Si appears to segregate to dislocations and grain boundaries. The obtained stress exponents for the two multiphase alloys suggest dislocation climb to be the controlling mechanism. Grain boundary precipitation of the T2 phase during creep deformation is observed and the precipitation kinetics appears to be affected by the test temperature and applied stress. A preliminary study was conducted on Mo-Nb-Si-B quaternary alloys also to evaluate and understand the effect of Nb addition on the microstructure and deformation behavior of Mo-Si-B alloys.
Notes:
Thesis (Ph.D.) -- Brown University (2010)

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Citation

Jain, Padam, "Deformation Response of Mo (Si,B) Solid Solution and Multiphase Mo-Si-B Alloys" (2009). Engineering Theses and Dissertations, Materials Science Engineering Theses and Dissertations. Brown Digital Repository. Brown University Library. https://doi.org/10.7301/Z0QR4VCZ

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