Processing of Nb-Si alloys using mixed elemental and pre-alloyed powders

Nb-silicide based alloys with the correct combination of alloying additions and processing methods could have applications in the ultra-high temperature environments of the aerospace industry. This family of alloys have high melting temperatures and impressive high temperature properties, and with improved oxidation resistance and room temperature toughness their successful introduction could reduce the inefficiency in jet turbine engines which is caused by the current need to cool Ni-based superalloys. This thesis looked at processing conditions for the reactive liquid phase sintering of a binary alloy (EG2) from mixed elemental powders (MEP), the reactive sintering from elemental powders and sintering using pre-alloyed powders (PA) for a quaternary alloy (EG1), and the sintering from pre-alloyed powder of a septenary alloy (EG3) all using spark plasma sintering (SPS). The pre-alloyed powders were made by comminuting an arc melted 600 g alloy with a microstructure consisting of Nbss and βNb5Si3 phases. The MEP powders were prepared by mixing elemental powders for 90 minutes in a 3D powder mixer. Densification in the elemental powder compacts occurred due to solid state and liquid phase reactive sintering as Nb and Si formed the αNb5Si3 indirectly and directly. The microstructure consisted of this continuous Nb silicide and Nb grains. There was evidence of coarsening after a longer dwell time or higher sintering temperature, and this also resulted in formation of the metastable Nb3Si and Nb carbides at the compact edges close to the graphite foil used in SPS. Finer powder size in the pre-alloyed sintered powders was shown to improve densification and sintering was also driven by the chemical gradients between the different powders. The microstructure of the as cast alloys coarsened in the pre-alloyed powder compacts made from EG3, and in a heat-treated sample and in cylinders cut from the EG3 alloy which were then SPS processed (TM= thermomechanical samples). In all PA, TM, and heat-treated samples the Nb3Sn formed and β Nb5Si3 transformed to the α Nb5Si3 and Nbss precipitates formed in some Nb5Si3, forming a sub grain structure.