Doctor of Philosophy
School of Mechanical, Materials & Mechatronic Engineering - Faculty of Engineering
Ahmadian-Najafabadi, Mehdi, Sintering, microstructure and properties of WC-FeAl-B and WC-Ni3Al-B composite materials, PhD thesis, School of Mechanical, Materials and Mechatronic Engineering, University of Wollongong, 2005. http://ro.uow.edu.au/theses/546
The effect of boron on microstructure and mechanical properties of the intermetallic matrix composites (WC-FeAl-B and WC-Ni3Al-B) were investigated. The results were compared with those obtained from WC-Co composite which had same binder volume fraction and fabricated under identical sintering conditions. Boron doped FeAl (FeooA14o) and NhAl alloys were selected as potential new alternative binders in place of cobalt for WC composites due to their particular combination of mechanical properties.
Sub micron WC-40vol%(FeAl-B), WC40vol%(NhAl-B) and WC-4Ovol%Co composites were synthesized by powder processing followed by uniaxial hot-pressing at the optimized temperature of 1500 °C under a pressure of20 MFa in argon atmosphere. Doped aluminide binders, with boron levels ranging from 0 to O.l°W'fOIO, were prepared in ultrafine form using controlled atmosphere arc melting and then ring-grinding under argon atmosphere. Hardness and indentation fracture tests were performed on composite specimens and the microstructures were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Phase and compositional characterization were studied by XRD, TEM-EDS and electron diffraction. Abrasive wear performance was evaluated using a pin-on-drum apparatus and the worn surfaces studied by SEM.
The results showed that WC sufficiently wet by the boron doped aluminide binders which resulted in sintered composites with near full densities. WC is also relatively stable in WC-4Ovol%(.FeAI-B) and WC-40vol%(NhAl-B) systems up to 0.1moJO B with no new phases observed at sintering temperature of 1500 "C. The microstructures exhibited faceted WC grains with no significant grain growth, with continuous aluminide binders and similar morphology to WC-Co. Increasing the amount of boron in the aluminide binders up to 0.1 moJO resulted in more faceting in WC particles and increasing amount of dissolved W in the aluminide binders.
Furthermore, the WC contiguity (WC/WC contact) and grain sizes of aluminide binders decreased with increasing the amount ofboron in the aluminide binders. Hardness of WC-FeAl was found to be higher than that of WC-NiJAl but the we-co had lowest hardness. WC-FeAl showed the highest wear resistance and lowest fracture toughness compared to those of WC-NiJAl and WC-Co. Fracture toughness and wear resistance of WC-NbAl were found to be comparable to those for WC-Co. Boron addition up to 0.1wtO,Io in both WC-FeAl-B and WC-NhAl-B had no significant effect on the hardness but the boron addition showed significant enhancement in their fracture toughness (42% for WC-FeAl-B and 38% for WC-NhAl-B) and better wear resistance 24% for WC-FeAl-B and 14% for WC-NiJAl-B) which were found to be higher than those of WC-Co. Increasing boron content also resulted in decreasing of surface abrasion during abrasive wear testing by brittle processes such as pull out of particular phases and cracking of carbide particles. The improvement of the fracture toughness and wear resistance with addition of boron could be due to combination of factors including increasing WC solubility in the aluminide binders and reduction in WC contiguity as well as aluminide binder's grain sizes. This in turn could lead to improve the strength of WC/aluminides interfaces and wettability of WC with aluminides.
Finally, the boron doped FeAl and NhAl alloys is proposed as alternative binders to replace Co in conventional WC-Co composites and it is envisaged that WC-FeAI-B and WC-Ni3Al-B composites could be considered as potential candidates in some specific applications.