Development, processing and fabrication of a nickel based nickel-chromium-iron alloy

An optimal powder metallurgy (P/M) approach to produce a nickel base Superalloy similar in composition to INCONEL(TM) 600 was carried out utilising a simple uniaxial pressing process. The efficiencies of a lubricant addition, binder, sintering times and temperatures were measured in terms of green and sintered densities as well as microstructural changes that occurred during processing. It was observed that with increasing % polyvinyl alcohol (PVA), an overall decrease in density of compact was obtained and that using 0.75wt % of lubricant (microwax) green densities in excess of 70% can be obtained.;The samples were subsequently sintered in air at 1270°C for times ranging from 0.5h to 5h and also in vacuum (6 millitorr) with temperatures ranging from 1260 through to 1400°C. The air sintering was carried out to optimize sintering time, whereas the vacuum sintering was employed to optimize sintering temperature. On sintering for 5h in air, chromium enrichment occurred at the grain boundaries with subsequent depletion of nickel and iron; this was not noted for 2h sintering or for sintering under vacuum. The optimum sintering conditions were determined to be at 1300°C sintering for 2h in vacuum. The samples processed under the optimum conditions were successfully cold rolled to 40% of the original thickness without cracking.;An investigation was also undertaken to determine the effect of Al concentration (1-12w/o) on the microstructure of the powder metallurgically (P/M) processed Ni-Cr-Fe ternary alloy, with a view to determine the concentration of aluminium that would yield a homogenously distributed and optimum volume fraction of the intermetallic-gamma'(Ni3Al) phase without the formation of topologically closed packed phases in the ternary alloy.;The phases that were likely to form with the variation in concentration of Al were first simulated by JMatPro(TM) thermodynamic software package, and then Ni-Cr-Fe alloys with varying concentration of aluminum were produced by P/M processing. The microstructure of the alloys was characterized by X-ray diffractometer (XRD), optical microscope (OM) and scanning electron microscope (SEM) equipped with energy dispersive spectrometer (EDS) and then compared with the thermodynamic predictions. It was observed that the experimental results matched reasonably closely with the thermodynamic predictions in terms of the phases present when Al was added to the ternary alloy, but not the volume fraction of the phases present in the microstructure. It was also observed by SEM image and quantitative EDS analysis that the optimum amount of well distributed Ni3Al phase formed when the concentration of aluminium was 6w/o. These results suggest that despite potential problems encountered in high temperature powder processing of Superalloys that often tend to influence the feasibility of using thermodynamic predictions to model such alloy systems, the software and predictions used in this study offer a way to simulate both design and characterisation of the experimental alloy.;To characterize the phases that formed in the 6w/o Al modified ternary Ni-Cr-Fe alloy during heating to the sintering temperature, a differential scanning calorimetric study was carried out to study sequence of phase transformations, their reaction modes and products on heating a green compact from room temperature to the sintering temperature of 1300°C. Two different heating rates were employed for the DSC study, 2.5°C/min and 10°C/min. Transformation reactions were also studied by heating the samples in a DSC to the points of exo/endothermicity and quenching in argon followed by phase identification by X-ray diffraction, and microstructural analysis by SEM equipped with EDS capability. A series of AlxNiy and AlxFe y type intermetallics were observed to form by phase transformation at temperatures from 540°C to 1120°C.;The sequence of the formation of intermetallics by these phase transformations closely replicated the intermetallics that were predicted by the binary Ni-Al and the Fe-Al equilibrium phase diagrams, with the final microstructure being Ni3Al (gamma'), AlNi, AlFe in a Ni-Cr-Fe (gamma) matrix. Most of these phase transformations were diffusion controlled, but at 640°C AlNi formed by combustion synthesis. Selected mechanical property of the alloy at 1120°C which corresponds to the completion of transformations was estimated in terms of micro-hardness and compared to the as-sintered Ni-Cr-Fe ternary.