Effect of tungsten concentration on microstructures of Co-10Ni-6Al-(0,2,4,6) W-6Ti (at%) cobalt-based superalloys

The effects of reducing the W concentration (and concomitantly mass density) of Co-10Ni-6A1-xW-6Ti at% Co-based superalloys, with a gamma(f.c.c.) plus gamma'(L1(2)) microstructure, is investigated for a series of alloys with W concentrations of 6 at% (baseline alloy), 4 and 2 at% (W-reduced) and 0 at% (W-free). The gamma'(L1(2)) solvus temperature decreases strongly (by 46 degrees C per 1 at% reduction in W), while the liquidus and solvus temperatures decrease mildly (3-5 degrees C per 1 at% W reduction) as measured by differential scanning calorimetry. Scanning electron microscopy was used to image phase formation in all alloys. When aging at 900 or 1000 degrees C for 256 h, the W reduction does not result in the appearance of additional phases within the grains beyond the gamma(f.c.c.) and gamma'(L1(2)) phases present in the original alloy. Grain-boundary precipitates are present for all alloy compositions: W-rich or W-free precipitates after aging at 1000 degrees C, and coarsened gamma'(L1(2)) precipitates after aging at 900 degrees C. The composition of grain boundary precipitates were measured with energy-dispersive X-ray spectroscopy. Vickers microhardness values decrease with decreasing W content, due to a reduction in gamma'(L1(2)) precipitation and 'reduced solid-solution strengthening.