Experimental Investigation on Isothermal Sections at 1273 and 1473 K in the Co-Ti-W System

The microstructures of 25 annealed alloys and XRD patterns of partial critical alloys in the Co-Ti-W system were investigated by using scanning electron microscopy (SEM) with energy dispersive spectrometer (EDS) and X-ray diffraction (XRD) methods. The isothermal sections at 1273 and 1473 K of the Co-Ti-W system were established. Five three-phase regions and five two-phase regions at 1273 K and eight three-phase regions and two two-phase regions at 1473 K were experimentally determined. The maximum solubilities of W in Co3Ti, alpha Co2Ti, beta Co2Ti and CoTi were determined to be similar to 7.9 at%, similar to 1.5 at.%, similar to 5.9 at.% and similar to 1.8 at.% at 1273 K, respectively. The maximum solubilities of Ti in Co7W6 and Co3W were determined to be similar to 11.9 at.% and similar to 15.2 at.% at 1273 K, respectively. The compound Co3Ti with L1(2) crystal structure was found to be stable at 1473 K in the Co-Ti-W system owing to the addition of W element, which confirmed that W can improve the stability of gamma ' with L1(2) crystal structure in the Co-based superalloys. The composition range of W in Co3Ti was measured to be similar to 7.4-10.4 at.% at 1473 K. In addition, the maximum solubilities of Ti in Co7W6 and W in beta Co2Ti and CoTi were similar to 15.4 at.%, similar to 7.6 at.% and similar to 3.1 at.% at 1473 K, respectively. No ternary compounds were found in the Co-Ti-W system at 1273 and 1473 K.