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 Co 3 Ti, αCo 2 Ti, βCo 2 Ti and CoTi were determined to be ∼7.9 at%, ∼1.5 at.%, ∼5.9 at.% and ∼1.8 at.% at 1273 K, respectively. The maximum solubilities of Ti in Co 7 W 6 and Co 3 W were determined to be ∼11.9 at.% and ∼15.2 at.% at 1273 K, respectively. The compound Co 3 Ti 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 γ′ with L1 2 crystal structure in the Co-based superalloys. The composition range of W in Co 3 Ti was measured to be ∼7.4–10.4 at.% at 1473 K. In addition, the maximum solubilities of Ti in Co 7 W 6 and W in βCo 2 Ti and CoTi were ∼15.4 at.%, ∼7.6 at.% and ∼3.1 at.% at 1473 K, respectively. No ternary compounds were found in the Co–Ti–W system at 1273 and 1473 K.