Experimental determination of phase equilibria in the Y–Co–Ti system through diffusion couples and equilibrium alloys

Abstract

Two isothermal sections of the Y–Co–Ti system at 600 °C and 800 °C were constructed for the first time using the diffusion couple technique and the equilibrium alloy method in combination with scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), electron probe microanalysis (EPMA), and X-ray diffraction (XRD). The stable ternary intermetallic compound YCo12-xTix was detected and was confirmed to have a ThMn12-type structure. The composition range in this ternary compound was measured to be 8.3–18.2 at.% at 600 °C and 8.9–19.1 at.% at 800 °C, resulting in the stable formation of YCo12-xTix with x = 1.1–2.4 at 600 °C and x = 1.2–2.5 at 800 °C. The experimental results measured by EDS and EPMA demonstrate that the maximum solubilities of Ti in YCo2, YCo3, Y2Co7 and Y2Co17 compounds at 600 °C are 3.3, 5.6, 5.7 and 6.6 at.%, respectively, while the maximum solubilities of Y in Co3Ti, Co2Ti(h), Co2Ti(c) and CoTi compounds are 2.7, 2.1, 2.6, 3.8 and 1.1 at.%. Meanwhile, the maximum solubilities of Ti in YCo3, Y2Co7, YCo5 and Y2Co17 compounds at 800 °C were determined to be 5.4, 3.2, 2.5 and 5.4 at.%, respectively, while the maximum solubilities of Y in Co2Ti(c), Co2Ti(h) and Co3Ti compounds were measured to be 2.5, 2.1 and 3.8 at.%. The phase equilibria of the Y–Co–Ti system obtained in this work would provide the experimental information for phase stability of YCo12-xTix compound and then explore the design of Y–Co–Ti based magnetic alloys with good magnetic properties.