Solid-state phase equilibria in the Co–Pt–Ho ternary system at 1173 K

The solid-state phase equilibria in the Co–Pt–Ho ternary system at 1173 K(Ho ≤ 75%) were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and energy dispersion spectroscopy (EDS) techniques. It is found that the 1173 K isothermal section consists of 14 single-phase regions, 24 two-phase regions, and 12 three-phase regions. At 1173 K, it is observed that the maximum solid solubility of Pt in Co17Ho2 is below 1 at.% Pt, whereas in Co7Ho2, Co3Ho and Co2Ho it is about 1.5, 1 and 1.5 at.% Pt, respectively. The maximum solubility of Ho in α-(Co, Pt) is below 1.5 at.% Ho, and for Co it is below 1 at.% Co in HoPt5, HoPt3, Ho3Pt4, HoPt, Ho5Pt4, Ho5Pt3, Ho2Pt and Ho3Pt with uniquely being about 18.5 at.% Co in HoPt2. Co5Ho was not found at 1173 K, and no new ternary compounds were observed.

► The 1173 K isothermal section of the phase diagram of the Co–Pt–Ho ternary system (Ho ≤ 75%) is determined.
► The maximum linear solid solubility of Co in HoPt2 is found to be 18.5 at.%.
► The maximum solubility of Ho in α-(Co, Pt) is below 1.5 at.% Ho.
► The maximum solid solubility of Pt in HoCo2 is about 1.5 at.% Pt.
► There exists an invariant section of the four-phase region in the Co–Pt–Ho ternary system at about 1183 K.