Phase equilibria and phase transformation of the body-centered cubic phase in the Cu-rich portion of the Cu-Ti-Al system

The phase equilibria and phase transformation of the body-centered cubic (bcc) phase in the Cu-Ti-Al system were investigated by the diffusion couple method, metallographic examination, differential scanning calorimetry, and x-ray diffraction. The isothermal sections at 700 and 900 °C and vertical sections at 18 at.% Al, 22 at.% Al, and 25 at.% Al in the Cu-rich portion were determined. These results indicate that (i) the Cu₂TiAl compound with the L2₁ Heusler structure has a larger solubility range; (ii) the stable B2 + L2₁ miscibility gap of the ordered bcc phase exists until the liquid phase, and the tie lines of this miscibility gap are almost parallel with the Cu-Ti side; (iii) the composition and temperature for the eutectic reaction (L ↔ B2 + L2₁) are about 7 at.% Ti and about 970 °C, respectively, and (iv) the velocity of the eutectoid decomposition [bcc ↔ face-centered cubic (fcc) + D8₃] of the bcc phase with martensitic morphology in the Cu-Ti-Al alloys is slower than that of the Cu-Al alloys.