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hnuma, R. Kainuma, and K. Ishida Department of Materials Science, Graduate School of Engineering, Tohoku University, Sendai, 980-8579, Japan (Received 4 January 2008; accepted 24 June 2008)

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.

I. INTRODUCTION

Cu–Ti binary and Cu–Ti–Al ternary alloys are known to exhibit microstructures with modulation structures and remarkable potential for precipitation hardening. Considerable research has been contributed to the mechanical properties, microstructure, and phase equilibria of the Cu–Ti–Al alloys.1–8 Panseri and Leoni first studied the phase equilibria on the Cu–Ti–Al system using thermal analysis.1 Following this, Virdis and Zwicker determined some isothermal sections and the existence range of three intermetallic phases (Cu2TiAl, CuTiAl, and CuTi2Al5) on the basis of metallography and x-ray diffraction (XRD).4 Although the crystal structures of these compounds have been studied, different conclusions for the crystal structure of the Cu2TiAl phase have been reported,9–13 as follows: some studies reported that the ternary compound Cu2TiAl has the B2 structure,9–12 while Raman and Schubert13 pointed out that the crystal structure of the Cu2TiAl compound is the same as that of the Cu2MnAl phase with the L21 Heusler type and sug-

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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0334 2674

J. Mater. Res., Vol. 23, No. 10, Oct 2008

gested that incomplete ordering is the reason for reporting B2 structure in earlier works.9–12 The investigations on the phase equilibria in the Cu– Ti–Al system were reviewed by Ran and Stadelmaier.14 The crystal structures of the phases and compounds in the Cu–Ti–Al system are summarized in Table I.14–18 Figure 1 shows the phase equilibria at 800 °C determined by Virdis and Zwicker,4 where the Cu2TiAl phase is in equilibrium with each phase in the Cu-rich portion. Although the phase equilibria in the Cu–Ti–Al system were basically determined, many uncertainties still remain. In particular, no studies on the phase separation and phase transformation of