KINETICS OF THE AMORPHOUS TO ICOSAHEDRAL PHASE-TRANSFORMATION IN AL-CU-V ALLOYS

Single phase icosahedral samples are obtained by annealing melt-spun Al75Cu15V10 amorphous alloys. The kinetcs of this amorphous to icosahedral phase transformation were measured isothermally and nonisothermally by differential scanning calorimetry and from changes in the electrical resistivity. Transmission electron microscopy and energy dispersive X-ray analysis indicate that the transformation proceeds polymorphically by nucleation and growth, ruling out a "micro-quasicrystal" model of the glass in this system. A standard Johnson-Mehl-Avrami analysis of the isothermal, transformation data yields Avrami exponents in the range 2-2.5, which are inconsistent with a polymorphic transformation. These anomalous Avrami exponent arise from an inhomogeneous distribution of quenched-in nuclei. Fits are made to a kinetic model assuming a constant nucleation rate and growth on these quenched-in nuclei. An analysis of the nucleation rates obtained from these fits gives an estimate for the interfacial energy between the icosahedral phase and the glass of 0.002 J/m2 less-than-or-equal-to alpha less-than-or-equal-to 0.015 J/m2, demonstrating that the short range order must be similar on both sides of the interface.