GROWTH OF STABLE AL-PD-MN ICOSAHEDRAL PHASE

We have carried out a detailed investigation of the Al71Pd19Mn10 alloy by using a combination of experimental techniques (neutron diffraction, electron diffraction, high resolution electron microscopy, and electron probe microanalysis). This alloy contains only the perfect icosahedral phase. This icosahedral phase is stable from room temperature up to its melting, though a transient cubic metastable phase coexists with it during the heating. It grows directly from liquid without involving any crystalline phase. Its isothermal growths at different temperatures can well be described by the Avrami equation with exponent n = 1.85. Comparisons with the Al-Cu-Fe I phase have also been made. The growth velocity of both I phases is approximately 1-mu-m/min with DELTA-T around 10 K, much slower than that of the crystalline phases growing under the same condition. After the sample was maintained at near liquidus temperature for more than 10 h, element losses and Si contamination from the crucible changed the sample composition. In consequence, crystalline phases have formed during the subsequent cooling and made the growth of the icosahedral phase a eutectic reaction.