Transition from nucleation controlled to growth controlled crystallizationin Pd43Ni10Cu27P20 melts

Crystallization of undercooled Pd43Ni10Cu27P20 melts is studied in a differ ential scanning calorimeter. Isothermal experiments allow us for the first time to determine the entire crystallization kinetics of a metallic liquid as a function of time from the liquidus temperature to the glass transition temperature. The results are summarized in a time-temperature-transformati on (TTT) diagram that reveals two time scales. One is given by the time to reach 1% of crystallized volume fraction and reflects the typical "nose" sh ape of the TTT-diagram. The other is the width of the crystallization event itself, which increases with decreasing temperature from 90 s at 793 K to 10,200 s at 623 K. Additional information about the crystallization process is gained by dividing the sample into about 300 particles that are process ed simultaneously and crystallization of each individual particle can be de tected. At high temperatures the onset of crystallization of individual par ticles are spread out over 1.5x10(5) s, whereas all particles crystallize s imultaneously below the nose and the crystallization is not distinguishable from that of one large sample. The results suggest that the dominant cryst allization mechanism changes in a very narrow temperature range from a nucl eation-controlled process at high temperatures to a growth-controlled proce ss at low temperatures. (C) 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.