Transition from nucleation controlled to growth controlled crystallization in Pd43Ni10Cu27P20 melts

Crystallization of undercooled Pd43Ni10Cu27P20 melts is studied in a differential 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-transformation (TTT) diagram that reveals two time scales. One is given by the time to reach 1 percent of crystallized volume fraction and reflects the typical 'nose' shape 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 processed simultaneously, and the crystallization of each individual particle can be detected. At high temperatures the onset of crystallization of individual particles are spread out over 1.5 x 10 exp 5 s, whereas all particles crystallize simultaneously below the nose, and the crystallization is not distinguishable from that of one large sample. The results suggest that the dominant crystallization mechanism changes in a very narrow temperature range from a nucleation-controlled process at high temperatures to a growth-controlled process at low temperatures. (Author)