Complex Avrami kinetics of TiB2 transformation into TiB whiskers during sintering of Ti-TiB2 nanocomposites

The kinetics of transformation of 5 w% TiB2 nano-particles (50 nm diameter) dispersed and pressed (1.5 GPa) in the of Ti matrix (150 µm diameter) into TiB nano-whiskers was studied as a function of temperature (800–900–1000 °C) and time (5–720 min) in 25 experiments in total, by vacuum sintering. The classical one-step Avrami kinetics does not properly describe this transformation due to the formation of oxide inhibitors from contaminants. However, the same results could be perfectly described by a complex Avrami kinetics supposing two parallel steps of transformations: i). fast transformation along the initially metallic Ti/TiB2 interface and ii). delayed transformation along contaminated Ti/TiB2 interface. Data for kinetic calculation was obtained by X-ray diffraction and Rietveld refinement on the sintered samples. Extrapolating the experimental results by the complex Avrami model beyond the measured temperature interval it was found that at and above about 1050 °C, the second step is missing due to the fast dissolution of oxide films in the beta-Ti matrix, leading to very fast kinetics.