Thermal stability and recrystallization of nanocrystalline Ti produced by cryogenic milling

The grain growth, thermal stability, and recrystallization behavior of a cryomilled Ti alloy with a grain size of about 21.2 nm were examined using differential scanning calorimetry, X-ray diffraction, and transmission electron microscopy. Isochronal heat treatments at different temperatures were applied to study the thermal stability and recrystallization behavior of this alloy system. The average grain size increased from 20 to 80 nm in the temperature range of 200 °C to 350 °C, and then significantly decreased to 15 nm during annealing at 400 °C to 450 °C. This phenomenon was rationalized on the basis of a recrystallization mechanism. When the annealing temperature increased from 450 °C to 720 °C, the grain size increased slightly from 15.2 to 27.5 nm. In addition, the isothermal grain growth behavior in this alloy was investigated in the temperature range of 150 °C to 720 °C, and the resulting grain growth activation energy was analyzed to rationalize the underlying grain growth mechanisms. An interesting scientific question that arises from the present work is whether a decrease in grain size can be obtained in nanocrystalline (nc) materials via a recrystallization mechanism. The present results show that indeed a smaller grain size is obtained after annealing at elevated temperatures (500 °C to 720 °C) in cryomilled nc Ti, and the experimental results are explained on the basis of a recrystallization mechanism.