Phase stability and its intrinsic conditions in nanocrystalline materials

The critical grain (particle) sizes of high-temperature phases being stable at room temperature for Co, Fe80Ni20 and ZrO2, which have martensitic transformations in their conventional bulk, coarse-grained samples, are calculated respectively as 35, 35 and 13 nm by our universal theory. However, our experiments on the as-prepared state of nanosized materials seem to be in conflict with our prediction. Based on the concept of phase stability, intrinsic conditions of determining the stability of high-temperature phases were proposed, namely, the high-temperature phase must first form during preparation, then, during cooling, the grain (particle) size of the high-temperature phase must be smaller than its critical grain size. The intrinsic conditions of high-temperature phase stability are verified by experiments on Co nanogranular film sputtered at 773 K, Fe80Ni20 nanocrystalline powders annealed at 953 K, and nanosized ZrO2 amorphous powders annealed at 1573 K. The critical grain size (14 nm) of the high-temperature phase determined by experiments of ZrO2 is in good agreement with the theoretical value (13 nm) since ZrO2 does not undergo oxidation during annealing. The origin of low-temperature phase formation in the as-prepared state of nanosized materials is also discussed.