Martensite and its reverse transformation in nanocrystalline bulk Co

A nanostructured surface layer of Co with the thickness of about 20 μm, considered as a bulk sample, was prepared by means of surface mechanical attrition treatment (SMAT). The average grain sizes of the samples prepared by 30 and 60 min SMAT are determined as 26 and 23 nm, respectively, by X-ray diffraction, and confirmed by transmission electron microscopy. Differential scanning calorimetry analysis for the above samples and a coarse-grained sample reveals that start temperature As of the α (hcp) → β (fcc) reverse martensitic transformation can be described as: T A S = 456–293/d (in °C, 15 nm ≤ d ≤ 100 nm, d is grain size). The nanocrystalline high-temperature β (fcc) phase with grain size smaller than about 35 nm obtained by heating SMAT samples for proper duration exhibits thermal stability during cooling from 500 °C to ambient temperature even at −196 °C. However, these thermally stable nanocrystalline β (fcc) phase samples can undergo the β (fcc) → α (hcp) martensitic transformation when treated by SMAT again. Thermal stability of the nanocrystalline low-temperature phase α (hcp) was observed in SMAT Co, that is, when the grain sizes are smaller than 15 nm, the reverse transformation will not occur until to 815 °C.