Martensitic transformation in nanostructured Fe-Ni alloys

In Fe-Ni thin films prepared by magnetron sputtering at room temperature, the bcc structure can exist stably in a larger range of Ni-content than that in Fe-Ni equilibrium diagram. Furthermore, the experiment verified that the bcc structure forms directly from collision of atoms during the sputtering process, rather than the product of fcc-->bcc martensitic transformation. The starting temperature for bcc-->fcc transformation in thin films is near to that of bulk Fe-Ni alloys. Theoretical calculations show that the nucleation barrier of martensitic transformation and the critical size of the martensitic embryo increase with decreasing grain size within the nanometer scale, implying that the martensitic transformation in nano-sized grains would be suppressed. Moreover, the autocatalytic tendency of martensitic transformation is significantly weakened within a transformed grain or within its neighboring grain due to the reduction of probability of martensite nucleation or the drop of the stress field at grain boundary produced by martensitic transformation.