DOI : 10. 1051/jp4 : 2003893 Martensitic transformation in nanostructured Fe-Ni alloys Q. Meng, Y. Rong and T. Y. Hsu (Xu Zuyao) School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200030, China Abstract. In Fe-Ni thin films prepared by magnetron sputtering at room temperature, the bec 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 fccobcc martensitic transformation. The starting temperature for bec-foc 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. 1. INTRODUCTION During the past two decades the structure and phase transformations in alloys with nano-sized grains have been investigated because of the progress and the request of some technologies. Kajiwara et al [l] indicated that the martensitic transformation (MT) was obviously suppressed in Fe-Ni and Fe-Co alloys with various compositions and grain sizes (20~200nu), being analogous to the Cech and Turnbull's results[2] on Fe-Ni alloys with small grains(25-100(im). Tadaki et a![3, 4] showed that the reverse temperature of MT in Fe-Ni with nano-sized grains was almost the same as that in alloys with normal coarse grains. The aim of the present work is to characterize experimentally the structure and MT of nano-sized particles in Fe-Ni films and explain theoretically the relative phenomena. 2. EXPERIMENTAL PROCEDURES A series of Fe,-xNix (0 : 5 x<60 at %) granular films on the substrate both at room temperature and heated up to 773K were prepared using a SPC350 multitarget magnetron sputtering system, which can mix two or over kinds of elements into a solid solution[5], specially for elements with the negative heat of mixing, such as Fe and Ni elements. The granular films were deposited on KCI substrates by alternative sputtering with pure Fe (99. 9%) and Ni (99. 9%) targets for near 7 minutes with 20 circles per minute under gas (Ar) pressure 4 xl10-1 Pa. The film thickness is about 50nm. The resultant compositions were determined using EDAX in S-520 scanning electron microscope. The specimens for transmission electron microscope (TEM) were obtained by putting KCI substrates into water with a little acetone and using copper grids to support the separated Fe-Ni films. The TEM observations of particles in as-sputtered films were