DISCONTINUOUS PRECIPITATION IN A NI-IN ALLOY STUDIED BY ANALYTICAL FIELD-ION MICROSCOPY

Discontinuous precipitation (DP) in a Ni-7.5 at% In alloy has been stu died by means of atom probe field ion microscopy (APFIM). The morpholo gy of the lamellar microstructure and the growth kinetics of the decom position reaction have been investigated by optical and scanning elect ron microscopy (LM, SEM). A back polishing method has been developed t o prepare APFIM specimens in a systematic manner for transmission elec tron microscopy (TEM), in order to obtain a definite inclination of di fferent interfaces in the tip apex for subsequent analysis of their ch emical composition. APFIM results of the Ni- and In-concentration acro ss the interfaces are presented. Based on the APFIM results it is poss ible to distinguish between the theoretical description of the concent ration profiles given by the models of Cahn and Hillert. It is shown b y both FIM images and concentration depth profiles that the alpha/beta -interface has a ''thickness'' of about 0.5 nm. In addition it is show n for the first time that the concentration across a beta-lamella is n ot constant. This experimental finding was used to develop a generalis ation of Cahn's model on discontinuous precipitation including the sol ute flux within the interface between beta and the parent phase alpha( 0). (C) 1997 Acta Metallurgica Inc.