MOSSBAUER INVESTIGATION OF INTERMIXING DURING BALL-MILLING OF FE0.3CR0.7 AND FE0.5W0.5 POWDER MIXTURES

The intermixing of Fe and T (T=Cr,W) during ball milling of elemental powder mixtures Fe1-xTx, with x=0.70 for T=Cr and x=0.50 for T=W, has been followed by Fe-57 Mossbauer spectroscopy at room temperature (RT) and by magnetization measurements for T=W. The chemical compositions have been chosen to yield final alloys or compounds which are nonmagne tic at RT to better follow the evolution of magnetic phases with milli ng times. For a long period of milling time t(m) before reaching the f inal stationary state, the hyperfine magnetic field distributions rema in stationary in shape for both T=Cr and T=W. Only the relative weight of the magnetic contribution decreases with t(m). For T=W, the averag e moment of magnetic Fe atoms is further shown to remain constant with t(m). Stationary hyperfine field distribution shapes are found to be similar not only for T=Cr and W but also for T=Si (x=0.50) while publi shed spectra suggest to add T=Al, Ti, V, Ta, Re to the latter nonexhau stive list. The stationary shape is characterized by a narrow peak loc ated at a field close to the field of alpha iron at RT (330 kG) and by a broad, almost featureless, band from similar to 50-100 kG to simila r to 300-320 kG. The broad band represents about 2/3 of the normalized field distribution. We deduce that the interpretation which consists in attributing the x-ray diffraction peaks of Fe-based bcc solid solut ions to a single Fe-rich homogeneous solid solution must be done with care for intermediate milling times. We cannot infer from such hyperfi ne measurements a detailed description of the regions of the powders w hich are responsible for such magnetic features. We argue however that irregular interfaces between nanometer-sized Fe-rich zones and T-rich zones may play a role to explain the observed shape of the hyperfine field distributions. The general conditions (process and materials) in which such phenomena may occur remain to be clarified.