MICROSTRUCTURE AND NANOSCALE COMPOSITION ANALYSIS OF THE MECHANICAL ALLOYING OF FEXCU100-X (X=16, 60)

The microstructures of Fe16Cu84 and Fe60Cu40 (atomic percent) during m echanical alloying (MA) were studied by high resolution electron micro scopy (HREM). Nanoscale composition distribution in Fe16Cu84 was deter mined using a HF 2000 FEG TEM. In the Fe16Cu84 specimen, a number of d eformation twins were observed. In the Fe60Cu40 specimen, shear band a nd generation of nanocrystals in the shear band were observed, which i s shown to be a typical mechanism for grain size reduction during MA. In both specimens, the b.c.c. grains tend to be very small (<5 nm) bef ore alloying, which is shown to be a prerequisite condition for the di ssolution of Fe in Cu and is also direct evidence to support the therm odynamical model proposed by Yavari et al. Nanoscale composition analy sis in Fe16Cu84 specimen shows that the average Fe contents in both th e interior of grains and the grain boundaries (GBs) are close to the d esigned composition, thus proving that a supersaturated solid solution has really formed. However, the Fe contents in both cases are rather inhomogeneous, indicating that the mixing of Fe and Cu during MA is in homogeneous. The process of MA is suggested to be divided into two sta ges: at the early stage, the grain sizes reduce quickly to a steady va lue due to the mobility of dislocations; further deformation can be fu lly accommodated by GBs. As a result, very fast volume diffusion and G B diffusion are achieved. NC-structure and greatly enhanced diffusion coefficients allow the formation of supersaturated solid solutions in immiscible systems with positive enthalpy of mixing. Copyright (C) 199 6 Acta Metallurgica Inc.