Microstructure and nanoscale composition analysis of the mechanical alloying of Fe xCu 100 − x ( X = 16, 60)

Abstract

The microstructures of Fe 16Cu 84 and Fe 60Cu 40 (atomic percent) during mechanical alloying (MA) were studied by high resolution electron microscopy (HREM). Nanoscale composition distribution in Fe 16Cu 84 was determined using a HF 2000 FEG TEM. In the Fe 16Cu 84 specimen, a number of deformation twins were observed. In the Fe 60Cu 40 specimen, shear band and generation of nanocrystals in the shear band were observed, which is 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) before alloying, which is shown to be a prerequisite condition for the dissolution of Fe in Cu and is also direct evidence to support the thermodynamical model proposed by Yavari et al. Nanoscale composition analysis in Fe 16Cu 84 specimen shows that the average Fe contents in both the interior of grains and the grain boundaries (GBs) are close to the designed 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 inhomogeneous. The process of MA is suggested to be divided into two stages: at the early stage, the grain sizes reduce quickly to a steady value due to the mobility of dislocations; further deformation can be fully accommodated by GBs. As a result, very fast volume diffusion and GB 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.