MICROSTRUCTURE INVESTIGATIONS OF BALL-MILLED MATERIALS

HREM and FEG TEM were emphasized and extensively used to follow the mo st subtle changes in the structure and composition of ball-milled Cu, Fe-Cu, and thermally decomposed Fe60Cu40. Some significant results are obtained and summarized as follows: HREM shows that the deformation o f ball-milled copper proceeds mainly by twinning and shear bands (SBs) formation. The nano-grains formed during ball milling (BM) contain a high density of dislocations. The grain boundaries (GBs) of nanocrysta lline (NC) Cu prepared by BM are ordered, curved, and strained, but di sordering, lattice distortion, and nanovoids in local regions were fre quently observed. Nanoscale composition analysis on mechanically alloy ed Fe16Cu84 shows that the average Fe content in both the interior of grains and the GBs is close to the designed composition, which proves that a supersaturated solid solution has really formed. However, the F e content is rather inhomogeneous between the larger and smaller grain s, which infers the inhomogeneous mixing of Fe and Cu during mechanica l alloying (MA). NC structure and the mechanical force-enhanced fast d iffusion are the reasons of the formation of supersaturated solid solu tions in immiscible systems with positive enthalpy of mixing. HREM obs ervations carried out with the thermally decomposed Fe60Cu40 solid sol ution show that the Nishiyama (N-W) or Kurdyumov-Sachs (K-S) orientati on relationships exist between alpha-Fe and Cu. Energy dispersive X-ra y spectra (EDXS) results show that the Cu content in these alpha-Fe gr ains reaches as high as 9.5 at.% even after heating to 1,400 degrees C , which is even higher than the maximum solubility of Cu in gamma-Fe a t 1,094 degrees C. (C) 1998 Wiley-Liss, Inc.