Abstract
The microstructure of an Fe–I.7 wt.-% Cu alloy has been studied in the pre-peak to peak hardness range of ageing, when a fine-scale distribution of copper-rich precipitates occurs. These precipitates are not visible in the electron microscope below ∼ 6 nm in dia.; field-ion microscopy has thus been employed to elucidate the microstructure. Precipitation was confirmed to occur randomly in the matrix, on line defects, and preferentially at grain boundaries where there was enhanced growth. The mean size of particles was found to be approximately constant until the volume fraction of precipitates exceeded 1%, while the number density of precipitates increased initially until particle coarsening began to take place. The maximum hardness state corresponded to a mean precipitate size of ∼ 3 nm, when the maximum number density of particles occurred.
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