Mechanical milling using a high energy ball mill was applied to a commercial pure iron powder to introduce a large deformation strain into the matrix. Microstructural changes during deformation and annealing were investigated for the mechanically milled powder, by means of X-ray diffractometry, transmission electron microscopy and hardness testing, in order to clarify the work-hardening mechanism of iron during super-heavy plastic deformation. Through the mechanical milling of 360 ks, nanocrystalline grains of 20-50 nm were formed within the powder, and the resultant powder was work-hardened to Hv950. Hardness of the mechanically milled powder mainly depends on the crystallite size(d); the size of dislocation cells, subgrains and nanocrystalline grains. In the mechanically milled powder, a linear relationship was confirmed between hardness and the reciprocal of crystallite size (d-1), supporting the strain hardening to Hv500. While it was also confirmed that the work-hardening mechanism of iron gradually changes from strain hardening to grain refining strengthening in the hardness range of around Hv600, where crystalline grains are refined less than 100 nm through heavy deformation. In the hardness range above Hv600, the Hall-Petch relationship was realized to the crystalline grain size.
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