Papers by Keyword: Dislocation Strengthening

Abstract: The limit of dislocation density was investigated by means of mechanical milling (MM) treatment of an iron powder. Mechanical milling enabled an ultimate severe deformation of iron powder particles and dislocation density in the MM iron powder showed the clear saturation at around the value of 1016m-2. On the other hand, the relation between hardness and dislocation density was examined in cold-rolled iron sheets, and the linear Bailey-Hirsch relationship; HV[GPa]=0.7+3×10-8ρ1/2 was obtained in the dislocation density region up to 3×1015m-2. Extrapolation of the Bailey-Hirsch relationship indicated that the dislocation strengthening should be limited to about 3.7GPa in Vickers hardness which corresponds to about 1.1GPa in 0.2% proof stress.

Abstract: Work hardening behavior and microstructure development during deformation by cold rolling were investigated in iron with different grain size. Grain refinement makes the introduction of dislocation easier. For instance, under the same deformation condition (5% reduction in thickness), dislocation density is the order of 1014m-2 in a coarse grained material (mean grain size; 20μm), while it reaches 7×1015m-2 in an ultrafine grained material (0.25μm). It is well known that the yield stress of metals is enlarged with an increase in dislocation density on the basis of the Bailey-Hirsch relationship. However, it should be noted that the ultrafine grained material never undergoes usual work hardening although the dislocation density is surely enhanced to around the order of 1016m-2: 0.2% proof stress is almost constant at 1.4 ~ 1.5GPa regardless of the amount of deformation. The dislocation density of 1016m-2 is thought to be the limit value which can be achieved by cold working of iron and the yield stress of iron with this dislocation density (ρ) is estimated at 1.1GPa from the Bailey-Hirsch relationship; σd [Pa] = 0.1×109 + 10 ρ1/2. On the other hand, yield stress of iron is enhanced by grain refinement on the basis of the Hall-Petch relationship; σgb [Pa] = 0.1×109 + 0.6×109 d-1/2 as to the grain size d [μm]. This equation indicates that the grain size of 0.35 μm gives the same yield stress as that estimated for the limit of dislocation strengthening (1.1GPa). As a result, it was concluded that work hardening can not take place in ultrafine grained iron with the grain size less than 0.35 μm because dislocation strengthening can not exceed the initial yield stress obtained by grain refinement strengthening.