Papers by Author: Y. Futamura

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.

Abstract: The effect of copper (Cu) addition on the grain growth behavior of austenite was investigated in a low carbon steel and a Cu bearing low carbon steel. Cu addition to the steel does not affect the nucleation rate of reversed austenite on heating in the martensitic structure but markedly retards the grain growth of the austenite during holding at 1173K (austenitization). As a result, the grain size of austenite in the Cu bearing steel becomes about one-third times smaller than that in the base steel after austenitization for 14.4ks. TEM observations in the Cu bearing steel revealed that Cu particles precipitated during aging treatment had completely dissolved in 1.2ks of austenitization. Therefore, the retardation of grain growth of austenite can not be explained by the grain boundary pinning effect of Cu particles but by the dragging effect of Cu atoms in the austenitic solid solution.

Abstract: Microstructural change and soft ening behavior during annealing were investigated for deformed ferrite and lath martensite in an ultralow carbon 1.5mass%Mn-0.0018mass%B steel, and then the difference in recrystallization behavior between the materials was discussed in terms of the nucleation site of recrystallized grains. The ferritic and martensitic materials were obtained by furnace-cooling and water-quenching, respectively, after solution treatment. The ferritic material was cold-rolled at a reduction of 80% to give the same dislocatio n density as of the martensitic material. The deformed ferritic material contains a large number of geometrically necessary boundaries with large misorientations, while the martensitic material does only contain original grain boundaries such as prior austenite grain boundaries, packet boundaries and block boundaries. The recrystallization during annealing is markedly retarded in the martensitic material compared with the deformed ferritic material. As a result, the time for completing the recrystallization was roughly a hundred times longer in the martensitic material than in the deformed ferritic material. This is due to the difference in nucleation site of recrystallized grains, that is, the geometrically necessary (GN) boundaries introduced by the deformation for the ferritic material, and only the original grain boundaries for the martensitic material.