Papers by Author: Taku Sakai

Abstract: A Ni-30 mass% Fe alloy with dispersed coarse oxide particles, ranging from 1 to 3 μm in diameter, was multi-directionally forged (MDF) at temperatures between 773 K and 873 K at a true strain rate of 1 x 10-3 s-1 in vacuum. For comparison, a particle-free Ni-Fe alloy was also prepared and MDFed. At 873 K, an obvious flow softening occurred during MDF especially in the alloy with dispersed particles, while work hardening followed by steady-state flow appeared at 773 K in both alloys. With increasing cumulative strain, the average grain size became gradually finer. However, the evolved microstructure was quite different depending on temperature and dispersion of particles. At 873 K, in the alloy with particles, equi-axed fine grains of about 1.3 μm in average was uniformly evolved at a cumulative strain of Σε = 2.4, while at 773 K the microstructure was still inhomogeneous. The above experimental results suggest important role of coarse particles to stimulate grain refinement.

Abstract: Microstructural changes taking place in an as-cast coarse-grained 7475 Al alloy was studied by using multidirectional forging (MDF) at a temperature of 250oC and at a strain rate of 3 × 10-4 s-1. The samples were deformed by MDF with a strain of 0.7 per pass up to cumulative strain (Σε) of 8.4. In the earlier stages of deformation, microstructural changes are mainly characterized by development of dislocation subboundaries with low-to-moderate misorientation angles. The misorientation angle initially increases with straining and reaches a plateau of around 3.7o in the strain range from 0.7 to 2.1, where new grain formation scarcely takes place in the original grain interiors. With further straining, grain fragmentation starts to occur accompanying with deformation bands developed at various directions, followed by rapid evolution of a new fine grain structure at large strain. The average grain size is around 1 μm at large strains and the average misorientation angle approaches a value of about 25o at Σε = 8.4.

Abstract: Grain refinement taking place in a magnesium alloy AZ31 was studied in a single- and multi-directional compression at a temperature of 573K. The structural changes observed by SEM/EBSD analysis can be characterized by the evolution of many mutually crossing kink bands at low strains, continuous increase in their number and misorientation angle in moderate strain and finally full formation of a fine-grained structure in high strain. The characteristics of new grain evolution process are sensitively affected by initial grain size (D0) and strain path. New grains are developed faster with decrease in D0. Multi-directional compression accelerates the evolution of fine grains and the improvement of plastic workability. The mechanism of new grain formation is discussed in detail.

Abstract: Effect of strain rate on grain refinement was studied in multidirectional forging (MDF) of a coarse-grained 7475 Al alloy at 490oC under strain rates of 3 × 10-4 s-1 and 3 × 10-2 s-1. At a strain rate of 3 × 10-4 s-1, the stress – strain ( σ - ε) behavior shows significant work softening just after yielding and a steady-state flow at higher strains. The structural changes are characterized by development of deformation bands at early stages of deformation, followed by formation of a fine grain structure in high strain in the whole material. The volume fraction of new grains increases with strain and approaches a value of about 0.85 over a strain of 3. At a higher strain rate of 3 × 10-2 s-1, in contrast, a steady-state flow following small flow softening appears at a relatively low strain. New grains are formed during steady state flow along original grain boundaries and the volume fraction reaches below 0.2 even in high strain. The occurrence conditions and the mechanisms of grain refinement are discussed in detail.

Abstract: Microstructural evolution in pure copper during multi-directional forging (MDF) at temperature of 77 K was studied. Flow stress during MDF at 77 K showed a monotonical increase at all strain. Ultra fine (sub)grains of 0.15μm in diameter were evolved, which was accompanied by deformation twinning, at strain of Σε = 2.4. In higher strain region, Σε = 6.0, lamellar-look structure of twins extensively appeared. The lamellar spacing was 10-100nm. For comparison, samples were also MDFed at 300 K. The flow stress curves showed an apparent steady state flow at above strain of Σε =2.0, which implies occurrence of dynamic recovery. The evolved (sub)grain size was 0.3 μm at high strain of Σε = 6.0. Therefore, grain refinement seems to take place more easily by MDF at 77 K compared with that at 300 K due to effect of deformation twin. Microstructures evolved under MDF at 77 K and 300 K showed different annealing behavior. Static recrystallization started earlier and faster in the samples MDFed at 77K than those MDFed at 300K.

Abstract: Low temperature superplasticity (LTSP) was studied in a fine-grained magnesium alloy AZ31 which was processed by multi-directional forging (MDF) under decreasing temperature conditions. Tensile specimens were cut from MDFed Mg alloy parallel to the final compression axis (CA), and the tensile axis perpendicular to the CA. Tensile tests was carried out at temperatures from 393K to 473K and at various strain rates. Superplasticity appears even at a low temperature of 393K with a stress exponent (n) of about 0.56 and a total elongation of 370%. The relative large stress exponent can be connected with grain coarsening taking place during deformation. The initial texture hardly takes place during deformation. This suggests that grain rotation does not occur during superplasticcity.

Abstract: Microstructural evolution during equal channel angular extrusion (ECAE) was investigated in a coarse-grained dilute aluminum alloy, Al-3%Cu, at a temperature of 250oC. Scanning electron microscopy (SEM) with electron back scattering diffraction (EBSD) and optical metallography (OM) was used to reveal the structural changes in the alloy deformed up to a strain of ε=12. The microstructural evolution at initial and moderate stages of deformation is characterized by the formation of low angle boundaries and deformation bands with moderate misorientations in grain interiors. With further deformation the number and the misorientation of the deformation bands increase, finally leading to the subdivision of original grains by these bands and then the development of fine grains with an average size of about 6 μm at ε=12. The evolution of deformation bands in initial grain interiors and their role on new grain formation are discussed in detail.

Abstract: Strain-induced grain refinement in a magnesium alloy AZ31 was studied in multi-directional forging (MDF) at a temperature range from 423K to 623K and at a strain rate of 3x10-3s-1. MDF with a pass strain of 0.8 was carried out to high cumulative strains of around 5 with changing of the loading direction during decreasing temperature from pass to pass. The structural changes can be characterized by the evolution of many mutually crossing kink bands at low strains followed by increase in their number and misorientation, finally resulting in a fully developed fine-grains at high strains. MDF with decreasing temperature can accelerate the evolution of much finer grains and the improvement of plastic workability. An average grain size of 0.3 μm is formed at an accumulative strain of 4.8 and at 423K. It is concluded that grain refinement under MDF conditions occurs by a series of deformation-induced continuous reactions; that is essentially similar to continuous dynamic recrystallization (cDRX).