Papers by Author: Hiromi Miura

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).

Abstract: The preferential initiation of dynamic recrystallization (DRX) at triple junctions (TJs) in stainless steel polycrystals was investigated in compression at 1123 K to 1323 K at a strain rate of 2 x 10-4 s-1. Nucleation appeared at TJs at strains as low as 0.1. This strain is only about 1/5 to 1/2 of the peak strain at which DRX is conventionally believed to occur extensively. Furthermore, DRX nucleation was not observed to take place at grain boundaries or in the matrix at this strain. The probability of DRX nucleation at TJs increased monotonically with strain and temperature. It also depended on the angle, y, between the compression axis and the sliding boundary. That is, when the angle, y, approaches 45 degrees, the probability of DRX nucleation at TJs is higher. These results reveal the important role of grain-boundary sliding (GBS) on DRX nucleation at TJs. It should also be noted that more than 90% of the grains nucleated at TJs were twins. Such dynamic twinning suggests that the essential DRX nucleation mechanism is twinning.

Abstract: Grain refinement in a magnesium alloy AZ31 was studied in multi-directional forging (MDF) at a strain rate of 3×10-3s-1 with decreasing temperature from 623K to 423K. The MDF was carried out up to large cumulative strains with changing the loading direction during decreasing temperature from pass to pass. The structural changes were characterized by generation of many mutually crossing kink bands at low strains, followed by development of very fine grains at large strains. The results showed that MDF with decreasing temperature can accelerate uniform generation of much finer grains, resulting in the minimal grain size of 0.36µm in a cumulative strain of 4.8 at 423K. The mechanism of grain refinement was discussed.

Abstract: Hot deformation and associated structural changes were studied in compression of a magnesium alloy AZ31 with initial grain sizes (D0) of 22 µm and 90 µm at a temperature of 573K. D0 influences significantly the flow curve and the kinetics of grain refinement during hot deformation. For D0 = 22 µm, grain fragmentation takes place due to frequent formation of kink bands initially at corrugated grain boundaries and then in grain interiors in low strain, followed by full development of new fine grains in high strain. For D0 = 90 µm, in contrast, twinning takes place in coarser original grains, and then kink bands and new fine grains are formed mainly in finer ones at low strains. Then new grains are formed in necklace along the boundaries of coarse original grains, followed by their development into the grain interiors. Grain refinement in the Mg alloy can be concluded to result from a series of deformation-induced continuous reactions, they are essentially similar to continuous dynamic recrystallization (cDRX).

Abstract: Orientation-controlled Mg single crystals were tensile tested at temperatures between 473 K and 673 K at a strain rate of 4.2 x 10-4 s-1 in vacuum. Though all the single crystals showed high ductility compared with that of polycrystals, the ductility of the single crystals strongly depended on the crystal orientation. The [27 -1 -26 1] single crystal showed 0.57 fracture strain, while the [3 8 -11 -1] single crystal showed superplastic behavior of ductility over 1.8 strain. The observed strong orientation dependence of ductility seemed to be caused by orientation dependence of ease occurrence of dynamic recrystallization (DRX) in the single crystals during high-temperature deformation. The orientation dependence of ductility of Mg single crystals will be discussed in detail concerning crystallographical orientations of the single crystals, occurrence of DRX and fracture.

Abstract: The deformation behavior and structure changes of magnesium alloy AZ31 were studied in compression at temperatures ranging from 523K to 673K and at a strain rate of 3 x 10-3 s-1. They depend sensitively on deformation temperature. At high temperatures, grain fragmentation takes place due to frequent formation of kink bands initially at corrugated grain boundaries and then in grain interiors, followed by full development of new grains in high strain. At lower temperatures, in contrast, twinning takes place in rather coarse grains and kink bands are formed mainly in finer original ones in low strain. It is concluded that new grain evolution can be controlled by a deformation-induced continuous reaction resulting in grain fragmentation by kink bands, i.e. continuous dynamic recrystallization (cDRX). The latter is discussed comparing with conventional, i.e. discontinuous, DRX.

Abstract: Strain-induced grain refinement in a coarse-grained 7475Al alloy was studied by means of multidirectional forging (MDF) carried out at T = 490oC under a strain rate of 3 x 10-4 s-1. Integrated flow curves exhibit significant work softening just after yielding, followed by steady-state-like behavior at high strains. The evolution of new fine grain structure during deformation can be assisted by grain-boundary sliding, resulting in frequent formation of high strain gradients and subsequently microshear bands in grain interiors. Microshear bands developed in various directions are intersected with each other, subdividing original grains into misoriented small domains. The number and the misorientation angle of microshear bands progressively increase during deformation, finally followed by their transformation into high-angle boundaries. It is concluded that grain refinement under hot MDF conditions occurs by a series of deformation-induced continuous reactions; that is essentially similar to continuous dynamic recrystallization.