Hirobumi Tobe

Mantle region accommodating two-dimensional grain boundary sliding in ODS ferritic steel

Two-dimensional grain-boundary sliding (GBS) was achieved microscopically in an oxide-dispersion-strengthened ferritic steel with an elongated and aligned grain structure, which was deformed perpendicular to the long axis. At the border between superplastic regions II and III, microscopic deformation was observed using sub-micron grids drawn on the material surface using a focused ion beam. GBS was accommodated by intragranular deformations in narrow areas around grain boundaries, which has been predicted by earlier researchers as characteristics of the core–mantle model. These observations suggest that dislocations slip only in the mantle regions around wavy boundaries to relax the stress concentration caused by GBS during superplasticity.

Two-Dimensional Observation of the Core–Mantle Model for Superplastic Flow in an ODS Ferritic Steel

Two-dimensional grain movements were microscopically observed in high-temperature shear deformation of an oxide-dispersion-strengthened ferritic steel with an elongated and aligned grain structure that was sheared in a direction perpendicular to the grain long axis. The microstructure was analyzed using electron back-scattered diffraction and electron channeling contrast imaging techniques before and after the shear deformation. Clear grain switching events, which are assumed to occur via grain-boundary sliding (GBS), were observed and the switching mechanism was characteristic of the core–mantle superplasticity model proposed by Gifkins; dislocation densities got much higher in narrow areas near the grain boundaries (mantles) than the grain interiors (cores). The mantle regions typically appeared in protruding portions of grains that was likely resistant to GBS, and low-angle boundaries were found to emerge at the core–mantle boundaries via slipping of dislocations within the mantle regions.

Effect of Heat-treatment Conditions on Microstructure and Shape Memory Properties of Ti–4.5Al–3V–2Fe–2Mo Alloy

The effect of heat treatment conditions on microstructure and superelasticity of Ti–4.5Al–3V–2Fe–2Mo (mass%) alloy was investigated by scanning electron microscopy with electron backscattered diffraction (SEM/EBSD) and loading-unloading tensile tests. The presence or absence of solution treatment (ST) at 1223 K before heat treatment at 1073 K was considered. The SEM analysis showed that the specimen without solution treatment had globular α and β grains while the solution-treated specimen had needle-shaped α in coarsened β grains. The loading-unloading tensile tests revealed that the stress for inducing martensitic transformation and shape recovery strain were almost the same in both the specimens, although the grain sizes of β austenite were significantly different.

Low-temperature creep mechanism in ultrafine-grained aluminum

This study investigated the low-temperature creep mechanisms in ultra-fine grained aluminum made by accumulative roll bonding. The low-temperature creep behaviors in ultra-fine grained aluminum with grain size of 0.39 μm were divided into four regions by three certain stress values, σmy, σmulti and σy. σmy is stress for dislocation movement, σmulti is stress for dislocation multiplication, and σy is yield stress. First, below σmy, plastic deformation was negligible. Second, from σmy to σmulti, creep deformation with n=2.5 occurred by grain boundary sliding. Third, from σmulti to σy, creep deformation with n=7.2 occurred by intragranular recovery of dislocations. Last, above σy, power-law breakdown was confirmed. (Received February 3, 2017 Accepted March 16, 2017)