Papers by Author: Koichi Tsuchiya

Abstract: We have investigated {332}<113> twinning and detwinning mechanisms in β-Ti alloys. Microstructure-twinning relations were evaluated in a β-Ti-15Mo (wt.%) alloy by statistical analysis of the evolving twin structure upon deformation by in-situ SEM testing and electron backscattering diffraction (EBSD). We find that most of the primary twins (~80%) correspond to the higher stressed variant and follow Schmid’s law with respect to the macroscopic stress. Detwinning mechanism was evaluated in a multilayered β-Ti-10Mo-xFe (x: 1-3 wt.%) by EBSD. We find that the detwinning process consists of two independent events that occur at two different microstructural regions, i.e. twin tips located at grain interiors and grain boundaries. Both detwinning modes can be explained from a thermodynamic standpoint where the boundary dissociation processes minimize the boundary free energy.

Abstract: The microstructure and aging behavior of Cu-1.8wt%Be-0.2wt%Co alloy specimens processed by high-pressure torsion (HPT) at room temperature (RT) and 150°C after solution treatment have been studied. Application of HPT processing at RT and 150°C under an applied pressure of 5 GPa for 10 revolutions at 1 rpm to alloy specimens (RT-and 150°C-specimen) produces an ultra-fine grained structure with a grain size of 70 nm. The hardnesses of the RT-and 150°C-specimens increase with equivalent strain up to 7 and then saturate at constant values of 400 and 430 Hv, respectively. Annealing the RT-specimen at 150°C for 10 min increases the hardness from 400 to 430 Hv. Transmission electron microscopy observations of the 150°C-specimen and the RT-specimen annealed at 150°C reveal that there are no intragranular and intergranular precipitates. It is suggested that the higher hardness of the 150°C-specimen than the RT-specimen is ascribed to the segregation of Be atoms on dislocations during HPT processing at 150°C. The RT-and 150°C-specimens harden rapidly and exhibit maximum values of hardness at 3 min during aging at 320°C. The increase in the hardness is attributed to the precipitation of finely dispersed G.P. zones.

Abstract: Swirly segregation of Mo was introduced in Ti-12 mass% Mo alloys through hot caliber rolling. After isothermal aging of the alloys, ω phase precipitated heterogeneously in the alloys due to the segregation of Mo. The effect of the swirly segregation and isothermal aging condition on room temperature tensile properties of Ti-12Mo alloy was investigated. Tensile strength has been slightly affected by the swirly segregation. However, total elongation has been extremely improved from 4~10 % to around 20 % in the samples with tensile strength of between 1000 and 1100 MPa. Under all aging conditions, samples with the swirly segregation show larger reduction in area.

Abstract: Process of nanostructure formation and amorphization by high pressure torsion (HPT) were studied for various intermetallic compounds. In ZrCu after HPT deformation, optical microscopy revealed that numerous shear bands formed running nearly parallel to the shear direction. Partial amorphization was confirmed by X-ray diffraction and TEM observations. Detailed TEM observations revealed localized amorphization within the nano-scale shear bands. For HPT deformation of zone-melted Zr50Cu40Al10 the preferential amorphization of ZrCu phase was observed. On the contrary, amorphization was not observed for Ni3Al even after HPT deformation of 100 turns; the sample remained to be disordered nanocrystalline of about 50 nm. The process and mechanism of the grain refinement and amorphization will be compared and discussed for these intermetallic compounds.

Abstract: Among the various severe plastic deformation (SPD) processes, high pressure torsion (HPT) has several unique characteristics. These are applicability of very large strain and deformation under high pressure. Due to these abilities of HPT, several unique phenomena have been observed. In the present paper, three topics were reviewed; 1) work-softening in pure Cu, 2) high pressure phase formation in pure Ti and 3) synthesis of Cu-NbC composite. Work softening in pure Cu was observed when low strain rate and high pressure were applied. In Ti high pressure ω phase is obtained after unloading only when the deformation at high pressure was applied. The volume fraction of ω phase increased with the increase in the amount of strain. In pure Fe, high pressure ε phase was not retained at ambient pressure. The bulk Cu-NbC composite was synthesized starting from elemental powders. This demonstrates that HPT is an efficient tool for mechanical alloying and cold consolidation.

Abstract: The microstructure and the mechanical properties of pure Fe after HPT-straining at a rotation-speed of 0.2 rpm under a compression pressure of 5 GPa were investigated. The elongated grains with 300 nm thick and 600 nm long were observed at r = 1.5 mm away from the disk center regions after HPT-straining for 5 turns ( εeq = 45). The obtained Vickers microhardness in the submicrocrystalline Fe after 5 turns was around Hv 3.6 GPa. The engineering tensile strength and total elongation of the HPT-processed Fe for 10 turns were 1.9 GPa and 30 %. These facts suggest that HPT-straining leads to significant refinement of microstructure and increase in strength with good ductility.

Abstract: SUS 304 austenitic stainless steel was processed by HPT at room temperature with different rotation speed. It was found that the microstructure evolution and composed phases along the progress of HPT were sensitive to the strain rate (rotation speed). During deforming with the low strain rate, the deformation-induced dynamic phase transformation (DPT) from austenite (γ) to martensite (α’) occurred and the microstructure is characterized by elongated submicron α’ grains after 10 revolutions. While the euqiaxed nanocrystalline α’ grains were produced after HPT at the continuously alternative low and high strain rate. XRD analyses showed that multiple DPT of γ→α’→γ→α’ took place during HPT at the continuously alternative low and high rotation speed. Based on the experimental results, it was proposed that the euqiaxed ultrafine grained structure were produced by multiple DPT under the high strain and strain gradient.

Abstract: Effect of isothermal aging on martensitic transformation temperatures, mechanical properties and microstructure was investigated for a Ni-rich Ti-Zr-Ni shape memory alloy at temperatures ranging from 673 K to 773 K. The aging behaviour was two stage process: the first stage associated with an increase in the Vickers hardness and a decrease in martensitic transformation temperatures and the second stage with a decrease in the hardness and increase in the transformation temperatures. Second stage was also characterized by the appearance of nano-scale precipitates, which has never been reported.

Abstract: Effect of chemical composition was investigated on martensitic transformation temperatures, Curie temperature, magnetization and microstructures for Ni-(Mn, Fe, Co)-Ga and Cu-Mn-Ga systems. In the Ni-(Mn, Fe, Co)-Ga alloys, which is a modification of Ni-Mn-Ga systems, the Af and TC over 400 K were achieved. Cu-Mn-Ga alloy exhibited shape memory effect at temperatures above 373 K and had TC over 400 K. Furthermore, Cu-Mn-Ga exhibits good ductility even in polycrystalline condition unlike the case of Ni-Mn-Ga. Effect of addition of the fourth element to improve the magnetic property is under investigation.

Abstract: Formation of nanocrystalline structure by severe plastic deformation has studied extensively. Although ultra fine grained structure (grain size larger than 100 nm) had been obtained in many processes such as heavy cold rolling, equal channel angular pressing (ECAP) or accumulative roll bonding (ARB), the formation of nano grained structure (< 100 nm) is limited to processes such as ball milling, shot peening or drilling. In the present study, high pressure torsion (HPT) deformation and drilling were carried out to understand the conditions necessary to obtain nano grained structure in steels. The results of HPT experiments in pure Fe showed that HPT has superior ability of strengthening and grain refinement probably due to a strain gradient but the saturation of grain refinement occurs before reaching nano grained structure. Drilling experiments in high carbon martensitic steel revelaed that nano grained ferrite forms at the drilled hole surface only when the transformation from ferrite to austenite takes place during drilling. Considering various other processes by which nano grained ferrite was produced, it is proposed that heavy strains with large strain gradients together with dynamic transformation are necessary to reach nano grained ferrite structure.