Minoru Umemoto

Tensile Property of Submicrocrystalline Pure Fe Produced by HPT-Straining

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.

Nanocrystalline Structure in Steels Produced by Various Severe Plastic Deformation Processes

The formation of nanocrystalline structure in steels by ball milling, shot peening and drilling were studied. In ball milling and shot peening, nanocrystalline layers form with sharp boundaries from deformed structure regions. Nanocrystalline layer showed extremely high hardness. By annealing, nanocrystalline layer showed substantially slow grain growth without recrystallization. The temperature of the specimen during deformation is low and deformation is done in ferrite state. In drilling, several μm thick nanocrystalline layers form at the top surface of a drill hole. Nanocrystalline layers showed high hardness and good thermal stability. The fresh martensite and retained austenite near a drill hole indicate that the temperature reached above Ac3 and nanocrystalline layers are produced in austenite condition. It is recognized that nanocrystalline layers produced in the processes studied in the present investigation has similar characteristics irrespective of the temperature it produced. It is proposed that deformation with a large strain gradient is an important condition to produce nanocrystalline structure.

Nanostructured Shape Memory Alloys for Biomedical Applications

Application of TiNi shape memory alloy in biomedical field is rapidly expanding. Some of the applications calls for non-conventional properties, which may require new methods of thermomechanical treatment and surface modification. In the present study, the effect of nanocrystallization/amorphization by various method of severe plastic deformation, such as, shot peening, cold rolling and high pressure torsion, was investigated on properties of TiNi shape memory alloys. Shot peening using iron based metallic glass media was found to be an effective method to obtain the amorphous surface. Surface amorphization improved the corrosion resistance. Nanocrystalline TiNi exhibited peculiar superelastic properties. Correlation between the microstructure and phase transformation in nanostructured TiNi was discussed.

Formation of Nanocrystalline Structure by Shot Peening

The effects of the shot peening (SP) condition and the initial hardness of specimens on the formation and thickness of nanocrystalline (NC) layer were investigated. The NC structure is found to be independent of the SP techniques, air blast, impeller and ultrasonic SP. In the SP condition, the increase in the kinetic energy per one shot is effective to increase the thickness of NC layer. It is also found that there is a certain critical initial hardness of specimens to produce the NC structure by SP. The NC structure forms when the specimen hardness is lower than the shot hardness.

Plastic Flow and Grain Refinement under Simple Shear-Based Severe Plastic Deformation Processing

In the present work, effects of loading scheme and strain reversal on structure evolution are studied by using high pressure torsion (HPT) and twist extrusion (TE) techniques. High purity aluminum (99.99%) was processed at room temperature up to a total average equivalent strain of ~4.8 by TE and HPT with two deformation modes: monotonic and reversal deformation with a step of 12˚ rotation. It was revealed that microstructural change with straining observed in pure Al was a common consequence of the SPD processing and was not affected significantly by the loading scheme. At the same time, it was found that strain reversal retarded grain refinement in comparison with monotonic deformation.

Phase Transformation and Annealing Behavior of SUS 304 Austenitic Stainless Steel Deformed by High Pressure Torsion

SUS 304 austenitic stainless steel was subjected to severe plastic deformation (SPD) by the method of high pressure torsion (HPT). From a fully austenitic matrix (γ), HPT resulted in phase transformation to give a two phase structure of austenite (γ) and martensite (α) by the transformation γuf067α. The phase transformation was accompanied by an increase in hardness (Hv) from 1.6 GPa in the as annealed form to 5.4 GPa in the deformed state. Subsequent annealing in temperature range 250oC to 450oC resulted in an increase in both α volume fraction and hardness (6.4 GPa). Annealing at 600oC resulted in a decrease in α volume fraction hardness.

Work-Softening, High Pressure Phase Formation and Powder Consolidation by HPT

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.

Influence of strain amount on stabilization of ω-phase in pure Ti by severe plastic deformation under high-pressure torsion

In pure Ti, the influence of shear deformation on the uf061 to uf077 transformation and the development of texture in the uf077-phase under high-pressure torsion (HPT) straining were investigated by means of X-ray and neutron diffractions. The fraction of uf077-phase increased with strain in the uf077-phase state. Bulk submicrocrystalline uf077-Ti was fabricated by HPT- straining under the compressive pressure P = 5 GPa with the equivalent strain uf065eq > 110 at the rotation speed of 3.3 × 10 -3 rev. per sec. (0.2 rev. per min.) at room temperature. The texture of uf077-phase evolved by HPT-straining with the prismatic planes parallel to the shear direction of HPT-straining and the basal planes perpendicular to it.

Reversal Straining to Manage Structure in Pure Aluminum under SPD

All the SPD techniques introduce reversal straining principally, but effects of the reversal deformation on structure evolution were not studied directly yet. In the present work, an attempt was made to manage structure in pure (99.99%) Al by strain reversal through high pressure torsion (HPT). Total accumulated deformation up to equivalent strain ~8 was used. General trend of the grain refinement is similar for both deformation modes; and it is typical with all other SPD processed FCC metals. At the same time, the difference in microstructure evolution at the vicinity of the specimen axis and with increasing distance in the radial direction introduces microstructural heterogeneities which are specific features of the reversal straining. In the monotonic deformation process the A ({111}<011>) fiber is gradually substituted by the C component ({ 0 0 1}< 1 1 0>) with increasing strain before it is found to weaken. In the reverse straining process the A fiber is found to dominate the deformation texture in the low strain region. In the reverse straining process at high strain level, a {001}<100> component appear.

Tensile and fatigue properties of sub-microcrystalline ultra-low carbon steel produced by hpt-straining

Abstract The tensile and fatigue properties of ultra-low carbon steel after HPT-straining at a rotation-speed of 0.2 rpm under a compression pressure of 5 GPa were investigated. Elongated grains with 300 nm thickness and 600 nm length with high dislocation density were formed by the HPT-straining. The obtained Vickers microhardness was around 3.6 GPa. The engineering tensile strength of the HPT-processed ultra-low carbon steel for 5 and 10 turns was 1.9 GPa, which is similar to the value of maraging high-alloy steels. The elongation increased with strain (at 5 to 10 turns). The increase in elongation is caused by the reduction of the stress concentration due to the existence of continuously recrystallized grains. The fatigue strengths of HPT-processed specimens were twice as high as those of the 90 % cold-rolled specimen in the low-cycle fatigue region, whereas in the high-cycle fatigue region the fatigue strengths were not so different due to the high notch sensitivity of the HPT-processed specimens.