Papers by Author: Minoru Umemoto

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.

Abstract: 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.

Abstract: The mechanical behavior of the sintered nanostructured intermetallic alloys, Al67Ti25Mn8 and Al67Ti25Fe8 has been investigated by means of compression tests as a function of temperature. These intermetallic materials are produced by mechanical alloying and spark plasma sintering. The sintered alloys have been characterized by X-Ray Diffraction and Transmission Electron Microscopy. Their nanostructure consists of a single-phase with an L12 (cubic) structure and an average grain size in the nanoscale (lower than 30 nm). These nanostructured intermetallics show considerably high ductility in compression at high temperature but are brittle at temperatures below 400 °C. In such cases the compressive fracture strength can reach values as high as 1.8 GPa (Al67Ti25Fe8). At 500 °C, some ductility is found together with a relatively high flow stress (around 1 GPa), the corresponding deformation curve shows strain hardening and in some cases stress serrations (strain aging). At 600 °C, a low flow stress is measured (~300 MPa) with high ductility. At 700 and 800 °C, a quasi-superplastic behavior is found with a total deformation of around 45 and very low flow stresses. No evidence of dislocation motion is found at temperatures above 700 °C suggesting a deformation mechanism based on grain boundary sliding.

Abstract: 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.

Abstract: Dissolution behavior of cementite in eutectoid steels with pearlitic and spheroidite structures by severe plastic deformation was studied. Applying a long time milling, cementite dissolved completely and matrix turned out to be nanocrystalline ferrite. By a ball drop deformation (at high strain rates), heavily deformed layers in which cementite dissolves completely or partially were produced. By applying pulsed laser irradiation, re-austenitized zone which transformed to fresh martensite during quenching was produced. The boundary between the re-austenitized zone and matrix exhibited similar microstructure with that observed in specimens subjected to a ball drop deformation. It was suggested that the dissolution of cementite by heavy deformation at high strain rates are probably due to thermal effect, that is, re-austenitization.

Abstract: Thermal- and stress-induced martensitic transformation was investigated on TiNi shape memory alloys subjected to severe plastic deformation (SPD) by cold rolling. TEM observation revelaed the sample is a mixture of nanocrystalline and amorphous after 40% cold rolling. DSC analysis suggested that the martensitic transformation was suppressed when the thickness reduction was over 25% reduction. Aging at lower temperatures (573 ~ 673 K, 3.6 ks) restores the phase transformations, but to a limited extent. The stress-strain curves of nanocrystalline/amorphous TiNi are characterized by absence of stress-plateau and small hysteresis.

Abstract: 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.