Papers by Author: Minoru Umemoto

Abstract: Using the ultrafine laminate method, thin foils (50 µm) of Ni-rich TiNi shape memory alloys were produced. Overall composition of the Ti/Ni laminate is Ti-50.7%Ni. TiNi (B2) phase was obtained after different diffusion treatments at 1073 K for 36 ks. Aging treatment at 773 K for 3.6, 18, 36, 72 and 144 ks were also performed. Phase transformation behavior of aged foils changed from two-step, to three-step and then to two-step transformation, which is similar to the case of bulk TiNi alloys. Uniform distribution of Ti3Ni4 phase was observed for aged samples by transmission electron microscopy. Two-way shape memory effect of the aged TiNi foil was also demonstrated.

Abstract: Surface nanocrystallization in various steels by shot peening (both air blast and ultrasonic) and particle impact processing was investigated. Nanocrystalline layers with several µm thick were successfully fabricated by these methods. In all the case, the nanocrystalline layers have extremely high hardness and separated from adjacent deformed structure regions with sharp boundaries. By annealing, the nanocrystalline layers showed substantially slow grain growth without recrystallization. Those characteristics are similar to those observed in the samples experienced ball milling and ball drop deformation.

Abstract: Half metallic ferromagnets are candidate materials for novel electronic devices, which can control the spin character of the conducting electrons. Electronic band calculation predicted that Co2MnGe Heusler compounds may has the half metallic band structure. In the present study, structure and magnetic properties have been investigated of the Co2MnGe for bulk and thin films. In the bulk sample, plate defects have been found.

Abstract: Deformation and dissolution behavior of the cementite in eutectoid steels with pearlitic and spheroidite structures were studied using high resolution SEM and AFM. Cementite lamellae in deformed pearlite exhibited inhomogeneous slip, smooth thinning or necking, fragmentation and cleavage fracture. Complete dissolution of cementite lamellae and spheroidal cementite particles was observed in specimens deformed with large strains at high strain rates. The dissolution mechanism of cementite by heavy deformation was discussed.

Abstract: Surface amorphization by shot peening in intermetallic compounds was examined. The formation of amorphous layer has been verified in Ti-Ni and Zr-Co-Ni alloys. In Ti-Ni alloy, an amorphous phase and nanocrystalline grains were detected by TEM for 120 s peening duration. However, recrystallization and grain growth occurred, when peening duration was up to 400 s. In Zr-Co-Ni alloy, an amorphous phase coexisting with nanocrystalline grains was observed in Zr-37Co-13Ni alloy. For other alloy compositions (Zr-50Co and Zr-50Ni), an amorphous phase was not observed, and nanocrystalline grains were observed. All the observed nanocrystalline grains may be recrystallized from amorphous phase, because their shape was spherical.