Koichi Tsuchiya

Formation of nanocrystalline structure in steels by ball drop test

Abstract Formation of nanocrystalline structure in a eutectoid steel by severe plastic deformation has been studied by a `ball drop test. The microstructures and hardness similar to those of nanocrystalline structures produced by ball milling have been obtained near the surface of specimens. The high strain rate of around 104 s−1 is proposed to be an essential condition to produce nanostructure by deformation.

Electron-microscopy investigation on nanocrystal formation in pure Fe and carbon steel during ball milling

Abstract Investigations on ball milled pure iron and Fe–0.89C spheroidite alloy powders indicated that the formation of nanoscaled dislocation cells and/or subgrain boundaries is responsible for the grain refinement to nanocrystals. The possible recovery and/or recrystallization due to adiabatic heating induced by high-strain rate is suggested to aid this process.

Formation of Nanocrystalline Structure in Steels by Air Blast Shot Peening and Particle Impact Processing

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.

Martensitic Transformation and Mechanical Behavior of TiNi Shape Memory Alloys after Severe Plastic Deformation

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.

Property of TiNi Shape Memory Foils Produced by Ultrafine Laminates Method

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.

Comparison of Nanocrystallization in Steels by Ball Milling and Ball Drop Test

Nanocrystallization of steels by ball milling and by a ball drop test was compared using specimens with different carbon content and starting microstructures. The nanocrystalline structure produced by ball milling and ball drop test has essentially the same characteristics; nano-sized ferrite grains, dissolution of cementite, high hardness (about 10 GPa), and absence of recrystallization and slow grain growth by annealing. The present ball drop test confirmed that nanocrystallization by ball milling is due to severe plastic deformation and not due to contamination. Low test temperature and pre-strain enhanced the nanocrystallization in a ball drop test. The amount of true strain necessary to obtain nanocrystalline regions was estimated to be larger than 3 using the shear band produced by one time of ball drop.

Surface Amorphization in Intermetallic Compounds by Shot Peening

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.