Papers by Author: Hiroyuki Nakayama

Abstract: Characteristics of high temperature shape memory alloy in Ti50-Pd(50-x)-W(x) at%, x = 3 and 5 system has been examined. The prepared ingot was homogenized at 1273 K for 86.4 ks followed by water quenched. The homogenized samples were hot-rolled at 1273 K to a plate shape with thickness of 3.5 mm and 1.0 mm. Transformation temperatures were determined by 4-probe electrical resistivity measurement. The transformation behavior in the both samples exhibited similar trend. The first heating curve showed no significant step. In contrast, the first cooling curve exhibited clear step due to martensitic transformation. After second cycle, the transformation behavior was stabilized, thus the clear steps in the cooling and heating curves were seen. Stressstrain curve of hot-rolled plate with 3.5 mm was examined by compression in Ti50-Pd47-W3 at% alloys. The sample exhibited 1 % shape memory effect. By bending test, nearly perfect SME effect was observed in hot rolled Ti50-Pd45-W5 at% alloy plate with thickness of 1 mm.

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: 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.