T. Hara, National Institute for Materials Science, Tsukuba, Japan; Y. Yamabe-Mitarai, National Institute for Materials Science (NIMS), Tsukuba, Japan; M. Nishida, Kyushu University, Kasuga, Japan; E. Okunishi, JEOL Ltd., Tokyo, Japan
The alloys combining with Ti and Platinum group metals have been widely studied as high-temperature shape memory alloy. We found that Iridium added Ti-Pt alloys show higher transformation temperature than that of TiPt binary alloy. We have studied them regarding shape memory effect and pseudoelasticity. The transformation temperatures increase with increasing Ir contents; in the case of Ti50Pt12.5Ir37.5 alloy, Ms temperature is 1448K, that is one of the highest transformation temperature in such materials. As for the shape recovery, about 4% was found in Ti50Pt25Ir25 alloy. Such mechanical properties also change with the composition as well as the transformation temperatures. In order to understand and to improve these properties, information on the microstructure and crystal structure is essential. In this study, microstructure and crystal structure of martensite phase in Ti(Pt,Ir) alloys are investigated by TEM and Cs-corrected STEM to understand the compositional change in such properties. TEM specimens with four different compositions, Ti50PtxIr(50-x) (x=12.5, 25.0, 37.5, 50)(at%), were prepared. Observations were done at room temperature using TEM(JEM-2010F) and STEM(JEOL ARM-200 and JEM-2100 with Cs-corrector). In the previous TEM study, we reported that the stacking sequence for each alloy changes with the composition. However, because of the complexity in diffraction patterns, diffuse scattering and strong streak, precise structure could not be determined. In this study, HAADF-STEM observations with Cs-corrected STEM were performed to analyze stacking structure of these alloys directly. In such observation, since Ti and Pt(Ir) column can be distinguished by Z-contrast, stacking sequence can be analyzed directly. As the result of such observation and analysis, for example, the structure of Ti50Pt25Ir25 alloy is found that it has a complex 7-layered long period stacking order structure. Since TiPt binary alloy is confirmed approximate 2H structure, stacking order becomes irregularly from 2H with increasing Ir content.
Summary: Microstructure and crystal structure of ternary Ti50PtxIr(50-x) (x=12.5, 25.0, 37.5, 50)(at%) martensite phase are investigated by TEM and STEM. In all alloys, the microstructure is twinned one but changes with Ir content. Cs-corrected HAADF-STEM observation reveals that crystal structure of martensite has long period stacking order and the stacking sequence also changes with the composition.
