The Characteristics of Ribbons and the Self-accommodation of R-phase in TiNi-based Shape Memory Alloys

Autor: Chih-Hsuan Chen, 陳志軒
Rok vydání: 2015
Druh dokumentu: 學位論文 ; thesis
Popis: 103
Characteristics of aging treatments, transformation sequence, phase identification, microstructures, precipitation hardening and shape memory performances of melt-spun Ti50.4Ni49.5Si0.1, Ti50.1Ni49.7Si0.2, Ti50.6Ni39.4Cu9.8Si0.2 and Ti50Ni25Cu25 ribbons were studied. Ti50.4Ni49.5Si0.1, Ti50.1Ni49.7Si0.2 and Ti50.6Ni39.4Cu9.8Si0.2 ribbons show significant precipitation hardening by GP zones and/or nanoscale Ti2Ni/Ti3Ni4 precipitates after aging treatment, which result in excellent shape memory performance. The recovery of both Ti50.4Ni49.5Si0.1 and Ti50.1Ni49.7Si0.2 ribbons during pseudoelasticity tests are higher than 78 % under nanoindentation load of 50 mN, which is significantly higher than conventional bulk Ti49.8Ni50.2 alloy. Additionally, the microstructure, precipitation phenomenon and shape memory properties of TiNiSi ribbons are found closely related to the slight variation of Ti/Ni ratio and Si content. For Ti50.6Ni39.4Cu9.8Si0.2 ribbon, shape memory effect with 6.2 % recoverable strain is obtained after crystallized and aged at 550 ºC for 15 min. Amorphous Ti50Ni25Cu25 ribbon becomes well-crystalized after being treated at 500 ºC for 15 min and shows pronounced pseudoelasticity response under average contact pressure between 2.5-3.0 GPa. The structure of GP zones in Ti-rich Ti50.92Ni48.94Si0.14 ribbon was studied and a hybrid structure composed of C11b Ti2Ni and Ti5Ni3 substructures is proposed to explain the appearance of these extra diffuse streaks. Experimental results indicate that performance of TiNi-based shape memory alloys can be effectively enhanced by fabricating the alloys with rapid solidification process. The self-accommodation morphologies of R-phase variants studied by an EBSD system provide a fast and high resolution method to study twinning relationship between R-phase variants.
Databáze: Networked Digital Library of Theses & Dissertations
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