Aging Effect on Multi-step Transformation Behavior in Ni-rich TiNi shape memory alloys
Date Issued
2007
Date
2007
Author(s)
Tsai, Tsung-Jung
DOI
zh-TW
Abstract
Ti49.2Ni50.8 and Ti48.7Ni51.3 shape memory alloys (SMAs) melt, homogenized and aged without any hot/cold rolling have grain size >50μm. The distribution of Ti3Ni4 precipitates directly affects the transformation behavior of these alloys. After 350℃ and 400℃ aging, Ti49.2Ni50.8 shows a normal three-step transformation. At 450℃×6hr, the nucleation rate of Ti3Ni4 is so high that the supersaturated Ni atoms in the grain center can not diffuse such long distance to grain boundaries and thus, there are three distinct regions resulted in the grain, i.e., regions I, II and III. Four-step transformation of B2Ⅰ,Ⅱ,Ⅲ→RⅠ,Ⅱ,Ⅲ, RⅠ,Ⅲ→B19’Ⅰ,Ⅲ, B2→B19’(no ppt) and RⅡ→B19’Ⅱ is induced from these regions. Experiment results show that the aging temperature decides the diffusion rate of Ni atoms, the growth rate and the distribution of Ti3Ni4 precipitates in the grain. The Ti3Ni4 precipitation rate has its maximum at around 450℃ aging, but can hardly be observed at 350℃ and 650℃ aging. According to SEM observations, after longer aging time, the precipitates distribute more homogeneously in the grain and the multi-step transformation no longer exists. For Ti48.7Ni51.3 alloys, its higher Ni supersaturation causes the Ti3Ni4 nucleation having higher driving force and causes the precipitation rates being almost the same at grain boundaries and grain interior. This feature results in a simple B2→R→B19’ two-step transformation in aged Ti48.7Ni51.3 SMA. The multi-step transformation behavior of Ti49Ni41Cu10 SMA is also investigated in this study.
Subjects
富鎳鈦鎳形狀記憶合金
多階相變態
Ti3Ni4析出物
時效處理
DSC
SEM
Ni-rich TiNi SMAs
multi-step transformation
Ti3Ni4 precipitate
age treatment
Type
thesis