Studies on Multi-stage Transformations and Thermal Effects Exhibited in TiNi and TiNiCu Shape Memory Alloys

• Main Authors: Kai-Nan Lin, 林凱南
• Other Authors: 吳錫侃
• Format: Others
• Language: en_US
• Published: 2009
• Online Access: http://ndltd.ncl.edu.tw/handle/91246584425379311029

博士 === 國立臺灣大學 === 材料科學與工程學研究所 === 97 === Multi-stage transformations (MST) exhibited in annealed Ti51Ni49 melt-spun ribbon, in severely cold-rolled and annealed Ti51Ni40Cu9 alloy and in annealed Ni-rich Ti49Ni41Cu10 alloy are investigated in the first part of this thesis. The grain-size mixed as-spun and annealed Ti51N49 ribbons show MST behavior. The MST transformation peaks are associated with B2→R transformation, R→B19’1 transformation for large grains and R→B19’2 transformation for small grains during cooling, and B19’1→B2 transformation for large grains and B19’2→B2 transformation for small grains during heating. For cold-rolled Ti51Ni40Cu9 alloy annealed at 500oC × 72h and 650oC × 1h, the specimens have small grains near the rolling surfaces and large grains in the central region. The MST peaks are associated with B21←→B191 transformation of large grains, B22←→B192 transformation of small grains and (B191+B192)←→B19’ transformation of both large and small grains. For annealed Ni-rich Ti49Ni41Cu10 alloy, Ti(Ni,Cu)2 precipitates are formed in 500oC annealed specimens. Specimens annealed at 500oC for 6h ~ 24h exhibits MST which is confirmed to be composed of B21←→B191←→B19’1 and B22←→B192←→B19’2 transformations corresponding to the regions near and far from Ti(Ni,Cu)2 precipitates, respectively. Thermal effects exhibited in TiNiCu(Pd) alloys are investigated in the second part of this thesis. First, for the annealing effect, different recovery behaviors of B2→B19 and B19→B19’ transformations of cold-rolled and annealed Ti50Ni40Cu10 alloy are studied. The change of internal friction values of these two transformations affected by annealing is mainly due to the difference in the change rate of transformation volume between them, which is related to their different recovery behaviors and microstructures. Second, the thermal cycling effect on B2→B19→B19’ transformations of cold-rolled and annealed Ti50Ni40Cu10 alloy is studied. The transformation peak temperature and its tanδ peak value of B19→B19’ transformation are more suppressed by thermal cycling than those of B2→B19 which is also owing to the different microstructures between B19 and B19’ martensites. Third, the isothermal oxidation behavior of Ti50Ni40Cu10 alloy in 700oC ~ 1000oC air is investigated. The multi-layered oxide scale is formed, consisting of an outermost Cu2O(Ni,Ti) layer, a layer of the mixture of TiO2, TiNiO3 and irregular small pores, a layer of the mixture of Ni(Ti,Cu), TiO2 and irregular large pores, a Ti(Ni,Cu)3 layer and an innermost Ti30Ni43~47Cu27~23 layer. The apparent activation energy for the oxidation reaction is determined to be 180kJ/mol. Fourth, cold-rolling effect on martensitic transformation of annealed Ti50Ni40Cu10 alloy is also investigated. Cold-rolling defects can act as nucleation sites. Particle-like Ti(Ni,Cu)2 precipitates formed in cold-rolled and annealed alloy are more and smaller with higher volume than plate-like Ti(Ni,Cu)2 precipitates formed in hot-rolled and annealed alloy. This feature affects the alloy’s mechanical properties, transformation temperatures, chemical composition in matrix and the formation of relaxation peak. Finally, the annealing effect and martensitic transformation of Ti50Ni25-XPd25-YCuX+Y quaternary alloys with X, Y ≦ 10at.% which exhibit B2←→B19 transformation with Ms temperature in the range of 57oC ~ 180oC are studied. The substitution of Ni/Pd by Cu affects the lattice constants, the hardness and the cold-rolling workability. Ti50Ni15Pd25Cu10 alloy has quite good thermal stability. However, Ti50Ni15Pd25Cu10 alloy annealed in between 450oC ~ 650oC shows obvious decrease for both Ms temperature and △Hc value, especially at 550oC, due to Ti2Pd and Ti(Cu,Pd)2 formation.