TiNi及TiNiCu形狀記憶合金多階段相變態及熱效應之研究

Abstract

本研究第一部份探討存在於Ti51Ni49薄帶、大量冷加工之Ti51Ni40Cu9合金及富鎳Ti49Ni41Cu10合金中的多階段相變態。首先，多階段相變態會出現在大小晶粒混合的快速凝固Ti51Ni49薄帶之中，此處之多階段相變態的各變態階段在冷卻時依序是整體的B2→R變態、對應大晶粒的R→B19’1變態及對應小晶粒的R→B19’2變態；在加熱時依序是對應小晶粒的B19’2→B2變態及對應大晶粒的B19’1→B2變態。其次，大量冷加工後經500oC退火72小時及650oC退火1小時的Ti51Ni40Cu9合金試片中，亦有大小晶粒之分布。其中小晶粒分布在試片表面，大晶粒分布在試片中心區域。此試片之多階段相變態的各變態階段依序分別是對應大晶粒的B21←→B191變態、對應小晶粒的B22←→B192變態及試片整體的(B191+B192)←→B19’變態。最後，多階段相變態亦會出現在500oC退火之富鎳Ti49Ni41Cu10合金中。此多階段相變態之成因與退火時所產生之Ti(Ni,Cu)2析出物相關，乃是由接近析出物附近區域的B21←→B191←→B19’1變態和遠離析出物區域的B22←→B192←→B19’2變態組合而成。本研究第二部分探討TiNiCu(Pd)合金之熱效應。首先探討經大量冷加工之Ti50Ni40Cu10合金中退火對其B2→B19和B19→B19’變態不同回復情形的影響。由實驗結果可知，兩者內耗值回復程度的不同主要和兩者單位溫度內可參與變態之體積不同有關，此不同來自B19和B19’兩種麻田散體的不同微結構。其次探討熱循環對Ti50Ni40Cu10合金中B2→B19和B19→B19’變態的不同影響程度。比較變態溫度及內耗值的改變可知，熱循環對B19→B19’變態的壓抑程度較為顯著，此不同壓抑程度亦源於B19和B19’兩種麻田散體不同的微結構。接下來探討Ti50Ni40Cu10合金在700oC ~ 1000oC空氣環境中的氧化性質，發現Ti50Ni40Cu10合金表面會形成多層結構的氧化層，由外到內的第一層是Cu2O(Ni,Ti)層；第二層是TiO2、TiNiO3和小孔洞的混合層；第三層是Ni(Ti,Cu)、TiO2和大孔洞的混合層；第四層是Ti(Ni,Cu)3層；最內層是Ti30Ni43~47Cu27~23層。由實驗結果同時可以量測出Ti50Ni40Cu10合金在大氣環境下氧化的活化能為180kJ/mol。此外，本實驗亦比較退火對有無冷軋延之Ti50Ni40Cu10合金的不同影響，研究結果顯示，經冷軋延之Ti50Ni40Cu10合金退火後會析出許多細小顆粒狀Ti(Ni,Cu)2析出物，和未經冷軋延直接退火析出板片狀Ti(Ni,Cu)2析出物之試片相比，其機械性能較佳、B19→B19’變態溫度略微上升並且出現弛豫峰特徵。最後，本實驗探討退火對Ti50Ni25-XPd25-YCuX+Y (X, Y ≦ 10at.%)合金的影響，實驗結果得知，合金試片皆為B2→B19變態，變態開始溫度在57oC到180oC之間，而以銅取代鎳和鈀會影響晶格常數、硬度及冷加工性。進一步針對Ti50Ni15Pd25Cu10合金進行退火即熱循環研究可知，此合金擁有極佳的熱穩定性，然而在450oC ~ 650oC退火時會因為Ti2Pd及Ti(Cu,Pd)2析出，使得變態溫度跟變態潛熱下降。

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.