TiNi基形狀記憶合金制振能及非晶質薄帶之研究

Abstract

本研究主要是探討TiNi基形狀記憶合金在持溫狀態下本質內耗(IFPT+IFI)的性質。經過冷加工以及退火之Ti50Ni50形狀記憶合金在降溫過程中會產生兩階段相變態的(IFPT+IFI)B2→R及(IFPT+IFI)R→B19’內耗峰，該內耗峰的tan δ值皆和 成線性正比的關係，表示Ti50Ni50形狀記憶合金在持溫狀態下制振能的機制和應力促使雙晶界面的移動有關。在不同參數下所量測到 (IFPT+IFI)R→B19’的tan δ值皆較(IFPT+IFI)B2→R來得高，主要原因是因為R→B19’相變態的變態應變較大且雙晶界面較多的緣故。單一R相及B19’相的本質內耗IFI皆是由靜態的IFS以及動態的IFD所組成。IFSR及IFSB19’的tan δ值皆和 成線性正比的關係，表示其制振能機制和應力促使雙晶界面的移動有關。在不同參數下所量測到IFSR的tan δ值皆較IFSB19’來得高，這這個特性和R相具有較低的storage modulus有關。IFDB19’的tan δ值和 成線性正比的關係，且在-60 oC出現的relaxation peak僅和IFSB19’有關，而和IFDB19’無關。在B2→B19’一階段相變態中，所量測到本質內耗峰(IFPT+IFI)B2→B19’的 tan δ值同樣和 成線性正比的關係，但由於(IFPT+IFI)B2→B19’在相變態過程中沒有R相的產生，因此其tan δ值低於有R相產生之(IFPT+IFI)B2→R及(IFPT+IFI)R→B19’的值。Ti51Ni39Cu10形狀記憶合金經過固溶處理後，因為試片沒有冷加工所導入的缺陷、差排以及Ti3Ni4析出物，因此具有較Ti50Ni50 形狀記憶合金高的tan δ值及較廣的變態溫度範圍。 利用melt-spinning製備的Ti50Ni25Cu25薄帶為完全非晶質狀態，且因為Ti50Ni25Cu25薄帶具有較Ti50Ni50薄帶高的Cu含量，因此其Qp值較非晶質Ti50Ni50薄帶低。此外，Ti50Ni25Cu25薄帶的結晶活化能Ea以及結晶起使溫度Tx都較非晶質Ti50Ni50薄帶低。當Ti50Ni25Cu25薄帶在500 oC退火3分鐘，初始結晶的晶粒含Cu量較低，因此展現較佳的形狀記憶效應。Ti50Ni25Cu25薄帶中結晶晶粒的數量隨著退火時間的增長而逐漸增加，但是Ti50Ni25Cu25薄帶的可回復應變卻逐漸降低並且開始脆化。這個特性是因為已結晶晶粒中的Cu含量亦隨著退火時間而逐漸增加。因此，Ti50Ni25Cu25薄帶只有在適當地控制退火條件下才會具有良好的形狀記憶性質。Ti50Ni25Cu25非晶質薄帶利用持溫DSC實驗以及Johnson-Mehl-Avrami方程式所得到的Avrami exponent 值約為3.0，表示 Ti50Ni25Cu25非晶質薄帶主要的結晶機制為界面控制，三維等向成長及早期成核飽和。根據Arrhenius relation，Ti50Ni25Cu25非晶質薄帶的結晶活化能為314 kJ/mol，這個結果和利用Kissinger方法所得到的結果相仿，表示Ti50Ni25Cu25薄帶在固定升溫速率以及持溫狀態下的結晶機制相當類似。

The inherent internal friction (IFPT+IFI) of cold-rolled and annealed Ti50Ni50 alloy is studied under isothermal conditions. The tan δ values of two-stage (IFPT+IFI)B2→R and (IFPT+IFI)R→B19’ are both proportional to and thus the damping mechanism of (IFPT+IFI)B2→R and (IFPT+IFI)R→B19’ is related to stress-assisted martensitic transformation and stress-assisted motions of twin boundary. The tan δ value of (IFPT+IFI)R→B19’ is larger than that of (IFPT+IFI)B2→R because the larger transformation strain and the greater twin boundaries associated with R→B19’ transformation. The intrinsic internal friction IFI of R-phase and B19’ martensite are composed of static internal friction IFS and dynamic internal friction IFD. The tan δ values of IFSR and IFSB19’ are both proportional to and are related to the stress-assisted motions of twin boundaries. The tan δ values of IFSR are higher than those of IFSB19’ is owing to the softer storage modulus E0 in R-phase. The tan δ values of IFDB19’ are linear proportional to . The occurrence of relaxation peak at -60 oC is found to come from the IFSB19’, instead of the IFDB19’. The tan δ value of (IFPT+IFI)B2→B19’ is also proportional to and its damping mechanism is thus related to stress-assisted martensitic transformation and stress-assisted motion of twin boundaries. The tan δ value of one-stage (IFPT+IFI)B2→B19’ is smaller than those of (IFPT+IFI)B2→R and (IFPT+IFI)R→B19’ because the former shows no R-phase. The solution-treated Ti51Ni39Cu10 alloy displays a higher tan δ value and a wider transformation temperature range than cold-rolled and annealed Ti50Ni50 SMA because there are no cold-rolled defects or dislocations, as well as no Ti3Ni4 precipitates in the former. As-spun Ti50Ni25Cu25 ribbon is fully amorphous with a lower wavenumber Qp than the amorphous Ti-Ni alloys owing to its high Cu content. Both crystallization activation energy Ea and onset temperature Tx for Ti50Ni25Cu25 ribbon are lower than those for Ti50Ni50 ribbon. When Ti50Ni25Cu25 ribbon is annealed at 500 oC for 3 min, the initial as-crystallized grains contain a low Cu content and perform a prominent shape memory effect. Through prolonging the annealing time, more grains are crystallized in the ribbon but it becomes more fragile and its recoverable strain decreases. This characteristic is due to the increasing Cu content in the crystallized grains. Crystallized Ti50Ni25Cu25 ribbon can exhibit a good shape memory effect only under appropriate annealing conditions. The Avrami exponent of Ti50Ni25Cu25 amorphous ribbons during isothermal annealing derived from the Johnson-Mehl-Avrami equation is about 3.0. This indicates that the main crystallization mechanism of Ti50Ni25Cu25 ribbons is interface-controlled three-dimensional isotropic growth with early nucleation-site saturation. According to the Arrhenius relation, the activation energy for crystallization is 314 kJ/mol. This value is similar to that obtained using the Kissinger method, which implies that the crystallization during continuous heating or isothermal annealing follows a similar crystallization mechanism.