"Ti49.6Ni50.4棒材,Ti48.9Ni51.1及Ti50Ni48Fe2形狀記憶合金塊材之彈熱效應研究"

Abstract

本研究針對Ti49.6Ni50.4 棒材、Ti48.9Ni51.1 及 Ti50Ni48Fe2 形狀記憶合金塊材進行研究，探討熱循環及應力循環對於形狀記憶合金之變態溫度與彈熱效應的影響，以及不同操作溫度下之彈熱效應。經過時效處理之Ti49.6Ni50.4_275℃×125 hr與TiNi51.1_400 ℃×50 hr試片具有最良好的熱循環、應力循環穩定性以及彈熱效應性能，是因為在時效處理時產生之Ti3Ni4析出物，具有析出強化的效果，能夠有效阻擋循環時所產生之差排，提高材料之循環穩定性。並使Ti49.6Ni50.4_275℃×125 hr與TiNi51.1_400 ℃×50 hr試片於應力循環的過程中保持較高之溫度變化量，不易因循環次數增加而下降，顯示經析出硬化後，可擁有較佳之超彈性性能，並提供較高之溫度變化量。然而，由熱循環與應力循環之結果中可以發現，TiNi基形狀記憶合金經時效處理產生Ti3Ni4析出物後，雖然可有效提高其熱循環穩定性，並於超彈性加壓時減少差排生成，提高應力循環之穩定性，但於應力循環的過程中仍能觀察到相變態溫度有大幅度的改變，顯示析出硬化雖可使材料於單純熱循環下有良好的穩定效果，但對於應力循環之抵抗力較不足夠，表示應力對於形狀記憶合金變態行為的劣化影響，大於單純之熱循環之影響。不同溫度下之彈熱效應性能同樣以經時效處理之Ti49.6Ni50.4_275℃×125 hr與TiNi51.1_400 ℃×50 hr試片最佳，於 -40 ℃至150 ℃之溫度範圍內具有良好之彈熱效應表現，最高可達10 ℃~12 ℃。此外可以利用中間相R相之變態，擴展可產生彈熱效應之溫度範圍，即使溫度低於Rf變態溫度，仍可利用R↔B19’之間之相變態提供溫度變化，顯示導入B2↔R↔B19’之二階相變態可有效擴展彈熱效應之使用溫度範圍。透過數位影像關係法與紅外線之影像分析，可以有效地觀察出TiNi基形狀記憶合金於超彈性變態時之相變態趨勢、應力場分布以及溫度變化量分布，可以發現所有試片皆為均勻之麻田散體相變態。

In this study, the effects of thermal/stress cycles on transformation temperature, the effect of stress cycles on elastocaloric effect and the elastocaloric effect at different operating temperatures in Ti49.6Ni50.4 bar, Ti48.9Ni51.1, and Ti50Ni48Fe2 shape memory alloy blocks were investigated. The Ti49.6Ni50.4_275℃×125 hr and TiNi51.1_400 ℃×50 hr aged specimens exhibited the best thermal/stress cycles stability and elastocaloric effect performance, because of the strengthing effect of Ti3Ni4 precipitations formed during aging treatment. The precipitates could effectively hinder the dislocation movement during thermal/stress cycles and improved the functional stability of the material. The Ti49.6Ni50.4_275℃×125 hr and TiNi51.1_400 ℃×50 hr aged specimens maintained a high-temperature change during the stress cycles, indicating that they could contribute better superelasticity after precipitation hardening. However, it is noted from the results of thermal cycles and stress cycles that, although precipitation hardening could effectively improve the thermal cycles stability of TiNi-base shape memory alloys, the phase transition temperatures obviously shifted during the stress cycles. This features indicated that the precipitation hardening could improve the functional stability during thermal cycling but was less effective to resist dislocation motion during stress cycling. As a consequence, the effect of stress on the deterioration of the transformation behavior of shape memory alloy was found greater than the effect of thermal cycling. The Ti49.6Ni50.4_275℃×125 hr and TiNi51.1_400 ℃×50 hr aged specimens also showed better elastocaloric performance. At the temperature range of -40 °C to 150 °C, these two samples showed higher temperature change of cooling during unloading than the other specimens, the temperature change up to 10 ℃~12 ℃. In addition, it was found that the operating temperature range could be expanded by introducing R phase transformation, even if the operating temperature was lower than the Rf, the phase transformation between R↔B19' could be utilized to provide a temperature change, indicating that the two-stage phase transition of B2↔R↔B19' can effectively extend the operating temperature range of the elastocaloric effect. In this study, by utilizing digital image correlation(DIC) and infrared image analyses, the strain field distribution and temperature distribution of TiNi-based shape memory alloy superelastic can be directly observed and measured, and all specimens ware found to undergo uniform phase transformation.