- 學位論文
晶體方位對單晶CuAl17Mn12與CuAl17Mn11Mo0.5形狀記憶合金之超彈性與彈熱效應影響之研究
Research on the Superelasticity and the Elastocaloric Effect of CuAl17Mn12 and CuAl17Mn11Mo0.5 Single-crystal Shape Memory Alloys
摘要
本研究透過循環熱處理誘發異常晶粒成長來製成不同方位之CuAl17Mn11Mo0.5單晶與CuAl17Mn12單晶之形狀記憶合金塊材，以研究晶體方位對超彈性與彈熱效應之影響。此外，也研究[ 1 2 5 ]、[ 4 4 5 ]、[ 3 3 25 ]與[ 5 7 18 ]方位之CuAl17Mn11Mo0.5單晶與[ 3 6 10 ]方位之CuAl17Mn12單晶於不同操作溫度下之超彈性與彈熱效應。DSC結果顯示，不同晶體方位CuAl17Mn11Mo0.5單晶之相變態溫度相近。超彈性實驗結果顯示，相變態應變與臨界應力對晶體方位有很大的相關性；當晶體方位接近[ 0 0 1 ]的相變態應變較小，但臨界應力最大；而晶體方位接近[ 1 1 1 ]的相變態應變較大，但臨界應力最小，實驗結果趨勢與理論吻合。其中，部分試片之應力-應變圖在相變態平台後，會有一段應力隨著應變增加而急劇增加的部分，該部分代表試片之麻田散體發生彈性變形。透過DIC分析[ 1 2 5 ]與[ 7 8 49 ]方位之CuAl17Mn11Mo0.5單晶之應變場，發現麻田散體彈性變形與麻田散體相變態會同時發生。彈熱效應實驗結果顯示，大部分的試片在同成分且在相近的相對壓縮應變下，會有相近的卸載溫差。變溫實驗結果顯示，在最低操作溫度至Md溫度區間中，臨界應力會隨著操作溫度上升而增加。不同晶體方位之Clausius-Clapeyron equation之斜率與Md不一樣，同成分當中，接近[0 0 1]方位之Clausius-Clapeyron equation之斜率為當中最小的，而其Md為最高的；而接近[1 1 1]方位之Clausius-Clapeyron equation之斜率為當中最大的，而其Md為最低的；至於其他晶體方位之Clausius-Clapeyron equation之斜率與Md則介於中間。此外，CuAl17Mn11Mo0.5單晶在Md至約140°C時，臨界應力會隨著操作溫度上升而減少，推測是因為差排或殘留麻田散體之應力場促使變態所需之應力下降。在約140°C以上時，臨界應力又會隨著操作溫度上升而增加，推測是因為omega相析出而阻礙應力誘發麻田散體相變態所導致。
並列摘要
In this research, cyclic heat treatment was applied to induce abnormal grain growth in order to prepare single-crystal CuAl17Mn11Mo0.5 and CuAl17Mn12 bulk shape memory alloys with different grain orientations. The purpose of preparing single-crystal samples was to investigate the dependence of the superelasticity and the associated elastocaloric effect on crystal orientations. Among these single-crystal samples, the following grain orientations, including [1 2 5], [4 4 5], [3 3 25], and [5 7 18] orientated single-crystal CuAl17Mn11Mo0.5 alloys and [3 6 10] orientated single-crystal CuAl17Mn12 alloy, were tested at different operation temperatures. The martensite transformation temperatures of differently orientated CuAl17Mn11Mo0.5 single-crystals were similar by DSC measurements. The transformation strains of these alloys were closely dependent on crystal orientations. Samples closed to [0 0 1] orientation had low critical stresses of martensite transformation and large transformation strains, while samples closed to [1 1 1] orientation sample had high critical stresses and small transformation strains. These results matched well with predicted theoretical strains. DIC analysis showed that martensite transformation and elastic deformation happened simultaneously at the end of the superelastic plateau. Tests on the elastocaloric effect showed that most smaples (with the same composition) exhibited similar temperature drops under a similar relative compressive ratio. Experimental results showed that the slopes of Clausius-Clapeyron relation of differently orientated samples were different. Grains near [0 0 1] orientation had a lower slope and higher Md, whereas grains near [1 1 1] orientation had a higher slope and lower Md. The other samples with intermediate orientations exhibited slope and Md between the values of [0 0 1] and [1 1 1] oriented grains. In addition, it was noted that while the operating temperature was increased to between its Md and about 140 degree Celsius, the critical stress for martensitic transformation decreased with increasing temperature, which may be caused by the dislocation slip during loading and residual martensite after unloading. In contrast, while the operating temperature was above about 140 degrees Celsius, the critical stress for martensitic transformation increased with increasing operating temperature. This feature might originate from the precipitation of the omega phase at these temperatures, which impeded stress-induced martensitic transformation, and thus higher stress was required for triggering superelasticity.