NiTi之鋁化層生成機構和氧化行為

Abstract

本實驗探討NiTi利用表面改質粉浴法中的鋁化，將NiTi鋁化以後所形成的鋁化層利用SEM/EDS分析微觀結構及其生成機構，另外在對其塗層做氧化試驗來檢測其抗氧化性。鋁化處理的參數以溫度為主，分別為800°C/4 hr和1000°C/4 hr鋁化處理，另外一組以仿照鎳基超合金的鋁化處理兩階段製程，先800°C/4 hr鋁化處理再1000°C/4 hr熱處理。由鋁化的結果顯示，在800°C/4 hr和1000°C/4 hr鋁化層相似，由外向內依序皆形成(Ni2Al3+TiAl3)、Al2NiTi、AlNi2Ti的反應層，但1000°C/4 hr在Al2NiTi和AlNi2Ti之間有Al3NiTi2形成，而在800°C/ 4 hr鋁化處理再1000°C/4 hr熱處理則由外向內依序形成(NiAl+Al2NiTi)、Al3NiTi2、AlNi2Ti。800°C/4 hr鋁化處理和800°C/4 hr鋁化處理再1000°C/4 hr熱處理的氧化層相似，氧化層皆分為內部氧化層和外部氧化層，外部氧化層以TiO2氧化層為主，內部氧化層以Ni與TiO2的兩相結構為主；而1000°C/4 hr鋁化處理後，則生成連續且具有保護性的Al2O3。在氧化試驗中，800°C /4 hr的鋁化塗層在氧化50小時後，基材即被完全氧化；兩階段製程的鋁化塗層則在氧化10小時後即基材被完全氧化；而1000°C的鋁化塗層皆未被氧化，且生成Al2O3連續保護層。前兩個試驗抗氧化不佳可能是鋁化塗層厚度不夠且不均(厚度10~20μm)。

Aluminide coatings of NiTi were prepared using pack cementation method under an argon atmosphere at temperatures ranging 800°C to 1000°C for 4 hr. The cross section and chemical compositions of the reaction layers at NiTi interfaces were characterized using analytical scanning electron microscopy. The reaction layers are (TiAl3 + Ni2Al3 ), Al2NiTi and AlNi2Ti in sequence (to NiTi substrate) after aluminization at 800°C/4 hr. The reaction layer of Al3NiTi2 was formed between Al2NiTi and AlNi2Ti after aluminization at 1000°C/4 hr.. The reaction layers using two stages (800°C/4 hr-aluminization; 1000°C/4 hr-annealing) are (NiAl + Al2NiTi), Al3NiTi2 and AlNi2Ti in sequence (to NiTi substrate). Whatever aluminide coating at 800°C/4 hr or two stages coating proceeds, the nonprotective oxide layers comprised of TiO2 and (TiO2 + Ni) after oxidation at 1000°C. However, a protective Al2O3 was formed (1000°C/4 hr-aluminization) after oxidation at 1000°C.