鎳基合金之電化學鑄造研究

The Electroforming of Nickel-based Alloys

本研究利用不同電流密度（3～13A/dm2）所得之電鑄鎳及不同錳含量之電鑄鎳錳合金（Mn %為0.14及0.2%），進行各種溫度（300、500和700及時間（4和22hrs）退火處理，再測試室溫及高溫（300℃～600℃）之抗拉強度及延伸率，最後以電子顯微鏡（SEM和TEM）觀察其顯微組織，以期進一步了解電化學參數、機械性質及顯微結構三者之關係。研究結果顯示，電流密度越高之電鑄鎳，其抗拉強度下降，延伸率則上昇，當電流密度為3A/dm2，其抗拉強度892MPa，此為較佳之操作條件，當拉伸溫度超過400℃時，有明顯之沿晶（Intergranular）硫脆現象及靱性（Toughness）下降趨勢，裂口呈脆性特徵。鎳錳合金（Mn=0.25%）抗拉強度可達1233.8MPa，且無高溫硫脆現象及韌性下降情形，裂口呈延性特徵。電鑄鎳經由TEM觀察其顯微組織，含細小的晶粒（0.2～0.7µm）與高密度的差排及雙晶，此為其強化之原因，而電鑄鎳錳合金更增加了錳原子的固溶強化效果。

The purpose of this study is to understand the relationship among the electrochemical parameters, mechanical properties and microstructures of electroforming nickel-based alloys. This study investigated the tensile properties of the electroforming (EF) nickel at different current densities (3A～13A/dm2) and EF nickel-manganese alloys with different Mn content (Mn%=0.14, 0.25%) which were annealed in an argon-purged furnace at 300~700℃ for 4 or 22 hrs at room and elevated temperatures (300~600℃). Finally, microstructures were examined by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The results of investigations showed that the higher the applied current density, the lower the tensile stress and the higher elongation of EF Ni are. The tensile stress was 892 MPa at current density of 3 A/dm2. As the test temperature was higher than 400℃, the fracture surface revealed intergranular brittle cracking, caused by sulfur embrittlement. The tensile stress of an EF Ni-0.25%Mn alloy achieved 1233.8 MPa, and its fracture surface showed no high temperature sulfur embrittlement and belonged to ductile fracture characteristic. Based on TEM observations, the microstructure appeared fine grains (0.2~0.7 µm)、high density dislocations and twins, which strengthen the EF Ni alloys. In addition, Mn had the solid solution strengthening effect for the EF Ni-Mn alloys.