Be與Fe含量對A357合金腐蝕及應力腐蝕性質之影響

Effects of Be and Fe Content on the Corrosion and Stress Corrosion Cracking Susceptibility of A357 Alloys

本研究主要目的在利用電化學方法與慢應變速率拉伸試驗，探討A357合金中不同Be與Fe 含量，對其微結構、腐蝕及應力腐蝕性質之影響。實驗結果顯示，不加Be的A357合金含有大量的針狀富鐵相及文字型含Mg的富鐵相，Be的添加可減少富鐵相的含量，並改善其成分與形狀，使針狀富鐵相與含Mg的文字型富鐵相被較無害的不含Mg結節狀富鐵相取代。由於含Mg 文字型富鐵相的減少，使固溶於基地內的Mg含量增加，促進β'-Mg2Si的析出動力與析出量。富鐵相數量的增加使材料的抗腐蝕能力降低，然而在晶界處並沒有大量富鐵相的存在，富鐵相的數量與形態對合金抗應力腐蝕性質無明顯之影響。A357合金晶界析出物數量少，此少量析出物彼此沒有連結，且其活性與基地相近，可有效阻擋應力腐蝕裂縫成長，致使A357合金具有良好的抗應力腐蝕性質。

Effects of Be and Fe content on the corrosion properties and stress corrosion cracking (SCC)of A357 alloys in 3.5%NaCl solution were investigated by electrochemical test and slow strain rate tension (SSRT) technique. The variation of microstructure of the alloys was analyzed to compare with the variation of mechanical properties and stress corrosion properties. A larger amount of acicular shape and scrip morphology of Mg-containing structure of iron-bearing phases were found in Be-free alloys. These structures are replaced by nodular shape Mg-free structure of iron-bearing constituents when Be is added. Addition of Be reduces the amount of iron-bearing phases, increases the amount of solid solution of Mg. Be can enhance the precipitation kinetics and increase the amounts of β'-Mg2 Si precipitations to promote the tensile properties. With the increasing the amount of iron-bearing phases, the corrosion resistance is reduced. The amounts, composition and shape of iron-bearing phases are insignificant about stress corrosion cracking (SCC) resistance. Less amounts, discontinuous arrangement and similar activity to matrix of the precipitations at grain boundary are unfavorable for grow of the crevices, which make A357 alloys possess excellent SCC resistance.