添加稀土元素對鐵基形狀記憶合金 腐蝕與沖蝕特性影響之研究

Abstract

本研究利用真空電弧重熔法(VAR)，分別配製 Fe–25Mn–6Si，Fe–25Mn–6Si–0.03RE，Fe–25Mn–6Si–5Cr和Fe–25Mn–6Si–5Cr–0.11RE形狀記憶合金，並分析其抗腐蝕與抗沖蝕特性，藉以闡明添加稀土元素對於鐵基形狀記憶合金腐蝕與沖蝕特性之影響。 實驗結果顯示浸泡在3.5% NaCl 溶液中，Fe–25Mn–6Si–5Cr合金有最佳的抗腐蝕性，Fe–25Mn–6Si–5Cr 和 Fe–25Mn–6Si–5Cr–0.11RE合金在浸泡實驗時其表面會有腐蝕產物覆蓋，使曲線小幅改變，而後發現有孔蝕的現象，形成許多腐蝕坑。電化學腐蝕試驗顯示，Fe–25Mn–6Si–5Cr合金的腐蝕電位最高，即抗腐蝕性最好。而應力腐蝕試驗中，無論是在鹽水中還是大氣環境中，Fe–25Mn–6Si–5Cr合金均有最大的破裂應力。由此可知， Fe-Mn-Si記憶合金添加Cr、RE等元素皆可幫助記憶效應，但Cr增加抗腐蝕性而RE卻略下降其抗腐蝕性。 經由坑穴沖蝕及氣沙沖蝕試驗得知， Fe-Mn-Si記憶合金中，添加Cr會強化材料而抗坑穴沖蝕，而添加RE則因較易應力誘發麻田散鐵而略為增加抗坑穴沖蝕性。靶材重量損失越大者，其表面平均粗度值也越大，鐵基記憶合金沖蝕後呈現加工硬化的現象。Fe–25Mn–6Si、Fe–25Mn–6Si–0.03RE合金經沖蝕後的表面已大部分被破壞；而Fe–25Mn–6Si–5Cr和Fe–25Mn–6Si–5Cr–0.11RE合金則僅呈現局部沖蝕損傷。在固定的氣沙沖擊速度下，Fe–25Mn–6Si、Fe–25Mn–6Si–0.03RE合金的沖蝕速率均大於添加Cr的合金，而沖蝕速率也隨沖蝕時間增加而增加，添加RE會使本身抗氣沙沖蝕的能力略微下降。選擇不同的角度衝擊試片時，在角度30度時，呈現沖蝕速率的最大值。其顯微組織依沖擊角度增加，表面形態皆由狹長犁溝流線痕跡逐漸轉變為突起屑片重疊覆蓋之形態。

The Fe–25Mn–6Si, Fe–25Mn–6Si–0.03RE, Fe–25Mn–6Si–5Cr and Fe–25Mn–6Si–5Cr–0.11RE shape memory alloys are prepared by VAR technique. The effects of slight addition of rare-earth element (RE) on the corrosion and erosion characteristics of Fe-based shape memory alloys are systematically investigated. Experimental results show that in a 3.5% NaCl solution, the Fe–25Mn–6Si–5Cr alloy has the best chemical corrosion resistance. The corrosion product of the Fe–25Mn–6Si–5Cr and Fe–25Mn–6Si–5Cr–0.11RE alloys will cover the specimen surface and exhibit a slight weight loss during the immersion tests. In addition, the Fe–25Mn–6Si–5Cr and Fe–25Mn–6Si–5Cr–0.11RE alloys will be locally attacked and introduce the pitting corrosion. During the electrochemical corrosion test, the Fe–25Mn–6Si-5Cr alloy has higher corrosion potential than Fe-25Mn-6Si, Fe-25Mn-6Si-0.03RE and Fe-25Mn-6Si-5Cr-0.11RE alloys. It indicates that the Fe-25Mn-6Si-5Cr has a better corrosion resistivity. In the stress-corrosion cracking test, the Fe–25Mn–6Si–5Cr alloy has the highest fracture stress among these alloys both in the atmosphere and 3.5% NaCl solution. It is also found that the RE addition, which can increase the alloy’s shape memory effect, will slightly degrade the corrosion resistance of Fe-Mn-Si-Cr alloys. Based on the results of cavitation erosion test, Fe-based shape memory alloys with addition of Cr and RE elements will exhibit an excellent erosion resistance. The surface roughness increases with increasing the erosion weight loss. The work hardening phenomenon at the surface occurs during the cavitation erosion tests for Fe-based memory alloys. After cavitation erosion tests, the surface of the Fe-25Mn-6Si-5Cr and Fe-25Mn-6Si-5Cr-0.11RE are only locally attacked, but the Fe–25Mn–6Si and Fe–25Mn–6Si–0.03RE alloys exhibit severely attacked surfaces with peeling-off big area. In the gas-sand impingement test, the erosion rates of Fe–25Mn–6Si and Fe–25Mn–6Si–0.03RE alloys are higher than the Fe–25Mn–6Si–5Cr and Fe–25Mn–6Si–5Cr–0.11RE alloys. The erosion rate increases with increasing impingement velocity. The addition of RE into Fe-based memory alloys will degrade their erosion resistance of gas-sand impingement. The maximum of erosion rate occurs at the impingement angle of 30o for Fe-based shape memory alloys. The impinged surface morphologies of Fe-based alloys exhibit a lot of long and narrow furrows at lower impingement angles, but the overlapped chips at higher impingement angles.