鎂錫合金於不同腐蝕環境中之應力腐蝕性質探討及其改善

Abstract

鎂合金為近來受到矚目的輕型結構性材料，廣泛的應用於汽車、航太…等工業應用。再加上鎂合金於人體環境中易發生降解，近來於生醫材料上的應用逐漸受到重視。因此，本研究將針對鎂錫合金在不同環境中的應力腐蝕性質進行研究，並研究其改善方法。 針對不同錫合金添加對鎂合金於3.5% NaCl 溶液中的應力腐蝕性質做探討。本研究係以重力澆鑄法製成鎂錫合金鑄錠，錫添加重量百分比為2、5以及8 (S2、S5、S8)，分別對各材料進行均質化(homogenization annealing heat treatment, HA)以及固溶熱處理(solution heat treatment, SHT)，以分析錫析出物以及錫於鎂基地對應力腐蝕之影響。研究結果顯示， HA-S2、HA-S5以及HA-S8試片之平均晶粒大小約為90-100μm，因此本實驗忽略晶粒大小之影響。鎂錫合金之析出物為Mg2Sn，主要沿著晶界析出。Mg2Sn的平均體積分率於HA-S2、HA-S5以及HA-S8試片分別為0.07 ± 0.02%, 1.98 ± 0.42%, and 5.06 ± 0.92%。又，由能量色散X射線分析顯示，三試片之基地之錫含量不同，因此本實驗中，主要影響鎂錫合金之應力腐蝕性質因素有二: Mg2Sn析出相以及鎂基地中之錫含量。SHT試片經三點彎曲試驗顯示鎂基地中之錫含量對應力腐蝕之影響甚小，可視為實驗誤差。接著，針對HA試片進行應力腐蝕試驗，可發現平均孔蝕密度隨著錫含量的添加隨之上升，其原因為析出相與鎂基地之電位差所導致的伽凡尼腐蝕。此推論可藉由HA以及SHT試片之平均孔蝕密度比較得到證實，SHT試片之孔蝕密度均低於HA試片。裂縫之延伸圖顯示，本實驗之鎂錫合金應力腐蝕機制為沿晶腐蝕。HA-S8試片具有最高之腐蝕速率，其原因為高體積分率第二相所導致之伽凡尼腐蝕。反之，固溶處理之試片消除第二相之影響，降低裂縫之延伸速率，SHT-S8具有最高之抗應力腐蝕能力。 接著，藉由先前之研究結果，選用S2試片作為鎂錫合金於生物體內之應力腐蝕試驗材料。由先前研究發現鎂錫合金之應力腐蝕機制為沿晶腐蝕，因此本實驗希望藉由等通道轉角擠型(equal–channel angular extrusion, ECAE)來改善鎂錫合金於模擬體液中應力腐蝕性質。ECAE製程同時改變晶粒以及第二相分布。經過四道次ECAE製程，平均晶粒大小由S2試片之123.61 μm降至34.32 μm。且第二相之體積分率隨著ECAE道次增加而增加，顯微結構隨著ECAE製程變得更加均勻。三點彎曲試驗顯示，隨著ECAE製程，平均裂縫密度增加但降低平均裂縫深度。ECAE製程增加第二相體積分率，增加第二相所導致之伽凡尼腐蝕效應，進而增加平均裂縫密度。然而，高裂縫密度之試片，裂縫之互相影響，阻礙裂縫之成長，ECAE製程可有效降低裂縫延伸速率。

Magnesium alloys as one of the lightest structure are widely applied in material automotive field and aerospace construction. Moreover, since the degradation property of magnesium alloys, the application as bio-materials becomes attractive. In this study, we studied the stress corrosion behaviors of magnesium-tin alloys in various environment and its modifications. We studied the effect of Sn addition on stress corrosion behavior in 3.5% NaCl solution. Three Mg–Sn alloys were prepared with added Sn concentrations of 2, 5, and 8 wt.%, which denoted as S2, S5 and S8. Samples underwent homogenization annealing and solution heat treatment, denote as HA and SHT, to study the effect of second phase and matrix Sn on stress corrosion behaviors. The average grain size of samples HA-S2, HA-S5, and HA-S8 ranged from 90 to 100 μm. The result revealed that the difference in average grain size among the three samples is small; thus, the effect of grain size on the stress corrosion behavior was disregarded for HA samples. The average volume fractions of the Mg2Sn phase were 0.07 ± 0.02 %, 1.98 ± 0.42 %, and 5.06 ± 0.92 % for samples S2, S5, and S8, respectively. To evaluate the effect of matrix Sn on the stress corrosion behaviors, we analyzed each HA sample using EDS. The differences could be observed in the matrix Sn contents among the three HA samples. Thus, in the case of this study, we proposed that these two variables—the Sn content in the matrix and the presence of the Mg2Sn second phase—affect the SCC behaviors. The average pit density remained essentially constant for SHT samples. Thus, the effect of matrix Sn on pit density was slight and could be ignored in this study. The average pit density of HA samples increased with increasing Sn content. This revealed that increasing pit density of HA samples were correlated with increasing volume fraction of second phase. An intergranular cracking mode was observed. The solution heat treatment dissolved the Mg2Sn phase and eliminated the micro-galvanic corrosion due to Mg2Sn, thereby enhancing stress corrosion resistance. Mg-8%Sn sample through solution heat treatment showed the best stress corrosion resistance. The stress corrosion behavior of S2 sample in simulated body fluids (SBF) was systematically investigated to determine its performance in a physiological environment. The effect of equal-channel angular extrusion (ECAE) on the stress corrosion behavior of S2 sample was investigated using three-point bending test. The ECAE process changed both grain size and Mg2Sn second phase distribution. The grain size under homogenization annealing condition (about 123.61 μm) was significantly reduced to 34.32 μm after four ECAE passes. The microstructure became more homogeneous with further ECAE passes. In bent-beam stress corrosion test, the average crack depths of samples underwent ECAE process were significantly lower than that of sample under homogenization annealing condition. This showed the evidence that the ECAE process could reduce crack propagation rate. The enhancement of stress corrosion resistance in this case was attributable to uniform microstructure and the grain refinement via ECAE process.