生物可降解 Mg-5wt.%Sn 合金經等通道轉角擠型與鹼處理後在人體模擬液中應力腐蝕裂縫之研究

Abstract

本實驗將重力澆鑄法製成的生物可降解 Mg-5wt.%Sn 合金，藉由 ECAE 製程改變其顯微結構，同時透過鹼處理的方式，在合金表面形成一保護膜，利用三點彎曲試驗法的定性分析，觀察其在人體模擬液中應力腐蝕裂縫的情況。 研究結果發現，ECAE 製程可藉由動態再結晶機制有效地細化晶粒，經過兩次和四次擠型後，晶粒尺寸由 135.9µm 下降至 47.8 µm 與 18.4µm。同時因受到溫度與時間的影響，第二相 Mg2Sn 會逐漸從 α-Mg 基地中析出，使第二相的體積分率上升。經 ECAE 擠型後的 Mg-5wt.%Sn 合金，在人體模擬液中應力腐蝕裂縫的長度與深度明顯較短，因 Mg-5wt.%Sn 合金的應力腐蝕裂縫為沿晶走向，晶粒細化可使裂縫的傳播路徑更長，使裂縫難以傳播至試片深處，趨向於均勻腐蝕，而細小的晶粒加上分部相對均勻的第二相在一定程度上亦可阻礙合金的腐蝕。 經鹼處理後的 Mg-5wt.%Sn 合金因表面形成 Mg(OH)2 保護層，故可延緩合金的腐蝕，且經 ECAE 擠型四次後再經鹼處理有最佳的 抗應力腐蝕能力，可形成較為緻密的保護層，晶粒細化可補償氧化物與鎂合金之間莫耳體積的不匹配，提升鈍化膜的鈍化程度。 另外，分析 Mg-5wt.%Sn 合金在人體模擬液應力腐蝕後之產物，發現其主要成分為 Mg(OH)2、CaCO3、Ca3Mg3(PO4)4，能在合金表面上沉積 Ca-P產物，顯示其具有好的生物相容性。

In this study, biodegradable Mg-5wt.%Sn alloy showed different microstructure after Equal Channel Angular Extrusion (ECAE). In addition, a protective layer can be formed on the surface of Mg-5wt.%Sn alloy via alkali treatment. We observed the stress corrosion crack of Mg-5wt.%Sn alloy after ECAE and alkali treatment in simulated body fluid (SBF) by 3-point bending test. The results indicated that ECAE process can effectively refine the grain by dynamic recrystallization. After two and four ECAE passes, the average grain size of Mg-5wt.%Sn alloy can be refined from 135.9 µm to 47.9 µm and 18.4 µm, respectively. At the same time, more Mg2Sn phase gradually precipitated on grain boundary from a-Mg matrix due to the effect of time and temperature. Both depth and length of stress corrosion crack for ECAE processed Mg-5wt.%Sn alloy in SBF are short and tend to show a uniform corrosion behavior. Since the route of stress corrosion crack of Mg-5wt.%Sn was intergranular type, grain refinement can extend the crack propagation route and inhibit the crack propagation rate. Besides, small grain sizes allow the second phase become a corrosion barrier, and increase corrosion resistance to some extent. Alkali treatment can form a Mg(OH)2 protective layer on the surface of Mg-5wt.%Sn alloy, and the corrosion can be postponed. Mg-5wt.%Sn alloy after four passes ECAE and alkali treatment existed a more compact protective layer and showed best stress corrosion resistance. Grain refinement may help compensate for oxide/magnesium mismatch and enhance the passivity of the protective layer. The stress corrosion product of Mg-5wt.%Sn alloy in SBF is mainly Mg(OH)2, CaCO3 and Ca3Mg3(PO4)4, indicating a good biocompatibility.