Effects of Intermetallic Microstructure on Degradation of Mg-5Nd Alloy
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AGNESIUM is chemically very active. It can interact with other alloying elements to form intermetallic phases. Many of the current developed biodegradable magnesium alloys contain intermetallics.[1–7] When these alloys are immersed in corrosive media, the intermetallic compounds can couple with a-Mg to result in possible galvanic corrosion.[8–10] The corrosion resistance of Mg alloys is closely related with the distribution of these intermetallic compounds. Previous investigations indicated that such intermetallic phases with a continuous network distribution can work as a barrier to retard corrosion propagation.[11] For YAPING ZHANG, YUANDING HUANG, FRANK FEYERABEND, SARKIS GAVRAS, REGINE WILLUMEITRøMER, KARL ULRICH KAINER, and NORBERT HORT are with the Institute of Materials Research, Helmholtz-Zentrum Geesthacht, Max-Planck-Strasse 1, 21502 Geesthacht, Germany. Contact e-mail: [email protected] YULING XU is with the Shanghai Spaceflight Precision Machinery Institute, 201600 Shanghai, China. Manuscript submitted November 30, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
example, in the alloy AZ91, Mg17Al12 phase exhibits a dual purpose during corrosion in simulated body fluid, namely acting as micro-cathodes to accelerate corrosion and/or simultaneously supplying barriers to inhibit corrosion due to its different distributions.[4,12,13] The discontinuous Mg17Al12 phase can work as cathodes and deteriorate the corrosion resistance. Contrary to this, the continuous network of Mg17Al12 phase formed after T6 treatment can act as barriers to the corrosion attack.[14] Unlike in Mg-Al alloys, in Mg-Zn alloys, the continuous nanosized MgxZny phase precipitated during T6 treatment cannot form a tight barrier to inhibit the corrosion propagation. Both the discrete and continuous MgxZny phases behave as micro-cathodes to accelerate the corrosion.[15] Similar features were found in ZK40-2Nd.[16] The semi-continuous intermetallic phase in ZK40-2Nd leads to the severe corrosion attack. Nevertheless, in ZK60-3.6Nd, the honeycomb intermetallic phase restrains the propagation of corrosion.[17] Thus, whether corrosion of magnesium alloys is retarded or accelerated is highly influenced by the distribution of intermetallic compounds.
Intermetallics can be formed at high temperatures during casting or by solid-state transformation at relatively low temperatures.[18] Their precipitations are influenced by the solid solubility of alloying elements in the Mg matrix. Rare-earth (RE) elements are widely used as the major alloying elements in biodegradable magnesium alloys to improve mechanical properties and to enhance corrosion resistance. Among these RE elements, Nd has a low solid solubility (3.6 wt pct at the eutectic temperature and almost zero at the room temperature) in Mg.[19] It was reported to be a suitable element for medical applications.[20] Nd-containing biodegradable magnesium alloys, such as WE43[21,22] and WE54,[23,24] are well known for their significant aging response due to its limited solid solubility at lower temper
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