Influence of alloying and processing on the degradation properties of Mg-Dy-Zn alloys under physiological conditions

Alloying and processing are critical strategies for enhancing the mechanical properties of Mg-based biomaterials. However, the intermetallics can precipitate during these procedures and subsequently introduce the galvanic corrosion, which may accelerate the degradation of Mg alloys. To investigate the types, amounts and distributions of intermetallics and their effects on the degradation behavior, Dy and Zn were selected as alloying elements, which have been reported to have good cytocompatibility and improve mechanical properties. Three Mg-xDy-1.5Zn (x = 5, 10, and 15 wt. %) alloys were prepared via permanent mold casting, followed by two distinct post-processing routes: direct extrusion and extrusion with a 48 h preheating treatment. The degradation behavior was investigated through immersion tests in DMEM + 10% FBS under cell culture conditions. In the as-cast alloys, Mg-5Dy-1.5Zn contained only small amount of intermetallics which were composed of W, γʹ and 18R LPSO phases and acted as galvanic cathodes, accelerating the degradation of the Mg matrix. With the Dy content increasing to 10 and 15 wt. %, large amounts of intermetallics including 18R LPSO and dense γʹ phases were formed, which can instead serve as a continuous network barrier to retard degradation propagation. It was revealed that the existence of intermetallics exhibited different effects on the degradation behavior of the alloys. At the early stage of immersion, the Mg-10Dy-1.5Zn alloy suffered the most serious degradation among these three alloys, owing to its more severe micro galvanic corrosion. With the immersion proceeding, the degradation rate of the Mg-5Dy-1.5Zn alloy continuously increased because of the scattered distribution of a small number of intermetallics. In contrast, the continuous network structure of intermetallics and a compact degradation layer provided protection from further degradation for the Mg-10Dy-1.5Zn and Mg-15Dy-1.5Zn alloys. In the extruded as-cast alloys, the Mg-5Dy-1.5Zn alloy primarily contained a small amount of fragmented W and LPSO phases, both acting as galvanic cathodes. With the Dy content increasing to 10 and 15 wt. %, the fragmented and extrusion-aligned 18R LPSO, 14H LPSO and γʹ phases were formed. The fragmented W phase, contained in the Mg-5Dy-1.5Zn alloy, exhibited a significantly higher volta potential difference than the LPSO phase in the Mg-10Dy-1.5Zn and Mg-15Dy-1.5Zn alloys, leading to a more severe galvanic corrosion. Consequently, its degradation rate continuously increased from 1 day to 14 days and remained significantly higher than those of Mg-10Dy-1.5Zn and Mg-15Dy-1.5Zn alloys. In comparison, the Mg-10Dy-1.5Zn alloy exhibited a lower degradation rate than the Mg-15Dy-1.5Zn alloy, primarily due to its lower intermetallic content, which decreased the extent of galvanic corrosion. In the alloys extruded with a 48 h preheating treatment, the volume fraction of intermetallics increased with increasing Dy content. The Mg-5Dy-1.5Zn alloy consisted of precipitated γʹ phase, both nano-size and micro-size W phases. In contrast, the intermetallics in the Mg-10Dy-1.5Zn alloy were composed of LPSO phase, W phase and precipitated γʹ phase, which were more densely distributed within dynamic recrystallized grain than in the Mg-5Dy-1.5Zn alloy. The 18R and 14H LPSO phases formed during the 48 h heat treatment in the Mg-15Dy-1.5Zn and showed a fragmented and extrusion-aligned distribution in corresponding extruded alloy. Attributed to the high volta potential of the W phase, the Mg-5Dy-1.5Zn alloy exhibited the highest degradation rate among the three alloys. Meanwhile, the W phase in the Mg-10Dy-1.5Zn alloy did not trigger severe local corrosion. It could be attributed to the denser compact degradation layer on the surface of the γʹ phase, which retarded the corrosive ions for further penetration into the Mg matrix. In addition, such γʹ phase could act as cathode that accelerates the degradation of the Mg matrix; therefore, the Mg-10Dy-1.5Zn exhibited a higher degradation rate than Mg-15Dy-1.5Zn alloy. From the as-cast to extruded as-cast alloy, the intermetallics were fragmented during the extrusion, which could play two roles in influencing the degradation rate. On the one hand, the contact area between the intermetallics and the Mg matrix could be increased, which can provide more corrosion sites, thus accelerating the degradation of Mg. On the other hand, the network structure could be destroyed during the extrusion for the Mg-10Dy-1.5Zn and Mg-15Dy-1.5Zn alloy, leading to decreased corrosion resistance. The extruded as-cast alloys showed more detrimental degradation behavior compared with their as-cast counterparts. Furthermore, in the alloys extruded with a 48 h preheating treatment, the dissolution of intermetallic could lead to the decreased galvanic corrosion thus enhancing corrosion resistance. In addition, the homogenized distribution of alloying elements effectively decreased galvanic corrosion between the segregation region and the Mg matrix, thereby increasing the corrosion resistance. Mg-5Dy-1.5Zn alloy and Mg-15Dy-1.5Zn alloy displayed decreased degradation rates compared to their extruded as-cast counterparts. Especially, nano-size fragmented W phase in the Mg-5Dy-1.5Zn alloy had a minimal effect on the galvanic corrosion. In addition, the Mg-10Dy-1.5Zn alloy displayed a slightly increased degradation rate, which could be attributed to the precipitation of dense γʹ phase within dynamic recrystallized grains, accelerating the degradation of the Mg matrix.

Author dc:creator

• Jiang, Genzhi

Advisor dc:contributor.advisor

• Tolnai, Domonkos

Licence dc:rights.uri

• https://creativecommons.org/licenses/by-nc-nd/4.0/

Language dc:language.iso

en

Identifier URI

https://doi.org/10.14279/depositonce-25172

OAI identifier oai:identifier

oai:depositonce.tu-berlin.de:11303/26344