Mn‐promoting formation of a long‐period stacking‐ordered phase in laser‐melted Mg alloys to enhance degradation resistance

Abstract

Magnesium (Mg) alloys are promising candidates for use as biomedical implant materials. However, their very fast degradation rate greatly restricts their application. A rare earth phase possessed with a long‐period stacking‐ordered (LPSO) structure was in favor of enhancing the degradation resistance of Mg alloys. In fact, the formation of the LPSO phase in Mg alloys depends on their stacking fault energy. The lower the stacking fault energy, the more the LPSO phase formed. In this study, manganese (Mn) was alloyed to Mg alloy ZK30–10Gd (containing 3 wt.% Zn and 10 wt.% Gd) via selective laser melting to promote the formation of the LPSO phase. As an alloying element, Mn could be in favor of reducing stacking fault energy due to the fact that the large difference between the atomic radius of Mn and that of Mg induced large lattice distortion to facilitate forming stacking faults. The results showed that as the Mn content increased from 0 to 1.2 wt.%, the area fraction of LPSO phase increased from 12.22% to 22.37%, meanwhile the area fraction of (Mg,Zn)3Gd phase decreased from 9.31% to 2.32%. The ZK30–10Gd–0.6Mn possessed the highest degradation resistance (weight loss rate 0.38 mg · cm−2 · day−1). The enhancement of degradation resistance had two reasons. On the one hand, more LPSO phase provided more sites for nucleation of degradation products, which could promote the formation of a homogeneous and compact degradation product film to protect the Mg matrix. On the other hand, the inhibition of (Mg,Zn)3Gd phase precipitation could reduce galvanic corrosion.

Additive manufacturing of magnesium and its alloys: process-formability-microstructure-performance relationship and underlying mechanism

“…Chemical composition of material is another key factor affecting grain size. Up to now, the effects of Al [43,79], Mn [93][94][95], Zn [44], Cu [92,96], Ce [97], Dy [98] on the grain size of additively manufactured magnesium alloys have been investigated. The results demonstrated that the addition of Al, Cu, Zn, Ce and Dy can significantly refine the grains, and the grain size decreases monotonically with the content increase of these elements.…”

“…Higher SFE leads to easier formation of LPSO structure. Shuai et al found that Mn addition could lower the SFE of ZK30-10Gd alloy and promote the precipitation of LPSO structure in as-built LPBF microstructure [95]. However, the precipitation behavior of LPSO structure was still not adequate during the LPBF process.…”

“…Wei et al found that the volume fraction of Mg 7 Zn 3 was increased with increasing Zn content, and the morphology of Mg 7 Zn 3 transformed gradually from granular shape to nearly reticular structure [44]. In addition, it is reported that Ag, Cu, Mn + RE, Al, Ca and Dy additives can induce the formation of Mg 54 Ag 17 [143], MgZnCu [144], LPSO structure [95], Mg 17 Al 12 [79], amorphous phase [17] and Mg-Zn-Dy phase [98] in LPBF-processed Mg-Zn alloys, respectively. The emergence of these phases caused by alloying elements addition in Mg-Zn alloys will undoubtedly change the morphology, size, and volume fraction of the eutectic Mg 7 Zn 3 phase at grain boundaries, thereby reducing crack sensitivity and improving performances.…”

“…(c) Induce the formation of dense LPSO structure. Shuai et al indicated that dense LPSO structure lead to the formation of a homogeneous and compact degradation product film by providing substantial sites for nucleation, which isolated the Mg matrix from the corrosive solution and protected the samples from further degradation [95]. (d) Forming a continuous net-like structure.…”

“…For instance, Ti addition can promote grain refinement, reduce undesirable Mg 17 Al 12 phase, and induce the formation of net-like protective layer composed of low-potential eutectic α phase [153]. Mn addition can refine grain structure [93], induce the formation of dense LPSO structure [95], and produce a relatively protective manganese oxide film [94], to decrease the degradation rate.…”