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Microstructures and mechanical properties of Mg-6Gd-xY-0.5Zr (wt.%, x = 0, 0.5, 1 and 3) alloys with different conditions (solution-treated and 200 °C peak-aged) are investigated. Species of precipitates in Mg-6Gd-0.5Zr alloys are modified by Y addition. In 0Y, 0.5Y and 1Y alloys, β′ precipitates and tentacle-like structures with specific crystallographic directions around β′ body are the main phases. The crystallographic directions of tentacle-like structures are {112‾0}Mg. When 3Y is added, β′ and β1 precipitates become the main phases. Except that, the size and area number density of β′ precipitate are also influenced by adding Y. With Y addition, the size of β′ precipitate is refined, and its area number density increases. This phenomenon has been explained by theoretical method in this paper. According to the study, Mg-6Gd-1Y-0.5Zr alloy with good combination properties is developed. The yield strength, ultimate tensile strength and elongation of Mg-6Gd-1Y-0.5Zr alloy are 164.4 MPa, 280.6 MPa and 16.1 %, respectively. The strengthening precipitates in the peak-aged Mg-6Gd-Y-0.5Zr alloys are β′ precipitates, and the strengthening effect of the β′ precipitates on the yield strength of the Mg-Gd-(Y)–Zr alloys with low Gd/Y content is more obvious than the Mg-Gd-(Y)–Zr alloys with high Gd/Y content.

Due to their capability of fabricating geometrically complex structures, additive manufacturing (AM) techniques have provided unprecedented opportunities to produce biodegradable metallic implants—especially using Mg alloys, which exhibit appropriate mechanical properties and outstanding biocompatibility. However, many challenges hinder the fabrication of AM-processed biodegradable Mg-based implants, such as the difficulty of Mg powder preparation, powder splash, and crack formation during the AM process. In the present work, the challenges of AM-processed Mg components are analyzed and solutions to these challenges are proposed. A novel Mg-based alloy (Mg–Nd–Zn–Zr alloy, JDBM) powder with a smooth surface and good roundness was first synthesized successfully, and the AM parameters for Mg-based alloys were optimized. Based on the optimized parameters, porous JDBM scaffolds with three different architectures (biomimetic, diamond, and gyroid) were then fabricated by selective laser melting (SLM), and their mechanical properties and degradation behavior were evaluated. Finally, the gyroid scaffolds with the best performance were selected for dicalcium phosphate dihydrate (DCPD) coating treatment, which greatly suppressed the degradation rate and increased the cytocompatibility, indicating a promising prospect for clinical application as bone tissue engineering scaffolds.

This study evaluates the heat treatment effects on microstructural and mechanical properties of A357 alloy. Solution treatments performed at 540 °C for 6, 8, and 10 h represent identical microstructures and show that the eutectic phase is partially dissolved into the matrix, resulting in Si particles of similar shape and size. Contrarily, heat treatment parameters influence Mg2Si precipitates length rather considerably; 8 h solution treated-peak aged condition produces the smallest sized needle-shaped βʺ precipitates and contributes principally in the achievement of higher ductility. It is identified that over-aging not only enhances the length of the precipitates; however, it also causes the partial transformation of needle-shaped βʺ into rod type βʹ precipitates. This phenomenon is the core reason for the reduction in hardness and ductility simultaneously, while enhancement in strength characteristics slightly upon over-aging. Entropy values of differential scanning calorimetry thermograms and calculated dimples diameter of fractured surfaces are fairly aligned with mechanical properties and agree with microstructural characterizations of nano-sized precipitates analyses. Developed relationships among precipitates size (length), ductility, and quality index indicate that precipitation mechanism does not merely contribute to the achievement of superior strength properties; nevertheless, it also plays a crucial role in determining the ductility of A357 cast alloy. Keywords: Solution and aging treatments, Mg2Si precipitates size, Mechanical properties, DSC, Ductility of A357 alloy