Microstructures and mechanical properties of Mg-10Gd-6Y-2Zn-0.6Zr(wt.%) alloy

Microstructures and tensile mechanical properties of Mg-10Gd-6Y-2Zn-0.6Zr alloy were systematically studied. Four phases were found in the as-cast specimen: α-Mg, Mg₃(GdYZn), Mg₁₂(GdY)Zn and Mg₂₄(GdYZn)₅. The long-period stacking order (LPSO) structure is found, which is the phase of Mg₁₂(GdY)Zn. The LPSO structure has two existing forms: lamellar structure in the inner grains and block-like structure at grain boundaries. 6H-type LPSO structure with a stacking sequence of ABCBCB′ is defined in homogenized specimen, where A and B′ layers are significantly enriched by Gd, Y and Zn. The ageing hardening behavior of as-extruded specimens at 200 °C has been investigated. The ultimate tensile strengths of the as-extruded and peak-aged alloys are 360 MPa and 432 MPa, and the elongations are 18% and 5% respectively. The effective strengthening models have been considered to predict the strength. The results suggested that the sub-micron metastable β′ phase was the main strengthening factor of the peak-aged alloy.     

具有长周期结构的Mg–Gd–Zn–Zr合金的微结构和摩擦学行为

利用传统的熔铸法制备Mg-14.28Gd-2.44Zn-0.54Zr合金,研究铸态和固溶态合金的微结构。利用销-盘装置研究铸态和固溶态合金的室温润滑滑动摩擦磨损行为研究。在外载荷为40 N,滑动速度为30-300 mm/s以及滑行路程为5000 m情况下,测量磨损率和摩擦因数。研究结果表明：铸态合金主要由α-Mg固溶体、分布在基体内的层片状的14H型长周期结构（LPSO）和β-[（Mg,Zn）3Gd]相组成。经过温度为773 K固溶处理35 h后,大量的β相转变成具有14H型X相LPSO结构。由于固溶处理后大量β相转变为热稳定的韧性X-Mg12Gd Zn长周期结构相,固溶合金呈现较低的抗磨损能力。     

The structure of long period stacking/order Mg-Zn-RE phases with extended non-stoichiometry ranges

We propose structural models of the unique long period stacking/order (LPSO) phases formed in Mg-Zn-RE alloys, based on Z-contrast scanning transmission electron microscopy observations and first principles calculations. The LPSO structures are long period stacking derivatives of the hcp Mg structure, and the Zn/RE distributions are restricted at the four close-packed atomic layers forming local fcc stacking (i.e. a local ABCA stacking). Chemical order is well developed for the LPSO phases formed in Mg₉₇Zn₁Er₂ (14H type) and Mg₈₅Zn₆Y₉ (18R type) alloys with pronounced superlattice reflections, and the relevant Zn/RE distributions clearly emerge in the Z-contrast atomic images. Initial ternary ordered models were constructed by placing all the atoms at the ideal honeycomb sites, leading to plausible space groups of P6₃/mcm for the 14H type and C2/m, P3₁12 or P3₂12 for the 18R type. The characteristic ordered features are well represented by local Zn₆RE₈ clusters, which are embedded in the fcc stacking layers in accordance with the L1₂ type short-range order. Energy favored structural relaxations of the initial model cause significant displacement of the Zn/RE positions, implying that strong Zn-RE interactions may play a critical role in phase stability. The LPSO phases seem to tolerate a considerable degree of disorder at the Zn and RE sites with statistical co-occupations by Mg, extending the non-stoichiometric phase region bounded along the Zn/RE equiatomic line from ∼Mg_94.0Zn_2.0Y_4.0 to ∼Mg_83.3Zn_8.3Y_8.3.     

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

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.     

Mg97Y2Zn1合金中18R型长周期有序相的微观结构（英文）

通过第一性原理计算研究Mg97Y2Zn1合金中18R型长周期有序相(LPSO)的微观结构,从理论上确定Zn和Y原子在LPSO相中的排列。结果表明:添加原子首先分布在18R型LPSO相两端的层错层,然后向内部的层错层延伸。计算结果与实验现象非常吻合。同时,也揭示了18R与其他LPSO相之间的微观结构关系;结合能和形成焓表明了18R型LPSO相的稳定性与Y和Zn原子含量之间的关系。计算得到的电子结构揭示了18R型LPSO相微观结构和稳定性潜在的机理。     

Effect of precipitates on microstructures and properties of forged Mg-10Gd-2Y-0.5Zn-0.3Zr alloy during ageing process

The precipitate behavior during forging and ageing process of Mg-10Gd-2Y-0.5Zn-0.3Zr alloy has been investigated by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The mechanical properties of the alloy after forging and ageing process have been evaluated using Vickers hardness and room-temperature tensile tests. The results show that precipitation of 14H-type long period stacking order (LPSO) phase is the main strengthening phase in the as-forged alloy. The LPSO phase and refinement of grains contribute to the strength improvement of the alloy after forging process. The optimal mechanical properties of the alloy are obtained when it is aged at 200 °C for 60 h, which mainly owes to the precipitation of large amounts of β′ and 14H-type LPSO phases on the α-Mg matrix. The growth of secondary phases, widening of soft precipitate free zones and coarsening of grains during subsequent ageing process at higher temperature lead to the decrease of mechanical properties of the alloy.     

Enrichment of Gd and Al atoms in the quadruple close packed planes and their in-plane long-range ordering in the long period stacking-ordered phase in the Mg–Al–Gd system

The crystal structure of a long period stacking-ordered (LPSO) phase newly found in the Mg–Al–Gd ternary system has been investigated by scanning transmission and transmission electron microscopy. The LPSO phase in the Mg–Al–Gd system is found to form by stacking structural blocks, each of which consists of six close-packed atomic planes. In each of the structural blocks long-range ordering occurs for the constituent Mg, Al and Gd atoms, with enrichment of Gd atoms occurring in four consecutive planes of the six close-packed atomic planes. The ideal chemical composition of the structural block is determined to be Mg₂₉Al₃Gd₄ (Mg–8.3 at.% Al–11.1 at.% Gd). However, the stacking of structural blocks is largely disordered. Strictly speaking, the technical term LPSO cannot be used to describe this phase because of the long-range ordering of the constituent atoms in each structural block. The crystal structure of the LPSO phase can thus be crystallographically described as one of the order–disorder structures, and either the C2/m, P3₁12 or P3₂12 space group is assigned when the simplest stacking of structural blocks is assumed.     

Ultrasonic Vibration and Rheocasting for Refinement of Mg–Zn–Y Alloy Reinforced with LPSO Structure

In this work, ultrasonic vibration (UV) and rheo-squeeze casting was first applied on the Mg alloy reinforced with long period stacking ordered (LPSO) structure. The semisolid slurry of Mg–Zn–Y alloy was prepared by UV and processed by rheo-squeeze casting in succession. The effects of UV, Zr addition and squeeze pressure on microstructure of semisolid Mg–Zn–Y alloy were studied. The results revealed that the synergic effect of UV and Zr addition generated a finer microstructure than either one alone when preparing the slurries. Rheo-squeeze casting could significantly refine the LPSO structure and α-Mg matrix in Mg_96.9Zn₁Y₂Zr_0.1 alloy without changing the phase compositions or the type of LPSO structure. When the squeeze pressure increased from 0 to 400 MPa, the block LPSO structure was completely eliminated and the average thickness of LPSO structure decreased from 9.8 to 4.3 μm. Under 400 MPa squeeze pressure, the tensile strength and elongation of the rheocast Mg_96.9Zn₁Y₂Zr_0.1 alloy reached the maximum values, which were 234 MPa and 17.6%, respectively, due to its fine α-Mg matrix (α1-Mg and α2-Mg grains) and LPSO structure.     

Thermodynamic stability of Mg–Y–Zn ternary alloys through first-principles

To clarify the thermodynamic stability of a Mg-based long-period stacking ordered (LPSO) structure, we systematically study the energetic preference for alloys on multiple stacking sequences with different compositions for random mixing of constituent elements, Mg, Y, and Zn, based on special quasirandom structure (SQS). Through calculation of the formation free energy of SQSs, it was found that the Mg–Y–Zn alloy exhibits phase separation into Mg- and Y–Zn- rich phases, which is consistent with previous theoretical studies. The bulk modulus of SQSs for various compositions, stacking sequences, and atomic configurations is approximately 35 GPa, i.e., it does not show significant dependence on the atomic arrangements, which therefore means that there are not significant differences among the effects of phonon on the stability of each structure in the LPSO structure. Introducing a stacking fault into hcp stacking sequence results in the acquisition of a “negative” energy, which indicates the profound relationship between the introduction of stacking faults and the formation of an LPSO structure.     

含长周期堆垛有序结构Mg94Zn2Y4挤压态合金的显微组织与力学性能

采用SEM和TEM等分析方法研究包含长周期堆垛有序结构的挤压态Mg94Zn2Y4合金的显微组织和力学性能。结果表明：铸态Mg94Zn2Y4合金由18R-LPSO和α-Mg两相组成。挤压后,长周期相分层,并形成宽度为50~200 nm的α-Mg 薄片。合金经498 K时效处理36 h后达到时效峰值,在其组织中析出β′相,该析出相的出现显著提高了α-Mg基体的显微硬度,从HV108.9增加到HV129.7;而LPSO结构的显微硬度稳定在HV145左右。TEM分析及其电子衍射花样表明,β′相与α-Mg和LPSO结构具有独特的位相关系,其原子最密排面的堆垛方向垂直于α-Mg和LPSO相最密排面的堆垛方向。由于β′相和18R-LPSO相的共同存在,处于时效峰值态的Mg94Zn2Y4合金的抗拉强度达到410.7 MPa。