Mg-Y Alloys Research Articles

Effect of preheating treatment in Ar atmosphere on oxidation resistance of Mg-Y alloys and formation of oxide film

Magnesium and its alloys occupy a vital role in the aerospace, automotive and communications industries due to their low density, high specific strength and electromagnetic shielding capabilities. However, due to the strong affinity of Mg for O and the fact that MgO is generally not protective, application of Mg and its alloys at high temperature often results in catastrophic oxidation. One promising approach is based on the addition of Y in combination with a suitable pretreatment to achieve a protective oxide. Therefore, the oxidation resistance (OR) of Mg-xY (x = 0.5, 2.5 and 5.5 wt%) was investigated after using a preheating treatment in Ar atmosphere. The oxidation resistance of preheated Mg-xY alloys is greatly improved compared to unpreheated alloys and pure Mg, particularly for x = 2.5. This improvement is attributed to the enrichment of Y to the surface caused by outward diffusion of Y, and subsequent selective oxidation of Y under low oxygen partial pressure to generate a dense and compact protective film, composed of an outer thin MgO/Y2O3 composite layer and an inner thick Y2O3 layer. Some flocculent MgO/Y2O3 composites were observed on the surface of preheated Mg-5.5Y during the oxidation process, lowering the OR of this alloy.

Investigation of Direct Preparation of Mg-Y Alloy Using Ca-YF3 Thermal Reduction

Investigation of Direct Preparation of Mg-Y Alloy Using Ca-YF3 Thermal Reduction

A comprehensive theoretical scheme for understanding the core structure and the mechanical behavior of basal dislocations with solute atmosphere in Mg alloys

The basal dislocations play a key role in the deformation of hexagonal Mg alloys. However, its core structure and mechanical behavior have thus far not yet been understood adequately, largely owing to the fact that the basal dislocation, often decorated with segregated solute atmosphere and dissociated into two partials, cannot be modeled accurately. Significant discrepancies exist between theoretically calculated and experimentally observed structures for these dissociated dislocations. Here we present a comprehensive theoretical scheme to model them in the Mg alloys with alloying elements of Y, Zn, Al and Li, respectively. Firstly, solute–dislocation interactions were computed by appropriate first-principles calculations. Secondly, the statistic solute concentration at a dislocation is estimated by a Fermi–Dirac distribution. Finally, all these effects on dislocations are integrated into an improved two-dimensional Peierls–Nabarro model to predict their core structures and mechanical behaviors in Mg alloys. It is shown that although Zn, Al and Li atoms have little effect on basal dislocations, Y atoms can largely increase their dissociated width. Furthermore, when it moves from its ground state configuration, the dissociated dislocation can further be widened, such that its dissociated widths in a Mg–0.8Y (at%) alloy at 300 K can reach 12–36 nm, in rather good agreement with the experimentally observed values of 20–30 nm. Besides, the Peierls and yield stresses of Mg alloys shall increase with increasing added solute concentration, while they are decreased drastically with increasing the temperature from 300 to 800 K, but with the Mg-Y alloy as somewhat exception, which is again consistent with experimental observations.

Effect of grain size on tensile behavior and the underlying deformation modes in a Mg-5Y sheet

This work aims to further understand the grain size effects on individual slip/twining activities at grain scale and their correlation with the tensile behavior, for a Mg-5Y sheet with mean grain size range of 3–130 μm, based on slip trace/electron backscatter diffraction (EBSD) analysis. The correlation between tensile yield strength and the corresponding grain size fitted well with the Hall-Petch relationship, i.e., the k value was 191 MPa·μm1/2, with an R2 of 0.96. According to the identified 1004 sets of slip traces in 2477 grains, basal <a> slip was always dominant and tended to increase (from 47.2% to 71.5%) with grain size increased from 3 to 85 μm. Meanwhile, the percentage of pyramidal II <c+a> slip exhibited a marked decline from 34.5% to 8.9%, while that of prismatic <a> and pyramidal I <c+a> slip changed slightly. The decreased pyramidal II <c+a> slip activity was consistent with the decreased tensile yield strength as grain size increased. The twinning activity was limited and insensitive to grain size. In addition, the geometrically necessary dislocation (GND) density was increased with increasing grain size. Based on the slip activity statistics, the critical resolved shear stress (CRSS) ratios were estimated, which exhibited a significant grain size effect. Specifically, CRSSpris/CRSSbas showed negative grain size dependence, while the opposite was true for CRSSpyrIICA/CRSSbas. This study provides valuable grain-scale experimental evidence for the grain size effect on individual slip activity and CRSS ratios, which contributes to a better understanding of the Hall-Petch relationship in Mg-Y alloy.

Enhanced age hardening response of a Mg-Y alloy by Ca addition at an elevated temperature

The effect of Ca addition on the age hardening response and precipitation behavior of a Mg-7Y-0.5Zr (wt%) alloy during isothermal aging at 350 ℃ has been studied by hardness test and high-angle annular dark-field scanning transmission electron microscopy. The addition of 0.6 wt% Ca can effectively enhance the age hardening response of Mg-7Y-0.5Zr alloy at 350 ℃, and the enhanced hardness is mostly attributed to the uniform precipitation of a novel strengthening precipitate phase, designated βp. The βp phase forms as plates on {101̅0}α planes. It has a Cd45Sm11-type structure (space group F4̅3m, lattice parameter a = 2.24 nm) and a composition of Mg5Y0.8Ca0.2. Its orientation relationship with the α-Mg matrix phase is such that [1̅11]βp // [21̅1̅0]α and (110)βp // (0001)α. After prolonged aging at 350 ℃, the metastable phase βp can transform in-situ to the equilibrium phase β-Mg24Y5 with a cube-to-cube orientation relationship. The βp → β phase transformation is associated with the transition of constituent atomic clusters and the re-distribution of selected atoms within clusters.

Achieving stable coherent pure Y nanoprecipitates in a Mg-Y alloy

The introduction of stable nanoprecipitates in magnesium (Mg) alloys is challenging. Herein, densely dispersed pure Y nanoprecipitates are successfully introduced into a binary Mg-Y alloy via in situ heating in a transmission electron microscope. Two reproducible orientation relationships between the nanoprecipitates and the Mg matrix are unraveled, which are confirmed to be energetically favorable by crystallographic matching calculations. The characteristic shape and size of the nanoprecipitates warrant the interfacial coherency to avoid easy coarsening. The nucleation and growth of the nanoprecipitates are further revealed at the atomic scale. Our findings provide new insights into strengthening strategies for Mg alloys.

Quantitative investigation on the individual contribution of twinning and dislocation slip to creep in dilute Mg-Y alloys with various grain sizes

This work investigated the individual contribution of twinning and dislocation slip to creep in Mg-2 wt%Y alloys with different grain sizes. Three samples with the grain sizes of d = 31.2 μm, d = 63.5 μm and d = 127.7 μm were prepared by regulating the annealing treatment conditions, and the compressive creep tests were conducted at 473 K under 80 MPa along the rolling direction. It was found that the volume fraction of twins and the density of dislocations were continuously increased with increasing creep time regardless of grain size. During the first 50 h, the contribution of twinning was gradually increased while the contribution of dislocation slip was gradually decreased. Afterwards, the contributions of twinning and dislocation slip remained stable with increasing creep time in all samples. In addition, it was suggested that decreasing grain size could reduce the contribution of twinning but enhance the contribution of dislocation slip to creep. The results may provide reliable reference to the understanding and modeling of creep behaviors in Mg-RE alloys.

Role of micro-alloying element in dynamic deformation of Mg-Y alloys

The microscopic mechanism of the effect of alloying element on the shock behavior for alloy materials is still limited. In this work, based on our developed Finnis–Sinclair (F–S) interatomic potential of Mg-Y alloy, using nonequilibrium molecular dynamics (NEMD) simulations, we gave new insights into the role of alloying element Y in dynamic deformation of Mg-xat.%Y (x = 1, 3, 6, 8 and 10) alloys, and further revealed the deformation mechanisms of these solid solution alloys, which is dependent on the compositions of Y and the crystallographic orientations. The results confirm that the lattice instability caused by Y atoms results in the reduced yield stress. Under shock along the [0001] direction, the improved plasticity is mainly attributed to the increasing amorphous activity and decreasing amorphous annihilation rate with Y additions, but the other mechanisms, e.g., the dislocation nucleation and phase transition, are suppressed. Furthermore, for the [101¯0] direction, the energy barriers of stacking faults and disordering can be overcome at high Y content, so these two deformation modes are activated accompanying with the lattice reorientation to release the large internal stress, and the enhanced plastic activity is mainly related to the inhibited phase transition. This work can broaden the knowledge of the dynamic response of Mg-Y alloys at atomic-scale, which is significant for the shock experiments and material design.

Achieving exceptional strength and ductility combination in a heterostructured Mg-Y alloy with densely refined twins

Metals and alloys with heterogeneous microstructures are an emerging class of materials that exhibit exceptional mechanical properties, owing to the novel scientific principle of hetero-deformation induced (HDI) strengthening and hardening. For magnesium alloys, due to their low recrystallization temperature, poor ductility at room temperature, limited cold workability, and the tendency to generate strong basal texture during deformation, it is difficult to obtain heterostructures without relying on precipitation of the second phases. Here, three heterostructured Mg-2.9Y (wt.%) materials with varying accumulative equivalent true strains, i.e., 5 %-5 cycles, 7.5 %-5 cycles, and 10 %-5 cycles materials were fabricated via applying five complete triaxial compression cycles to the bulk alloy. The 5 %-5 cycles material with an accumulative equivalent true strain of 0.37 is featured with long twin lamellae embedded in coarse grains. When the accumulative true strain increases to 0.72, a heterogeneous structure composed of long and short twin lamellae is formed inside the 7.5 %-5 cycles material. As the equivalent true strain further increases to 1.01, the 10 %-5 cycles material exhibits a mixed structure with densely refined twin lamellae embedded in the coarse-grained matrix. The room-temperature uniaxial tensile tests show that the yield strength of the materials processed by triaxial cyclic compression (TCC) has been significantly improved compared to that at the initial state, whereas ductility was not significantly sacrificed without the subsequent heat treatment. The dense and refined twin lamellae that serve as hard domains in this material provide a high density of interfaces and impede dislocation motion effectively. This results in significant HDI strengthening and hardening. These findings provide new insight into the design of heterostructured hexagonal close-packed materials with both high strength and good ductility.

Revealing the effect of misorientation on the deformation behavior of a Mg-Y alloy from the perspective of local strain

The addition of rare earth (RE) elements in magnesium alloys may significantly enhance the ductility of magnesium alloys, which is generally attributed to the decrease of stacking fault energy, the reduced critical resolved shear stress (CRSS) differences among different deformation modes and the weakening of basal texture. Rare earth elements also change the misorientation between adjacent grains of magnesium alloy, whose effects on the deformation behavior and thus the ductility of magnesium alloy were not emphasized before. In this work, ex-situ high-resolution digital image correlation (HR-DIC) coupled with electron backscatter diffraction (EBSD) was used to reveal the effect of the misorientation on the deformation behavior of a Mg-Y binary alloy from the perspective of local strain. It is shown that the heterogeneity of local strain is largely dependent on the misorientation between adjacent grains. For the intergranular deformation, three scenarios are observed, namely strain transfer, strain concentration and suppressed deformation, which mainly occurs at low misorientation, moderate misorientation and high misorientation, respectively. In addition, much more non-basal slip is found to be activated near the grain boundaries with moderate misorientation angles due to the high geometric compatibility factor (m’B-Pr/Py), which is favorable for the intergranular deformation compatibility, thus reducing the strain concentration near grain boundaries and achieving high ductility. For the intragranular deformation, a wide range of misorientation between adjacent grains in the Mg-Y alloy causes multiple basal slip systems to be activated within a grain, which also facilitates work hardening, thus contributing to improving ductility. Therefore, tailoring the misorientation between adjacent grains may be an effective way to improve the ductility of magnesium alloys.

Kinetic simulation of early-stage precipitation behavior in Mg-RE binary alloys during aging process

The precipitation behavior of magnesium-rare earth (Mg-RE) alloys plays a crucial role for their properties. However, the precipitation happens at small-length and long-time scales, making it challenging to be analyzed by state-of-the-art experimental techniques. Here, we combine the advantages of both molecular dynamic force fields on describing atom interactions and Monte Carlo method on describing diffusive events to develop an embedded atom method (EAM) potential based kinetic Monte Carlo (KMC) model. Using the proposed model, we simulated the formation and evolution of Y clusters in Mg-Y alloy formed by the vacancy mechanism, and rationalize the simulation results using aberration-corrected scanning transmission electron microscopy characterize. We conducted a systematic analysis of the atomic structure, the evolution kinetics and path of the Y cluster by tracing Y atoms and comparing with density function theory (DFT) calculations. Our work reveals that, all solute columns in a same cluster trend to grow along the [0001]Mg direction synchronously. The method presented is not only used for the Mg-Y alloy but also other Mg-RE alloys such as Mg-Gd as illustrated in the last part of the paper.

Discharge performance of Mg-Y binary alloys as anodes for Mg-air batteries

In this work, the electrochemical properties and discharge performance of Mg-xY (x = 0, 0.5, 1, 2, 3, and 5 wt.%) binary alloys have been systematically investigated as anodes for Mg-air batteries, using electrochemical techniques and Mg-air battery tests. The results reveal that the Mg24Y5 and Y-rich phases in Mg-Y alloys play the micro-cathode and micro-anode roles, respectively. By adding a small amount of Y (0.5 and 1 wt.%), the electrochemical activity of the anode can be effectively enhanced, and hydrogen evolution during discharge can be inhibited. Among the investigated anodes, the Mg-1Y anode possesses the highest discharge voltage at all current densities. The Mg-0.5Y anode exhibits excellent comprehensive discharge performance, with the highest anodic efficiency, specific capacity, and energy density of 63%, 1376 mAh·g− 1, 1899 mWh·g− 1 at 10 mA·cm−2, respectively. The discharge voltage and anodic efficiency decrease as the Y content increase further, attributed to the excessive second phases accelerating the self-corrosion and causing the "chunk effect".

Microstructure, Texture Evolution, and Strain Hardening Behaviour of As-extruded Mg-Zn and Mg-Y Alloys under Compression

Microstructure, Texture Evolution, and Strain Hardening Behaviour of As-extruded Mg-Zn and Mg-Y Alloys under Compression

Local structure analysis around Y in Mg99.7Y0.3 single crystal using x-ray fluorescence holography

Mg-Y alloys exhibit a good room-temperature ductility even at fairly large grain sizes and a moderately strong basal texture. Since a clarification of this structural origin is contributed for developing high-performance, affordable, lightweighted structural metals, it is desired to reveal the structural origin. The objective of this study is to reveal the structural origin of an enhanced ductility in a Mg99.7Y0.3 single crystal using an x-ray fluorescence holography (XFH) analysis. It was found that a part of Y atoms was located at the ideal positions in pure Mg but others formed Y dimers in the hexagonal close-packed structure of Mg99.7Y0.3. It was considered that the dimer formation plays an important role in activating non-limited the plastic slip in the crystallographic z-direction of Mg-Y alloys although the crystal structure is still hexagonal by close-packed.

Formation of I1 stacking fault by deformation defect evolution from grain boundaries in Mg

I 1 stacking faults (SFs) in Mg alloys are regarded as the nucleation sites of 〈 c+a 〉 dislocations that are critical for these alloys to achieve high ductility. Previously it was proposed that the formation of I 1 SFs requires the accumulations of a large number of vacancies, which are difficult to achieve at low temperatures. In this study, molecular dynamics (MD) and molecular statics (MS) simulations based on empirical interatomic potentials were applied to investigate the deformation defect evolutions from the symmetric tilt grain boundaries (GBs) in Mg and Mg-Y alloys under external loading along 〈 c 〉 -axis. The results show the planar faults (PFs) on Pyramidal I planes first appear due to the nucleation and glide of 〈 1 2 c + p 〉 partial dislocations from GBs, where 〈 p 〉 = 1 3 〈 10 1 ¯ 0 〉 . These partial dislocations with pyramidal PFs interact with other defects, including pyramidal PFs themselves, GBs, and 〈 p 〉 partial dislocations, generating a large amount of I 1 SFs. Detailed analyses show the nucleation and growth of I 1 SFs are achieved by atomic shuffle events and deformation defect reactions without the requirements of vacancy diffusion. Our simulations also suggest the Y clusters at GBs can reduce the critical stress for the formation of pyramidal PFs and I 1 SFs, which provide a possible reason for the experimental observations that Y promotes the 〈 c+a 〉 dislocation activities.

The chemical environment and structural ordering in liquid Mg-Y-Zn system: An ab-initio molecular dynamics investigation of melt for the formation mechanism of LPSO structure

In an effort to clarify the formation mechanism of LPSO structure in Mg-Y-Zn alloy, the chemical environment and structural ordering in liquid Mg-rich Mg-Y-Zn system are investigated with the aid of ab-initio molecular dynamics simulation. In liquid Mg-rich Mg-Y alloys, the strong Mg-Y interaction is determined, which promotes the formation of fivefold symmetric local structure. For Mg-Zn alloys, the weak Mg-Zn interaction results in the fivefold symmetry weakening in the liquid structure. Due to the coexistence of Y and Zn, the strong attractive interaction is introduced in liquid Mg-Y-Zn ternary alloy, and contributes to the clustering of Mg, Y, Zn launched from Zn. What is more, the distribution of local structures becomes closer to that in pure Mg compared with that in binary Mg-Y and Mg-Zn alloys. These results should relate to the origins of the Y/Zn segregation zone and close-packed stacking mode in LPSO structure, which provides a new insight into the formation mechanism of LPSO structure at atomic level.

Anisotropic cyclic deformation behavior of an extruded Mg-3Y alloy sheet with rare earth texture

Mg-RE (magnesium-rare earth) alloys exhibit pronounced in-plane anisotropy of mechanical response under quasi-static monotonic loading resulting from the RE texture, as extensively reported. In this work, an obvious in-plane anisotropy of cyclic deformation behavior was observed in an extruded Mg-3Y alloy sheet during strain-controlled tension-compression low-cycle fatigue (LCF) at room temperature. The extrusion direction (ED) samples displayed better fatigue resistance with almost symmetrical hysteresis loops and longer fatigue life compared with the transverse direction (TD) samples. The influences of texture on the deformation modes, cracking modes, and mechanical behavior of Mg-Y alloy sheets under cyclic loading were studied quantitatively and statistically. The activation of various slip/twinning-detwinning systems was measured at desired fatigue stages via EBSD observations together with in-grain misorientation axes (IGMA) analysis. The results indicate that the activation of deformation modes in the TD sample was featured by the cyclic transition, i.e., prismatic slip (at the tensile interval) →{10–12}tension twinning (at the compressive reversal) → detwinning + prismatic slip (at the re-tensile reversal). In the case of the ED sample, the cyclic deformation was dominated by the basal slip throughout the fatigue life. For cracking modes, intergranular cracking and persistent slip bands (PSB) cracking were the primary cracking modes in the ED sample while the TD sample showed a high tendency of {10–12} tension twinning cracking (TTW cracking). The underlying mechanisms influencing the activation of various slip/twinning-detwinning systems, as well as cracking modes and cyclic mechanical behavior, were discussed.

Redistribution and refinement of the dendrites in a Mg-Y alloy by laser surface remelting and its influence on mechanical properties

Laser surface remelting (LSR) was performed on a Mg-6.2Y (at.%) magnesium alloy to enhance mechanical properties of its surface. The results show that both grains and secondary phase were greatly refined by LSR. The ε phase (Mg24+xY5) was redistributed from α-Mg grain boundaries to interior of the grains. The Y content of ε phase decreased from 14.3 to 9.3 at.% due to the LSR. Some metastable Y-rich nano particles were formed on the ε phase due to the rapid solidification. Mechanical properties of treated surface were significantly improved due to grain refinement and phase redistribution. Compressive strength of the treated region was increased to 606 MPa and exhibited a moderated ductility compared to the initial material. The increase of ductility by LSR is related to the uniformed redistribution of ε phase, which led to a homogeneous distribution of strain.

A study of high damping capacities and mechanical properties of cast Mg-Gd binary alloys

In this study, we aim at the inherent contradiction that the addition of elements with strong solid solubility strengthening effect in Mg leads to a sharp deterioration of damping capacities. It is proposed that utilizing the element with the low growth restrict factor (GRF) can form parallel distribution of second phases in solidification microstructure to impede the sharp deterioration of damping capacities with increasing element content. In cast Mg-xGd (x = 1, 1.5, 3, 6 wt%) binary alloys, with increasing Gd (GRF = 1.03) content from 1 wt% to 6 wt%, the damping value Q−1 decreases from 0.16 to 0.05 due to the existence of the parallel distribution of second phases at the strain amplitude of 0.8 × 10−3, while the yield strength increases 30 MPa to 84 MPa. Compared with the reported sharp decrease in damping properties of high-damping Mg-Y alloys with increasing Y content, Mg-Gd alloys could be promising potential candidates for developing high-performance damping magnesium alloys (Q−1 ≥ 0.01).

Dislocation behavior in a polycrystalline Mg-Y alloy using multi-scale characterization and VPSC simulation

In this study, the dislocation behavior of a polycrystalline Mg-5Y alloy during tensile deformation was quantitatively studied by an in-situ tensile test, visco-plastic self-consistent (VPSC) modeling, and transmission electron microscopy (TEM). The results of the in-situ tensile test show that <a> dislocations contribute to most of the deformation, while a small fraction of <c + a> dislocations are also activated near grain boundaries (GBs). The critical resolved shear stresses (CRSSs) of different dislocation slip systems were estimated. The CRSS ratio between prismatic and basal <a> dislocation slip in the Mg-Y alloy (~13) is lower than that of pure Mg (~80), which is considered as a major reason for the high ductility of the alloy. TEM study shows that the <c + a> dislocations in the alloy have high mobility, which also helps to accommodate the deformation near GBs.