Microstructure,mechanical and bio-corrosion properties of Mn-doped Mg–Zn–Ca bulk metallic glass composites

The effects of Mn substitution for Mg on the microstructure, mechanical properties, and corrosion behavior of Mg_69 ? xZn₂₇Ca₄Mn_x (x = 0, 0.5 and 1 at.%) alloys were investigated using X-ray diffraction, compressive tests, electrochemical treatments, and immersion tests, respectively. Microstructural observations showed that the Mg₆₉Zn₂₇Ca₄ alloy was mainly amorphous. The addition of Mn decreases the glass-forming ability, which results in a decreased strength from 545 MPa to 364 MPa. However, this strength is still suitable for implant application. Polarization and immersion tests in the simulated body fluid at 37 °C revealed that the Mn-doped Mg–Zn–Ca alloys have significantly higher corrosion resistance than traditional ZK60 and pure Mg alloys. Cytotoxicity test showed that cell viabilities of osteoblasts cultured with Mn-doped Mg–Zn–Ca alloys extracts were higher than that of pure Mg. Mg_68.5Zn₂₇Ca₄Mn_0.5 exhibits the highest bio-corrosion resistance, biocompatibility and has desirable mechanical properties, which could suggest to be used as biomedical materials in the future.     

Development of high-performance quaternary LPSO Mg–Y–Zn–Al alloys by Disintegrated Melt Deposition technique

In the present study, new quaternary MgY_1.65Zn_0.74Al_0.53 and MgY_3.72Zn_1.96Al_0.45 alloys (wt.%) were synthesized employing the Disintegrated Melt Deposition (DMD) casting technique followed by hot extrusion. Microstructural characterization revealed the presence of 14H long-period stacking ordered structure (LPSO) and Mg₄Y₂ZnAl₃ phases aligned along the direction of extrusion in both alloys. Refined grains (⩽5 μm) due to the effect of dynamic recrystallization (DRX) were also observed to co-exist with larger worked grains (⩾20 μm) in the extruded microstructures. Compared to monolithic Mg, significant increase in the microhardness (∼67–88%), tensile yield strength (∼245–290%) and ultimate tensile strength (∼113–144%) were observed in the Mg–Y–Zn–Al alloys. Despite the significant increase in strength of materials, failure strains of both Mg–Y–Zn–Al alloys were comparable to monolithic Mg. Ignition temperatures of both Mg–Y–Zn–Al alloys were found to outperform commercially available AZ31, AZ80 and WE43 (high-temperature) Mg alloys, and the highest ignition temperature of 770 °C was achieved in the MgY_3.72Zn_1.96Al_0.45 alloy.     

Solid state amorphization of Mg-Zn-Ca system via mechanical alloying and characterization

Magnesium based bulk metallic glasses have attracted significant attention of researchers due to better mechanical and corrosion properties when compared to their crystalline counterparts especially for biomedical applications. Scaling up the part size and production volumes of such materials through liquid metallurgy route is challenging. In this work amorphous Ca₅Mg_60+xZn_35?x (X = 0, 3 and 7) alloys have been successfully synthesized through solid state amorphization using a high energy planetary ball mill. X-ray diffraction was used to identify the crystalline phases of the powder during reaction. Evolution of amorphous phase was analysed using a parameter involving the ratio of integral area of peaks to the integral area of background (I_PB) obtained from XRD patterns. Results showed reaction time increases with decreasing Zn content in Ca₅Mg_60+xZn_35?x (X = 0, 3 and 7) alloy to obtain maximum amorphous structure with a small amount of residual crystalline phase. Prolonged milling of these powders, to eliminate residual crystalline phases, resulted in the nucleation of Mg_102.08Zn_39.6 phase. The composition dependent characteristic temperatures and thermal stabilities were studied using differential scanning calorimetry.     

Interface microstructure and mechanical properties of zinc–aluminum thermal diffusion coating on AZ31 magnesium alloy

The zinc–aluminum (Zn–Al) alloy coating with excellent wear and corrosion resistance was fabricated on the surface of magnesium substrate (AZ31) using thermal diffusion technique. The microstructure, phase constitution and chemical composition were investigated. The experimental observation exhibited that the interfacial microstructures were composed of network eutectic structures and lamellar eutectoid structures at heating temperature of 350 °C for holding time of 30 min under 0.1 MPa in a vacuum of 10⁻³ Pa. X-ray diffraction (XRD) pattern analysis identified that α-Mg, Mg₇Zn₃ and MgZn phases were formed in the diffusion layer. The interdiffusion of Mg and Al atoms were restricted by Mg–Zn intermetallic compounds (IMCs). The value of microhardness at the diffusion layer increased due to the formation of Mg–Zn eutectic phases. This technique is beneficial to improving poor wear and corrosion resistance of magnesium alloy.     

X-ray photoelectron spectroscopy investigations of zinc–magnesium alloy coated steel

The coating layer composition depth profiles and element chemical states of zinc–magnesium alloy coated steel were investigated by X-ray photoelectron spectroscopy depth profiling. Through the analysis of photoelectron signals and Auger signals of different elements on different depth planes of the coating layer, it can be found that the surface of the coating layer contains MgCO₃, MgO, Mg(OH)₂, metallic Mg, metallic Zn and some complex zinc compounds. Under the surface, there is a Zn₂Mg alloy layer with the thickness of about 300 nm accompanied with MgO and Mg(OH)₂ in the layer. There is a transitional layer with the thickness of about 200 nm between the Zn₂Mg alloy layer and the pure Zn layer, whose components consist of zinc–magnesium alloy without fixed stoichiometry, a little MgO and a little Mg(OH)₂.     

Mg–Zn–Y alloys with long-period stacking ordered structure: In vitro assessments of biodegradation behavior

Using Dulbecco's modified eagle medium (DMEM) with 10% fetal bovine serum (FBS) as simulated body fluid, degradation behavior of Mg_100 ? 3x(Zn₁Y₂)_x (1 ≤ x ≤ 3) alloy series with long period stacking order (LPSO) structures was investigated. As indicated, with increasing the volume fraction of LPSO phase, degradation rate of the alloys is accelerated. Further refining the grain size by microalloying with zirconium and warm extrusion has a significant effect to mitigate the degradation rate of the Mg₉₇Zn₁Y₂ alloy. Time-dependent behavior during degradation of the magnesium alloys can be described using an exponential decay function of W_R = exp(a + bt + ct²), where W_R is normalized residual mass/volume of the alloy. A parameter named as degradation half-life period (t_0.5) is suggested to quantitatively assess the degradation rate. For the localized-corrosion controlled alloys, the t_0.5 parameter physically scales with electrochemical response ΔE which is a range between corrosion potential (E_corr) and pitting potential (E_pt). In comparison with conventional engineering magnesium alloys such as the AZ31, WE43, ZK60 and ZX60 alloys, extruded Mg_96.83Zn₁Y₂Zr_0.17 alloy with LPSO structure exhibits a good combination of high mechanical strength, lower biodegradation rate and good biocompatibility.     

Crystalline structure of epitaxial CaxMg1−xF2 alloys on Si(100) and (111) substrates

Epitaxial growth of Ca_xMg_1−xF₂ alloy composed of cubic fluorite-type CaF₂ and tetragonal rutile-type MgF₂ on Si substrates was investigated as a possible method for fabricating lattice-matched ultra-thin dielectric layers. The crystalline structure of 1.2-nm-thick Ca_xMg_1−xF₂ alloy layers grown on Si(100) and Si(111) substrates by molecular beam epitaxy was characterized. On both Si(100) and Si(111), the grown Ca_xMg_1−xF₂ layers exhibited a cubic structure over a wide range of alloy composition (0.2 ≤ x ≤ 0.9). The surface was particularly smooth in the case of x = 0.9 on Si(111). It was found that, for both Si(100) and Si(111), the first two monolayers grown were composed of pure MgF₂ with a cubic structure. This is partly why the cubic Ca_xMg_1−xF₂ alloy was stable on the Si substrates.     

Transient liquid phase (TLP) bonding of Al7075 to Ti–6Al–4V alloy

TLP diffusion bonding of two dissimilar aerospace alloys, Ti–6Al–4V and Al7075, was carried out at 500 °C using 22 μm thick Cu interlayers for various bonding times. Joint formation was attributed to the solid-state diffusion of Cu into the Ti alloy and Al7075 alloy followed by eutectic formation and isothermal solidification along the Cu/Al7075 interface. Examination of the joint region using SEM, EDS and XPS showed the formation of eutectic phases such as, ?(Al₂Cu), T(Al₂Mg₃Zn₃) and Al₁₃Fe along grain boundaries within the Al7075 matrix. At the Cu/Ti alloy bond interface a solid-state bond formed resulting in a Cu₃Ti₂ phase formation along this interface. The joint region homogenized with increasing bonding time and gave the highest bond strength of 19.5 MPa after a bonding time of 30 min.     

Microstructure characterisation and mechanical properties of rapidly solidified Mg–Zn–Ca alloys with Ce addition

Abstract

The rapidly solidified (RS) Mg–Zn–Ca based alloys with Ce addition were produced via atomising the alloy melt and subsequently splat quenching the atomised droplets on the water cooled copper twin rollers in the form of flakes. The alloys were characterised by XRD, SEM, HRTEM, DSC and microhardness. All the alloys are characteristic of fine grains in the size of 1–5 μm. The RS Mg–6Zn–5Ca ternary alloy is composed of α–Mg, Mg₂Ca, Ca₂Mg₆Zn₃ and a small quantity of Mg₅₁Zn₂₀, Mg₂Zn₃ and MgZn₂. With the increment of Ce, the Mg₁₂Ce phase in the alloy increases and the Mg₅₁Zn₂₀, Mg₂Zn₃ and Ca₂Mg₆Zn₃ phases decrease, seemly indicating that the proper Ce content is beneficial to the suppression of the latter phases. Moreover, the addition of Ce to the Mg–Zn–Ca alloy contributes to the enhanced thermal stability of the phases in the alloys, especially for the Mg–6Zn–5Ca–3Ce alloy. The microhardness of the RS alloys increases with the increment of Ce, and the strengthening mechanism is discussed.     

Optimizing the tensile properties of Al–Si–Cu–Mg 319-type alloys: Role of solution heat treatment

The work presented in this study was carried out on Al–Si–Cu–Mg 319-type alloys to investigate the role of solution heat treatment on the dissolution of copper-containing phases (CuAl₂ and Al₅Mg₈Cu₂Si₆) in 319-type alloys containing different Mg levels, to determine the optimum solution heat treatment with respect to the occurrence of incipient melting, in relation to the alloy properties. Two series of alloys were investigated: a series of experimental Al–7 wt% Si–3.5 wt% Cu alloys containing 0, 0.3, and 0.6 wt% Mg levels. The second series was based on industrial B319 alloy. The present results show that optimum combination of Mg and Sr in this study is 0.3 wt% Mg with 150 ppm Sr, viz. for the Y4S alloy. The corresponding tensile properties in the as-cast condition are 260 MPa (YS), 326 MPa (UTS), and 1.50% (%El), compared to 145 MPa (YS), 232 MPa (UTS), and 2.4% (%El) for the base alloy with no Mg. At 520 °C solution temperature, incipient melting of Al₅Mg₈Cu₂Si₆ phase and undissolved block-like Al₂Cu takes place. At the same time, the Si particles become rounder. Therefore, the tensile properties of Mg-containing alloys are controlled by the combined effects of dissolution of Al₂Cu, incipient melting of Al₅Mg₈Cu₂Si₆ phase and Al₂Cu phase, as well as the Si particle characteristics.     

Nanostructured Al2O3–TiO2 coatings for high-temperature protection of titanium alloy during ablation

Plasma-sprayed nanostructured Al₂O₃–13 wt.%TiO₂ coatings were successfully fabricated on titanium alloys (Ti–6Al–4V) using as-prepared feedstock. Ablation experiments for the titanium alloy samples with or without a coating were carried out using a Metco 9MB plasma gun. The microstructure, phase constituents and mechanical properties of the titanium alloys before and after ablation were investigated by scanning electron microscope (SEM), X-ray diffractometer (XRD) and Vickers hardness tester. The surface morphologies, cross-sectional microstructure and hardness of titanium alloys with coatings are similar before and after ablation. In contrast, the microstructure and mechanical properties of the titanium alloy without coating are significantly changed after ablation. The surface coating is found to serve as a protective coating during ablation.     

Microstructure evolution,mechanical properties and diffusion behaviour of Mg-6Zn-2Gd-0.5Zr alloy during homogenization

The microstructure evolution and mechanical properties of Mg-6Zn-2Gd-0.5Zr alloy during homogenization treatment were investigated. The as-cast alloy was found to be composed of dendritic primary α-Mg matrix, α-Mg + W (Mg₃Zn₃Gd₂) eutectic along grain boundaries, and icosahedral quasicrystalline I (Mg₃Zn₆Gd) phase within α-Mg matrix. During homogenization process, α-Mg + W (Mg₃Zn₃Gd₂) eutectic and I phase gradually dissolved into α-Mg matrix, while some rod-like rare earth hydrides (GdH₂) formed within α-Mg matrix. Both the tensile yield strength and the elongation showed a similar tendency as a function of homogenization temperature and holding time. The optimized homogenization parameter was determined to be 505 °C for 16 h according to the microstructure evolution. Furthermore, the diffusion kinetics equation of the solute elements derived from the Gauss model was established to predict the segregation ratio of Gd element as a function of holding time, which was proved to be effective to evaluate the homogenization effect of the experimental alloy.     

A comparison study of hydrogen storage properties of as-milled Sm5Mg41 alloy catalyzed by CoS2 and MoS2 nano-particles

The influences of the catalysts of CoS₂ and MoS₂ nano-particles on microstructure and hydrogen storage behaviors of as-milled Sm₅Mg₄₁ alloy have been compared in this work. The Sm₅Mg₄₁ + 5 wt.% M (M = CoS₂, MoS₂) alloys were prepared by milling the mechanical ground as-cast Sm₅Mg₄₁ alloy powders (particle size ≤ 75 μm) with 5 wt.% CoS₂ or MoS₂ nano-particles (particle size ≤ 30 nm), respectively. The results demonstrate that the CoS₂ and MoS₂ nanoparticles are embedded into the alloy surface, which is nanostructure containing some crystal defects, such as dislocation, grain boundary and twin etc. Those microstructures play a beneficial role in reducing the total potential barrier that the hydrogen absorption or desorption reactions must overcome, hence improving the hydrogen storage kinetics of the alloys. The as-milled alloys are composed of Sm₅Mg₄₁ and SmMg₃ phases, and ball milling refines their crystal grains. The MgH₂ and Sm₃H₇ phases appear after hydrogenation, while Mg and Sm₃H₇ phases exist after dehydrogenation. The dehydriding activation energy of M = CoS₂ and MoS₂ alloys are 101.67 and 68.25 kJ/mol H₂ respectively. The initial hydrogen desorption of M = CoS₂ and MoS₂ alloys are 252.9 °C and 247.8 °C. The hydrogenation and dehydrogenation enthalpy changes of M = MoS₂ alloy are a little smaller than that of M = CoS₂ alloy. Therefore, the catalyst MoS₂ can improve the as-milled Sm₅Mg₄₁ alloy in hydrogen storage property more effectively than CoS₂.     

Effect of Y on the bio-corrosion behavior of extruded Mg–Zn–Mn alloy in Hank's solution

The bio-corrosion properties of Mg–Zn–Mn alloys with and without Y in Hank's solution at 37 °C were investigated by using electrochemical test and electrochemical impedance spectra (EIS). The results of open circuit potential (OCP) and polarization tests indicated that Y could reduce the cathodic current density. A passivative stage appeared in the Tafel curve of the Y containing magnesium alloy, indicating that a passivative film was formed on the surface of the Y containing magnesium alloy. EIS results showed that the Y containing alloy had higher charge transfer resistance and film resistance, but lower double layer capacity than the alloy without the Y element. The surface reaction product identification by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) showed that the surface corrosion products were hydroxide and phosphate (Mg₃Ca₃(PO₄)₄) for Mg–Zn–Mn alloy and phosphate (MgNaPO₄) for the Y containing Mg–Zn–Mn alloys. The XPS results also showed that a Y₂O₃ protective film was formed on the surface of the Y containing magnesium alloy which contributed mainly to the low cathodic current density and the high resistance.     

Dynamic recrystallization and texture evolution of Mg–Y–Zn alloy during hot extrusion process

The microstructure and texture evolution of Mg_98.5Y₁Zn_0.5 and Mg_92.5Y₅Zn_2.5 (atomic percent) alloys during hot extrusion were systematically investigated. The coarse LPSO phases with higher volume fraction (~ 57%) suppressed the twinning generation in the initial stage of extrusion, and accelerated the dynamic recrystallization through the particle deformation zones. Therefore, the volume fraction of DRXed grains in as-extruded Mg_92.5Y₅Zn_2.5 alloy was much higher than that of Mg_98.5Y₁Zn_0.5 alloy. The intensive recrystallization process resulted in the conventional basal texture weakening, although the texture evolution was mainly dominated by flow behavior. The dynamic recrystallization behavior in Mg_92.5Y₅Zn_2.5 alloy restricted the formation of deformation texture, and thus the more random texture was observed during the whole extrusion process.     

Development of a new magnesium alloy ZW21

A new magnesium alloy named ZW21 has been developed through orthogonal experiment method and the effects of heat treatment on the alloy’s tensile properties have also been investigated. The results indicate that the alloy only has one Mg–Zn–Y(Nd) phase of Mg₃Zn₃(Y, Nd)₂ (named W phase) and has higher mechanical properties, lower cost and lighter weight compared with the other congeneric alloys. Its microstructure is composed of small equiaxed dendrites and interdendritic discontinuous net-like eutectic structures. The eutectic structures appear in divorced W phase laths in the thin regions between the dendrites and in regular W + α-Mg lamellar structures in the triangle regions. The eutectic structures, especially the W phase, with such distribution are harmful to the tensile properties and thus proper heat treatment can improve its properties through changing the W phase distribution. Solution treatment at 525 °C for 4 h (T₄ treatment) increases the elongation from 17.75% to 26.5%. Subsequent ageing treatment at 250 °C for 24 h (T₆ treatment) improves the ultimate tensile strength from 210 MPa to 243 MPa. The fracture modes of the as-cast, T₄- and T₆-treated alloys all obey the quasi-cleavage regime. The fracture of the as-cast alloy belongs to a mixed mode of intergranular and transgranular forms, but those of the T₄- and T₆-treated alloys follow the transgranular mode due to the relatively high bonding strength between the grains.     

Hydrogen-storage characteristics of Cu,Nb2O5, and NbF5-added Mg–Ni alloys

From a Mg–23.5 wt%Ni–5 wt%Cu alloy synthesized by the gravity casting method in a large quantity (7.5 kg), Mg–23.5 wt%Ni–x wt%Cu (x = 2.5, 5 and 7.5) samples for hydrogen storage were prepared by melt spinning and crystallization heat treatment. The samples were ground under H₂ in order to obtain a fine powder. These alloys contained crystalline Mg and Mg₂Ni phases. The Mg–23.5Ni–2.5Cu alloy had the highest hydriding and dehydriding rates after activation among these alloys. The dehydriding curve under 1.0 bar H₂ at 573 K exhibits two stages; the dehydriding rate is high for about 2.5 min (the decomposition of Mg₂Ni hydride and Mg hydride in small particles), and then it becomes lower (the decomposition of Mg hydride). The hydriding and dehydriding properties of another sample 88 wt%(87.5Mg–10Ni–2.5Cu)–5 wt%Nb₂O₅–7 wt%NbF₅ were also investigated.     

Enhancing mechanical properties of Mg–Sn alloys by combining addition of Ca and Zn

We developed new wrought Mg–2Sn–1Ca wt.% (TX21) and Mg–2Sn–1Ca–2Zn wt.% (TXZ212) alloys with high strength and ductility simultaneously, produced by conventional casting, homogenization and indirect extrusion. A partial dynamically recrystallized microstructure, with the micron-/nano-MgSnCa particles and G.P. zones dispersing, was obtained in TX21 alloy extruded at 260 °C (TX21-260). The TX21-260 alloy exhibited yield strength (YS) of 269 MPa, ultimate tensile strength (UTS) of 305 MPa, while those of the TX21 alloy extruded at 300 °C decreased to be 207 MPa and 230 MPa respectively. For TXZ212-260 alloy, on the other hand, MgSnCa, MgZnCa and MgZn₂ phases were observed, and the average grain size increased to be ∼5 μm. The YS and UTS of TXZ212-260 alloy evolved to be 218 MPa and 285 MPa, and the elongation (EL) reached as high as 23%. The high strengths of TX21-260 alloy were expected due to the high number density of nano-MgSnCa phases, G.P. zones and ultra-fine grain size (∼0.8 μm). The high EL of 23% in TXZ212-260 alloy was consistent with the high work-hardening rate, which was attributed to the larger grain size, more high angular grain boundaries, presence of more nano-particles and the weaker texture.     

Influence of the substituting Ni with Fe on the cycle stabilities of as-cast and as-quenched La0.7Mg0.3Co0.45Ni2.55 − xFex (x = 0–0.4) electrode alloys

The electrode alloys La_0.7Mg_0.3Co_0.45Ni_2.55 ? xFe_x (x = 0, 0.1, 0.2, 0.3, 0.4) are fabricated by casting and rapid quenching techniques. The effects of the substitution of Fe for Ni on the cycle stabilities as well as the structures of the alloys have been investigated thoroughly. The results indicate that the substitution of Fe for Ni significantly enhances the cycle stability of the alloys. Furthermore, the positive impact of such substitution on the cycle stability has been observed to be more pronounced for the as-quenched alloy as compared to that for the as-cast one. Scanning electron microscopy (SEM) studies demonstrate that all the alloys exhibit a multiphase structure comprising of two major phases (La, Mg)Ni₃ and LaNi₅ along with a residual phase of LaNi₂. The substitution of Fe for Ni has been observed to facilitate the formation of a like amorphous structure in the as-quenched alloy. With an increase in Fe contents, a significant grain refinement of the as-quenched alloy and an obvious enlargement in the lattice constants and the cell volumes of the alloys have been noticed.     

Fabrication of biodegradable nano-sized β-TCP/Mg composite by a novel melt shearing technology

Biodegradable magnesium-matrix composites have attracted increasing interest for application in implant material fields. In this study, a new type of nano-sized β-tricalcium phosphate (β-TCP)/Mg–3Zn–Ca composite was proposed and produced using a novel melt shearing technology combined with high-pressure die casting (HPDC) process. The effect of the mixing methods on the distribution of β-TCP particles was investigated. Microstructure evolution during solidification process was analysed and the mechanical properties of the composite were also evaluated. Compared with the conventional mechanical stirring, the agglomerate phenomenon of the β-TCP particles in the matrix can be decreased by using the high shear unit and further decreased by melt shearing in the MCAST unit. The results also showed that the main constitutes in the matrix of the β-TCP/Mg–3Zn–1Ca composite are α-Mg and Ca₂Mg₆Zn₃ phase and most of the β-TCP particles are adjacent to the eutectic Ca₂Mg₆Zn₃ phase around the grain boundary. The average Vickers hardness, yield strength (0.2% YS), ultimate tensile strength (UTS), elastic modulus and elongation of as-cast of this composite are 79.0, 125.4 MPa, 150.0 MPa, 45.3 GPa and 2.85%, respectively.