Search

Magnesium (Mg) alloys that have both antibacterial and osteogenic properties are suitable candidates for orthopedic implants. However, the fabrication of ideal Mg implants suitable for bone repair remains challenging because it requires implants with interconnected pore structures and personalized geometric shapes. In this study, we fabricated a porous 3D-printed Mg-Nd-Zn-Zr (denoted as JDBM) implant with suitable mechanical properties using selective laser melting technology. The 3D-printed JDBM implant exhibited cytocompatibility in MC3T3-E1 and RAW267.4 cells and excellent osteoinductivity in vitro. Furthermore, the implant demonstrated excellent antibacterial ratios of 90.0% and 92.1% for methicillin-resistant S. aureus (MRSA) and Escherichia coli, respectively. The 3D-printed JDBM implant prevented MRSA-induced implant-related infection in a rabbit model and showed good in vivo biocompatibility based on the results of histological evaluation, blood tests, and Mg2+ deposition detection. In addition, enhanced inflammatory response and TNF-α secretion were observed at the bone-implant interface of the 3D-printed JDBM implants during the early implantation stage. The high Mg2+ environment produced by the degradation of 3D-printed JDBM implants could promote M1 phenotype of macrophages (Tnf, iNOS, Ccl3, Ccl4, Ccl5, Cxcl10, and Cxcl2), and enhance the phagocytic ability of macrophages. The enhanced immunoregulatory effect generated by relatively fast Mg2+ release and implant degradation during the early implantation stage is a potential antibacterial mechanism of Mg-based implant. Our findings indicate that 3D-printed porous JDBM implants, having both antibacterial property and osteoinductivity, hold potential for future orthopedic applications.

Based on the 3 factors and 3 levels orthogonal experiment method, compositional effects of Mg, Si, and Ti addition on the microstructures, tensile properties, and fracture behaviors of the high-pressure die-casting Al-xMg-ySi-zTi alloys have been investigated. The analysis of variance shows that both Mg and Si apparently influence the tensile properties of the alloys, while Ti does not. The tensile mechanical properties are comprehensively influenced by the amount of eutectic phase (α-Al + Mg2Si), the average grain size, and the content of Mg dissolved into α-Al matrix. The optimized alloy is Al-7.49 Mg-3.08Si-0.01Ti (wt%), which exhibits tensile yield strength of 219 MPa, ultimate tensile strength of 401 MPa, and elongation of 10.5%. Furthermore, contour maps, showing the relationship among compositions, microstructure characteristics, and the tensile properties are constructed, which provide guidelines for developing high strength and toughness Al–Mg–Si–Ti alloys for high-pressure die-casting.

Progressive solidification is usually considered an effective strategy to reduce the hot tearing susceptibility of a cast component. In this study, special constrained plate castings with progressive changes in cross-section were designed, which enabled progressive solidification. The hot tearing behavior of a newly developed NZ30K Mg alloy (Mg-3.0Nd-0.2Zn-Zr, wt.%) was studied under progressive solidification using various mold temperature distributions and constraint lengths. Of these, a homogeneous mold temperature distribution is found to be the best option to avoid hot tearing, followed by a local low mold temperature distribution (with a chiller), then a gradient mold temperature distribution. Unexpectedly, compared with the homogeneous mold temperature distribution, adding a chiller does not provide any further reduction in the hot tearing susceptibility of the NZ30K Mg alloy. A high mold temperature and a short constraint length increase the hot tearing resistance of cast Mg alloys. Progressive solidification is not a sufficient and necessary condition to avoid the formation of hot tearing. The two key factors that determine the occurrence of hot tearing under progressive solidification are the maximum cooling rate and the constraint length. Decreasing these values can reduce the incidence of hot tearing.

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.

Current hot-tearing predictions for shape castings are suitable for well-investigated alloys with complete mechanical property databases, but are not applicable to newly developed alloys. This study proposed a simplified hot-tearing criterion for shape casting based on the hot-tearing sensitivity to strain rate during solidification. The criterion is simplified to a specific form: M = LRG1.5 < Mcritical, based on the constrained rod casting method that is used to evaluate the hot-tearing susceptibility of Mg alloys, where L (length of the constrained rod), R (the cooling rate), and G (the temperature gradient) can be calculated by numerical simulation, while Mcritical can be determined by comparison of simulation and experimental results in the constrained rod casting. The prediction of the hot-tearing criterion for a 14-inch (0.35 m) Mg wheel casting was in good agreement with the experimental results, confirming its practicability. The proposed hot-tearing criterion is suitable for newly developed alloys without need of consideration of their mechanical properties.

In order to design a high strength sand cast Mg–Gd–Zn alloy, the microstructures and mechanical properties of cast Mg–12Gd–xZn–0.4Zr (x = 0, 0.4, 0.8) (wt%, signed as GZ1200K, GZ1204K and GZ1208K) alloys were studied. Zn addition changes the constitution of the strengthening phases in Mg 12Gd alloy. Small amount of Zn addition leads to the formation of basal precipitates of γ″ and γ′, accelerated-precipitation of the β1 phases and lower number density of the prismatic β′ phases, and modifies the morphology of the prismatic β′ precipitates. Compared with the Zn-free alloy, the Zn-containing alloys (GZ1204K & GZ1208K) having higher yield strength in peak-aged and over-aged conditions are mainly due to the extra prismatic β1 precipitates, which do not precipitate in the Zn-free alloy. Under sand casting condition, GZ1208K alloy strengthened by the co-precipitates of the prismatic β′ and β1 phase indicates a good combination of strength and ductility at room temperature: yield strength of 270 MPa, ultimate tensile strength of 348 MPa and elongation of 2.6%.

The quench sensitivity of a long period stacking ordered (LPSO) phase containing Mg alloy, Mg–12Gd–0.8Zn–0.4Zr (GZ1208K) (wt%), was characterized under the water cooling (WC) and air cooling (AC) conditions. GZ1208K alloy indicates significant quench sensitivity, that is, the yield strength (YS) of GZ1208K-WC alloy is 25.2% and 21.6% higher than GZ1208K-AC alloy, when aged at 225 °C for 8 h and 32 h. The significant quench sensitivity is due to the formation the long basal precipitates during cooling in the air, which probable are a mixture of basal γ′ precipitate and the LPSO phases, and called as fine-LPSO precipitates at the present study. The strengthening effect of solution atom, fine-LPSO precipitate and prismatic precipitate in GZ1208K alloy can be listed in following order: prismatic precipitate >> fine-LPSO precipitate > solution atom. Their influence on the ductility was also discussed.

The effects of different Zn addition (0, 0.2, 0.5, 1.0 wt%) on the microstructure and mechanical properties of cast Mg–1Nd–1Ce–Zr alloy in as-cast, solution-treated and 200 °C peak-aged conditions were studied. Precipitates in cast Mg–1Nd–1Ce–Zr alloy are significantly modified by the Zn addition. In the Zn-free alloy, the disk-shaped prismatic precipitates and the point-like precipitates are the main strengthening phases. When 0.2 Zn is added, the disk-shaped precipitates are refined and very fine basal precipitates form additionally. When 0.5 Zn is added, the basal precipitates become the main strengthening phase. Further increasing the Zn addition to 1.0%, only spare basal precipitates and point-like precipitates exist. The 0.5 Zn addition alloy has the highest strength at room temperature, whose yield strength, ultimate tensile strength and elongation in T6 condition are 136 MPa, 237 MPa and 9%, respectively. Keywords: Mg alloy, Precipitate, Rare earth, Zn addition, Mechanical properties, Mg wheel

Low fatigue strength has been a major barrier for structural applications of cast magnesium alloys. It is important to understand the fatigue mechanisms and improve the fatigue lives of these alloys. Stress-controlled high-cycle fatigue behaviors of cast Mg-8Gd-3Y-Zr (wt.%) alloy in as-cast, solution treated (T4) and aged (T6) conditions were studied at room temperature, and the fatigue damage morphologies were carefully characterized. During high-cycle fatigue, only basal slips were observed on the surface of fatigue samples under different stress amplitudes, which suggests that basal slip is the dominant fatigue damage mechanism. The T4 and T6 alloys show distinguishing different damage morphologies at grain interiors: there are serried PSMs in the T4 alloy and only sparse PSMs in the T6 alloy. Basal slip deformation can be transferred among basal planes in the “soft” T4 alloy, while only limited on several PSBs in the “hard” T6 alloy. In the T4 alloy, a great number of basal slip planes in a single grain and most of the grains participate in the fatigue deformation, resulting higher fatigue strength (80 MPa, for 107 cycles). For T6 alloy, the fatigue deformation only happens in several PSBs and most of the grains have no plastic deformation, resulting lower fatigue strength (70 MPa).

Polycaprolactone (PCL) is widely used in bone tissue engineering due to its biocompatibility and mechanical strength. However, PCL is not biologically active and shows poor hydrophilicity, making it difficult for new bones to bind tightly to its surface. Magnesium (Mg), an important component of natural bone, exhibits good osteo-inductivity and biological activity. Therefore, porous PCL/Mg scaffolds, including pure PCL, PCL/5%Mg, PCL/10%Mg, and PCL/15%Mg, were prepared to elucidate whether the porous structure of scaffolds and the bioactivity of PCL may be enhanced via 3D printing and incorporation of Mg powder. Compared with the control group (pure PCL only), the hydrophilicity of composite PCL/Mg scaffolds was greatly increased, resulting in the scaffolds having decreased water contact angles. Tests for adhesion and proliferation of rat bone marrow mesenchymal stem cells (rBMSCs) indicated that the PCL/10%Mg scaffold showed superior compatibility. Furthermore, as indicated by alkaline phosphatase (ALP) activity and semiquantitative analysis of alizarin red staining, PCL/10%Mg scaffolds exhibited significantly stronger osteogenic activity than the other scaffolds. Animal experiments demonstrated that PCL/10%Mg scaffolds displayed pro-osteogenic effects at an early stage (4 weeks) and produced more new bone mass 8-12 weeks following implantation, compared with the control group. Visceral and blood parameter analyses indicated that PCL/10%Mg scaffolds did not exert any noticeable toxic effects. PCL/10%Mg composite scaffolds were found to promote bone defect repair at an early stage with good cytocompatibility. This finding revealed a new concept in designing bone tissue materials, which showed potential as a clinical treatment for bone defects.

Magnesium (Mg) alloys that have both antibacterial and osteogenic properties are suitable candidates for orthopedic implants. However, the fabrication of ideal Mg implants suitable for bone repair remains challenging because it requires implants with interconnected pore structures and personalized geometric shapes. In this study, we fabricated a porous 3D-printed Mg-Nd-Zn-Zr (denoted as JDBM) implant with suitable mechanical properties using selective laser melting technology. The 3D-printed JDBM implant exhibited cytocompatibility in MC3T3-E1 and RAW267.4 cells and excellent osteoinductivity in vitro. Furthermore, the implant demonstrated excellent antibacterial ratios of 90.0% and 92.1% for methicillin-resistant S. aureus (MRSA) and Escherichia coli, respectively. The 3D-printed JDBM implant prevented MRSA-induced implant-related infection in a rabbit model and showed good in vivo biocompatibility based on the results of histological evaluation, blood tests, and Mg2+ deposition detection. In addition, enhanced inflammatory response and TNF-α secretion were observed at the bone-implant interface of the 3D-printed JDBM implants during the early implantation stage. The high Mg2+ environment produced by the degradation of 3D-printed JDBM implants could promote M1 phenotype of macrophages (Tnf, iNOS, Ccl3, Ccl4, Ccl5, Cxcl10, and Cxcl2), and enhance the phagocytic ability of macrophages. The enhanced immunoregulatory effect generated by relatively fast Mg2+ release and implant degradation during the early implantation stage is a potential antibacterial mechanism of Mg-based implant. Our findings indicate that 3D-printed porous JDBM implants, having both antibacterial property and osteoinductivity, hold potential for future orthopedic applications., Graphical abstract Image 1, Highlights • Porous JDBM implants promising mechanical properties was fabricated by selective laser melting. • 3D-printed JDBM implant exhibited excellent antibacterial property, osteoinductivity, and biocompatibility. • Temporally enhanced immunoregulatory effect in early stage was a potential antibacterial mechanism of Mg-based implant.

Simultaneous effects of boron (B) additions on grain refinement, morphological changes in Fe-intermetallic compounds, and consequent variations in mechanical responses of the cast Al-7.50Si-0.55 Mg (A357) alloys are evaluated in this research work. The contents B are varied from 0.002 to 0.40 wt. % and subsequent changes in the grain size and Fe-intermetallics morphologies are assessed through polarized light images, optical micrography, and scanning electron microscopy. A small addition of B (0.04 wt. %) yields identical refinement result (620 µm) as noted for commercial grain refiner Al-5Ti-1B (642 µm); whereas, the higher amount (0.40 wt. % of B) reduces the grain size up to 44% (360 µm). In as-cast state, the structure of the π-Fe phase is transmuted from plate/needle-type to Chinese-script particles for increased quantities (0.10 ~ 0.40 wt. %) of B. Contrarily, the heat-treatment (T6) causes the depletion of Mg and B from the π-Fe phase Chinese-script morphologies and disintegrates them by creating segregated regions of Fe particles. These agglomerated areas of Fe particles act as stress concentration points during tensile loading and considerably decrease the ductility values from 6.7 to 1.8%. Therefore, it implies that great care must be taken while developing B containing grain refiners.

Based on the hot tearing index |∆T/∆(fs)0.5| recently proposed by Kou and the thermodynamic calculations of Pandat software, Al, Cu, and Mn elements were picked up and their influence on hot tearing susceptibility of Mg–xZn (x = 6, 8, 10, wt%) alloys was studied by experiments. The results indicate that Al addition can significantly reduce the hot tearing susceptibility of Mg–Zn alloys. Either 0.5Cu or 0.3Mn addition individually can reduce the HTS of the Mg–6Zn–(1, 4) Al alloys, while adding together increases the susceptibility. The addition of 0.5Cu and 0.3Mn both individually and together increases the HTS of Mg–8/10Zn–1Al alloys. Based on the experimental and calculation results, the index can be modified to |∆T/∆(fs)0.5|(d)2 for more accurate prediction on the hot tearing resistance of Mg–Zn based alloys. Grain refinement significantly improves the hot tearing resistance of Mg–Zn based alloys. Keywords: Hot tearing susceptibility, Mg–Zn, Alloying element, Thermodynamic calculation, Grain size

Mg-10Gd-3Y-0.4Zr (wt%) (GW103K) Mg-RE based alloy was manufactured in Laser melting deposition (LMD) process and its microstructure and mechanical properties were studied in as-fabricated and T6-treated conditions. As-fabricated LMDed GW103K alloy consists of equiaxed grains about 19 µm in size and Mg3Gd, GdH2 particles. Under conventional solution and aging treatment parameters, grains of LMDed alloy coarsen significantly, from 19 to 158 µm. Plenty of GdH2 particles can be observed after solution treatment. Yield strength (YS), ultimate tensile strength (UTS) and elongation of as-fabricated and T6-treated LMDed GW103K alloy at room temperature are 118–232 MPa-13.9% and 191–298 MPa-8.9%, respectively. Compared with sand cast alloy, as-fabricated LMDed GW103K alloy has comparable YS (−5 MPa), much higher UTS (+51 MPa) and elongation (+12.1%). Unfortunately, after T6 treatment under conventional heat treatment parameters, YS and UTS were about 40 MPa lower than those of sand cast alloy, with relatively higher elongation (+5.8%). Reduction of Gd, Y content during LMD process, grain growth and formation of GdH2 particles during heat treatment are believed to be the main reasons of the reduced YS and UTS.

This paper studied the tensile crack initiation behaviors in cast NZ30K alloys under as-cast, T4- and T6-treated conditions. Results indicate that tensile failure of cast magnesium alloys mainly nucleates from the tensile-sensitive microstructural constituent such as eutectic particles, grain boundaries and twin grain boundaries. For the as-cast alloy, the micro-cracks form mainly by cracking of the eutectic phases (~83%). In addition, twin grain boundaries are also found to be preferential sites for crack nucleation (~17%). The tensile failure in the T4- and T6-treated alloys is mainly caused by twin nucleation (T4~60% and T6~44%) and grain boundaries cracking (T4~28% and T6~52%). Interactions among dislocation-slip, twinning and grain boundaries as well as eutectic phases determine the crack initiation behavior of cast magnesium alloys.

The effect of Sm on the microstructure of a Mg-3Al alloy has been investigated where it was found that the size of the primary alpha-Mg grains was significantly coarsened by the addition of 0.7 wt. % Sm, but dramatically refined by 2.1 wt. % Sm. It is proposed that the coarsening effect of Sm is due to a decrease in the nucleation potency of the native Al-Fe-C-O particles when they transform to the lower potency Al-Fe-Sm-C-O particles. The grain refinement observed when 2.1 wt. % Sm is added, is caused by the formation of potent Al2Sm nucleant particles prior to the solidification of alpha-Mg. The "Interdependence Theory" was then applied to describe the nucleant selection process in the Mg-3Al-2.1Sm alloy. (C) 2015 Elsevier B.V. All rights reserved.

Different solution treatments at 480, 500 and 520 °C were carried out on the semi-continuously cast Mg 96.34 Gd 2.5 Zn 1 Zr 0.16 alloy. It was found that different solution temperatures lead to formation of different phases. Besides α-Mg matrix and Zn–Zr compounds, the secondary phases along the grain boundaries change with solution temperature. At 480 °C and 500 °C, long period stacking ordered (LPSO) structured X phases and residual eutectic compounds (Mg, Zn) 3 Gd are observed, while at 520 °C, only residual eutectic compounds (Mg, Zn) 3 Gd exist. The different microstructures solution treated at different temperatures lead to different aging response and mechanical properties. Higher solution temperature results in higher aging response and better mechanical properties. The alloy solution treated at 520 °C for 8 h and aged at 200 °C for 64 h shows the best tensile properties at room temperature: ultimate tensile strength of 405 MPa, yield strength of 292 MPa and elongation of 5.3%. The influences of existing forms of Gd and Zn elements on tensile properties of Mg–Gd–Zn alloy were discussed, which indicates that the existing of Gd and Zn elements as precipitates leads to better strengthening effect than that in the form of LPSO structured X phase.

Laser scanning confocal microscopy (LSCM) and Electron back-scattered diffraction (EBSD) were applied to the study of surface morphology variation of as-cast Mg–3.0Nd–0.2Zn–Zr (NZ30K) (wt.%) alloy under tension-compression fatigue test at room temperature. Two kinds of typical damage morphologies were observed in fatigued NZ30K alloy: One was parallel lines on basal planes led by the cumulation of basal slips, called persistent slip markings (PSMs), and the other was lens shaped, thicker and in less density, led by the formation of twinning. The surface fatigue damage morphology evolution was analyzed in a statistical way. The influences of stress amplitude and grain orientation on fatigue deformation mechanisms were discussed and the non-uniform deformation among grains and the PSMs, within twinning were described quantitatively.

The relationships between fatigue strength and ultimate tensile strength as well as hardness have been studied in high-strength NZK alloys (Mg– y Nd– z Zn– x Zr) and other magnesium alloys. In the absence of casting defects, clear linear relationships have been found between the fatigue strength and the ultimate tensile strength and the hardness values in these magnesium alloys in both T4 (solutionized) and T6 (solutionized and aged) conditions. The fatigue strength models developed in this work alloys can be directly applied to other defect-free magnesium alloys.

In order to analyze the strengthen behaviors of the Y element, the microstructures and mechanical properties of Mg-10Gd-0.2Zn-xY-0.4Zr (x = 0, 1 wt%) two alloys were investigated. Based on the experimental results, Y addition could largely refine the size of β′ precipitate and increase its number density. This phenomenon has been analyzed by theoretical studies in this paper. Due to the fine and dense β′ precipitates, the mechanical properties of the Y-containing alloy was highly improved. In addition, β1 precipitate also has important contribution to the mechanical property. Based on the TEM observation, after tensile test, the β′ precipitates were obviously sheared by the " open=" 11 2 ¯ 0 α slip dislocations in Y-containing and Y-free alloy, then, it illustrates that the strengthen effect in both two alloys was mainly derived from the shearable mechanism.

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

The recovering behavior of Nd rare earth (RE) element from industrial neodymium-iron-boron (NdFeB) magnet scarps has been investigated through the liquid metal extraction (LME) method. It was found that when the holding time of melt was above 15 min, it had a negligible influence on the recovery efficiency of Nd as the Nd diffusion was rapid. The optimal temperature should be below 830 °C to avoid the formation of unreducible Nd2O3 within the fine NdFeB powders. The total recovery ratio of Nd was about 80% through a twice-recycling route. The impurity levels of Fe and B that generated during the reaction were acceptable due to spontaneous settling, as below 0.22% and 0.002%, respectively.

The effects of samarium (Sm) on the grain refinement, microstructures and mechanical properties of as-cast AZ31 (Mg–3Al–1Zn–0.3Mn) magnesium (Mg) alloy have been investigated. Very serious grain coarsening happens when Sm content is between 0.16% and 1.18%. This is due to both the reactions between Al and Sm which reduce the constitutional undercooling effect and the lack of Al2Sm heterogeneous nuclei. However, excellent grain refinement effect is achieved at Sm content above 2.17%, which is because the in-situ formed Al2Sm particles significantly promote heterogeneous nucleation. The main phases in AZ31–xSm alloys include α-Mg, β-Mg17Al12, Al11Sm3 and Al2Sm. The Mg17Al12 phase is gradually suppressed by the increase in Sm content, and the Al2Sm is present at a higher Sm content. Because of grain refinement strengthening and secondary phase strengthening effects, the room temperature tensile property of AZ31–3.13Sm alloy has the optimal value of YS78.7MPa–UTS216.7MPa-EL13.6%.

An ultra-high strength casting Mg–15.6Gd–1.8Ag–0.4Zr alloy has been developed by conventional ingot metallurgy and heat treatment. The alloy exhibits outstanding ultimate tensile strength of 423 MPa and tensile yield strength of 328 MPa in the peak-aged condition. The main factors contributing to the ultra-high strength are the high number density of the prismatic β′ plates and basal γ″ plates and their relative perpendicular distribution. Besides, a newly intermetallic compound is found in the as-cast microstructure other than Mg5(Gd, Ag) compound. Although, the retained intermetallic compound at the grain boundaries in the alloy solution-treated at 480 °C for 18 h and at 500 °C for 8 h has a similar composition to Mg5(Gd, Ag) compound, its crystal structure is demonstrated to be different to the two types of intermetallic compounds in the as-cast condition. Lastly, the effects of intermetallic compounds and precipitation phases on the fracture behavior of the alloy have been investigated, namely dominant inter-granular fracture in the as-cast and peak-aged conditions and mainly trans-granular fracture in the solution-treated condition.

High cycle fatigue behaviors of Mg–3Nd–0.2Zn–2Zr (abbrev. NZ30K20) magnesium alloys were studied using porosity-free low pressure sand mold castings bars (abbrev. LSM). The results show that the LSM–NZ30K20 alloys show a significant response to heat treatment, achieving 51%, 39% and 17% increases in YS, UTS and fatigue strength (FS), respectively, in the peak-aged condition, compared with those of the as-cast alloy. The fatigue failure of the porosity-free LSM–NZ30K20 alloy is mainly originated from the localized shearing near the specimen surfaces during cyclic loading. In the T6 condition, the increased fatigue strength is attributed to the strong precipitation hardening in addition to the grain boundary constraints. The fatigue fracture behavior of the LSM–NZ30K20 alloy castings is well described by the Weibull modulus and the characteristic fatigue life. Fatigue lives of the LSM–NZ30K20 alloy can be quantitatively predicted using the classical Basquin equation.

The tensile and fatigue strength of cast Mg–xNd–0.2Zn–0.45Zr alloys (x = 0, 1, 2, 3 wt%) in both solution-treated (T4) and solution + 200 °C peak-aged (T6-PA) conditions were investigated in the present study. The results indicate that Neodymium (Nd) is an effective element to improve both the tensile and fatigue properties of cast Mg–0.2Zn–Zr alloys. The strengthening effect depends on its content in a way of power function (σ = σ0 + K C Nd n ), where the power exponent n is about 0.52–0.54 for yield strength (YS) and 0.59–0.61 for fatigue strength. The yield strengthening effect of Nd element in the form of precipitates (T6-PA) is about three times of that as solution atoms (T4), while the fatigue strengthening effect of Nd element in the form of precipitates is only about 50 % higher than that as solution atoms. The improved strength (both YS and ultimate tensile strength) can lead to the same amount improvement of the fatigue strength in T4-treated alloys, while only can cause less than half improvement of the fatigue strength in T6-PA-treated alloys.

An intermediate frequency magnetic field (IFMF) was imposed upon Gd–Mg diffusion couples that were annealed at 773 K. It was found that the average thickness of diffusion layer under the magnetic field was much wider than that without the magnetic field, indicating that the atomic diffusion ability was enhanced. The magnetic field was then applied to the solution treatment of Mg–16.66Gd–0.088Zr (wt%) alloy. Compared with the samples without magnetic field, the area fraction of the eutectic phase (Mg 5 Gd phase) was evidently lower at each time point when the IFMF was imposed during the solution treatment, i.e. the solution treatment process was accelerated by the magnetic field. These two phenomena are mainly due to the enhanced atomic diffusion ability in Mg–Gd alloy under IFMF.

High cycle fatigue behavior of casting Mg 96.34 Gd 2.5 Zn 1 Zr 0.16 alloy was investigated for its improvement by heat-treatment. After solution treatment (T4, 10 h@773 K) or solution treatment plus artificial aging (T6, 10 h@773 K+128 h@473 K), fatigue strength of this alloy was found to be enhanced. The T6-treated alloy achieved the highest fatigue strength, 130 MPa, being 25 MPa and 18 MPa greater than those of the as-cast and T4-treated alloy, respectively. The average fatigue life of the heat-treated alloys is longer than that of the as-cast alloy a given stress amplitude. For the distribution of fatigue life, a fatigue life gap spanned from 10 5 to 10 7 can be observed in the as-cast and T4-treated alloy. Such a gap is absent after the alloy received artificial aging. The mechanism for the high cycle fatigue behavior of the casting alloy after heat-treatment was also discussed.

In this investigation, a simple and feasible cylindrical geometry was selected for determination of interfacial heat transfer coefficient (IHTC) between AZ91D magnesium alloy and silica sand mold. IHTC was calculated by inverse method based on the experimental data. Calculating results show that IHTC experiences three distinct variation stages as a function of casting surface temperature ( T cs ) during mushy zone region. When T cs is between 595 °C and 593 °C, IHTC decreases rapidly from 525 W/(m 2 K) to 410 W/(m 2 K) due to the onset of air gap; When T cs is between 593 °C and 554 °C, as well as solid fraction of casting center lower than critical solid fraction (CSF), IHTC decreases slowly for the durative feeding of liquid metal to casting surface; When T cs is between 554 °C and 470 °C, as well as solid fraction of casting center higher than CSF, IHTC decreases rapidly to a constant value of 62 W/(m 2 K) for the absence of feeding. The same solidification process was simulated by Anycasting™ software and the cooling curve of the casting surface was obtained using the inverse-calculated IHTC as a function of T cs . It is observed that the absolute values of Δ T errors of the measured cooling curve and the calculated cooling curve fluctuate from 0 °C to 7.5 °C, proving the accuracy of the calculated IHTC by using inverse method.

The microstructure and mechanical properties of cast Mg–9Al–2Sn–xMn (x = 0, 0.1, 0.3 wt.%) alloys in as-cast, solution treated and aged conditions are investigated. The results reveal that Mn addition into Mg–9Al–2Sn alloy leads to the formation of Al8(Mn,Fe)5 phases, the grain refinement and the increase of discontinuous Mg17Al12 precipitates along the grain boundaries. 0.1 wt.% Mn addition has an obvious effect on accelerating the aging behavior of Mg–9Al–2Sn alloy at the early aging period up to 8 h, while 0.3 wt.% Mn addition alloy has an comparable aging behavior with the Mn-free alloy. Mn addition has less influence on the fracture behavior of Mg–9Al–2Sn alloy, while the fracture pattern is mainly determined by the thermal conditions. Mg–9Al–2Sn–0.1Mn alloy has the best combination of strength and elongation when aged at 200 °C for 8 h and the yield strength, ultimate tensile strength and elongation are 154 MPa, 292MPa and 5%, respectively.

This article presents the tension–compression high cycle fatigue behavior of as-cast Mg96.34Gd2.5Zn1Zr0.16 alloy produced by semi-continuous casting at ambient temperature. The relationship between stress amplitude and cycles to failure is established, which indicates that fatigue strength of this alloy is approximately 105±8 MPa. Fracture surface of specimens were examined using a scanning electron microscope, indicating that the fatigue cracks all initiate from the oxides located at the surface. Different from other cast Mg alloys, there exist two kinds of unique fatigue morphologies at the fatigue propagation region, which consists of fine steps. Meanwhile, there is a fatigue life gap between 105 and 107 cycles on the S–N curve, which probably demonstrates that the growth rate of the fatigue cracks of as-cast Mg96.34Gd2.5Zn1Zr0.16 alloy is relatively large, and once the fatigue cracks form, the samples could fails in less than 105 cycles.

This paper reports the significantly improved high cycle fatigue performance of a new magnesium alloy NZ30K-T6 (Mg–3Nd–0.2Zn–0.45Zr) compared with commercial AZ91-T6 alloy (Mg–9Al–1Zn). The test samples were taken from the bulkhead areas of automotive V6 engine blocks made of low pressure sand casting process, and tested at room and elevated (150 °C) temperatures. The room-temperature fatigue strength of the NZ30K-T6 alloy at 107 cycles is approximately 90 MPa, about 38% increase compared to that of AZ91-T6 alloy. Increasing test temperature (from 25 °C to 150 °C) reduces the fatigue strength of NZ30K-T6 alloy by as much as 23%. Oxide films are found to be the main cause of the fatigue crack initiation in all failed samples examined. Fatigue lives of the samples containing oxide films can be quantitatively predicted by a model using the sizes of the casting defects (i.e., oxide films in this study). The fatigue fracture behavior of the magnesium alloy castings is also well described by Weibull modulus and the characteristic fatigue life.

The influences of casting defects on the fatigue behaviors of Mg–3Nd–0.2Zn–Zr magnesium alloys were studied using porosity-free low pressure sand mold casting bars (LPS) and gravity permanent mold casting ingots (GPM) containing a few porosities. The results show that porosities have detrimental effect on the fatigue strength and life. The samples failed from the porosities show much lower fatigue strength and life in comparison with those failed from slip bands and twin bands. The fatigue strength increases with the yield strength increasing. Fatigue strength of the porosity-free T6-treated specimens made by LPS is determined by the threshold stress for basal slip, which is related to the interactions among slip bands, precipitates and grain boundaries. Both grain boundary constraints and cyclic deformation irreversibly caused by twinning are the crucial factors influencing the fatigue strengths of the porosity-free T4-treated specimens made by LPS.

The fluidities of commercial AZ91D and newly developed Mg–3Nd–0·2Zn–Zr (wt-%) (NZ30K) magnesium alloys were investigated at the same superheat temperature (Tpouring−Tliquidus) and temperature difference between solidus line and mould temperature (TDSM) (Tsolidus−Tmould) conditions. The results indicate that in the range of superheat 60–150°C and TDSM 400–100°C, the increases in both pouring temperature and mould temperature can effectively improve the fluidities of both AZ91D and NZ30K alloys. The flow lengths of AZ91D and NZ30K alloys could be estimated approximately by and (ΔT: superheat; T: pouring temperature; T0: mould temperature). In order to achieve better fluidity in permanent mould casting, the ranges of pouring and mould temperatures are suggested to be 718–748 and ≧268°C for AZ91D alloy, and 760–790 and ≧350°C for NZ30K alloy respectively.

This paper investigates the high cycle fatigue properties of a recently developed high-strength cast magnesium alloy [Mg-3Nd-0.2Zn (all compositions in wt pct except when otherwise stated)] with varied Zr contents for grain refinement (NZ30K) and the influence of heat treatment conditions. The NZ30K alloy containing 0.45Zr and heat treated to the peak-aged T6 condition [14 hours at 473 K (200 °C)] shows the highest fatigue strength, about 100 MPa, which is about 25 pct higher than that of commercial AZ91D-T6 alloy. In the absence of casting flaws, the high cycle fatigue properties of the NZ30K alloy strongly depend on its grain size and heat treatment conditions. The dependency of fatigue strength on grain sizes follows the Hall–Petch relationship. The NZ30K alloy also shows a significant response to heat treatments. The fatigue strength increases in a near linear fashion with increasing yield strength of the material through heat treatment.