The fatigue characteristics of the AZ91D-T6 alloy samples taken from engine blocks have been investigated at 20 °C and elevated temperature (150 °C). The fatigue strength and cyclic stress amplitude of the alloy significantly decrease with the increase of the test temperature, although cyclic hardening occurs continuously until failure for both temperatures. With the increase of the temperature, the decreased fatigue life of the alloy tested at the same stress amplitude is mainly attributed to the decreased matrix strength and the increased hysteresis energies. Fatigue failure of the engine blocks made of AZ91D-T6 alloy is mainly controlled by casting defects. For the defect-free specimens, the crack initiation behavior is determined by the single-slip (20 °C) and by environment-assisted cyclic slip (150 °C) during fatigue, respectively. The low-cycle fatigue lives of the alloy can be predicted using the Coffin-Manson relation and Basquin laws, the three-parameter equation and the energy-based concepts, while the high-cycle fatigue lives of the alloy fitted well with the developed long crack life model and MSF life models.

•Details structure of β′ (β L′ and β S′) and β″ (β″ (I) and β″ (II)) are characterized by HAADF-STEM and TEM.•The precipitation sequence of Mg-Sm series alloy should be modified as S.S.S.S → β″ (D019) → β L′/β S′(cbco) → β1 (fcc) → β (bct)•A phase transformation model between β″/β′ is proposed based on 〈a〉 dislocation shearing mechanism.Strengthening precipitate phases in a Mg-4Sm-0.4Zr (wt%) alloy, aged isothermally at 473 K, were characterized using transmission electron microscopy and high-angle annular dark-field scanning transmission electron microscopy. The β″ (D019) precipitates appear in continuous and discontinuous D019 structures, which are designated as β″ (I) and β″ (II) respectively and the β″ (II) is the dominant phase in the peak-aged state. A small number of β' (cbco) precipitates were observed in Mg-Sm-Zr alloy, and these phases have two types of lattice parameters where one is a = 0.64 nm, b = 1.14 nm and c = 0.52 nm, and the other is a = 0.64 nm, b = 2.28 nm and c = 0.52 nm, designated as β L′ and β S′ respectively. The precipitation sequence in Mg-Sm-Zr alloys can be presented as super-saturated solid solutions (S.S.S.S) → β″ → β' → β1 (fcc) → β (bct), indicating almost Mg-RE (RE = Nd, Sm, Gd, Y, Dy) alloys have the same ageing precipitation sequence. A phase transformation model between β″/β′ was proposed based on the experimental results.Download high-res image (218KB)Download full-size image

•FEM simulation was applied to evaluate actual extrusion temperatures.•The DTE enhanced precipitation hardening after extrusion.•The DTE with faster cooling speed resulted in finer recrystallized grains.•The DTE led to YS of 380 MPa which is 338 MPa after isothermal extrusion.A high-strength Mg-Gd-Zn-Zr alloy developed by a novel extrusion method called differential-thermal extrusion (DTE) was detailed characterized, to reveal the effects of the DTE on microstructure and mechanical properties. Different from traditional isothermal extrusion (ITE) with the same billet and mold temperature, the billet temperature for the DTE is remarkably higher than the mold temperature. The finer grains and less long period stacking ordered (LPSO) structure in the DTE sample resulted from its higher cooling speed after deformation. Meanwhile, the stronger precipitation strengthening effect was caused by the higher billet preheat temperature which reserved the precipitation ability before extrusion. As a result, the finer grains and stronger precipitation strengthening led to the strong tensile yield strength (380 MPa) and ultimate tensile strength (461 MPa) of the aged DTE sample, while those of the aged ITE sample are only 338 MPa and 420 MPa, respectively. The higher strengths indicate the DTE is a better extrusion process for producing high-strength Mg alloys than the traditional ITE.Download high-res image (388KB)Download full-size image

•Linear chain distribution of β′ particles forms under the cooperation of the applied stress and dislocations.•The applied stress promotes preferential growth of one of the three β′ variants during coarsening.•The dislocation line acts as heterogeneous nucleation sites for β′ particles.A linear chain distribution of β′ precipitates is found in a Mg-2.4Gd-0.1Zr (at.%) alloy crept at 250 °C for 155 h or longer under a uniform tensile stress in a range of 80–120 MPa, which is distinctly different from the random distribution of β′ precipitates in the same alloy before creep tests. In this work, the influences of the applied stress and dislocations on the distribution of β′ precipitates and the formation of the linear precipitate chains are investigated via the interaction energy calculation and phase field simulation. Our calculation and simulation results indicate that the applied stress promotes the preferential growth of one of the three β′ variants during coarsening. The applied stress can also activate the gliding and climbing of dislocations during creep. Among these dislocations, the a-type basal edge dislocations are most likely to act as heterogeneous nucleation sites for β′ precipitates. Compared with the other two variants, the variant that has zig-zag monolayers of Gd atoms perpendicular to the dislocation line is more favourable to nucleate and grow along the dislocation line and form precipitate chains. This variant is also the one favoured by the applied stress.Download high-res image (181KB)Download full-size image

•Reports for the first time the interactions and interfaces between β′ and β′F phases.•The β′ and β′F form concomitantly for their similar formation energy values.•The alternate distribution of β′ and β′F can reduce the total elastic strain energy.•Four different types of β'/β′F interfaces form in different proportions.This study reports the co-existence of β′ and β′F phases and their interactions in an aged Mg‒Gd alloy. Observations made from high-angle annular dark-field scanning transmission electron microscopy reveal that the β′ and β′F precipitates form linear chains along {11¯00}α and within each chain they have an alternate distribution. Four types of interfaces exist between the adjoining β′ and β′F precipitates, and one of them is the most frequently observed. The interaction between β′ and β′F precipitates is examined using first-principles density functional theory computations and phase field simulations. Our calculation and simulation results reveal that the β′ and β′F phases can form simultaneously because of similar values of their formation energy. The alternate distribution can effectively reduce the magnitude of the tensile stress around, and the compressive stress in and around the β′ precipitates. One of the four types of interfaces has a more negative formation energy, and as a result, it is most frequently observed.Download high-res image (433KB)Download full-size image

Precipitation of long-period stacking ordered (LPSO) structure in Mg–15Gd–0.8Zn–0.8Mn (wt%) alloy after solution treatment at 520 °C for 1 000 h has been investigated using high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). The stacking periodicity of the LPSO phase is lengthened 14-fold along the c-axis and the LPSO phase consists of Gd- and Zn-enriched solute clustering. Detailed HAADF-STEM imaging shows a clear evidence of a co-segregation of Gd and Zn within a LPSO phase. This paper provides a better understanding of the interaction between Gd and Zn, which can, therefore, provide useful guidelines to develop high performance Mg alloys.

•The Mg–3Al alloy was significantly coarsened by the addition of 0.7 wt. % Sm, but dramatically refined by 2.1 wt. % Sm.•Sm coarsened the Mg–3Al alloy by inhibiting the Al–Fe–C–O particles nucleant potency.•2.1% Sm refined the Mg–3Al alloy because of the solidification of Al2Sm particles.•“Interdependence Theory” was applied to describe the nucleant selection process in the Mg–3Al–2.1Sm alloy.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 α-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 α-Mg. The “Interdependence Theory” was then applied to describe the nucleant selection process in the Mg–3Al–2.1Sm alloy.

•The first time applying the in situ X-ray radiographic observation to the Mg–RE alloys.•The real time solidification of Mg–Gd alloys has been captured.•The dendrite growth direction of Mg–Gd alloys gradually rotates was obtained.•The average primary dendrites arm spacing decreases with increasing cooling rates.The real time microstructure evolutions of directional solidification in Mg–Gd alloys are obtained by synchrotron X-ray radiography, and the effects of different low cooling rates under a fixed thermal gradient are studied. Different from the organic alloys, the growth direction of columnar dendrites gradually rotates to the direction of the thermal gradient as the cooling rates increase, which is attributed to the difference of undercooling. Meanwhile, the interface velocity increases but the mean dendrite spacing decreases, and the morphology varies with implication for solute segregation.

•A novel bulk GdN /Mg-Gd nanocomposite was successfully fabricated.•GdN nanoparticles synthesized by in-situ reaction were distributed homogeneously.•The GdN/Mg-Gd nanocomposite exhibits phenomenal mechanical properties.A novel bulk Mg-Gd matrix nanocomposite reinforced with GdN nanoparticles has been successfully fabricated utilizing an advanced powder metallurgy route within in-situ reactions during sintering process. The as-prepared nanocomposite with nanocrystalline (NC) or ultrafine-grained (UFG) matrix and homogeneously dispersed nanoparticles exhibits phenomenal mechanical properties: ultimate tensile strength (UTS) of 521 MPa, yield strength (YS) of 506 MPa and tensile elongation (El) of 2.8%. The drastic improvement of the strength in comparison with that of conventional magnesium alloys is ascribed to fine grain strengthening of NC/UFG matrix, dispersion strengthening and load bearing mechanism of the GdN nanoparticles and β (Mg5Gd) or β′ (Mg3Gd) precipitates.

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 grain size on tensile and fatigue properties has been investigated in cast Mg alloys of Mg-2.98Nd-0.19Zn (1530 μm) and Mg-2.99Nd-0.2Zn-0.51Zr (41 μm). The difference between RB and push–pull fatigue testing was also evaluated in both alloys. The NZ30K05-T6 alloy shows much better tensile strengths (increased by 246 pct in YS and 159 pct in UTS) and fatigue strength (improved by ~80 pct) in comparison with NZ30-T6 alloy. RB fatigue testing results in higher fatigue strength compared with push–pull fatigue testing, mainly due to the stress/strain gradient in the RB specimen cross section. The material with coarse grains could be hardened more in the cyclic loading condition than in the monotonic loading condition, corresponding to the lower σf and the higher σf/σb or σf/σ0.2 ratio compared to the materials with fine grains. The fatigue crack initiation sites and failure mechanism are mainly determined by the applied stress/strain amplitude. In LCF, fatigue failure mainly originates from the PSBs within the surface or subsurface grains of the samples. In HCF, cyclic deformation and damage irreversibly caused by environment-assisted cyclic slip is the crucial factor to influence the fatigue crack. The Coffin–Manson law and Basquin equation, and the developed MSF models and fatigue strength models can be used to predict fatigue lives and fatigue strengths of cast magnesium alloys.

•Phase equilibria of the Mg–Gd–Ag system was experimentally studied.•A new ternary compound was found in the Mg–Gd–Ag system.•Thermodynamic description of Mg–Gd–Ag system was developed for the first time.Phase equilibria of the ternary Mg–Gd–Ag system focused on Mg-rich region at 400 °C and 450 °C are experimentally studied through evaluation of six selected compositions under the long annealed states. Analysis techniques, such as SEM/BSE, TEM and EPMA are employed for phase identification and chemical investigation. A previously unreported ternary compound, designated as X, is detected in the microstructures of all the samples. The crystal structure of this phase is determined as diamond-cubic structure with the lattice parameter of a=1.59±0.01 nm. Extensive solubility of the Mg5Gd phase along the direction parallel to the Gd–Ag binary in the ternary field is observed. Based on the thermodynamic descriptions of the three constituent binaries and the ternary experimental data obtained in the present study, a thermodynamic description for the Mg–Gd–Ag system is developed by using CALPHAD technique. Reasonable agreement between the calculated and experimental results is achieved.

•Production route of Mg-Nd master alloy from NdFeB magnet scraps was proposed.•The recovery was systematically studied and parameters were optimized.•The Nd content achieved above 20% and impurity levels were acceptable.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.

Fatigue behavior of magnesium castings has been investigated with NZK (Mg–Nd–Zn–Zr) alloys and other magnesium alloys including AZ91D, GW103, and AM-SC. Multi-scale fatigue (MSF) life models have been adapted to estimate fatigue lives of the magnesium castings studied. Results indicate that fatigue failure of magnesium alloys with few casting defects is dominated by crack initiation first within a grain close to the free surface and then propagation in trans-granular mode through either twin grain boundaries in the T4 heat treatment condition or from persistent slip bands in the T6/T7-treated condition. The fatigue life of cast magnesium alloys can be well predicted using multi-scale fatigue (MSF) models together with characteristic microstructure constituent – grain sizes.

•LPSO structure has first been found in as-cast Mg–6Gd–4Y–xZn–0.5Zr.•X-phase exists in as-cast Mg–6Gd–4Y–0.3(0.5)Zn–0.5Zr.•Zn content results in different β-phase in the studied alloys.Mg–6Gd–4Y–xZn–0.5Zr (x = 0.3, 0.5 and 0.7 wt.%) alloys were prepared via conventional ingot metallurgy (I/M) in this study. The as-cast microstructures of these alloys were established by X-ray diffraction (XRD) analyses, optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM) observations. Lamellar stacking order (SF) and 14H-type long period stacking order (LPSO) structure within α-Mg matrix are formed in the three as-cast alloys. The eutectic secondary phase is (Mg,Zn)24(Gd,Y)5 for the alloy containing 0.3 wt.% Zn, while, it is (Mg,Zn)3(Gd,Y) for the alloys containing 0.5 wt.% Zn and 0.7 wt.% Zn. Moreover, X phase-(Mg,Zn)12(Gd,Y) is formed in the latter two as-cast alloys.

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 effect of Friction stir process (FSP) parameters on the microstructure and mechanical properties of an extruded Mg–2.0Nd–0.3Zn–1.0Zr (wt.%) alloy was investigated in this paper. The alloy was friction stir processed with different passes: single-pass, three-pass and five-pass, under a tool rotation rate of 800 rpm and a traverse speed of 200 mm min−1. FSP results in remarkable grain refinement of the extruded alloy (average grain size ∼3.8 μm as 3 passes) and almost complete dissolution of the Mg12Nd phase in the matrix. With the increase of pass, the average grain size in the stir zone (SZ) is decreased firstly and then increases. The Vikers hardness of SZs in all FSPed samples is higher than that of the parent material (PM). Tensile tests at room-temperature show that the tensile strengths of the stir zones along the FSP advancing direction are slightly lower than those of PM. However, the elongations are remarkably improved from 13.0% for PM to 24.5% for SZ FSPed with three-passes. These improved tensile properties are attributed to the microstructure refinement, dynamic recrystallization and dissolution of the Mg12Nd phase.

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.

This paper reports the tensile properties and deformation behavior of a newly cast magnesium alloy Mg–3Nd–0.2Zn–xZr (NZ30Kx, x=0, 0.1, 0.3, 0.5, 1.0, 2.0 wt%) with different grain sizes and heat treatment conditions. Zirconium (Zr), as an effective grain-refiner in NZ30K alloys, is found to significantly improve the yield strength (YS), ultimate tensile strength (UTS) and elongation of these alloys. The NZ30K alloys show a significant response to heat treatment, achieving 60 and 47% increases in YS and UTS, respectively, in the peak-aged condition (14 h at 200 °C), compared with those of the as-cast alloy. Both grain refinement and age hardening significantly improve work-hardening at strains below 1%, leading to higher yield strength. At large strains, however, only grain refinement continues to show a slight strain hardening. The strain-hardening exponents of both NZ30Kx–T4 and NZ30Kx–T6 alloys decrease with reducing grain size at large strains. The Hall–Petch relationships of true flow stresses of NZ30Kx–T6 alloys at low strains (0.001–0.04) show a tendency of two-slope feature as plastic strains. All tensile failures occur prior to global instability, indicating the existence of localized damage.

This paper presents the results of a transmission electron microscopy study of the microstructure of extruded Mg–Nd–Zn alloys that exhibit improvement in strength, making them potential candidates for bumper beam applications. Three sets of extruded rods with 0%, 0.2% and 0.5% Zn addition are prepared to examine the effect of Zn on the precipitates in the as-extruded and aged condition. At least five different precipitate phases with different structures, morphologies and distributions were identified and correlated with the mechanical properties. Zn is seen to have no effect on the precipitation sequence of Nd-rich phases but seems to control the growth of those phases. It is found that small amount of Zn can lead to drastic changes in the nature of the precipitate phase due to the strong affinity between Zn and Nd in the alloy to form Mg–Nd–Zn intermetallics, affecting the strength and ductility. Control of both the Nd and Zn content are necessary to take advantage of the “rare earth texture” induced by Nd to improve ductility and precipitation strengthening by the Mg–Nd–Zn intermetallics when Zn is present.Highlights► Age-hardening effect of as-extruded Mg–3Nd (–Zn)–Zr alloys decreases with Zn. ► Increasing Zn, β″ in aged alloys decrease while basal plates increases. ► Aged NZ30K behave the highest UTS due to co-existence of basal and prism plates. ► The yield phenomena are connected to the random texture as well as dislocations.

A high-strength Mg-15.3Gd-1.8Ag-0.3Zr (GQ152K, mass fraction) alloy was prepared by conventional ingot metallurgy process. The solution and aging (denoted as T6) treated alloy exhibits remarkable mechanical properties with ultimate tensile strength of 421MPa and tensile yield strength of 309MPa. It has higher igniting temperature of 1 208 K. Moreover, it can stand against flame at 1 203K for over 6min in vertical burning tests, and its flammability behavior is very similar to that of 6101Al alloy. Vertical burning tests appear to be able to directly study the flammability behavior of Mg alloys and it appears to be a good approach to study the flammability behavior of Mg alloys in an aircraft fire accident.

In the present study, the microstructure, mechanical and wear properties of AXJ530 alloy under different solidification condition were investigated. AXJ530 alloys were cast in a multi-step permanent mould casting (PMC) with five different cooling rates, and also in high pressure die casting (HPDC). The effect of cooling rate was determined for the room temperature mechanical properties and the dry sliding wear resistance of the AXJ530 alloys. The results showed that grain size of AXJ530 alloy was refined and thinner lamellar eutectic phase formed at higher cooling rate. It was concluded that these changes led to the observed concurrent increases in ultimate tensile strength (σuts), yield strength (σ0.2) and elongation (δ) of the AXJ530 alloy. The relationship between grain size and yield strength/hardness agreed with Hall–Patch behavior. The dry sliding wear rate of the PMC specimens decreased with increasing of cooling rate, but micro-porosity/inclusion in the HPDC specimen decreased its wear resistance properties. Abrasion was determined to be the dominant wear mechanism for the AXJ530 alloys.

Short fiber reinforced magnesium matrix composite was fabricated by infiltration with molten AE44 (Mg–4.0Al–4.1RE–0.3Mn) alloy. The microstructures in the interfacial region and matrix of composite were characterized using scanning electron microscopy, electron probe micro-analyzer and transmission electron microscope. The fracture initiation and growth was observed by in situ scanning electron microscopy. It was shown that the distribution of the alloying elements was affected by the addition of fibers. The SiO2 binder in perform reacted with molten magnesium during infiltration, and the reaction product was identified as MgO. The Al–RE phases formed on the surface of fiber and in the matrix, in the form of lamella (Al11RE3) and particle (Al2RE), respectively. Microcrack initiated in the region of interface of composites, further failure included interfacial debonding add fiber breakage mode or only fiber breakage mode, depending on the fiber distribution to the tensile stress direction.

In present work, the NdH2 precipitates in Mg–2.5%Nd (wt.%) alloy were identified by various microstructural analyzing techniques including OM, XRD, TEM with EELS, SEM with EDS, etc. It was revealed that the as-cast alloy contained α-Mg solid solution and eutectic phase Mg12Nd, while there were NdH2 particles at or near the grain boundaries in 540 °C/6 h solution-treated state. As widely acknowledged, they worsen its mechanical property, especially, the cracking and fracture behaviors. Synchronously, the formation of hydride NdH2 is proved to be favored and practicable based on thermodynamics theory.