Zr Particles Research Articles

Effect of aging treatment on phase evolution and mechanical properties of selective laser melted Al-Mg-Er-Zr alloy

• The precipitation L1 2 -Al 3 Zr primary phase refines the grain size and forms bimodal grain structure. • Al 3 (Er,Zr) particles greatly improves the mechanical properties, with strength of 510 MPa and elongation of 16 %. • Dislocations, as a diffusion path to accelerate atomic migration, play an important role in the evolution of phases. Er and Zr modified Al-Mg alloy was manufactured by selective laser melting. The effect of aging treatment on phase evolution and mechanical properties of alloy has been studied. The results show that bimodal grain structures (2.8 ± 1.1 μm) could be obtained, thanks to the Al 3 Zr primary phases promoting the formation of equiaxed grains (0.8 ± 0.3 μm) at the boundary of molten pool. During the aging of 375 °C, Al 3 (Er,Zr) particles with the size of 2–5 nm were produced via synergistic precipitation of Er and Zr, which would greatly improve the strength and reach 510 ± 9 MPa. At the same time, the Mg-rich phase was dissolved, the Mn-rich phase was precipitated, and dislocations could accelerate the diffusion of solute atoms during the evolution of the phases.

High-temperature oxidation of sprayed alumina and zirconium‑yttrium-based coatings on Zircaloy-4 in air and oxygen: Investigation on microstructure evolution and phase changes

Zr-Y-C coating was prepared from APS, and its enhancement on the oxidation resistance of Zircaloy-4 in air and oxygen was compared with Al2O3 coating. Detailed characterization techniques including SEM, EDS, AFM, Raman spectroscopy and XRD were employed to analyze microstructure evolutions and phase components. Zr-Y-C coating is less susceptible to oxygen and Al2O3 coating is more resistant to air attack. Oxides and nitrides were both formed on these two coatings. Air ingress made Zr-Y-C coating exhibit a significant weight gain, while it maintained relatively stable in oxygen. As for Al2O3 coating, cracks developed and the porosity increased in air. When exposed to oxygen, metal ion diffusion accelerated, and numerous irregular Zr and Al oxide particles were produced.

Self-propagating high-temperature synthesis of heterophase materials in the Zr–Mo–Si–B system. Kinetics and mechanisms of combustion and structure formation

The paper focuses on the study of the combustion kinetics and mechanisms of elemental mixtures in the Zr–Mo–Si–B system, as well as the analysis of phase and structural transformation stages in the combustion wave. A thermodynamic analysis of potential chemical reactions occurring in the combustion wave was carried out. The reaction of ZrB2 formation is preferred in the range of 298–2500 K. Above 2200 K, the formation of MoB becomes more thermodynamically advantageous as compared to MoSi2. Phase stability estimates of combustion products showed that ZrB2, MoSi2 and MoB phases are in equilibrium. Experimental dependences Тc(Т0) and Uc(Т0) are linear, which implies an unchanged combustion mechanism at T0 = 298÷800 K. Preheating leads to an increase in Uc. Similarly, an increase in the proportion of Zr and B in the mixture has a similar effect, i.e. an increase in heat emission and Tc. With a minimum content of Zr and B, the interaction between Mo and Si with the formation of MoSi2 by the reaction diffusion mechanism is decisive. As the proportion of Zr and B increases, the rise of T0 to 750 K does not affect the Tc. Eeff values (50–196 kJ/mol) confirm the significant influence of liquid-phase processes on the combustion kinetics. The mechanism of structure formation was studied. A Si–Zr–Mo melt is formed in the combustion front. The primary grains of ZrB2 and MoB crystallize from this melt as it is saturated with boron. At the same time, the melt spreads over the surface of Zr and Mo particles. This leads to the formation of ZrSix, MoSix films. Core-shell structures are formed behind the combustion front, which disappear as they move towards the post-combustion zone. The phase composition of products is formed in the combustion front in less than 0.25 s.

Investigation on the Atomic Mechanism for Grain Refinement of Magnesium Alloys by Mg-Zr Master Alloy.

The valence electron structure, bond energy, and cohesive energy of Mg, Zr, and α-Mg containing Zr, and α-Zr containing Mg crystals were calculated using the empirical electron theory of solids and molecules (EET). The calculation results show that the bond and cohesive energies of Zr were much greater than those of Mg, so Zr particles could precipitate ahead of α-Mg in general magnesium alloy melts or insoluble Zr particles exist when the magnesium melt temperature is relatively low. The bond energy of α-Zr decreases with the increase in Mg content; therefore, at the end of the growth of Zr particles, the remaining Zr atoms in the melt exist in the form of Mg-Zr clusters. In order to reduce the surface energy of Zr particles, the outer surface of Zr particles tends to terminate with a Zr-Mg atomic layer, that is, a stable two-dimensional Zr-Mg atomic layer is formed first on the (0001) crystal surface of the outermost surface of Zr particles. Furthermore, on the basis of the calculated results, a complementary criterion to the edge-to-edge model of heterogeneous nucleation is also proposed. {ure and single Zr particles cannot become the heterogeneous nucleus of α-Mg, but when there is an atomic layer of two-dimensional Zr-Mg on its surface, the nucleation of particles can be activated. Mg atoms in the liquid phase preferentially attach to the Zr-Mg/Mg-Zr atomic layer on the surface of Zr particles to grow and form a stable ordered structure, which lastly transforms Zr particles into efficient heterogeneous cores.

Effect of annealing on microstructure and tensile properties of as-extruded Mg-5Gd-4Y-0.5Zn-0.5Zr alloy

In order to investigate the effect of heat treatment on the microstructure and tensile properties of an Mg-5Gd-4Y-0.5Zn-0.5Zr alloy, the phase diagram was calculated by Pandat software, and accordingly the as-extruded alloy was annealed at 450, 510 and 550 °C followed by water quenching. The microstructure and mechanical properties of the specimens were analyzed subsequently by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and tensile testing. The results showed that the average grain size of the as-extruded specimen was about 4 μm, in which Mg24Y5 was detected, and fine-grain strip was formed during the extrusion process. The grain size of the specimen annealed at 450 °C for 24 h was about 11 μm, and the main secondary phases were 14H-LPSO, δ-Zr3Zn2, β1-Mg3Gd and (HCP)Zr particles. The plate-like 14H-LPSO and cubic β1- Mg3Gd phases were coherent with the matrix, and the surface layer of (HCP)Zr was enriched with Zn atoms. When the annealing temperature reached 510 °C and 550 °C, the grain size increased to about 83 and 151 μm, respectively. The 14H-LPSO phases were gradually dissolved while the other secondary phases still existed. It was shown by uniaxial tensile testing that the ultimate tensile strength (UTS), yield tensile strength (YTS) and elongation (EL) of the as-extruded specimen were 291 MPa, 236 MPa and 24.7%, respectively. Increase of UTS, EL and decrease of YTS was resulted after annealed at 450 °C. However, both the strength and ductility dropped obviously after annealed at 510 °C and 550 °C. Ductile fracture features were found on the fracture surface of the as-extruded and 450 °C annealed specimens, whereas brittle topology, such as cleavage, was the main feature on the fracture surfaces of the specimens annealed at higher temperatures. Based on the strength analysis, it was concluded that the fine-grain strengthening was more important than solution strengthening and precipitation strengthening in this work.

Effects of solute atoms re-dissolution on precipitation behavior and mechanical properties of selective laser melted Al–Mg-Sc-Zr alloys

Selective laser melted (SLM) Al–Mg-Sc-Zr alloys still needs be post-treated to exhibit outstanding combination of strength and ductility. However, the influences of the different post heat treatments on the SLM Al–Mg-Sc-Zr alloys have not been systematically investigated. Hence, the SLM Al–Mg-Sc-Zr alloys were post-treated by a series of different heat treatments to investigate their influences on the SLM Al–Mg-Sc-Zr alloy, including the phase transitions, microstructural evolutions, and mechanical properties. Scanning electron microscopy (SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM) were used to study the emergence and effects of Al3(Sc,Zr) precipitates. Strengthening mechanisms of the different heat-treated samples were recognized and calculated in details based on the observations of cubic primary Al3(Sc,Zr) particles and spherical secondary Al3(Sc,Zr) precipitates behavior. Benefiting from the grain boundary strengthening and precipitation dispersion strengthening mechanisms, ultimate tensile strength of the aged samples increased from 353.7 MPa to 528.0 MPa.

Evaluation of Corrosion Inhibition of Plasma Sprayed FsHA/YSZ Coating on β-Titanium (Ti-13Nb-13Zr) Alloy Using Electrochemical Techniques

α + β titanium alloys especially Ti-6Al-4V alloy have dominated implant industry over the years due to their high corrosion resistance, strength, and flexibility. However, their high modulus (110GPa) compared to the human bone (18-30GPa) results in aseptic loosening of implants. Hydroxyapatite (HA) coating on Ti-6Al-4V alloys has been used to mitigate these demerits, nevertheless, more still need to be done. Hence, the present study aims at developing a natural and economical bioceramic coating on low modulus Ti-13Nb-13Zr alloy substrates using plasma spraying technique for biomedical applications. The bioceramic used was natural HA derived from fish scales (FsHA) and FsHA doped yttria stabilized zirconia at (10-20 wt.%). FsHA/YSZ powders and the coated samples were examined by XRD and SEM/EDX and the surface roughness, microhardness and corrosion resistance of the uncoated substrate and coated samples determined. The XRD pattern showed good crystalline FsHA/YSZ powders for all the compositions while the microstructure of the coatings revealed a fine splat lamellar morphology with partially melted and non-melted FsHA particles along with evenly dispersed Zr particles within the coating matrix for the FsHA/YSZ coatings. The maximum surface roughness (4.215 µm) was found with the FsHA coating while FsHA/YSZ coatings presented the highest hardness values (492.5-536.9 Hv) compared to the FsHA coating (467.8 Hv) and the uncoated substrate (385.9 Hv). Similarly, the corrosion resistance of the Ti-13Nb-13Zr alloy was significantly improved with the deposition of FsHA/YSZ bioceramic coatings.

High strain rate superplasticity and secondary strain hardening of Al-Mg-Sc-Zr alloy produced by friction stir processing

There have been many studies on superplasticity of Al-Mg-Sc-Zr alloy with middle magnesium content in friction stir processing (FSP), while few studies on superplasticity of FSPed Al-Mg-Sc-Zr alloy with low-magnesium were researched. In this work, the microstructure of Al-3Mg-0.1Sc-0.1Zr alloy with the grain size of ~1.6 µm and high-angle grain boundaries dominated was obtained by FSP. The high-temperature tensile test was carried out, which showed that Al-3Mg-0.1Sc-0.1Zr alloy had good superplasticity in the range of 400 ℃ ~ 500 ℃ and 3 × 10−3 s−1 ~ 3 × 10−1 s−1, and the optimal elongation reached 1850% at 475 ℃ and 10−2 s−1, showing a high strain rate superplasticity. The FSPed alloy had a significant secondary strain hardening ability with increasing flow stress in the failure fracture stage at higher temperature or relatively low strain rate, showing “H” type (continuous hardening) and “C” type + secondary hardening type (initial hardening then softening + secondary hardening) curves, which was mainly attributed to the concurrent grain growth at higher strain during high-temperature tension. By studying the microstructure evolution and constitutive equation, it was found that the main mechanism of superplastic deformation was grain boundary sliding accommodated by grain boundary diffusion, the coordination mechanism was lattice diffusion. Al3(Sc, Zr) particles hindered the coarsening of grains obviously with good thermal stability, which were conducive to the realization of excellent superplastic properties.

Effect of Er and Zr additions and aging treatment on grain refinement of aluminum alloy fabricated by laser powder bed fusion

In this work, Al-0.88Er-0.78Zr (wt%) alloy was fabricated via laser powder bed fusion (LPBF). The microstructure, precipitates distribution, electrical conductivity and hardness of as-built and aging treatment specimens were carefully characterized. Results show that LPBF process greatly expand the solid solubility of Er and Zr in Al. The addition of Er can significantly refine the grain size and form a bimodal grain structure consisting of fine equiaxed grains (grain size ~0.53 ± 0.15 µm) at the boundary of molten pool together with coarse columnar grains (width 2.57 ± 0.84 µm) at the center of molten pool. After 375 ℃ for 3 h aging treatment, the particles on the grain boundary inhibit the grain growth; at the same time, a large number of Al3(Er,Zr) particles with L12 structure and size of 2.25 ± 0.3 nm are precipitated, which plays a major role in enhancing the hardness with the peak hardness of about 89.24 ± 3.77 HV. Er and Zr elements are proved to be an alternative way of the additive elements for developing a new 3D printing high-strength aluminum alloy.

Extraordinary ductility enhancement of Mg-Nd-Zn-Zr alloy achieved by electropulsing treatment

Mg-Nd-Zn-Zr (JDBM) alloy was studied as a candidate for biodegradable implant material because of its moderate mechanical properties, good biocompatibility, and favorable uniform degradation behavior. To verify whether JDBM alloy exhibits electroplasticity effect and then study the mechanism of electropulsing treatment on JDBM alloy, in this study, homogenized and pre-tensile deformed samples were treated by electropulsing. After the electropulsing treatment, the average grain size was refined due to recrystallization, micron-scale Mg12Nd secondary phases precipitated slightly, while the morphology of nanoscale Zr particles changed from rodlike to ellipsoidal shape. The elongation to failure (EL) increased obviously for the homogenized and pre-tensile deformed JDBM alloy samples after electropulsing treatment, accompanying with no obvious sacrifice of the yield strength (YS) for the former and an evident decrease of YS for the latter, mainly due to the reduction of the dislocation density. The YS decrement and EL increment (77.57%) for the latter are more apparent attributed to the higher density of dislocations introduced by pre-tensile deformation. Therefore, the electropulsing treatment can obviously improve the mechanical properties of JDBM alloy, especially for the plasticity. The present work opens a new window for the fabrication of JDBM alloy profiles with high mechanical properties, especially for the plasticity, such as the cold drawing wires and tubes for biomedical applications. It also could provide theoretical references for other magnesium alloy processing.

Effects of ESMT on Microstructure and Mechanical Properties of Al-8Zn-2Mg-1.5Cu-0.15Sc-0.15Zr Cast Alloy in Squeeze Casting Process

Al-8Zn-2Mg-1.5Cu-0.15Sc-0.15Zr alloy with high-strength performance as well as good castability has been developed. In this study, effects of electromagnetic stirring melt treatment (ESMT) on microstructure and mechanical properties of the alloy in the squeeze casting process were investigated. The results show that solidification structure and mechanical properties are significantly improved by ESMT; compared with the conventional squeeze casting, the average grain size decreases from 112 μm without ESMT to 53 μm with ESMT. Meanwhile coarse primary Al3(Sc, Zr) particles unavoidably occurred in cases without ESMT disappear, and segregation degree of the main elements of Zn, Mg, Cu are greatly alleviated; the tensile strength increases from 590 MPa to 610 MPa, and the elongation increases from 9% to 11%. The structure refinement and homogenization should owe to uniform temperature and composition distribution by ESMT under squeeze casting with rapid solidification.

(Digital Presentation) Effect of Zirconium Addition on the Corrosion Behavior of Binary Magnesium-Zirconium Alloys

Magnesium (Mg) has the characteristics of low density and high specific strength, but its poor corrosion resistance is a fatal disadvantage. Therefore, various alloying elements are often added to Mg alloys to improve their properties. Among them, zirconium (Zr) is commonly added as a grain refiner for Mg alloys. However, in most cases, Zr was added as a secondary additive, by which the effect on the corrosion properties would be shadowed by other alloying elements. Therefore, this research aims to elucidate the effect of Zr addition on magnesium corrosion with two binary Mg-Zr alloys with different Zr concentrations (0.037wt% and 0.208wt%).In this study, scanning electron microscope (SEM) with X-ray energy dispersive spectroscopy (EDS) was used to study the Zr distribution and compositional difference in the alloys. Scanning Kelvin probe force microscope (SKPFM) was used to reveal the Volta potential difference between the Zr particles and the Mg matrix. In situ optical microscopic (OM) observation combined with open circuit potential (OCP) monitor was used to record the alloy corrosion morphology evolution. And through electrochemical impedance spectroscopy (EIS) analysis, the difference in the alloy corrosion properties was investigated. Hydrogen evolution tests were used to quantify the corrosion rates.Through SEM/EDS analysis, the size, distribution, and Fe content of the zirconium particles in the alloy are the most important differences between the two samples. For the sample with 0.208wt%Zr, the Zr particles are larger and contain more Fe. From SKPFM analysis, we also found that the Volta potential difference between the Zr particles and the Mg matrix is larger for the sample with higher Zr addition. Hydrogen evolution tests show that the corrosion rate of the 0.208wt%Zr sample is about 5 times compared to the 0.037wt%Zr sample, which is consistent with the EIS analysis results. The driving force of corrosion mainly comes from the microgalvanic effect provided by the Zr particles. The relationship between the Zr particles and localized corrosion behavior will also be discussed. Figure 1

Processing and in vitro corrosion analysis of sustainable and economical eggshell reinforced Mg and Mg-Zr matrix composite for biomedical applications

Although magnesium and magnesium alloys have low elasticity and biodegradable capabilities, the rapid rate of degradation of pure Mg in the body affects the mechanical and corrosion properties. In the current work, the Mg and Mg-1Zr matrix composite reinforced with Eggshell (ES) were synthesized using powder metallurgy. The elemental powders were blended using a milling machine, then compacted at 550 MPa and sintered in an inert atmosphere at 450° C for 2 hr. The synthesized composites were characterized in terms of structure, microstructure, densification, and microhardness. In vitro corrosion, performance was studied in Hank’s medium. The results showed that the sintered samples of Mg-1Zr, Mg-2.5ES, and Mg-1Zr-2.5ES had a relatively homogeneous distribution of reinforcement, with densifications of 99.65%, 98.9%, and 99.2%, respectively. The sintered Mg-1Zr and Mg-1Zr-2.5ES samples showed enhanced microhardness by 13% and 6% respectively compared to pure Mg. The biocorrosion study revealed the beneficial role of reinforcing Mg with Zr and ES particles and their synergetic effect by showing the higher polarization resistance and lower corrosion rate values in Hank’s medium. The finding demonstrates the potential of ES to be used as effective green reinforcement particles in the synthesis of Mg and Mg-Zr-based composite with improved in vitro corrosion properties for biomedical applications.

Operando X-ray diffraction study of thermal and phase evolution during laser powder bed fusion of Al-Sc-Zr elemental powder blends

Elemental powder blends are an emerging alternative to prealloyed powders for high-throughput alloy design via additive manufacturing techniques. Elemental Al+Sc(+Zr) powder blends were processed by laser powder bed fusion into Al-Sc and Al-Sc-Zr alloys, with operando X-ray diffraction at the Swiss Light Source extracting the structural and thermal history of the process. The pure Sc and Zr particles were found to react with the molten Al pool at 550–650 °C, well below their respective melting temperatures. Various scan areas (1 × 1, 2 × 2, 4 × 4, and 8 × 2 mm 2 ) were studied to compare (i) the base plate “preheating” effect caused by prior laser scans, (ii) the return temperature reached after the melting scan and before the following scan, (iii) the initial cooling rate immediately after solidification, and (iv) the time spent in the “intrinsic heat treatment range”, defined as 300–650 °C, where secondary Al 3 (Sc,Zr) precipitation occurs. Microstructural analysis of the as-built samples show 110–140 nm L1 2 -Al 3 (Sc,Zr) primary precipitates at the bottom of the melt pool. The 1 × 1 mm 2 samples exhibit the most elongated grains (long axis of 10 ± 5 µm), which correlates with the highest build plate temperature and the slowest initial cooling rate (3–5 × 10 5 K/s). In comparison, the 4 × 4 mm 2 samples exhibit the smallest equiaxed grains (2 ± 0.6 µm), corresponding to the lowest build plate temperature and the fastest initial cooling rate (6–7 × 10 5 K/s). These results indicate the need for establishing a minimum feature size during part design, or for modifying the laser parameters during processing, to mitigate microstructure and performance differences across features of different sizes.

Effect of minor Sc addition on high temperature tensile behavior of Al–7Zn–2Mg–2Cu–0.2Zr alloy

Alloys of Al–7Zn–2Mg–2Cu–0.2Zr and Al–7Zn–2Mg–2Cu–0.2Zr–0.2Sc were prepared by extrusion casting. Elongation–to–failure tests and strain–rate–change tensile tests were conducted at 300, 350, 400 and 450 °C, and deformation mechanisms were investigated based on microstructure, stress exponent and deformation activation energy. The results show that the 0.2 wt% Sc addition improves both tensile ductility and strength by refining grains and forming second phase particles. Primary Al3(Sc, Zr) particles precipitate during solidification and refine the as–cast grains, and secondary Al3(Sc, Zr) particles precipitate and interact with moving dislocations during high temperature deformation. The Sc addition has no significant effect on high temperature deformation mechanism. The deformation of both alloys is dominated by dislocation creep strengthened by second phase particles at 300 and 350 °C. Such strengthening effect weakens with the increase in temperature, and the deformation transfers to solute–drag dislocation creep at 450 °C.

Precipitation behavior, mechanical properties, and corrosion resistance of rare earth–modified Al-Zn-Mg-Cu alloys

In this study, we evaluated the microstructure, mechanical properties, and corrosion resistance of four types of 7055 alloys with and without the addition of rare earth elements (Er, Pr) elements. The results show a large number of coarse residual phase particles in the matrix of the 7055 alloy without the addition of rare earth elements, and the area proportion of residual phase is 3.56%; the proportion of low angle grain boundaries (LAGBs) after solid solution treatment is 31.1%. For the three other alloys that incorporate a rare earth element, the number of residual phases in the matrix is decreased, and the proportion of LAGBs is increased. As a result, the 7055 alloy with Pr (7055-Pr alloy) has the lowest number of residual phases, and the area proportion of residual phase is 1.41%; the 7055 alloy with Pr and Er (7055-Pr-Er alloy) has the highest proportion of LAGBs, which is 41.5%. Compared to the untreated 7055 alloy, the elongation and corrosion resistance of the three alloys with a rare earth element are increased. The increase in elongation is due to a reduction in the number of residual phases in the matrix, while the enhanced corrosion resistance is mainly due to the formation of Al3(Er, Zr) particles as a result of adding Er, which could pin the grain boundaries. Thus, the migration of LAGBs to high angle grain boundaries (HAGBs) is inhibited, the LAGBs are retained, and the corrosion resistance of the alloy is enhanced.

Dynamic recrystallization and precipitation in an Al–Mg–Sc alloy: effect of strain rate

Al–Mg–Sc alloy ingots were compressed 80% at different strain rates (1/s, 0.01/s and 0.0001/s) and different temperatures (300 °C and 450 °C). The microstructure was characterized by SEM, EBSD, XRD and TEM. The EBSD results show that the samples with a deformation rate of 0.01/s have the largest number of tiny equiaxed grains at the grain boundaries, but the equiaxed grains in the 0.0001/s sample are less and larger. This indicates that the number of new equiaxed grains does not always increase with decreasing strain rate. Furthermore, the TEM results confirmed that the recrystallization mechanism with a strain rate of 0.0001/s is similar to in situ recrystallization. The XRD results show that with the increase of temperature and the decrease of strain rate, there are fewer dislocations in the samples after thermal deformation. The particle stimulated recrystallization is also different in the 0.01/s sample and the 0.0001/s sample. Furthermore, at a strain rate of 0.0001/s, a large number of fine Al3(Sc,Zr) particles appear, which is due to the dynamic precipitation during slow thermal deformation.

Microstructure, mechanical property and thermo-stability of traditionally and severely deformed Al–Mg–Sc–Zr alloy

The effect of annealing temperature on microstructure and mechanical property of rolling and equal channel angular pressing Al–Mg–Sc–Zr alloy were studied. The ECAP passes were set as 2, 4 and 6, and the rolling deformation was set as 75%. Then, the samples were annealed at different temperature (from 100 °C to 550 °C). Results show that with the increase of the ECAP strain, the grain size becomes finer and finer, and the microstructure becomes more and more uniform. According to the XRD result, the average dislocation densities are 8.02 × 1014/m2, 1.67 × 1015/m2, 2.12 × 1015/m2 and 2.21 × 1015/m2 for CR, 2P, 4P and 6P sample. The Vickers hardness also increases with the ECAP strain. The yield strengths of CR, 2P, 4P, and 6P samples are 384 MPa, 420 MPa, 449 MPa and 462 MPa, respectively. Different samples have different degrees of hardness reduction after annealing. When the annealing temperature is over 400 °C, the hardnesses of samples are upside down. The strength of 6P samples are the lowest among all samples, while the those of CR samples are the highest. This is because the extreme high dislocation density leads to drastic recrystallization, making the grains grow up and many Al3(Sc, Zr) particles become large and incoherent, undermining the pinning effect.

Effect of deformation on evolution of Al3(Er,Zr) precipitates in Al–Er–Zr-based alloy

The effect of cold-rolling and high-temperature aging on precipitation processes in an Al–Er–Zr alloy was investigated by microhardness and resistivity measurements, scanning electron microscopy, (scanning) transmission electron microscopy and X-ray diffraction. Many similarities with the Al–Sc–Zr system have been shown. Based on the obtained results, following decomposition sequence of supersaturated solid solution of the Al–Er–Zr-based alloy has been proposed: Er-rich clusters → Al3Er phase → layer rich in Zr (Al3(Er,Zr) phase). Cold-rolling enhances the precipitation of Al3Er particles and accelerates the process identified as formation of the Zr-rich shell around the Er-rich core of Al3(Er,Zr) precipitates. The effect of precipitation strengthening is negligible compared to the effect of work hardening as the supersaturation of Er in the Al matrix was probably considerably lowered by formation of Er-rich primary precipitates observed in the as-prepared state of the alloys studied. The core–shell structure of the Al3(Er,Zr) particles formed after 2 and 4 h of aging at 600 °C was confirmed by Z contrast imaging and X-ray energy dispersive spectroscopy. The coherency strain-field around the precipitates is retained even after 4 h of aging at 600 °C. The Er/Zr atomic ratio of the precipitates was estimated based on their lattice constant.

Exceptional grain refinement of Mg-Zr master alloy treated by tungsten inert gas arc re-melting with ultra-high frequency pulses

The Zr distribution in a commercial Mg-30Zr master alloy was modified by tungsten inert gas arc re-melting with ultra-high frequency pulses (UHFP-TIGR) leading to the most substantial grain size reduction using a Mg-Zr master alloy. The results show that the Zr particle size is well refined, and higher contents of soluble Zr (Zrs) and nanoscale (∼10 nm) Zr particles (Zrnp) can be obtained. Zrs provides considerable constitutional supercooling (CS) for the nucleation of α-Mg grains on Zrp (∼100 nm particles). An exceptional improvement in the grain refinement efficacy (54%) of Zr master alloys is achieved with a holding time of 5 min. Consequently, a new approach to the design and fabrication of high-efficiency grain refiners is proposed.