Heterogeneous Nucleation Research Articles

Effects of Typical Inclusions on the As‐Cast Microstructure and Recrystallization of Low‐Density Steel

Herein, the effect of nonmetallic inclusions (NMIs) on the microstructure and dynamic recrystallization (DRX) of Fe–23Mn–6Al–0.7C low‐density steel is studied. The specimens are separately added sulfur (S) and nitrogen (N) to change the characteristics of NMIs. As a result, the addition of S significantly increases the number of MnS inclusions and AlN–MnS complex inclusions in low‐density steel. The addition of N results in a significant increase in the size of NMIs. In the results, NMIs act as the heterogeneous nucleation sites for austenite and refine the as‐cast grains in low‐density steel. In addition, the treated specimens are subjected to a thermal compression simulation experiment. During the thermal compression process, discontinuous DRX is considered to be the main recrystallization method in the experimental low‐density steels. Meanwhile, the typical NMIs significantly accelerate the DRX within grains in low‐density steel through the mechanism of particle‐stimulated nucleation and continuous DRX.

Effect of aging treatment on microstructure and corrosion properties of Al-Cu-Mn-Si-Mg-Er-Zr alloy

The microstructure, mechanical properties and intergranular corrosion resistance of Al-Cu-Mn-Si-Mg-Er-Zr alloy with squeeze casting in the peak aging state at different temperatures were studied. The alloy was aged at 165 °C, 175 °C and 185 °C, and reached peak aging at 18 h, 12 h and 8 h respectively, among them, the alloy treated with 175 °C/12 h aging has the highest hardness, reaching 149 HV, and the yield strength is also the highest, which is 387 MPa. It is because the Q phase precipitated during the aging process of the alloy provides a heterogeneous nucleation site for the θ′ phase, the average size of the θ′ phase is 73.38 nm. The intergranular corrosion depth of the alloy after aging treatment at 175 °C/12 h was the deepest, reaching 284.91μm. At this time, the width of precipitation-free zone(PFZ) is the widest in the grain boundary microstructure, which is 92.21 nm, and the grain boundary precipitation phase is continuously distributed.

In-situ nanoscale precipitation behavior and strengthening mechanism of WC/IN718 composites manufactured by laser powder bed fusion

Nickel matrix composites (NMCs) have received much attention as they show excellent high temperature performances, which could meet the demand for hot-end components in severe environments. In this study, the duplex in-situ nanoscale carbides and oxides reinforcing IN718 were fabricated by laser powder bed fusion (LPBF) and addition of submicron WC particles. The microstructural features and mechanical properties were investigated with variations of WC content (0, 0.5, 2, and 4 wt%). The columnar dendrites with strong <001> texture that boundary distributed continuous Laves phase were found in the LPBF-processed IN718. Incorporating WC led to higher laser absorption and processability, whereas it also decreased the anisotropy of the matrix featured by weak texture index. With an optimized content of 2 wt%, the WC was dissolved into W and C atoms under laser irradiation, which promoted the homogeneous precipitation of intergranular cubic (Nb, Ti)C (∼50 nm) and intragranular spherical Al2O3 (∼20 nm). These precipitates would for heterogenous nucleation to refine grains (∼12.6 μm) and bring about high-density dislocations. The yield strength (YS) and ultimate tensile strength (UTS) increased by 28.8% and 26.5%, respectively, while maintaining high elongation compared with reported NMCs, demonstrating a favorable strength-ductility combination. Finally, the mechanism of microstructure evolution and strengthening was elucidated.

An Algorithm for Modeling Thermoplastic Spherulite Growth Using Crystallization Kinetics.

Crystallization kinetics were used to develop a spherulite growth model, which can determine local crystalline distributions through an optimization algorithm. Kinetics were used to simulate spherulite homogeneous nucleation, growth, and heterogeneous nucleation in a domain discretized into voxels. From this, an overall crystallinity was found, and an algorithm was used to find crystallinities of individual spherulites based on volume. Then, local crystallinities within the spherulites were found based on distance relative to the nucleus. Results show validation of this model to differential scanning calorimetry data for polyether ether ketone at different cooldown rates, and to experimental microscopic images of spherulite morphologies. Application of this model to various cooldown rates and the effect on crystalline distributions are also shown. This model serves as a tool for predicting the resulting semi-crystalline microstructures of polymers for different manufacturing methods. These can then be directly converted into a multiscale thermomechanical model.

Crystallization of Calcium Carbonate and Calcium Phosphate Phases in Silica Hydrogel: Morphological and Compositional Characterization

The present study showcases a series of crystallization experiments using a specially designed double diffusion system to grow crystals belonging to the calcium carbonate–phosphate system. The experimental U-shaped device comprised two vertical solution containers, separated by a horizontal column of silica hydrogel. Each container was filled with 0.5 M CaCl2 and 0.5 M Na2CO3 solutions, which diffused through the gel column over time. Na3PO4 solutions, with 50 and 500 ppm concentrations, were incorporated into the gel in different experiments, resulting in a homogeneous distribution of phosphate concentrations within the diffusion column. After 15- and 30-day incubation periods post-nucleation, the crystals formed in different sections of the gel were carefully extracted and studied with scanning electron microscopy and electron microprobe. Additionally, Raman spectra were collected from the samples using a confocal Raman microscope, providing further insights into their molecular composition and structural properties. The obtained results show that under the induced experimental conditions (i) phosphate incorporates into calcite’s structure, and (ii) the growth of calcium phosphates in the presence of carbonate ions involves the sequential, heterogeneous nucleation of CO3-bearing OCP/HAP-like phases, with Raman spectral characteristics very similar to those of bioapatites.

Strengthening mechanisms of laser cladding TiC/FeCoCrNiCu high-entropy composite coatings: Microstructure evolution and wear behaviors

The HEA-TiC high-entropy composite coatings with TiC (5, 10, 15 and 20 wt%) reinforced FeCoCrNiCu matrix were fabricated by laser cladding to investigate their tribological performances and wear behaviors at RT and 600 ℃. As the percentage of TiC particles elevated, the microhardness was progressively enhanced to 531.65 HV0.5 and its surface energy weakened to 24.29 mN/m. Whereas the excess TiC particles added into the coating internally appeared serious agglomeration, resulting in performance rebound. Fine-grain strengthening of high melting point and high hardness TiC acting as a heterogeneous nucleation site, precipitation strengthening of the Laves phase and dispersion strengthening of re-solidified carbides, and the blocking effect of TiC particles on dislocation motion. These reinforcing behaviors synergize to confer the desired tribological properties on the 15 wt% TiC coating, with outstanding wear resistance (7.6 × 10−6 mm3/N·m) at RT. The highly dense oxide film composed of TiO2 and the friction film reconstituted by Cr2O3 act synergistically on the coating surface at 600 ℃, the wear rate guided by oxidative wear is reduced by 88.7 %, compared to the TiC-free coating.

On the formation trajectory of diverse corrosion morphologies associated with T phases in an Al-Cu-Li alloy

Four different types of localized corrosion morphologies associated with Al20Cu2Mn3 dispersoids (T phases) in an AA2195 alloy were traced using atom-resolved characterizations and quasi-in-situ experiments. At the atomic scale, we demonstrated that the rod-shaped T dispersoid is homogeneous within the grain but enriched with Cu on the side surfaces, leading to a depletion of T1 precipitates nearby and an abundance near the tips. This plays a significant role in the initiation of galvanic corrosion between T dispersoids and the matrix. Combined with heterogeneous nucleation, the subsequent redeposition of noble metal clusters results in four typical corrosion morphologies.

Introducing high-density growth twins in aluminum alloys by laser surface remelting via templated nucleation of grains

It is difficult to generate coherent twin boundaries in bulk Al alloys due to their high intrinsic stacking fault energy. Here, we report a strategy to induce high-density growth twins in aluminum alloys through the heterogeneous nucleation of twinned Al grains on twin-structured TiC nucleants and the preferred growth of twinned dendrites by laser surface remelting of bulk metals. The solidification structure at the surface shows a mixture of lamellar twinned dendrites with ultra-fine twin boundary spacing (∼2 μm), isolated twinned dendrites, and regular dendrites. EBSD analysis and finite element method (FEM) simulations have been used to understand the competitive growth between twinned and regular dendrites, and the solidification conditions for the preferred growth of twinned dendrites during laser remelting and subsequent rapid solidification are established. It is shown that the reduction in the ratio of temperature gradient G to solidification rate V promotes the formation of lamellar twinned dendrites. The primary trunk spacing of lamellar twinned dendrites is refined by the high thermal gradient and solidification rate. The present work paves a new way to generate high-density growth twins in additive-manufactured Al alloys.

Microstructure and wear performance of ex/in-situ TiC reinforced CoCrFeNiW0.4Si0.2 high-entropy alloy coatings by laser cladding

The W and extra/in-situ TiC reinforced CoCrFeNi high-entropy alloy coating was prepared on a 45 steel substrate by laser cladding. The synergistic effect of W and ex/in-situ TiC on the microstructure and wear properties of the coating was studied. The results showed that the ex/in-situ TiC promoted the selection of the FCC phase with a preferred orientation to the (111) crystal plane, and the increasing of in-situ TiC and W+ex-situ TiC/in-situ TiC resulted in the transformation of the FCC phase to the BCC phase. The microstructure of W+in-situ TiC coating was more uniform and two heterogeneous nucleation points Cr3C2 and TiC appeared in the coatings during the solidification process. The wear width, depth, and volume were minimized. The wear rate (2.54×10−6 mm3/(N·m)) was an order of magnitude lower than that of CoCrFeNi (4.97×10−5 mm3/(N·m)). The W+in-situ TiC coating shows the best wear resistance. There were a small amount of wear debris and shallow furrow-like grooves, and the wear mechanism was abrasive wear. The synergistic effect of W and in-situ TiC on wear resistance was significant.

Experimental and theoretical investigation on refining mechanism of Mg2Si by Y addition in the Mg-Al-Si alloys

Aiming at development of new lightweight and high-strength magnesium alloys, a combined experimental and theoretical study on refining mechanism of Mg2Si by Y addition in the Mg-Al-Si alloys was performed. Two alloys, Mg63.32Al22.83Si13.85 and Mg62.56Al22.55Si14.36Y0.52 (wt.%), were prepared and investigated by combining experimental characterization with quantum-mechanical and CALPHAD calculations. The experimental results indicated that Y can refine the Mg2Si phase in Mg-Al-Si alloys but it is rather sensitive to the preparation procedure of the alloys. It was observed that Al4MgY is surrounded by Mg2Si in the as-cast alloys with Y, but it undergoes separation after homogenization annealing. First-principles calculations revealed that the mechanism of Y refining the Mg2Si phase is direct adsorption on the Mg2Si surface rather than indirect adsorption in the form of the elements atomic substitution or entering the center interstitial sites of Mg2Si. The significant refinement of the Mg2Si phase after annealing is attributed to recrystallization within the grains, eliminating the original grain boundaries, and the surface activity is reduced with Y adsorption, resulting in the formation of new and smaller Mg2Si phases. The mechanism behind the Al4MgY phase being surrounded by Mg2Si in the as-cast state but subsequent separation after homogenization annealing is not heterogeneous nucleation, but maybe due to liquid entrapment.

A new concept of inoculation by isomorphic refractory powders and its mechanism for grain refinement

Isomorphic Inoculation (ISI) is a novel grain refinement technique during solidification by the addition of metallic powders to the melt rather than ceramics. The powders are designed to grow epitaxially rather than to nucleate new solid, increasing theoretically the ratio between solidified grains and inoculants to 1:1. This technique circumvents the energy barrier problems related to heterogenous nucleation in traditional grain refinement techniques. In this manuscript, we show through a combination of numerical and experimental techniques that this ratio could be reached. This is achieved through novel experimental techniques to replicate the effects of the melt on the powders. A new CaF molten salt experiment was used to, for the first time, show the recrystallization/grain growth of Ti powders at high temperature. In parallel, we calibrated a model on our experimental conditions and used it to calculate the dissolution rate of the powders in the melt. The combined effects of grain growth, preferential grain boundary dissolution (increasing the number of powders) and bulk dissolution (decreasing the number of powders) act together, bringing the number of particles present during solidification near that of the number of solidified grains found in the sample ingots. The per particle efficiency of this method is far in excess of traditional inoculation methods and has the potential for wide applications, especially in systems such as TiAl used here where the heterogeneous particles remaining in the as-cast material are undesirable.

"Hard" Emulsion-Induced Interface Super-Assembly: A General Strategy for Two-Dimensional Hierarchically Porous Metal-Organic Framework Nanoarchitectures.

Two-dimensional (2D) hierarchically porous metal-organic framework (MOF) nanoarchitectures with tailorable meso-/macropores hold great promise for enhancing mass transfer kinetics, augmenting accessible active sites, and thereby boosting performance in heterogeneous catalysis. However, achieving the general synthesis of 2D free-standing MOF nanosheets with controllable hierarchical porosity and thickness remains a challenging task. Herein, we present an ingenious "hard" emulsion-induced interface super-assembly strategy for preparing 2D hierarchically porous UiO-66-NH2 nanosheets with highly accessible pore channels, tunable meso-/macropore sizes, and adjustable thicknesses. The methodology relies on transforming the geometric shape of oil droplet templates within appropriate oil-in-water emulsions from conventional zero-dimensional (0D) "soft" liquid spheres to 2D "hard" solid sheets below the oil's melting/freezing point. Subsequent surfactant exchange on the surface of 2D "hard" emulsions facilitates the heterogeneous nucleation and interfacial super-assembly of in situ formed mesostructured MOF nanocomposites, serving as structural units, in a loosely packed manner to produce 2D MOF nanosheets with multimodal micro/meso-/macroporous systems. Importantly, this strategy can be extended to prepare other 2D hierarchically porous MOF nanosheets by altering metal-oxo clusters and organic ligands. Benefiting from fast mass transfer and highly accessible Lewis acidic sites, the resultant 2D hierarchically porous UiO-66-NH2 nanosheets deliver a fabulous catalytic yield of approximately 96% on the CO2 cycloaddition of glycidyl-2-methylphenyl ether, far exceeding the yield of approximately 29% achieved using conventional UiO-66-NH2 microporous crystals. This "hard" emulsion-induced interface super-assembly strategy paves a new path toward the rational construction of elaborate 2D nanoarchitecture of hierarchical MOFs with tailored physicochemical properties for diverse potential applications.

Modeling and validation of hydrogen porosity formation in aluminum laser welding

Laser welding is used to weld aluminum alloy 1100 for battery cell manufacturing in the automotive industry. However, these welds can have porosity that can decrease their performance in the battery packs. One type of porosity is gas porosity, commonly understood to be due to hydrogen entrapment in the weld during solidification. Hydrogen is dissolved in the molten aluminum at elevated temperatures during laser heating and can be entrapped in the weld pool upon solidification. When welding anodized aluminum, the amount of hydrogen porosity is increased because the anodized oxide layer is broken up and the broken-up oxide can act as heterogeneous nucleation sites for hydrogen pores. A cellular automaton model has been developed to predict the nucleation and growth of hydrogen porosity during laser welding. This model has been validated with microstructure analysis, LECO® hydrogen analysis, and X-ray micro-computed tomography. This model can be used by welding engineers to reduce porosity and control weld quality during laser welding of aluminum alloys.

Flexible and highly-loaded mixed-matrix membrane with bi-continuous metal–organic frameworks transfer pathway for gas separation

Highly-loaded metal–organic frameworks (MOFs) based mixed matrix membranes (MMMs) are essential for high-efficiency gas separation. However, the continuous distribution of MOFs across the membrane with intensified interface remains challenging. Herein, a flexible and defects-free MMM with a bi-continuous MOFs pathway is constructed by dopamine (DA)-assisted MOFs growth along nanofibers. Nanofibers are used to upload and distribute ZIF-8, which continues to grow via an epitaxial growth strategy to obtain ZIF-8 fibers, achieving ZIF-8 continuous both in long and short range. DA promotes heterogeneous nucleation and growth of ZIF-8 on fiber surface by chelating with Zn2+ through –OH and –NH2 groups. DA also ensures excellent interfacial compatibility by hydrogen bonding interaction between amino groups of DA and ether-oxygen groups of PEO. Finally, the highly-loaded MMM is constructed by in-situ photopolymerization of Poly (ethylene glycol) diacrylate) to embrace ZIF-8 fibers tightly. The resultant MMM presents ZIF-8 loading as high as 72.41 vol% with Young’s modulus of 33.42 MPa, and unchanged morphology and separation performances after bending for 180° over 50 times, revealing outstanding mechanical properties. The CO2 permeability and CO2/N2 selectivity are 280.3 % and 73.0 % higher than the PEO membrane, surpassing the 2019 McKeown upper bound.

Методика дослідження зародження кристалів у розплавах краплинним методом

The purpose of this work is to develop scientifically based ways of influencing the process of nucleation of crystallization centers in metal melts to control the formation of the cast structure and the properties of their blanks at the first stage of metal product production (at the casting stage). The possibility of controlling the nucleation of crystals in their melts was studied on transparent model environment (paraffin and salol) using the drop method, and the dependence of the crystallization kinetics of droplets with a dispersion of 100÷200 μm on their overheating temperature, the duration of exposure in an overheated state, and the degree of supercooling was established. Physical modeling on transparent environment made it possible to display images on a laptop screen with the help of a digital camera mounted on a microscope and to observe the nucleation and growth of crystals in real time, to determine the duration of solidification of all observed drops, depending on the experimental conditions. The criteria for evaluating the crystallization process of the investigated environment were the curves of the crystallization kinetics of drops from their melts. The nature of the change in the crystallization kinetics curves of the experimental environment, in our opinion, confirms the assumption about the decisive role of impurities in the crystallization processes of any alloys. The logical confirmation of the hypothesis about the decisive role of impurities in the processes of crystallization from the same positions of heterogeneous nucleation is the dependence of the number of solidified drops of experimental environment of the same diameter on the degree and duration of their overheating. It is likely that with greater overheating of the melt (or longer exposure to this overheating), partial deactivation (dissolution) of the impurities present occurs and the number of solidifying drops changes for the same supercooling. The analysis of the obtained results indicates a heterogeneous mechanism of crystal nucleation even in drops of microscopic size (~100 μm). The developed method of studying the processes of nucleation and growth of crystals in model alloys depending on their mass (size) can be applied in real conditions of mass crystallization. Keywords: model environment, droplet method, crystal nucleation, crystallization kinetics.

Relation between ice nuclei particles concentration and aerosol counting at different sizes

This work investigates the relation between the natural ice particle (IP) concentration measured between −30 °C and −48 °C and the aerosol number concentration in Córdoba, Argentina. Air mass back trajectories reaching Córdoba were calculated to also analyze the relation between the origin of the air masses and the behaviour of aerosol number concentration and IP concentration. For temperatures higher than −36 °C, results show a clear trend of an increase in IP concentration with both decreasing temperature and increasing concentration of aerosols with aerodynamic diameter ranging from 0.5 μm to 20 μm (naer,0.5). For temperature ranging from −30 to −34 °C a power law fit relating naer,0.5 and ice nuclei particle (INP) concentration is proposed. Based on the relationship between IP concentration and temperature for naer,0.5 at two specific concentration ranges, a change in trend was observed at temperatures below −36 °C compared to higher temperatures. The small slopes at temperatures lower than −36 °C indicate that heterogeneous nucleation is less important than at higher temperatures. Additionally, the fact that, at temperatures lower than −36 °C, the slopes for both ranges of naer,0.5 are similar, indicate that the nucleation processes are independent of the concentration and size of aerosols. From the comparison with values previously reported, it was found that measurements obtained in the present work are at least one order of magnitude higher than estimations following the parameterization proposed by DeMott et al. (2010). The observed differences can be explained considering the uncertainties in the estimated INP concentrations, the uncertainties related to the IP measurements, and the different characteristics of the sites where measurements were performed. Considering the scarcity of INP measurements and its relationship with aerosol concentration in the Southern Hemisphere, this work provides valuable data that can be compared against field measurements or used in empirical parameterizations, constituting in both cases important contributions to improve the representation of clouds in climate models.

Solidification path and precipitation mechanism of a Ni-Cr-co-Mo based heat-resistant alloy

The solidification path and precipitation mechanism of the C-HRA-3 Ni-Cr-Co-Mo-based heat-resistant alloy are investigated by methods such as differential scanning calorimetry analysis, in-situ observations, quenching experiments, and scanning transmission electron microscopy. The results confirm that the primary solidification precipitates in the center of the C-HRA-3 alloy VAR ingot consist of Ti(C,N), M6C-M23C6 symbiotic phases, Ti(C,N)-M6C-M23C6 symbiotic phases and surrounding dispersed M6C and M23C6 carbides. The precipitation temperatures for Ti(C,N), M6C, and M23C6 are 1310 °C, 1280 °C, and 1230 °C, respectively. The C-HRA-3 alloy exhibits a wide solidification temperature range of 176 °C (1390 °C ∼ 1214 °C). Within 1390 °C ∼ 1355 °C, the γ phase grows vigorously, accompanied by a significant reduction in the fraction of liquid phase. At 1355 °C ∼ 1214 °C, the decrease in liquid phase fraction slows down, accompanied by significant enrichment of Mo, Cr, C and Ti elements. There is a clear functional relationship between the liquid phase percentage and temperature. M6C carbides undergo two heterogeneous nucleation mechanisms in the liquid phase. Due to the degradation of M6C carbides discharging Cr and C atoms, M23C6 carbides form at their edge. Dispersed M6C and M23C6 carbides nucleate at misfit dislocations around the symbiotic phases. Appropriately reducing the cooling rate during the stage above the precipitation temperature and increasing the cooling rate below the precipitation temperature can significantly reduce the volume percentage and size of the symbiotic phases.

Effect of Ce addition on the nucleation and growth of austenite in ultra-high-strength steel

The effect of varying rare earth element cerium (Ce) contents (0∼0.0792 wt%) on the austenite grain size of ultra-high-strength engineering steel has been studied under different austenitization conditions (holding time of 5∼360 min at temperatures ranging from 900 °C to 1200 °C). Results show that the addition of Ce can refine grains, and the refining effect becomes more pronounced as the Ce content increases. The inhibition of austenite grain growth by fine Ce-containing inclusions during the holding process has been observed through laser scanning confocal microscopy (LSCM) and field-emission scanning electron microscope (FE-SEM) equipped with an energy dispersive spectrometer (EDS), and the pinning effect of Ce-containing inclusions on the austenite grain boundaries is the primary factor for the refinement of austenite grains. Besides, theoretical calculations have shown that the lattice misfits between the (100) plane of CeP and the (111) plane of the γ-phase is 7.83%, indicating that CeP could serve as a heterogeneous nucleation core for the γ-phase and was moderately effective. Furthermore, the uniformization effect of Ce on the grains is more prominent than the refining effect at the holding temperatures of 1000 °C and 1200 °C.

High temperature evolution of a confined silicon layer

The temperature-induced phase and morphology changes of a thin layer sandwiched between two substrates which it partially wets are investigated using transmission electron microscopy, scanning electron microscopy, and x-ray scattering techniques. For this, SiC wafers were bonded with Si layers of various thicknesses and annealed at temperatures below and above the Si melting point. Below the melting point of Si, solid-state dewetting occurs. It starts with the heterogeneous nucleation of pits at the Si/SiC interfaces and progresses to their partial transformation into voids crossing the whole film. The further growth of voids is accompanied with an increase in the Si film thickness. Final equilibrium is shown to be impacted by Si crystallographic state evolution. Above the Si melting temperature, liquid Si drives SiC interfaces step bunching. When high steps and large terraces are formed over the two SiC surfaces, Si is shown to be trapped within quasi-closed pockets. Eventually, the interface locally closes around these Si inclusions with the creation of SiC/SiC direct contacts. The influence of both annealing temperatures and Si film thickness on all these processes is discussed.

Influence of solute content and cooling rate on the preparation of semi-solid slurries for novel creep-resistant Mg-Gd-Ca-Zr alloys

The semi-solid Mg-Gd-Ca-Zr alloys slurries with fine and spherical α-Mg particles were successfully prepared. The influence of solute Gd and Zr contents, and cooling rate on the morphological evolution of α-Mg are systematically investigated. The results showed that addition of Zr will turn the coarse dendritic α-Mg particles into fine and globular particles. With the increase of Zr addition from 0 to 0.6 wt%, the particle size decreases, and the particle density and shape factor increase continuously. With the change of solute Gd content from 5 to 15 wt% in the Mg-yGd-2Ca-0.6Zr alloy, the particle size of α-Mg particles decreases continuously, and shape factor increases continuously. With cooling rate decreases from 3 to 1.5 °C/min, the α-Mg particles are slightly coarsened and the morphology are kept nearly globular, while with further decreasing into 0.5 °C/min, the α-Mg particles are severely coarsened and turned into irregular shape. The optimal Zr addition is ∼0.6 wt% and Gd addition is no less than 10 wt%, and the optimal cooling rate is 1.5–3 °C/min. The mechanisms for obtaining fine and spherical α-Mg particles through Zr and Gd additions are discussed to be different roles of heterogenous nucleation effects and growth restriction effects.