As-cast Structure Research Articles

Insight into effect of forced convection during slab casting on as-cast solidification structure

Insight into effect of forced convection during slab casting on as-cast solidification structure

Metastability-driven room temperature strain hardening in a nitrogen added FeMnCoCrN high-entropy alloy

This study deals with the strain hardening capability of a nitrogen added FeMnCoCr high-entropy alloy during room temperature tensile deformation with an emphasize on the mechanical stability of FCC phase. The heightened metastability of the FCC phase provides a proper condition for hierarchical evolution of dual-phase FCC-HCP structure which finally promotes the formation of 63 % HCP martensite. Initially favoring slip mechanisms, the texture of the FCC phase transitions to geometrically hard orientations, thereby reducing its deformation accommodation capacity. This transition prompts the involvement of the HCP phase, initially evidenced by the emergence of new FCC phase and ε-twins at HCP martensite intersections. Subsequently, the formation of thickened ε-twins within the primary HCP lathes further contributes to deformation accommodation, explaining the observed excellent hardening behavior in the as-cast structure.

Enhancing mechanical properties in Ti-Containing FeMn40Co10Cr10C0.5 High-Entropy alloy through Chi (χ) phase dissolution and precipitation hardening

This research investigates the effects of adding 2 at.% Ti on the microstructure, phase transformation, and mechanical properties of FeMn40Co10Cr10C0.5 high-entropy alloy (HEA) in both as-cast and homogenized conditions. The incorporation of Ti increased the lattice parameter of the FCC matrix and caused greater lattice distortion (∼5.9 %) due to the larger atomic radius of Ti. The negative mixing enthalpy of Ti resulted in segregation at interdendritic regions, forming a Ti-rich Chi (χ) phase during solidification. The as-cast structure displayed a dendritic microstructure consisting of 77.9 % FCC, 20.6 % χ, and 1.5 % TiC phases. After homogenization at 1200 °C for 2 h, the χ phase was dissolved, increasing the volume fraction of TiC to 24 %. The homogenized sample exhibited a substantial improvement in tensile properties, i.e., with the ultimate tensile strength (UTS) rising from 631 MPa in the as-cast condition to 689 MPa, and yield strength (YS) increasing from 346 MPa to 419 MPa, while maintaining an elongation of ∼ 54 %. These enhancements are attributed to solid solution strengthening, increased carbide precipitation, and the dissolution of the χ phase into the FCC matrix.

Microstructural refinement of an Al-Ce-Mg alloy via Shear Assisted Processing and Extrusion

Al-Ce alloys have attracted recent interest because of their high thermal stability due to the low solubility of Ce in the Al matrix. The Al11Ce3 eutectic phase gives excellent strain hardening behavior and moderate high-temperature strength in the as-cast state. However, its strengthening effect is limited by its coarse as-cast structure. Therefore, alternative manufacturing methods such as additive manufacturing or equal channel angular pressing have been applied to refine the Al11Ce3 phase to good effect. However, these techniques are both expensive and time-consuming. Therefore, this study aims to use Shear Assisted Processing and Extrusion (ShAPE), an emerging solid phase processing technique that is more easily scalable than the previously mentioned methods. ShAPE can produce useful cross-sections of an Al-8Ce-4Mg alloy while refining the Al11Ce3 phase to produce a higher strength material. It was found that a low temperature ShAPE process can improve the room temperature yield strength by ∼60 % compared to a binary Al-4Mg alloy. Additionally, the high-temperature yield strength of the Al-Ce alloys increased by 20 %, with a simultaneous 15 % improvement in ductility compared to the binary Al-Mg alloy. These results highlight the potential for ShAPE as a processing technique for Al-Ce alloys.

Study of minor Sc and Zr additions effect on silicon rich Al–Mg–Si aluminum alloy microstructure during Sc multistage thermal treatment

The study addresses the effect of multistage thermal treatment on microstructure formation of Al–Mg–Si series alloy with scandium and zirconium additions in Mg/Si = 0.3 proportion. Basic alloy AlM–Si without Sc and Zr and them modification AlMg1SiZrSc were cast. AlMgSiScZr alloy multi-stage thermal treatment included the following 4 annealing stages –550 °C 8 h + 440 °C 8 h + 500 °C 0.5 h + 180 °C 5 h. For AlMg1Si alloy it was performed following 550 °C 8 h + 180 °C 5 h practice. AlMgSiScZr microstructure was examined using scanning (SEM) and transmission (TEM) microscopy both in as-cast state and after each stage of thermal treatment, for AlMg1Si – both in as-cast state and after final thermal treatment. Coarse intermetallic particles were examined using SEM, while fine particles were examined using TEM. Besides, microhardness values were measured after various thermal treatment stages. It was found out, that coarse intermetallic compounds of Fe2Mg7Si10Al18 type are formed in both alloys during as-cast structure cooling down with such compounds partial dissolution during further thermal treatment. At the same time particles, that can be attributed either to (AlSi)3ScZr or to AlSc2Si2τ-phase, occur in inter-grain boundary of alloys with scandium-zirconium additions. No traces of scandium solid solution discontinuous decomposition have been identified during ingot cooling down. Thermal treatment during 8 h at 550 °C 8 and 8 h at 440 °C results in formation of the phase, which can represent both AlSc2Si2 and Al5SiZr2. At the same time nanoparticles (AlSi)3ScZr do not form during this thermal treatment stage. Heating to 500 °C during 30 min interval allows complete dissolution of magnesium in supersaturated aluminum solid solution. During the final stage β’’ particles (Mg5Si6), representing the major type, form in both alloys; in these conditions scandium does not produce significant effect on their formation.

Improved Uniformity of As-Cast Structure, Composition, and Mechanical Properties of 2524 Aluminum Alloy by Intensive Melt Shearing

Improved Uniformity of As-Cast Structure, Composition, and Mechanical Properties of 2524 Aluminum Alloy by Intensive Melt Shearing

Scalable DC casting of large-size AZ80 magnesium alloy slab under out-of-phase pulsed magnetic field: Experiment and simulation

The large-size AZ80 magnesium alloy slab was manufactured using direct chill casting assisted by out-of-phase pulsed magnetic field. Subsequently, the effects of OPMF on the as-cast structure and the Al content were investigated. By using numerical simulation, the macro physical fields including the flow field, the temperature distribution, and the sump profile were simulated and compared between the slab with and without OPMF. After introducing the OPMF, significant grain refinement occurred, and the macrosegregation degree of the Al element was alleviated. Compared with the case without OPMF, the simulations revealed that the melt flow rate and its range were greatly increased, accelerating the convective heat transfer in the sump. As a result, the temperature field was more uniformed distributed, the cooling rate was increased and the sump depth became shallower. The relevant mechanisms between the experimental results and the simulation results were discussed in detail.

Solidification and microstructure investigation on as-cast and annealed hypoeutectic Ti–Fe alloys

This work explored the solidification behavior, microstructure development, and resulting mechanical properties of hypoeutectic Ti–Fe alloys under both as-cast and annealed conditions. Ti-(27, 28, 29)Fe (wt%) alloys were synthesized through arc-melting, followed by annealing at 950 °C for 24 h and slow furnace cooling. The investigation procedures included thermodynamic calculations, differential scanning calorimetry, scanning electron microscopy, high-temperature laser-scanning confocal microscopy, X-ray diffraction, hardness measurements, and compression tests. The study demonstrated the efficacy of annealing treatments in reducing excess eutectic in the as-cast structure formed during solidification. Results from both thermodynamic calculation and experiment for the solidification process demonstrated reasonable agreement. The high β-stabilizing effect of Fe ensured the presence of the β phase at room temperature under both studied conditions. The solid-solution hardening of the β phase, coupled with the TiFe intermetallic, substantially contributed to the increased hardness and mechanical strength, although the increase in brittleness was noticeable in the as-cast state. Subsequent annealing enhanced plasticity but resulted in a reduction in strength, albeit maintaining yield strength at a considerable level, approximately 1450 MPa.

Effects of Co on the microstructure evolution and mechanical properties of Ti-4Al-6Cr-5Mo-8 Nb alloy by ultrasonic vacuum melting

An investigation was conducted into the impact of low cobalt content on the microstructure and mechanical properties of titanium alloys. Ti-4Al-6Cr-5Mo-8 Nb-xCo (x = 0, 2, 3, 4, and 5) alloys were prepared by ultrasonic-assisted melting and casting. The changes in microstructure, mechanical properties, and the strengthening and toughening mechanisms were researched. Results show that, when the cobalt content increases from 0 to 5 wt%, the as-cast structure is all β phase. The lattice parameter of the β phase decreases from 0.3663 to 0.3114 nm, a 15% decrease. The size of primary β grains decreases from 676.8 to 310.7 μm, a 54% decrease. When the cobalt content exceeds 3 wt%, the segregation energy of cobalt increases and grain boundary segregation occurs. Grain refinement is due to constitutional supercooling caused by the β-stable element cobalt, as well as the solute drag caused by the differences in intragranular and grain boundary compositions. In addition, the size mismatch between cobalt and titanium atoms leads to lattice distortion. As the cobalt content increases from 0 to 3 wt%, the tensile strength increases from 918.1 to 1147.8 MPa, a 25% increase, while the fracture toughness increases from 51.7 to 59.7 MPa·m1/2, a 15% increase. At 5 wt% cobalt, the tensile strength and fracture toughness diminished to 247.5 MPa and 14.8 MPa·m1/2, respectively. The decrease in lattice parameters mainly lead to an increase in strength. The secondary crack leads to an increase in toughness. The decline in strength and toughness results from cobalt segregating at grain boundaries, leading to embrittlement.

Effect of C content and annealing on dendrite structure evolution and corrosion resistance of CoCrFeMnNi alloys

In this study, a series of CoCrFeMnNiCx (0–0.8 at% C) high-entropy alloys (HEAs) were prepared by vacuum arc melting and subsequently vacuum annealed at 800°C or 1000°C for 120 h. The aim was to investigate the microstructure evolution and corrosion behavior of the as-cast and annealed CoCrFeMnNiCx alloys in a neutral corrosion solution. Experimental results revealed that the addition of C can refine the as-cast dendrite structure, with an optimal C content of 0.4 at%. Annealing treatment led to a decrease in C solubility in the face-centered cubic (FCC) phase, resulting in the precipitation of carbides. In a 3.5 wt% NaCl neutral solution, the as-cast alloy containing 0.4 at% C exhibited the highest corrosion resistance. After annealing at 1000°C, the alloy's pitting potential and corrosion resistance were further improved. The corrosion behavior of both the as-cast and annealed CoCrFeMnNiCx alloys was primarily composed of pitting and intergranular corrosion, although the dominant mechanism varied among these alloys. This study establishes a correlation between microstructure evolution and corrosion behavior, providing insights for the design of high-entropy alloys with superior corrosion resistance.

Изучение эволюции микроструктуры и механических свойств в алюминиевом сплаве 1570 с добавкой 0,5 % гафния

Introduction. Aluminum alloys are in high demand with the aerospace industry. From the viewpoint of various performance characteristic combinations, high-magnesium aluminum alloys with the addition of transition metals, such as Zr and Sc, are among the most future-oriented alloys. Alloy 1570 is one of the most popular in this group. Recent studies demonstrated the positive effect of 0.5 % hafnium addition on as-cast structure. Study objective is to study the effect of the addition of 0.5% hafnium on the structure and properties of aluminum alloy 1570 during thermomechanical treatment. The study addresses the effect of cold rolling, homogenization, and recrystallization annealing on mechanical properties and microstructure of the specimens from alloy 1570 and similar alloy with 0.5 wt. % hafnium addition. Study methodology: for the study, ingots were cast from alloy 1570 with and without additions of 0.5 wt. % of hafnium. The resulting ingots were homogenized for 4 h at 440 °С, followed first by hot rolling and then cold rolling. Cold-rolled specimens were annealed at temperatures 340 °С to 530 °С with a holding time of 3 hours. The homogenized, cold-rolled, and annealed specimens were examined using transmission and light microscopy. In addition, homogenized and cold-rolled specimens were subjected to uniaxial tensile tests to determine the mechanical properties of the studied alloy. Results and discussion. It is revealed that in an alloy containing hafnium, after homogenization annealing, there is a slight decrease in the average particle size and an increase in its total proportion in comparison with alloy 1570. In general, 0.5 % hafnium addition does not significantly affect the mechanical properties. The number of nanoparticles in both alloys increases, as does the yield strength compared to the as-cast state. When heated, both alloys demonstrate an increase in plasticity and a decrease in strength characteristics. Studies of the annealing effect on the grain structure of the studied alloys showed that hafnium increases the tendency of alloy 1570 to recrystallize. However, additional research is required to determine the reasons for this phenomenon.

Precipitation of β-NiAl phase in a high Ru-containing Ni-based single crystal superalloy

The precipitation of β-NiAl phase in a high Ru-containing Ni-based single crystal superalloys was investigated. The β-NiAl phase is mainly distributed in the interdendritic region of the as-cast structure. And it was found parts of Ru elements were polarized in β-NiAl phase as the existence of strong bonds between Ru atoms and Al atoms. In addition, the solidification path of the experimental alloy passes through the region of the L → L1 + γ → L2 + γ + β-NiAl → L3 + γ + β-NiAl + precipitated γ′ → γ + β-NiAl + precipitated γ′ + γ/γ′ eutectics. It provides experimental data and theoretical guidance for the composition design of the novel Ru-containing high-generation Ni-SCs.

Evolution of Microstructure and Mechanical Properties of Novel Al-Mg-Mn-Ag-Cr-Zr Alloy

In order to reinforce the mechanism of Ag in 5xxx aluminum alloys with low magnesium, research on the microstructure and mechanical properties of an Al-Mg-Mn-Ag-Cr-Zr alloy was conducted using optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), hardness measurement, and tensile testing. The as-cast microscopic structure of the alloy comprises the Al6(Mn, Fe) phase and the T-Mg32(Al, Ag)49 phase. Changes in the characteristics of the investigated alloy were clear during the aging process. Based on the findings obtained from TEM and SAED analysis, it was evident that the predominant strengthening phase during the peak-aged stage is the β″ phase, observed when the alloy is aged for 24 h at 160 °C. The β″ phase had a L12-type crystal lattice architecture and presented a completely coherent relevance with the Al-matrix. The lattice parameter, a, of the β″ phase was 0.408 nm. The mechanical properties of the peak-aged alloy increased greatly as compared to the as-quenched alloy. The tensile strength exhibited a rise from 410 MPa to 449 MPa, representing a 9.5% increase, while the yield strength demonstrated an increase from 185 MPa to 273 MPa, indicating a significant enhancement of 47.5%. The method used in the present study has solved the problem of 5xxx aluminum alloys not being heat treatable for strengthening to a significant degree, considerably improving the alloy strength. In addition, new methods and foundations for exploiting new-type Al-Mg based alloys and developing high-strength aluminum alloys are provided in this study.

Grain Refinement in 441 Ferritic Stainless Steel: High Versus Low Ti/Al Ratio after Casting and Hot Rolling

The effect of Ti/Al ratio (mass%) on the evolution of the microstructures after casting and hot rolling of Ca treated 441 dual stabilized ferritic stainless steel (FSS) was investigated in order to understand its effects on grain refinement mechanism. Industrially cast and lab simulated hot rolled samples, were subjected to similar processing conditions but with different Ti/Al ratios of 2.4 and 7.8. The microstructures and inclusions were analysed by the OM, SEM-EDS, SEM-EBSD and AzTecFeature software. The results showed that the steel with higher Ti/Al ratio exhibited finer grains after continuous casting and hot rolling, i.e., the initial finer as-cast structure resulted in finer grains and less substructure after hot rolling. The steel with higher Ti/Al ratio contained more Ti-rich complex inclusions and precipitates (especially TiN), which led to more heterogeneous nucleation of the -ferrite and grain refinement during solidification. On the contrary, the steel with low Ti/Al ratio exhibited coarser as-cast grain structure, less recrystallization and higher volume fraction of substructure after hot rolling. Therefore, it was deduced that the Ti/Al ratio is one of the essential parameters to achieving grain refinement in Ca treated 441 FSS during continuous casting.

Effect of Fe-Bearing Phases on the Mechanical Properties and Fracture Mechanism of Al–2wt.%Cu–1.5wt.%Mn (Mg,Zn) Non-Heat Treatable Sheet Alloy

The effects of Fe-bearing phases on the structure, mechanical properties, and fracture mechanism of a non-heat-treatable model sheet alloy (wt.%: Al–2%Cu–1.5%Mn(-Mg,Zn)), designed for Al20Cu2Mn3 dispersoids, was investigated. This involved a combination of thermodynamic modeling in the Thermo-Calc program and experimental studies of structure and mechanical properties. It has been shown that the addition of 0.5 and 0.4% iron and silicon leads to the formation of eutectic inclusions in the Al15(Mn,Fe)3Si2 phase. In addition to the Fe- bearing inclusions, the formation of the eutectic Al2Cu and Al2CuMg phases can be expected in the as-cast structure of the experimental alloys. Despite their relatively high fraction of eutectic particles, non-homogenized alloy ingots demonstrated sufficiently high deformation processability during the hot (400 °C) and cold rolling, which made it possible to obtain high-quality sheet alloys (with reduction degrees of 80 and 75%, respectively). The results of the tensile tests revealed that, after cold rolling, the addition of 1% Mg significantly increased the tensile and yield strengths, whereas the effect of 1% Zn was negligible. At the same time, the uniform distribution of Fe-bearing phases in the structure of the cold-rolled sheets contributes to the preservation of the dimple mechanism of the fracture toughness. This helps to maintain the same level of ductility for the cold-rolled sheet Fe-containing alloys as for Fe-free alloys. It has been shown, based on the data obtained, that adding Fe, Si, Mg, and Zn to the base Al–2%Cu–1.5%Mn alloy in a total amount of more than 3% makes it possible to retain the ductile fracture patterns of the base alloy and obtain a fairly higher level of mechanical properties. This suggests the fundamental possibility of using a variety of secondary raw materials (containing the main elements present in aluminum alloys of different alloying systems) to prepare a base alloy that does not require homogenization or thermal hardening.

Investigation on the Optimal Amount of Y and B Elements in High-Temperature Titanium Alloy Ti-5.9Al-4Sn-3.9Zr-3.8Mo-0.4Si-xY-yB

This article presents a novel and feasible approach for researching the quantity of the ceramic phase and component optimization in high-temperature titanium alloys with small trace amounts of ceramic phases. Different near-α titanium alloys with varying yttrium and boron contents were prepared through the utilization of a vacuum non-consumable arc furnace melting method. The alloy used was a Ti-5.9Al-4Sn-3.9Zr-3.8Mo-0.4Si base. Its microstructure, texture, mechanical properties, and fracture behavior were studied. The observation of the as-cast structure shows that the addition of different doses of trace Y and B elements significantly refines both the original β grains and α grains. Moreover, the addition of the B element transforms the Widmanstätten structure in the titanium alloy structure into a basketweave structure. The addition of Y can refine the grain structure, improve the uniformity of the matrix structure, and act as a strong deoxidizer, which will take away the oxygen in the matrix and purify it. The TiB whiskers generated with the addition of B promotes dynamic recrystallization behavior and leads to more equiaxed α grains being precipitated around them, resulting in a significant refinement of the microstructure of the as-cast alloy. After adding a small amount of B, the texture strength of the α phase is significantly reduced, indicating that TiB whiskers inhibit the formation of texture. After conducting performance screening and structure analysis, the study supplements the analysis of Y’s regulation of the titanium alloy structure. The regulation is primarily explained by combining the results of the analysis of boron content, phase diagram analysis, mechanical properties, and fracture analysis. The mechanical analysis introduces the unique load transfer strengthening of TiB whiskers combined with an analysis of high-temperature mechanical properties, as the threshold for addition. The optimal amounts of Y and B additions are 0.6 wt% and 0.8 wt%, respectively. The optimized alloy obtained under this condition can achieve a tensile strength of 950 Mpa at 500 °C without any plastic deformation or heat treatment.

Semi-macrosegregation and carbide banding in high-carbon chromium bearing steels: Characteristics, evolution, and control

Semi-macrosegregation in continuously cast blooms and its associated carbide banding defects in hot-rolled bars were investigated to explore the characteristics, evolution, and control methods in high-carbon chromium bearing steel. It was found that the semi-macrosegregation exists only in the central equiaxed crystal zone of the blooms, and C, Cr solute elements inside show typical spot-like positive segregation. Carbide banding appears as white bright bands along the rolling direction, composed of high number density M3C carbide particles, with a significant effect on the mechanical properties of the rolled products. The hereditary evolution from semi-macrosegregation to carbide banding was established by means of banded segregation. Morphologically, normal semi-macrosegregation and carbide-typed semi-macrosegregation are formed based on whether Cr-rich primary carbides are generated. These two types of semi-macrosegregation evolved into two kinds of carbide banding respectively, that is, dark bands and bright bands defined according to the ability to reflect light under OM. To alleviate the carbide banding defect in rolling products of high-carbon chromium bearing steel, the morphology and area ratio of equiaxed crystals in continuous cast blooms should be modified. It is observed that the equiaxed crystal ratio in the bloom can be decreased from 62.5 % to 28 % by increasing liquid steel superheat and inhibiting superheat dissipation in the mold through casting practices. Additionally, the morphology of equiaxed crystals is related to the as-cast solidification structure, and when the equiaxed crystal ratio is large, the equiaxed crystals turn out to be coarser ones with only cellular primary dendrite; when the columnar crystals are longer, however, the equiaxed crystals appear as a dendrite structure with fine secondary dendrites. As a result, semi-macrosegregation was improved obviously with the fine dendrite structure splitting the local residual liquid phase, leading to the decrease of the maximum width of carbide banding from 268.41 μm to 73.72 μm, and the increase of mechanical properties in the rolled bar of high-carbon chromium bearing steel.

Study on the Effect of SEMS on the As-Cast Structure of Steel in Thin Slab Continuous Casting Using an Equiaxed-Columnar Solidification Model

Study on the Effect of SEMS on the As-Cast Structure of Steel in Thin Slab Continuous Casting Using an Equiaxed-Columnar Solidification Model

A lightweight intermetallic Al-Cr-Ti medium-entropy alloy with good mechanical properties and oxidation resistance

Structure, mechanical properties, and oxidation behaviour of a Al55Cr23Ti22 (in at%) lightweight (ρ = 3.92 g/cm3) medium-entropy intermetallic alloy were studied. The initial as-cast structure consisted of an (Al, Ti)-rich L12 matrix and Cr-rich C11b lamellae with semi-coherent interfaces due to a (111)L12 || (110)C11b, [11̅0]L12 || [33̅1̅]C11b orientation relationship. The alloy demonstrated a combination of good plasticity (17–19%) and strength (600–610 MPa) both at 22 and 600 ℃; the latter gradually decreased to 155 MPa at 1000 ℃, while plastic strain exceeded 40%. Deformation at 600 and 800 ℃ led to the precipitation of (semi-) coherent C11b_1 and Al2Ti particles inside the L12 matrix and C11b phase, respectively, due to elemental redistributions within the initial phases and low lattice mismatch with a matrix. The alloy gained only 0.27 mg/cm2 after oxidation in the air at 1000 ℃ for 100 h thanks to an early formation of a protective Al2O3 layer. Compared to the current state-of-art lightweight and Al/Cr-containing refractory high/medium-entropy alloys, the Al55Cr23Ti22 alloy had a higher 1000 ℃-specific strength, room-temperature plastic strain, and demonstrated the best oxidation resistance. The obtained results suggest that the multicomponent L12-based trialuminides can be used as structural materials for potential high-temperature applications.

Mechanism for TiN refinement and resultant heterogeneous nucleation of δ-Fe in magnesium-rare earth-treated ultrapure ferritic stainless steel

The effect of magnesium rare-earth (Mg-RE) treatments on TiN epitaxial nucleation to refine the as-cast structure of ultrapure ferritic stainless steel was investigated. Four samples with different melt treatments were prepared, namely an initial sample, a low-RE-treated sample, a high-RE-treated sample, and a Mg-RE-treated sample. The results reveal that TiN can be refined after the RE/Mg-RE treatment because of the co-crystallization of TiN and RE-Mg-Al-O. Moreover, the size of the inclusions increased with the RE content owing to the decreasing number of oxides. Compared to the low-RE treated sample, the average inclusion size of the Mg-RE-treated sample was further reduced because of the presence of more oxides. These results suggest that the number of oxides is a key factor in TiN refinement. However, few changes were observed in the 3D morphology of the composite TiN under such circumstances. In addition, compared with the Initial sample, the RE/Mg-RE treatment yielded finer solidification structures, indicating that the addition of RE/Mg-RE modifiers decreased the nucleation barrier. The solidification structure coarsened with increasing RE content because of the decrease in the inclusion number density, particularly for TiN. The Mg-RE treatment yielded the finest solidification structure with completely equiaxed grains. In addition, high temperature laser confocal microscopy was successfully applied to observe the in-situ nucleation-growth of δ-Fe, showing that the average grain size after Mg-RE treatment decreased by 38.09%. To reveal these phenomena, a new interfacial wetting-lattice mismatch heterogeneous nucleation model is established, accounting for the high-temperature wetting angle (θ1). Using differential scanning calorimetry, the upper limit of the heterogeneous nucleation rate (Iheter.) of δ-Fe was determined from the number density of all the TiN inclusions. Crystallization of primary δ-Fe and Iheter. depend on the critical degree of undercooling (∆Tc∗).