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The present work aims to boost our understanding of factors governing the grain-refining efficiency of inoculation treatments by comparing the grain-refining efficiencies of two inoculators: Ti nanoparticles and LaB6 nanoparticles, in a 2024 Al alloy during additive manufacturing (AM). Experimental results obtained by scanning electron microscopy show that the LaB6 nanoparticle possessed almost no refining effect on the alloy, with the addition content ranging from 0.5 wt.% to 2 wt.%. Conversely, the Ti nanoparticle resulted in a more pronounced refinement and a fine, fully equiaxed microstructure at 1 wt.% Ti addition. Based on transmission electron microscopy analysis, the higher refining efficiency of Ti inoculation was ascribed to the incorporation of both Ti solute and the in situ-formed L12-Al3Ti nucleation particles. The former significantly increased the overall undercooling ahead of the growing Al grain, which ensured the activation of heterogeneous nucleation on the L12-Al3Ti nanoparticles, leading to grain refinement. This work highlights that despite the addition of nucleation particles, the incorporation of appropriate solutes to generate sufficient undercooling is the prerequisite for the activation of heterogenous nucleation in AM. [ABSTRACT FROM AUTHOR]

Abstract: CaO–Al2O3–MgO–SiO2 (CAMS)‐based glass‐ceramics were prepared using body crystallization method. Adding Cr2O3 into the ceramics not only effectively lowered the crystallization temperature, but also led to significant grain refinement of diopside that crystallized in the CAMS glass‐ceramic after crystallization treatment at 900°C for 2 hours. Experimental work verified that the epitaxial growth of the diopside on the spinel particles, which formed during nucleation treatment when fabricating the glass‐ceramics, facilitated the heterogeneous nucleation of diopside on the spinel and refined the diopside. In addition, two energetically favored crystallographic orientation relationships between the epitaxial growth diopside and spinel were experimentally observed. They are [ 1 ¯ 12 ] spinel//[001]diopside,// ( 1 1 ¯ 1 ) spinel//(200)diopside and [ 1 ¯ 12 ] spinel ]//[101]diopside, (311)spinel// ( 131 ¯ ) diopside. These two novel results can be potentially used to develop new glass‐ceramic materials with improved performance. [ABSTRACT FROM AUTHOR]

Grain size is one of the key microstructural factors that control the mechanical properties of steels. The present work aims to extend the theories of grain refinement which were established for cast light alloys to steel systems. Using a designed Fe-4 wt pct Si alloy (all-ferrite structure during whole solidification process), the solute effect on grain refinement/grain coarsening in ferritic systems was comprehensively investigated. Experimental results showed that boron (B), which is associated with the highest Q value (growth restriction factor) in ferrite, significantly refined the as-cast structure of the Fe-4 wt pct Si alloy. Cu and Mo with low Q values had no effect on grain refinement. However, although Y and Zr have relatively high Q values, addition of these two solutes led to grain coarsening in the Fe-4Si alloy. Understanding the results in regards to the growth restriction factor and the driving force for the solidification led to the conclusion that in addition to the grain growth restriction effect, the changes of thermodynamic driving force for solidification due to the solute addition also played a key role in grain refinement in ferritic alloys. [ABSTRACT FROM AUTHOR]

A new grain refiner, LaB, was identified for ferritic steels based on the crystallographic calculation using the edge-to-edge matching model. Addition of 0.5 wt pct LaB led to a reduction of the average grain size from 765 to 92 μm and the proportion of the columnar structure from 35 to 8 pct in an as-cast Fe-4Si ferritic alloy. Although LaB was supposed to act as an active inoculant for δ-ferrite, thermodynamic calculation indicated that LaB is not thermodynamically stable in the melt of the Fe-4Si alloy. It was subject to decompose into La and B solutes. Consequently, both La and B reacted with Fe, O and S, forming different compounds. Microstructural examination at room temperature observed LaSO and LaO particles within the ferrite grains and FeB along the grain boundaries in the samples. Through EBSD analysis, a reproducible orientation relationship between ferrite and LaSO was identified. In addition, the edge-to-edge matching calculation also predicted the high potency for LaSO to be an effective nucleant for δ-ferrite. It was considered that the grain refinement of LaB was attributed to the enhanced heterogeneous nucleation of δ-ferrite by LaSO, and the solute effect of B due to the high Q-value in ferrite. [ABSTRACT FROM AUTHOR]

Most studies in the area of grain refinement have always taken for granted that higher cooling rate results in finer grains. However, when microstructural variation of the as-cast Mg with cooling rate was investigated using a specially designed V-shaped copper mold, the results were different. Although fast cooling during solidification led to microstructural refining in pure Mg, grain coarsening was observed at a higher cooling rate in Mg alloys that were inoculation treated with 1.0wt pctZr and 1.4wt pctCaO, and in the Mg-Al binary alloys. It is considered that the grain coarsening at higher cooling rate was attributed to the smaller constitutional undercooling zone formed at fast cooling due to the high temperature gradient in the three Mg alloys. These results can help in redefining the role of cooling rate in the grain refinement process. [ABSTRACT FROM AUTHOR]

In order to clarify and articulate the long-standing problems associated with the role of various compounds in grain refinement of as-cast steels, a comprehensive crystallographic study on grain refiners in a number of low carbon steels has been conducted using the edge-to-edge matching (E2EM) model, which has been successfully applied to explain and predict effective grain refiners in light metals. Five commonly investigated compounds, namely NbO, CeS, TiN, Ce2O3and TiC, in steels were examined. According to the extent of crystallographic matching, the predicted grain refining potency of these five grain refiners is ranked in the order of NbO > CeS > TiN > Ce2O3 > TiC, which is consistent with previously reported experimental results. Four different orientation relationships between δ-ferrite and these grain refiners were predicted. One of them has been verified by previously published experimental data. The similarity and the advantages of the E2EM model over conventional Bramfitt’s model were also discussed. [ABSTRACT FROM PUBLISHER]

Grain refinement is known to be strongly affected by the solute in cast alloys. Addition of some solute can reduce grain size considerably while others have a limited effect. This is usually attributed to the constitutional supercooling which is quantified by the growth restriction factor, Q. However, one factor that has not been considered is whether different solutes have differing effects on the thermodynamic driving force for solidification. This paper reveals that addition of solute reduces the driving force for solidification for a given undercooling, and that for a particular Q value, it is reduced more substantially when adding eutectic-forming solutes than peritectic-forming elements. Therefore, compared with the eutectic-forming solutes, addition of peritectic-forming solutes into Al alloys not only possesses a higher initial nucleation rate resulted from the larger thermodynamic driving force for solidification, but also promotes nucleation within the constitutionally supercooled zone during growth. As subsequent nucleation can occur at smaller constitutional supercoolings for peritectic-forming elements, a smaller grain size is thus produced. The very small constitutional supercooling required to trigger subsequent nucleation in alloys containing Ti is considered as a major contributor to its extraordinary grain refining efficiency in cast Al alloys even without the deliberate addition of inoculants. [ABSTRACT FROM AUTHOR]

Based on electron backscatter diffraction, a novel and simple method was developed to determine orientation relationships (ORs) between phases in solids. The method was successful in explaining the crystallography of heterogeneous nucleation of Mg grains grown on Al2Y nucleation particles in an Mg–10wt.% Y alloy. Two reproducible ORs were identified and they completely agree with previous predictions from the edge-to-edge matching model. This confirms that Al2Y is an effective grain refiner for this type of alloy. [Copyright &y& Elsevier]

[Display omitted] In response to the critical need for lightweight designs and carbon neutrality, we introduce an innovative additively manufactured ultrafine-grained Al-Mg-Mn-Sc-Zr alloy reinforced with nano-structured planar defects via laser powder bed fusion (L-PBF), developed for complex-shaped parts that demand high strength and superior ductility. Owing to the uneven distribution of the L1 2 -ordered Al 3 (Sc, Zr) nanoparticles, the as-printed alloy demonstrates a hierarchically heterogeneous microstructure featuring a triple-modal grain distribution. Tailored planar defects comprising stacking faults, 9R phase and nanotwins are strategically introduced in the as-printed alloy. Beyond the nano-scaled planar defects and the triple-modal grain distribution, further direct ageing process augments the abundance of nanoprecipitates, collectively boosting the yield strength to 656 MPa, which is higher than almost all L-PBFed Al alloys hitherto reported, and a decent ductility of 7.2 %. This work paves the way for the near net shape forming of high-performance Al alloy components for advanced structural applications. [ABSTRACT FROM AUTHOR]

Refinement of as-cast structures is one of the most effective approaches to improve mechanical properties, formability, and surface quality of steel castings and ingots. In the past few decades, addition of rare earths (REs), lanthanum and cerium in particular, has been considered as a practical and effective method to refine the as-cast steels. However, previous reports contained inconsistent, sometime even contradictory, results. This review summaries the major published results on investigation of the roles of lanthanum or/and cerium in various steels, provides reviews on the similarity and difference of previous studies, and clarifies the inconsistent results. The proposed mechanisms of grain refinement by the addition of lanthanum or/and cerium are also reviewed. It is concluded that the grain refinement of steels by RE additions is attributed to either heterogeneous nucleation on the in-situ formed RE inclusions, a solute effect, or the combined effect of both. The models/theories for evaluation of heterogeneous nucleation potency and for solute effect on grain refinement of cast metals are also briefly summarized. [ABSTRACT FROM AUTHOR]

Abstract: Commercial Al-3Ti-1C and Al-5Ti-1B master alloys were added in order to refine the grains of Mg-Li-Al alloys. The effects of their addition levels on grain refinement of Mg-Li-Al cast alloy were investigated and the mechanism of the grain refinement was discussed. The results showed that the addition of Al-3Ti-1C master alloy reduced the grain size of LA141 cast alloy from 900 ?m to 400 ?m, while the addition of Al-5Ti-1B master alloy reduced the grain size of LA51 cast alloy from 500 ?m to 240 ?m. The grain refining mechanism was the heterogeneity nucleation of TiC and TiB2, because of less than 10% mismatches of TiC/?-Li and TiB2/?-Mg. [Copyright &y& Elsevier]

Grain refinement of additively manufactured titanium and titanium alloys can be promoted via adding foreign elements or particles, but it may lead to a reduction in ductility due to the formation of brittle intermetallic compounds. The present study shows that in-situ grain refinement of commercially pure titanium (CP-Ti) can be achieved through properly controlling the selective laser melting (SLM) parameters. It was found that higher input energy density worked in favour of grain refinement. Detailed microstructural characterization coupled with multi-physics simulation were performed to reveal the grain refinement mechanism. This was attributed to the intrinsic heat treatment (IHT) effect which resulted from the cyclic reheating inherent to the SLM process. As a result, the refined CP-Ti exhibited an exceptionally high ductility of 34.3 ± 0.5% without notable mechanical anisotropy. This work demonstrates the feasibility of utilizing thermal cycling of additive manufacturing (AM) to refine grains of metals without changing the composition. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Considerable studies on metal selective laser melting (SLM) have proved the necessity to refine microstructure parts fabricated by SLM in order to eliminate property anisotropy, hot-tearing and to increase the SLM-processability. In the present work, Ti nanoparticles, at the first time, were discovered to be an extremely effective inoculant for an SLMed 2024 aluminium alloy. 0.7 wt% addition of Ti nanoparticles was capable of substantially eliminating the hot-tearing cracks and columnar structure, and refining the grains in the SLMed 2024 alloy in a broad processing window. The substantial grain refinement in the Ti-inoculated 2024 alloy was attributed to the in-situ formation of Al 3 Ti nanoparticles with a L1 2 ordered structure, which formed a coherent interface with Al matrix and therefore significantly promoted the heterogeneous nucleation of the α-Al during solidification of melt pools in the SLM process. After a conventional T6 heat treatment, this SLMed alloy exhibited a superior balance of strength and ductility (tensile strength was up to 432 ± 20 MPa and elongation of 10 ± 0.8%), which was comparable to its wrought counterpart. This work can be considered as a breakthrough in research of fabricating high-strength aluminium alloys using SLM. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Industry applications of the current cryogenic high entropy alloys (HEAs) are limited by their prohibitive costs, relatively low yield strength and unknown corrosion resistance. Here, we present a cost-effective and facile approach to produce cryogenic HEAs with lower cost, exceptional mechanical properties and corrosion resistance. The key is to design a cost-effective Fe 35 Ni 35 Cr 20 Mo 5 Al 5 HEA and introduce ultrafine microstructure (UFM), including ultrafine grains (∼693 nm), high density of annealing twins and nanoprecipitates, into the alloy by manipulating the concurrent precipitation and recrystallization at 940 °C within 2 min. The cost-effective UFM-HEA exhibits a temperature-dependent strain-hardening capacity and a superior strength-ductility synergy at 77 K with a yield strength of ∼1165 MPa, a tensile strength of ∼1412 MPa and a uniform elongation over 20%. The superior tensile properties at cryogenic temperature are attributed to the twining-dominated multiple deformation mechanisms and the integrated strengthening effects, including grain refinement strengthening, twin strengthening and second-phase strengthening. At the fracture strain of ∼20%, extensive microcracks formed within the nanoprecipitates without propagating into the face-cantered-cubic matrix, suggesting a high crack tolerance of the UFM-HEA at both room and cryogenic temperatures. Moreover, the UFM-HEA has a superior corrosion resistance compared to the 316L stainless steel due to the larger passivity region and higher charge transfer resistance. Such outstanding cryogenic mechanical properties and corrosion resistance together with the lower cost make the UFM-HEA superior to most cryogenic HEAs. This strategy not only sheds light on the development of new-generation cryogenic HEAs but also significantly enhances their industrial application potential. [ABSTRACT FROM AUTHOR]

The edge-to-edge matching model was used to identify TiB 2 as a potential grain refiner to refine the high temperature δ-ferrite in low carbon steels. To effectively introduce the grain refiner, Al–5Ti–B master alloy (MA) was used to add TiB 2 particles into an A517 steel. However, there was no grain refinement in the as-cast A517 steel, attributed to the decomposition of TiB 2 particles in the molten A517 steel, into Ti and B. As a result, BN particles formed on the pre-existing Al 2 O 3 particles were detected within prior austenite grains. Nevertheless, there was significant grain refinement after reheating the A517 ingots to 1150 °C, isothermally holding for 2 h and air cooling. The austenite grain size of the reheated A517 steel was decreased from 318 ± 70 μm to 38 ± 4 μm using 0.1 wt% Al–5Ti–B MA. Grain refinement was caused by Ti-containing nanometre-sized precipitates along prior austenite grain boundaries inhibiting the grain growth. The grain refinement improved the strength and ductility of ingots after reheating and improved workability for the production of hot worked products. • TiB 2 has been identified as an effective grain refiner for low-carbon low-alloy steels. • Inoculation of the A517 steel with Al–5Ti–B master alloy could significantly refine the cast A517 steel after reheating. • With 0.05 wt% Al–5Ti–B master alloy addition, A517 steel showed the optimal strength-ductility trade-off. [ABSTRACT FROM AUTHOR]

The 4th generation of intermetallic γ-TiAl based alloy, the so-called β-solidifying TiAl, has attracted great attentions due to its promising high-temperature properties. But it is intrinsically brittle and crack-prone, which lowers the processability of laser additive manufacturing (LAM) that offers an effective pathway for material processing and design. Thus, improving the mechanical properties, particularly the room temperature plasticity, of LAMed β-solidifying TiAl alloy is of technical interest and theoretical importance. Based on previous results, the present work investigates the synergistic roles of La and B in grain refinement and crack elimination in a LAMed β-solidifying TiAl alloy with LaB 6 addition (0–2 wt%). Results show that La 2 O 3 and TiB in-situ formed via reaction between the β-solidifying TiAl and the added LaB 6 nanoparticles. Microstructural feature, phase transformation texture and mechanical properties at both room and high temperatures can be optimized through controlling the LaB 6 addition level. 0.5 wt% LaB 6 has been determined as the optimized addition, at which the balanced mechanical properties at both room temperature and 800 °C can be achieved for the LAMed β-solidifying TiAl alloy. Excessive LaB 6 addition led to agglomeration of in-situ formed precipitates, reducing density and therefore deteriorating mechanical properties. The study offers an in-depth understanding of the effective grain refining approach to overcome the intrinsic brittle nature of β-solidifying TiAl through optimized doping of LaB 6. [ABSTRACT FROM AUTHOR]

CaO was validated as an effective grain refiner for cast Mg alloys. 0.7 wt.% CaO addition led to significant reduction in grain size. As CaO particles were reproducibly found within the Mg grains, it was considered that CaO particles promoted heterogeneous nucleation. Additionally, some CaO particles were chemically reduced by Mg, which produced Ca solute and contributed to the grain refinement. The present work indicates that co-existence of active nucleants and solutes with high Q-values in the melt is essential to achieve the highest grain refining efficiency. For Mg alloys, chemical reactions should also be considered at inoculation treatment. [ABSTRACT FROM AUTHOR]

Recent studies have confirmed that the high brittleness and property anisotropy of the selective laser melting (SLM) fabricated H13 tool steel are attributed to the presence of elongated, unstable high carbon γ (austenite) thin films in between the columnar structures. To overcome this problem, TiN nanoparticles (0.5 wt.%) were used as an inoculator to promote heterogeneous nucleation of the prior γ-grains, converting the columnar structure into equiaxed and refined grains. As a result, the finer α'-blocks formed upon the subsequent martensitic transformation. This not only leads to the elimination of the property anisotropy, but also simultaneously increases the strength (tensile strength of 2051 ± 48 MPa) and ductility (elongation of 7.4 ± 0.7%) of the SLM-fabricated H13 steel. The present work forms a significant step towards to fabricating high-performance tool steels using the SLM technique. [Display omitted] [ABSTRACT FROM AUTHOR]

Grain refinement of cast magnesium alloys, particularly in magnesium–aluminium (Mg–Al) based alloys, has been an active research topic in the past two decades, because it has been considered as one of the most effective approaches to simultaneously increase the strength, ductility and formability. The development of new grain refiners was normally based on the theories/models that were established through comprehensive and considerable studies of grain refinement in cast Al alloys. Generally, grain refinement in cast Al can be achieved through either inoculation treatment, which is a process of adding, or in situ forming, foreign particles to promote heterogeneous nucleation rate, or restricting grain growth by controlling the constitutional supercooling or both. But, the concrete and tangible grain refinement mechanism in cast metals is still not fully understood and there are a number of controversies. Therefore, most of the new developed grain refiners for Mg–Al based alloys are not as efficient as the commercially available ones, such as zirconium in non-Al containing Mg alloys. To facilitate the research in grain refinement of cast magnesium alloys, this review starts with highlighting the theoretical aspects of grain refinement in cast metals, followed by reviewing the latest research progress in grain refinement of magnesium alloys in terms of the solute effect and potent nucleants. [ABSTRACT FROM AUTHOR]

Abstract: The mechanism underlying the grain refinement of cast aluminium by zirconium has been studied through examination of a range of Al alloys with increasing Zr contents. Pro-peritectic Al3Zr particles are reproducibly identified at or near the grain centres in grain-refined alloy samples based on the observations of optical microscopy, scanning electron microscopy and X-ray diffraction. From the crystallographic study using the edge-to-edge matching model, electron backscatter diffraction and transmission electron microscopy, it is substantiated that the Al3Zr particles are highly potent nucleants for Al. In addition, the effects of Al3Zr particle size and distribution on grain refinement has also been investigated. It has been found that the active Al3Zr particles are bigger than previously reported other types of active particles, such as TiB2 for heterogeneous nucleation in Al alloys. Considering the low growth restriction effect of Zr in Al (the maximum Q-value of Zr in Al is 1.0K), it is suggested that the significant grain refinement of Al resulting from the addition of Zr can be mainly attributed to the heterogeneous nucleation facilitated by the in situ formed Al3Zr particles. [Copyright &y& Elsevier]

Laser powder bed fusion (L-PBF) additive manufacturing was utilized to fabricate high-strength copper (Cu) alloys through inoculation of pure Cu powder with cobalt (Co) submicron particles. It was found that when the addition level of Co is lower than its maximum solid solubility in Cu (4.75 wt.%), the microstructure of Cu-Co alloys is characterized by coarse columnar grains. Further addition of Co over 4.75 wt.% led to a heterogeneous grain structure, with large equiaxed grains or columnar grains near the centers of melt pools whereas ultrafine equiaxed grains sitting on the melt pool boundaries. Microstructure characterization showed that the in-situ formed dual phase nanoparticles with Co shell and cobalt oxide (CoO) core acted as heterogeneous nucleation sites of Cu, which allowed ultrafine, equiaxed grains to form. The heterogeneously structured as-built Cu-Co alloy achieved a tensile strength of 381.4 ± 2.9 MPa and an elongation of 31.6 ± 1.3%. Co nanoprecipitation driven by post L-PBF heat treatment further increased the tensile strength of the Cu-Co alloy up to 491.1 ± 12.6 MPa without notably sacrificing the ductility. The combination of high strength and high ductility distinguishes Cu-Co alloys from most conventionally and additively manufactured Cu alloys. This study shows a strategy to produce Cu alloys with high performance through remarkable grain refinement by heterogeneous nucleation. [Display omitted] [ABSTRACT FROM AUTHOR]

Selective laser melting (SLM) has been successful in fabricating advanced engineering parts with high geometrical complexities. However, some metals or alloys with high strength, low weldability, and large freezing range, such as 7075 aluminium alloy, have low SLM-processability and are hard to be directly SLM-fabricated. Here, we proposed a novel strategy, combining substrate modification and inoculation treatment, to fabricate a crack-free and dense high-strength 7075 alloy using SLM at a broad processing window. Inoculation of the 7075 alloy powder with 1 wt% Ti submicron particles substantially refine the Al grains, effectively increasing the cracking resistance. Furthermore, the SLM substrate was modified, which allows integration of thermal insulation materials, such as vermiculite, to the substrate. As a result, the melt pool cooling rate and thermal gradient during solidification were significantly reduced. This directly led to a reduction in thermal stress within the melt pool. With combination of the substrate modification and Ti inoculation, crack-free, fine-equiaxed microstructure was obtained in the SLM-fabricated 7075 alloy that has the mechanical properties comparable to its wrought counterpart. This strategy can be implemented to SLM of other engineering alloys with low AM processability, providing a foundation for broadening industrial applications of SLM. [ABSTRACT FROM AUTHOR]

The effects of inoculation treatment with LaB 6 nanoparticles (0–2 wt% additions) on the microstructural evolution and mechanical performance in a selective laser melted AlSi10Mg alloy were comprehensively investigated. The addition of 0.2–0.5 wt% LaB 6 nanoparticles was identified to be optimal to achieve substantial grain refinement, microstructural homogeneity and thus remediation in the mechanical property anisotropy in the AlSi10Mg alloy. The substantial grain refinement was attributed to the coherent Al/LaB 6 interfaces, which facilitated the heterogeneous nucleation of Al on the LaB 6 nanoparticles during solidification. Increasing the LaB 6 addition up to 2 wt% only marginally further refined the equiaxed grains, which can be understood in terms of the concept of nucleation free zone formed in the liquid at front of the growing solid-liquid interfaces. The LaB 6 nanoparticles within the nucleation free zone could not be activated to be nucleants for α-Al. As a result, random orientation relationships between LaB 6 nanoparticles within the nucleation free zone and the Al matrix were determined. Those excessive LaB 6 nanoparticles weakened the melt pool boundaries, and therefore deteriorated the longitudinal ductility of the SLMed AlSi10Mg alloy. [ABSTRACT FROM AUTHOR]