Heterogeneous Nucleation Sites Research Articles

Enhancing reversible hydrogen storage performance of 2LiBH4–MgH2 via in-situ building heterogeneous nucleation sites

Enhancing reversible hydrogen storage performance of 2LiBH4–MgH2 via in-situ building heterogeneous nucleation sites

Construction of Cs2AgBiCl6/COF Heterojunction for Boosted Photocatalytic Thioester Oxidation.

Lead-free halide perovskites as a new kind of potential candidate for photocatalytic organic synthesis have attracted much attention recently. The rational heterojunction construction is regarded as an efficient strategy to delicately regulate their catalytic performances. Herein, a semi-conductive covalent organic framework (COF) nanosheet, C4N, is employed as the functional component to construct Cs2AgBiCl6/C4N (CABC/C4N) heterojunction. It is found that the C4N nanosheets with rich surface functional groups can serve as heterogeneous nucleation sites to manipulate the growth of CABC nanocrystals and afford close contact between each other, therefore facilitate the transfer and spatial separation of photogenerated charge carriers, as verified by in situ X-ray photoelectronic spectroscopy and Kelvin probe force microscopy. Moreover, the oxygen affinity of C4N endows the heterojunctions with outstanding aerobic reactivity, thus improving the photocatalytic performance largely. The optimal CABC/C4N heterojunction delivers a thioanisole conversion efficiency of 100% after 6h, which is 2.2 and 7.7-fold of that of CABC and C4N. This work provides a new ideal for the design and application of lead-free perovskite heterojunction photocatalysts for organic reactions.

Revealing intermetallic strengthening in bainitic steel designed for nickel aluminide formation

BainNiAlCu steel was designed to achieve a cementite-free bainitic structure and susceptibility to the formation of intermetallic phases at elevated temperatures. Based on Transmission Electron Microscopy (TEM) observations, the intermetallic strengthening occurring in the bainitic ferrite matrix was revealed. The B2-ordered phase are rich in Ni, Al, and Cu, which indicates the growth of NiAl/Cu co-precipitates. The intermetallic phase was characterized by a spherical morphology and nanometer size. The micro-segregation of Ni, Al and Cu was observed at the boundaries of cementite particles and the bainitic matrix. The redistribution of atoms during the growth of carbides forms a heterogeneous nickel aluminide nucleation site. Intermetallic strengthening is a promising strategy for controlling the thermal stability of bainitic structures. The results of this research constitute fundamental knowledge regarding the design of ultra-fine, cementite-free bainitic steels subjected to intermetallic strengthening.

Rapid-thermal-process pre-treatment promoted precipitation towards strengthening hard magnetism of Sm2Co17-type magnets

Dislocations can act as heterogeneous nucleation sites but their annihilations upon heating sacrifice point defects nearby, weakening the driving force for precipitate nucleation. In order to keep a high density of dislocations and point defects together, here rapid-thermal-process (RTP) pre-treatment was carried out to promote the precipitate nucleation in Sm2Co17-type permanent magnets that evolve gradual formation and dissociation of dislocations during concurrent precipitation and recrystallization. As exhibited in a model magnet Sm25Co49.3Fe17.1Cu5.6Zr3.0 (wt.%) with high-density dislocations at solution-treated state, the RTP pre-treatment can induce excess point defects owing to the migration of solute atoms towards equilibrium state and the quenching of point defects, and suppress the early-stage diffusion-controlled dissociation of dislocations. After whole-process isothermal aging and slow cooling, the RTP pre-treated magnet possesses much higher fraction of hexagonal SmCo5 (1:5H) nanoprecipitates than the non-pre-treated one, giving rise to effective enhancements in coercivity Hcj from 26.20 to 30.34 kOe and in knee-point field Hk from 14.43 to 19.20 kOe. These findings add insight into controlling precipitation in dislocation-bearing supersaturated solid solutions, which are feasible for strengthening the Sm-Co-based magnets.

AB-type dual-phase high-entropy alloys as negative electrode of Ni-MH batteries: Impact of interphases on electrochemical performance

High-entropy alloys (HEAs) and their corresponding high-entropy hydrides are new potential candidates for negative electrode materials of nickel-metal hydride (Ni-MH) batteries. This study investigates the cyclic electrochemical hydrogen storage performance of two AB-type HEAs (A: hydride-forming elements, B: non-hydride-forming elements) in Ni-MH batteries. TiV2ZrCrMnFeNi with a dual-phase structure shows a fast activation and a low charge transfer impedance with a discharge capacity of 150 mAhg−1, while TiV1.5Zr1.5CrMnFeNi with a single phase shows a slow activation and a capacity of only 60 mAhg−1. The better electrochemical performance of TiV2ZrCrMnFeNi was attributed to its higher vanadium/zirconium ratio and abundant interphase boundaries, which act as hydrogen paths and heterogeneous hydride nucleation sites. These results suggest the high potential of dual-phase HEAs as new active electrode materials for Ni-MH batteries.

The role of in-situ Al3Ti formed during solidification in improving the high-temperature properties of Al–Cu alloy

In this paper, in-situ Al3Ti-reinforced Al–Cu alloys were fabricated by Ti alloying and exhibit outstanding high-temperature properties. The results show that when the volume fraction of Al3Ti is 7.8 %, the ultimate strengths of the alloy can reach 203.9 MPa and 144.1 MPa at 300 °C and 350 °C, which are 31.0 % and 51.4 % higher than the base alloy, respectively. The analysis indicates that in addition to precipitation strengthening, dislocation strengthening, and load-transfer strengthening, low-angle grain boundaries also play an important role in improving the high-temperature properties of the alloy. When the Ti concentration in the alloy melt is high, Al3Ti grows into the two-dimensional lamellar dendrite with a complex structure and serves as the heterogeneous nucleation site of multiple α-Al cores, which results in the abnormal grain growth behavior and increase of low-angle grain boundaries. Moreover, fracture mechanism analysis suggests that the existence of Al3Ti increases the number of initial cracks, which increases the risk of splitting the matrix. The deformation behavior of Al3Ti is mainly controlled by twins, stacking faults, and dislocations. Under tensile stress, cracks first form in Al3Ti and then evolve into non-equiaxed voids in the matrix, which eventually connect and lead to the fracture of the alloy.

Preparation of CaO-MgO-Al2O3-SiO2 glass-ceramic with a high content of Cr2O3 using the spark plasma sintering (SPS)

2, 6, or 10 wt% of Cr2O3 was added into the CaO-MgO-Al2O3-SiO2 precursor glasses (PG), aiming to investigate the effects of high contents of Cr2O3 on the crystallization behavior and mechanical properties of glass-ceramics (GCs). The PGs were milled into powder and then sintered by spark plasma sintering (SPS) technology at 750∼850 °C under the pressure of 10 MPa. It was found that Cr2O3 induced the precipitation of spinel nuclei in the PGs, which served as the heterogeneous nucleation sites for the augite crystals. Augite and spinel were the main phases of the GCs, and the crystallinity increased by increasing the Cr2O3 addition amount and sintering temperature. The GC prepared at 850 °C with 6 wt% Cr2O3 addition showed excellent bending strength, with Vickers hardness, and fracture toughness of 214 MPa, 8.2 GPa, and 1.3 MPa·m1/2, respectively.

Microstructure and mechanical properties of a hypereutectic Al–Si–Cu alloy manufactured by laser powder bed fusion

A novel Al–Si–Cu alloy was designed for additive manufacturing of automotive components, such as pistons and supercharger rotors, requiring a combination of elevated temperature properties and high cycle fatigue strength. The alloy was designed with an Al–Si hypereutectic chemistry to exploit the formation of primary Si nucleants to enable an equiaxed as-printed microstructure during laser powder bed fusion (LPBF). The alloy design space was assessed using a combined integrated computational materials engineering (ICME) approach with melt-spinning experiments to select a chemistry exhibiting primary Si nucleants acting as heterogeneous grain nucleation sites. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) of the LPBF as-printed samples revealed primary Si particles with a fully equiaxed grain structure and no apparent texturing as evaluated using electron backscatter diffraction (EBSD). High-resolution scanning transmission electron microscopy (STEM) revealed sub-micron Si particles and Al2Cu θ-phase precipitates which result in enhanced room and elevated temperature mechanical strength (room temperature: UTS: 449 ± 10 MPa, YS: 336 ± 12 MPa) as compared to commercial Al10SiMg (UTS: 417 ± 3 MPa, YS: 236 ± 6 MPa).

Excellent oxidation resistance of rapidly crystallizing Cr2AlC coatings at high temperatures: Effect of microstructure

Two kinds of Cr2AlC coatings with different microstructures were prepared by direct crystallization during oxidation at 1000 ℃ and conventional crystallization at 700 ℃ for 1 h for amorphous Cr-Al-C coatings prepared by magnetron sputtering, respectively. The effect of microstructure on the oxidation behavior of the coatings at 1000 ℃ was investigated. The grain boundary density and dislocation density of the Cr2AlC coatings crystallized directly during oxidation were much higher, which provided much more sites for heterogeneous nucleation of α-Al2O3 and fast diffusion paths for Al. Therefore, the coating formed complete pure α-Al2O3 scales earlier with better antioxidant properties. More importantly, the present work demonstrates feasibility and advantages of directly applying amorphous Cr-Al-C coatings at high temperatures without additional crystallization treatments, which simplifies preparation process of Cr2AlC coatings, facilitates their industrial applications, and provides a new method for rapid preparation of MAX-phase coatings.

Dynamic Recrystallization Nucleation Mechanism and Precipitation Behavior of Homogeneous Al-Zn-Mg-Cu Alloy during Hot Deformation

The hot deformation behavior of Al-Zn-Mg-Cu alloy was investigated by flow stress curves in isothermal hot compression experiments with deformation temperatures of 350–450 °C and strain rates of 0.01 s−1 to 1 s−1, and the constitutive equation of homogeneous alloy was obtained. At the same time, the dynamic recrystallization and precipitation behavior during hot deformation and the relationship between them and the Z parameters were studied by using EBSD and TEM. DRV is the main mechanism of dynamic softening. With the decrease in Z parameter, the softening mechanism changes from dynamic recovery to discontinuous dynamic recrystallization or continuous dynamic recrystallization. At a higher Z parameter, the dislocation density and precipitated phase density are also higher because the high dislocation density provides heterogeneous nucleation sites of the precipitated phase. A large number of precipitates in the alloy also inhibit the nucleation and growth of dynamic recrystallization by hindering dislocation movement and grain boundary migration.

Simultaneously improved the strength and ductility of cast Al-3Li-2Cu alloy by Ti addition and heat treatment

This work systematically investigated the impact of minor Ti addition and heat treatment on grain structure, precipitates, precipitate-free zone (PFZ), and the performance of the Al-3Li-2Cu alloy. Minor Ti addition significantly refines the as-cast alloy, leading to a transition from columnar grains to equiaxed grains. The primary Al3Ti phases, with a low mismatch (0.99%) to the matrix, act as heterogeneous nucleation sites for α-Al, while the presence of Ti solutes restricts grain growth. As a result, the average grain size can be refined to 35.2 μm by adding 0.2 wt% Ti. Besides, minor Ti addition affects the corresponding precipitation behavior of the alloy. Detailed TEM observations reveal that the addition of Ti inhibits the growth of δ’(Al3Li) precipitates and the accompanying δ’-PFZ, which should be related to the interaction between Ti solutes and vacancies. Besides, reducing the aging temperature exerts a similar effect. Finally, the 0.2 wt% Ti-modified alloy exhibits excellent mechanical properties after aging at 175 °C for 8 h. It demonstrates a high yield strength (272 MPa), a high ultimate tensile strength (399 MPa), and an acceptable elongation (5.9%), which represent an increase of 8.8%, 34.8%, and 293.3%, respectively, compared to the alloy without Ti addition. The simultaneous improvement in strength and ductility is attributed to the presence of fine and equiaxed grains, high-density and homogenous δ’ precipitates, and a narrow δ’-PFZ. In-depth discussions are provided to elucidate the underlying mechanisms governing the microstructure and mechanical properties.

Solidification behavior of SiCf/Ti17 induced by fixed-point laser surface remelting

Taking advantage of laser additive manufacturing, continuous SiC fiber reinforced titanium matrix composites (SiCf/TMCs) with low density and excellent mechanical properties can be used to hybrid-fabricate large components with complex shape for aerospace applications. However, due to the anisotropy in both microstructure and heat transfer ability, the high-density and rapid heat input from laser during hybrid manufacturing process will cause the non-uniform damage of fibers and various solidification microstructure in the molten pool. In order to understand the solidification behavior of molten pool in SiCf/Ti17 composites, the fixed-point laser surface remelting was used to bring in rapid and symmetrical heat input. With the decrease of temperature from center to edge of the molten pool, both fiber damage degree and the contents of Si and C elements in melt decreased. The solidification microstructure contains TiCx, Ti5Si3 and β-Ti, and their morphology varies according to the melt composition. SiC fibers act as heterogeneous nucleation sites for first solidified phases and promote their preference growth along the radial direction of fibers resulting from the excellent heat transfer ability. The faceted Ti5Si3 grains solidified through peritectic reaction L+TiCx→Ti5Si3 exhibit strong <0001> texture.

The Performance Investigation of PLLA/PPAPH: The Influence of PPAPH as Heterogeneous Crystal Nuclei

To overcome PLLA�s poor crystallization capability, using nucleating agent as crystallization improvement strategy was performed in this study. PPAPH as PLLA�s an organic nucleating agent was firstly synthesized, and then PLLA was blended with different PPAPH loading through melting blend method, the resulting influences of PPAPH on PLLA�s performances were investigated using the relevant testing instruments. Melt-crystallization revealed that PPAPH played important role in promoting PLLA�s crystallization through providing effective sites of heterogeneous nucleation, and effect of PPAPH loading on PLLA�s melt-crystallization was very poor, indicating that low PPAPH loading could cause PLLA to possess powerful crystallization capacity. In addition, the relative low final melting temperature was beneficial for PLLA/PPAPH�s crystallization. However, an increase of cooling rate during cooling stage weakened PLLA/PPAPH�s crystallization capacity. PLLA/PPAPH�s cold-crystallization suggested that PPAPH had an inhibition effect on cold-crystallization process to some extent. Melting behaviors depended on heating rate and previous crystallization including melt-crystallization at various cooling rates and isothermal crystallization at various crystallization temperatures. PPAPH enhanced PLLA�s fluidity, tensile modulus and tensile strength. Unfortunately, PLLA�s transmittance was seriously weakened as PPAPH loading increased, as well as the elongation at break continuously decreased.

Enhanced strength-ductility synergy in an Al–Cu alloy via Cd-induced hybrid θ″+θ′ precipitation

In this work, trace Cd addition (∼0.2 wt%) was exploited in Al–Cu alloys to improve the mechanical properties by introducing the hybrid θ″+θ′ precipitates, as governed by the solid-solution temperature T. In detail, if the solid solution temperature is low (T = 500 °C), Cd microalloying merely initiates undesirable dual precipitates of θ″ and θ′ with similar size and a homogenous distribution. In such case, the Cd-microalloyed Al–Cu alloy suffers from limited optimization on the strength-ductility synergy in comparison to the Cd-free Al–Cu alloy. However, elevating T to 530 °C results in the formation of dense Cd-rich nanoparticles, significantly promoting the hybrid θ″+θ′ precipitation with a bimodal distribution by offering numerous heterogeneous Cd-rich nucleation sites. Such hybrid θ″+θ′ precipitation imparts the architecture of microstructural heterogeneity, which was found to be responsible for the rapid age-hardening response and enhanced strength-ductility synergy in the Cd-microalloyed Al–Cu alloy.

An experimental study of the effect of Ag, Cu, Li, Mg, Ni, Ti, Sc, Zr on grain refining in binary and multicomponent Al-based alloys

The objective of the current work is to establish the conventional mechanisms of grain refining, and the effect of the refiner-modifier interaction in Al-based alloys on the resultant grain refining obtained. The different castings were carried out in preheated metallic molds with solidification rates of around 0.15 °C/s and 7 °C/s. As the nucleation sites (Al3Ti) change in composition depending on the percentage of Si in the alloy, the term “poisoning” frequently used to explain the loss or weakening of the power of the refiner, is misused. Addition of L12 metals e.g., Zr and Sc would contribute to constitutional undercooling, restricting the coarsening of α-Al grains as well as act as suitable sites for heterogeneous nucleation. The use of transition or ultra pure (99.99) metals would increase the degree of undercooling as well as nucleation of new grains due to precipitation of the second phase leading to the formation of a heterogeneous grain distribution. Thus, grain refining of binary alloys - partially or fully, may not be produced only through constitutional undercooling. It is suggested that, the use of 0.18%Sc has more-or-less the same grain refining efficiency of 0.15%Ti added in the form of Al–Ti–B master alloy i.e., reduction in the grain size by about 85% and an increase in the alloy strength by about 30%. For a given solidification rate, there is a grain refining threshold beyond which no further refining can be achieved. Apparently grain refining mechanisms are independent of solidification rate, only the extent of refining.

Enhancing strength-ductility synergy in an extruded Al-Cu-Li-Mg-Ag alloy via homogeneous GP zones and dislocation configuration

Balancing strength and ductility proves challenging in high-strength wrought Al-Cu-Li alloys, particularly in extruded rods. The abundance of subgrain structures hinders dislocation movement, inevitably sacrificing the ductility. Herein, an effective strategy for enhancing the strength-ductility synergy in an extruded Al-3.7Cu-1Li-0.40Mg-0.75Ag alloy was reported by developing a novel combinatorial pretreatment (CPT), including pre-ageing after solution treatment, followed by pre-stretching. Subsequently, the CPT alloys undergo artificial ageing at 150 °C. Desirable comprehensive mechanical properties (YS = 668 MPa, UTS = 700 MPa, and EL = 12.5 %) were obtained through pre-ageing at 110 °C before 4 % pre-stretching. Notably, the elongation showed a remarkable improvement of 60.3 % compared to the non-CPT sample, without sacrificing the yield strength. Microstructure analysis indicates that the interactions between the GP zones and dislocations facilitated energy dissipation during pre-stretching, effectively mitigating local stress concentration. Consequently, promoted ductility was realized through a more uniform dislocation distribution. Furthermore, the dense GP zones formed during pre-ageing served as the heterogeneous nucleation sites for precipitates, increasing the volume fraction of θ′ phases. This adjustment in dislocation motion further contributed to ductility improvements. Our approach offers new insights into developing high-performance extruded Al-Cu-Li alloys with both high strength and desirable ductility.

Tailoring precipitate distribution in 2024 aluminum alloy for improving strength and corrosion resistance

For the traditional peak-aged (PA) AA2024 alloy, the formation of large S-phase precipitates within the grains, wide precipitate-free zones (PFZs) near the grain boundaries (GBs), and continuous distribution of grain boundary precipitates (GBPs) can be observed. As a result, the PA alloy exhibits relatively high strength but poor corrosion resistance. However, with the application of cyclic plasticity treatment, high-density 1–2 nm clusters form within the matrix, and no PFZs form near GBs. In this study, this treatment yields the optimal balance between strength–elongation characteristics and corrosion resistance. By combining cyclic plasticity and ageing heat treatment with different heating rates, the nanoscale clusters play a crucial role as heterogeneous nucleation sites, resulting in the formation of finer and higher number density of S precipitates within the matrix. Additionally, the presence of these clusters reduces the formation of GBPs and minimizes the width of PFZs. Consequently, compared to the traditional PA sample, this approach achieves a significantly higher yield strength (increased by 46 %) and ultimate tensile strength (increased by 18 %), along with superior corrosion resistance. Although the influence of ageing heat treatment with different rates on mechanical properties is not significant, it notably affects the formation of GBPs and corrosion resistance. Specifically, a slower heating rate leads to an increase in the spacing between adjacent GBPs, resulting in improved corrosion resistance. In summary, cyclic strengthening, as a novel method for alloy strengthening, when combined with ageing heat treatment, modulates the distribution of S precipitates within the matrix and GBs. This optimization maximizes the effects of precipitation strengthening and breaks the inverse relationship between strength and corrosion resistance.

Improvement of mechanical properties of Al-Si-Mg composites by in-situ formation of small-sized (Al, Si)3(Ti, Zr) particles

In this study, Al-Si-Mg composites were prepared by adding different combinations of titanium (Ti), zirconium (Zr), boron (B), and antimony (Sb)，and its micro-structure and mechanical properties were investigated. The results revealed that the average diameter of α-Al grains in the novel alloy, prepared through the combined addition of Ti, Zr, B, and Sb, was 110 µm, with a secondary dendritic arm spacing (SDAS) of 22.4 µm. This grain refinement is attributed to the in-situ formation of heterogeneous nucleation sites L12-(Al, Si)3(Ti, Zr). The alloy heat-treated at T6 exhibited notable mechanical characteristics, including a tensile strength of 354.5 MPa, a yield strength of 317.9 MPa, and an elongation at break of 5.6%. Significantly, the enhancement in mechanical properties is found to be independent of the modifications of eutectic silicon. This improved strength and elongation are attributed to both grain refinement and the in-situ formation of I4-(Al, Si)3(Ti, Zr). Overall, these findings provide an effective and direct method for obtaining high-performance Al-Si-Mg composites.

Effect of Cu and Sn additions on the thermal stability of Al–Mg–Si alloy

Age-hardening aluminum alloys generally exhibit poor thermal stability, making them unsuitable for prolonged use at temperatures around the aging range. This study investigated the effect of 0.08 wt% Cu and 0.05 wt% Sn on the thermal stability of peak-aging Al–0.9Mg–1.1Si (wt.%) alloy. After an exposure of 150 °C for 1000 h, the combined effects of Cu and Sn result in a higher yield strength (∼322 MPa) than that of the Cu/Sn-free alloy (∼311 MPa). This improvement is mainly attributed to the Sn-induced fine precipitate morphology. Specifically, precipitates with higher aspect ratios exhibit rapid coarsening rates, significantly promoting the response of experimental alloys to thermal exposure. In contrast, precipitates with lower aspect ratios coarsen at a slower rate and maintain a higher level of strengthening during thermal exposure. Furthermore, Sn atoms may also contribute to the formation of the thermostable Cu-containing phases (L phase). On the one hand, the Sn-induced higher precipitate density will increase the amount of L phase. On the other hand, the special Sn-containing sub-units may serve as heterogeneous nucleation sites to promote the formation of L phase. In summary, the combined effects of Cu and Sn enhance the thermal stability of the experimental alloys near the aging temperature range. The present study provides a straightforward and feasible approach to enhancing the thermal stability of Al–Mg–Si alloys.

PEGDA hydrogel microspheres with encapsulated salt for versatile control of protein crystallization

Due to their biocompatibility and adjustable chemical structure and morphology, hydrogels have great potential in many applications, and can be used to enhance protein crystal quality and crystallization efficiency, contributing to biomedicine manufacturing. Monodispersed PEGDA hydrogel microspheres (HMSs) were synthesized using a Lego-inspired microfluidic device. The generated droplets were then UV polymerized, partially hydrolyzed with 0.1 M NaOH solution to improve their absorption capacity, and soaked in a buffer solution containing 0, 0.5, 1, 2, and 4 M NaCl. Salt-loaded HMSs were used as the medium for the enhanced crystallization of hen egg white lysozyme from aqueous solutions. Different supersaturations were achieved in the protein solutions by releasing NaCl of different concentrations from HMSs, as confirmed by electrical conductivity measurements. HMSs with or without NaCl can both provide heterogeneous nucleation sites due to their nano-porous structure and wrinkled surface. The addition of NaCl-loaded HMSs to the protein solution can also increase or decrease the supersaturation in the whole solution or locally near the HMS, leading to controllable nucleation time and crystal size distribution dependent on the NaCl concentration loaded into HMSs.