TiB2 Surface Research Articles

In-melt electron analysis to accelerate process exploration of ceramics: Electron beam melting of TiB2

To enhance the versatility of electron beam powder bed fusion (EB-PBF), a widely utilized additive manufacturing (AM) technique for metallic materials, we propose a novel paradigm aimed at facilitating the exploration of the process parameters for less-studied materials, such as ceramics. The high melting points and poorly understood thermal properties of ceramics have constrained the comprehension of their melting behavior. In this study, titanium diboride (TiB2) sintered bodies were subjected to spot melting under four distinct electron beam currents and four different exposure times. By introducing a novel in-melt electron analysis (IMEA) approach, the various stages of melting were clearly identified. The analysis and interpretation of IMEA signals were found to be consistent with experimental observations on the spot-melted surface of TiB2. IMEA demonstrates significant potential for real-time process window optimization and quality assurance for challenging and novel materials.

Deuterium plasma induced preferential erosion in ultra-high temperature ceramics TiB2 and ZrB2*

Abstract Steady-state deuterium plasma exposures were performed on ultra-high temperature ceramics titanium diboride (TiB2) and zirconium diboride (ZrB2) using the PISCES-RF linear plasma device (LPD) as early screening for first wall, plasma-facing applications. Deuterium plasma exposures were performed using 40 eV ion energies at 240, 525, and 800 °C sample temperatures and 90 eV ion energies at 240 °C sample temperatures to analyze TiB2 and ZrB2 surface morphology and chemistry evolution behavior. Post-plasma exposure chemistry characterization of the near surface ( < 50 nm) region of the samples all show transition metal enrichment, indicating boron preferential erosion. Transition metal to boron fractions vary with plasma exposure temperature under the 40 eV ion energy; metal enrichment is maximized at 800 °C and then minimized at 525 °C. SEM micrographs of all plasma exposed sample surfaces show no significant or noticeable plasma induced damage from cracking or blistering.

On-the-Fly Machine Learning Force Field Study of Liquid-Al/Solid-TiB2 Interfaces.

Using the on-the-fly machine learning force field, simulations were performed to study the atomic structure evolution of the liquid-Al/solid-TiB2 interface with two different terminations, aiming to deepen the understanding of the mechanism of TiB2 as nucleating particles in an aluminum alloy. We conducted simulations using MLFF for up to 100 ps, enabling us to observe the interfacial properties from a deeper and more comprehensive perspective. The nucleation potential of TiB2 particles is determined by the formation of various ordered structures at the interface, which is significantly influenced by the termination of the TiB2 (0001) surface. The evolution of the interface during heterogeneous nucleation processes with different terminations is described using structural information and dynamic characteristics. The Ti-terminated surface is more prone to forming quasi-solid regions compared to the B-termination. Analysis of mean square displacement and vibrational density of states indicates that the liquid layer at the Ti-terminated interface is closer in characteristics to a solid compared to the B-terminated interface. We also found that on the TiB2 (0001) surface different terminations give rise to distinct ordered structures at the interfaces, which is ascribed to their different diffusion abilities.

Research on the electronic properties of TiC/γ-Fe and TiB2/γ-Fe interfaces based on first-principles calculations

Addressing the unclear understanding of the bonding mechanisms at the TiC/γ-Fe and TiB2/γ-Fe interfaces, the atomic structure, adhesive work, and electronic properties of the TiC (111)/Fe (111) and TiB2 (001)/Fe (111) interfaces were investigated using first-principles calculations. The results reveal that the C-terminated surface of TiC (111) and the B-terminated surface of TiB2 (001) exhibit high surface activity, facilitating their binding with the Fe (111) surface. The adhesive energy at the TiC (111)/Fe (111) interface is 13.04 J/m2, with a wetting angle of 41.32°, while at the TiB2 (001)/Fe (111) interface, the adhesive energy and wetting angle are 6.08 J/m2 and 74.69°, respectively. TiC demonstrates better wettability and stronger binding strength with γ-Fe surface. At the interface, the C-p orbitals and B-p orbitals hybridize with Fe-d orbitals to form bonds. The population of Fe-C bonds and Fe-B bonds is 0.46 and 0.02, respectively, indicating that Fe-C bonds exhibit covalent characteristics, while Fe-B bonds display ionic characteristics, with Fe-C bonds being stronger than Fe-B bonds. Statistical analysis of charge distribution and charge difference reveals electron accumulation near C and B atoms at the interface. Moreover, the electron transfer at the TiC (111)/Fe (111) interface is significantly greater than that at the TiB2 (001)/Fe (111) interface, which is the fundamental reason for the higher binding strength at the TiC (111)/Fe (111) interface.

Research on the electronic properties of TiB2/γ-Fe(1 1 1) and TiB2/Ni(1 1 1) interfaces

In order to explore the interfacial bonding mechanisms of TiB2/γ-Fe and TiB2/Ni in the composites, the adhesion work, electronic properties and fracture toughness of the TiB2(001)/ γ-Fe(111) and TiB2(001)/Ni(111) interfaces were investigated using first-principles calculations. The results reveal that the surface energy of the TiB2 surface at the B-terminated is the smallest and the constructed TiB2(001)/ γ-Fe(111) interface has the largest adhesive energy. The electronic structures of the TiB2(001)/γ-Fe(111) and TiB2(001)/Ni(111) interfaces reveal that bonding at the interfaces is provided by the B-2p orbitals with the Fe-3d and Ni-3d orbitals, respectively, and that the formation of Fe-B and Ni-B covalent/ionic bonds is the main source of bonding and interaction. The bonding and strength of Fe-B in the TiB2(001)/γ-Fe(111) interface is stronger than that in the TiB2(001)/Ni(111) interface, which is due to the higher charge density accumulation of Fe atoms at the TiB2(001)/γ-Fe(111) interface. Using Griffith’s theory, the TiB2(001)/γ-Fe(111) interface is inferred to have the strongest fracture toughness. This study suggests that the chemical bonding stronger Fe-B bonds result in a high bond strength and a more stable interfacial structure at the TiB2(001)/ γ-Fe(111) interface, leading to better resistance to cracking in practice.

The Effect of Zr and Li on the Microstructure of AlMg5Si2Mn-Type Casting Alloys

AbstractIn the article, the authors present results of microstructural studies of Al-Mg-Si-Mn casting alloys with Zr, Li, and TiB2 additions on a broad scale. Zirconium content was set on two levels: 0.34 and 1.58 wt%, and Li was set 1.2 and 2.0 wt%. It was found that the addition of Zr shifts the eutectic melting temperature to a higher level, up to 611.3 °C at 1.6 wt% Zr. At the same time, Li addition leads to the depression of eutectic melting temperature: down to 587.2 °C at 2.0 wt% Li, what is a common effect of eutectic modification which was confirmed by means of structural examinations. The complex addition of Li and AlTi5B1 resulted in a eutectic melting temperature close to the equilibrium eutectic temperature for the Al-Mg-Si system (596.2 °C). The grain refinement effect of Zr is due to the nucleation of α-Al on the Zr(Al1−x, Six)3 phase. Crystals of this phase were detected in the grain centers of Zr-containing alloys. The Li addition does not affect α-Al grain size but changes the morphology of eutectic colonies from petal-like to fibrous. Observation of TiB2 particles inside the primary Mg2Si crystals gives direct experimental confirmation of nucleation of the primary phase on the surface of TiB2 in the alloy after adding Li and AlTi5B1. Natural aging of the alloys resulted in the formation of fine precipitates detected close to dislocations. The most apparent supposition is that the mechanism responsible for their formation is heterogeneous nucleation in the stress field of dislocations. Hardness tests showed adding 2.0 wt% of Li is very effective, increasing hardness up to 113 HV0.2 in naturally aged condition, which is nearly double that of commercial Al-Mg-Si die-casting alloy. Several effects were proposed which may synergistically contribute to the rise of hardness in Li-containing alloys, such as solid solution strengthening, formation of primary LiAlSi phase and natural aging.

Simultaneous integration of anti Si-poisoning and anti-fading properties on an innovative Al grain refiner via Nb-modified TiB2

Si-poisoning and fading effect of grain refiners deteriorate the refining effect of Al-Si alloys (Si≥3 wt%), further worsening the performance of alloys. In this work, an innovative Al-Nb-Ti-B with anti Si-poisoning and anti-fading properties was prepared by introducing synthetic TiB2 into Al-Nb melt. At the cooling rate of 1.4 K/s, the average grain size of Al-7Si alloys inoculated by this Al-Nb-Ti-B was refined to 326.05 μm, 40% reduction compared to 549.06 μm by commercial Al-5Ti-1B. The grain size remained at 421.95 μm even after 120 min holding time. Anti-fading mechanism was investigated by size distribution analysis of TiB2 nucleating particles. Anti Si-poisoning mechanism is attributed to Nb element which accumulates on the (01—10) surface of TiB2 and reduces the lattice misfit between TiB2 and α-Al. Atomic-scale microstructure analysis and first-principles calculations reveal that the Nb enrichment on TiB2 increases the chemical affinity of TiB2/Al interface and decreases the Si adsorption propensity. In brief, appropriate size distribution and unique Nb-rich layer contribute to the excellent grain refinement of TiB2 particles modified by Nb. These findings may shed new light on the realm of innovative aluminum grain refiners.

First-principles study on interfacial property in MgB2-based reactive hydride composites

The underlying physico-chemical interactions between transition metal-based boride particles that formed during the dehydrogenation process and MgB2 in 2LiBH4+MgH2 reactive hydride composite at the atomic scale are still unknown. In this work, the properties of the TiB2/MgB2 interface were investigated by first-principles calculations utilizing density functional theory (DFT). Taking the two terminations of both MgB2 and TiB2 as well as four different stacking sequences into account, energies of the TiB2 and MgB2 (0001) surfaces as well as the work of adhesion and the electronic structure of the interfaces were studied. The results show that the interface between the B-terminated MgB2 (0001) surface and the Ti-terminated TiB2 (0001) surface is the energetically most favorable among all four stacking options and possesses the largest work of adhesion. Our results further show that the TiB2 particles possess good nucleation potency for MgB2 particles from the thermodynamic perspective.

Exploring the intricacies of machine learning-based optimization of electric discharge machining on squeeze cast TiB2/AA6061 composites: Insights from morphological, and microstructural aspects in the surface structure analysis of recast layer formation and worn-out analysis

Aluminium (Al) Alloy-6061/TiB2 was developed with Squeeze casting while varying composite quantities with titanium diboride (TiB2). The metallographic structure of the composite was investigated using EDS and scanning electron microscopy (SEM). The machining of developed composite is challenging with the conventional methods due to higher tool wear and surface roughness (Ra). This study analyses the output responses, and machinability of TiB2 of 15% volume reinforced AA6061 composites using Electrical Discharge Machining (EDM). The EDM operation variables such as current, pulse on time, and voltage gap were utilized to examine material removal rate (MRR), tool wear rate (TWR) and Ra employing Box–Behnken design of experiments. The MRR, TWR and Ra data acquired from various experiments were optimized considering single and multiple objectives by a hybrid approach. Machine learning (ML) was applied to predict responses using linear regression (LR), decision tree (DT) and random forest (RF). The samples' recast layer was examined using SEM, and the thickness of the recast layer was also documented. The Al6061 alloy's SEM micrographs reveal deep equiaxed dimples on its fracture surface, indicating a high level of plastic deformation before failure. TiB2 is evenly distributed as darker particles in the Al-matrix alloy with no directional-orientation. The Al6061-15 wt% TiB2 composites' fracture surfaces display deep dimples, ductile-fracture for Al6061 alloy due to higher plastic deformation, and cleavage facets in some areas. The aluminum alloy's wear pattern depicts long, extensive ploughing grooves with short cracks parallel to the sliding direction at the grooves' bottom. SEM micrographs in recast-layer formation have reported that molten metal ligaments break into droplets during flushing, exposing fresh surfaces. The aluminum alloy's groove width is larger compared to the composites. Thus, it can be observed that the wear tracks on the base Al6061 alloy are larger and deeper in comparison to the Al- 15 wt% TiB2 surface composites due to the presence of hard TiB2 particles. Also, the recast layer may be advantageous due to its wear-resistant properties. TiB2 has importance in an industry where components slide with each other it is used in disc brakes in automobiles. Compared to industry standards, single-objective optimization considerably enhanced the outcomes: MRR increased by 10.61%, TWR enriched by 25.91%, and Ra increased by 17.60%. According to an ANOVA, peak current contributed 87.88%, 98.18%, and 95.80% respectively to MRR, TWR, and Ra. It is requisite to investigate multi-objective optimization for EDM of Al6061-15 wt% TiB2 composites' owing to the diverse optimal factor combinations for responses. Ra was slightly increased by 1.14% due to simultaneous optimization utilizing hybrid techniques with Equal and Entropy weights, which diminished TWR by 11.74% and increased MRR by 11.66%. ML, particularly DT, improves machining performance by assisting productivity, durability, and cost-efficiency under changeable circumstances and raising forthcoming study possibilities.

Nitrogen Photoelectrochemical Reduction on TiB2 Surface Plasmon Coupling Allows Us to Reach Enhanced Efficiency of Ammonia Production.

Ammonia is one of the most widely produced chemicals worldwide, which is consumed in the fertilizer industry and is also considered an interesting alternative in energy storage. However, common ammonia production is energy-demanding and leads to high CO2 emissions. Thus, the development of alternative ammonia production methods based on available raw materials (air, for example) and renewable energy sources is highly demanding. In this work, we demonstrated the utilization of TiB2 nanostructures sandwiched between coupled plasmonic nanostructures (gold nanoparticles and gold grating) for photoelectrochemical (PEC) nitrogen reduction and selective ammonia production. The utilization of the coupled plasmon structure allows us to reach efficient sunlight capture with a subdiffraction concentration of light energy in the space, where the catalytically active TiB2 flakes were placed. As a result, PEC experiments performed at -0.2 V (vs. RHE) and simulated sunlight illumination give the 535.2 and 491.3 μg h-1 mgcat-1 ammonia yields, respectively, with the utilization of pure nitrogen and air as a nitrogen source. In addition, a number of control experiments confirm the key role of plasmon coupling in increasing the ammonia yield, the selectivity of ammonia production, and the durability of the proposed system. Finally, we have performed a series of numerical and quantum mechanical calculations to evaluate the plasmonic contribution to the activation of nitrogen on the TiB2 surface, indicating an increase in the catalytic activity under the plasmon-generated electric field.

Interfacial strain evolution and abnormal Poisson's ratios in Al nuclei for the Al/TiB2 heterostructure

The interfacial lattice mismatch and local elastic strains of Al/TiB2 heterostructures are quantitatively investigated by aberration-corrected transmission electron microscopy. Both in-plane atomic expansion and out-of-plane compression of Al layers stacked on (0001)TiB2 are uncovered and favor the further extension of deformed Al layers for realizing a successfully heterogeneous nucleation. These lattice strains can persist up to several Al atomic layers from the topmost (0001) surface of TiB2. Based on the compressive and expansive elastic strain relation, we estimate the Poisson's ratio in Al nuclei and find that large mismatch strains lead to abnormal changes in the Poisson's ratio. This study reveals the effect of lattice mismatch on the heterogeneous nucleation of Al on TiB2 and may have important implications in understanding the heterogeneous nucleation of Al.

Atomic insights into heterogeneous nucleation and growth kinetics of Al on TiB2 particles in undercooled Al-5Ti-1B melt

Titanium diboride (TiB2) is an effective grain refiner of Al alloys in the industry that facilitates casting processes by forming uniformly refined microstructures. Although our understanding of the underlying refinement mechanisms has advanced, the atomic kinetics of heterogeneous nucleation of Al on TiB2 remains unknown. Here, we report atomic-scale observations of the heterogeneous nucleation and growth kinetics of Al on self-formed TiB2 particles by in situ heating of undercooled Al-5Ti-1B films. We demonstrate that an ordered Al monolayer forms on the Ti-terminated {0001}TiB2 surface; then, the surrounding Al atoms are initiated to form an island-shaped Al nucleus with face-centered cubic {111} stacking without the assistance of a Ti-rich buffer layer. The interfacial lattice mismatch between {111}Al and {0001}TiB2 causes remarkable out-of-plane strain that decreases gradually with Al nucleus layers increasing to 6 atomic layers. The elastic strain energy originating from this interfacial strain increases the free energy of the Al/TiB2 heterostructure, hence impeding the rapid growth of the Al nucleus. We found that TiB2 particles stabilize the Al nuclei rather than activating their free growth into grains when the experimental undercooling ΔT is lower than the onset undercooling ΔTfgin Greer's free growth model. Our findings provide an atomic-scale physical image of the heterogeneous nucleation and growth mechanisms of Al with inoculator participation and elucidate the strain-dependent growth kinetics of Al nuclei.

Interfacial precipitation of the in-situ TiB2-reinforced Al–Zn–Mg–Cu–Zr composite

The interfacial precipitation of reinforced particles has a critical influence on particle-reinforced aluminum-matrix composites. In this study, TiB2 interfacial precipitation of 6 wt% TiB2/Al–10Zn–2.3 Mg–2Cu–0.12Zr composite was studied. The results demonstrate that the TiB2 surface has a considerable impact on precipitation during different states of the composite preparation process (including heat treatments). The TiB2 surface acted as a nucleation site for D023–Al3Zr and was wrapped with the T phase during solidification; a good orientation relationship existed between TiB2 and D023–Al3Zr: (0 00 1)TiB2//(1‾ 03‾)D023–Al3Zr, (1 01‾ 0)TiB2//(1 05‾)D023–Al3Zr, and [1 21‾ 0]TiB2//[0 1 0]D023–Al3Zr. After homogenization or solution, the TiB2 wall hindering solute atoms diffused into the matrix, particularly for Mg and Cu, causing the T phase to dissolve incompletely, and the residual T phase on the TiB2 surface transformed to the S phase layer at a longer holding time. Additionally, the hindering effect of the TiB2 walls results in the enhancement of the local high solute atom concentration and provides a high precipitation driving force. Therefore, the interfacial phase, Mg(Cu2-XZnX), preferentially precipitated at the D023–Al3Zr and S phase interface during peak aging. In this process, the periodic misfit dislocations of the interface facilitate the diffusion of solute atoms and reduce the nucleation energy barrier, promoting interfacial precipitation. Eventually, the interfacial precipitates layer relieved the local stress concentration but consumed the solute atoms, which was considered to be beneficial to plasticity at the cost of strength weakening to achieve excellent comprehensive mechanical properties.

The effects of Ti on the precipitation evolution and age-hardening behavior of Al–7Si–Mg alloy

The effect of Ti addition on the precipitation evolution for Al–7Si–Mg alloy was systematically investigated by combing high-resolution transmission electron microscopy (HRTEM) and first-principles calculations. At the early aging stage, the rod-shaped Al3Ti and ellipsoid Ti5Si3 phases precipitated firstly, and the stronger Si capture ability of them suppressed the precipitation of the Mg–Si phases (β'' and β′), leading to the deceleration of the aging hardening kinetics. With prolonging aging time, one Al atom in Al3Ti was easily replaced by Si atom to form a more stable Al2SiTi phase. It was demonstrated by Energy Dispersive Spectrometry (EDS) and the calculation of formation enthalpy. The accurate atomic structure of the Al2SiTi phase was also obtained, which provided a resonable speculation on the structure of Al3Ti two-dimensional compound (2DC) containing Si on the surface of TiB2. This shed light on a deep understanding of Si poisoning on grain refinement of Al–Si/Al–5Ti–B system. At the over-aged stage, the Mg–Si phases began to precipitate, preventing the decrease of the hardness. Finally, a new precipitation sequence in the Ti-rich Al–7Si–Mg alloy can be obtained: SSSS → Al3Ti + Ti5Si3 → Al2SiTi + Ti5Si3 → Al2SiTi + Ti5Si3 + β" → Al2SiTi + Ti5Si3 + β"+ β'.

Theoretical insights into the behaviors of sodium and aluminum on the cathode titanium diboride surfaces

Density functional theory calculations were adopted to systematically investigate the adsorption and diffusion behaviors of sodium and aluminum over TiB2 surfaces or in TiB2 crystal to characterize the interaction mechanism between sodium and TiB2 cathode in aluminum reduction cells. Results suggest that Na and Al will stably adsorbe on the low-index TiB2 (0 0 0 1) surface, and the presence of vacant defects can significantly strengthen this adsorption. The migration of Na and Al over pristine TiB2 is anisotropic, with the largest energy barrier of 0.024/0.32 eV for Na and 0.28/1.57 eV for Al over Ti/B-terminated surfaces. The Ti vacancy in Ti-terminated surface is more effective to hinder Na and Al migration with the large diffusion barriers of 0.36 eV for Na and 2.07 eV for Al. Specially, for the B-terminated surface, B vacancy will promote the Na and Al diffusion with the lower barriers. Additionally, it is difficult for Na and Al to form interstitial defects and diffuse in covalent TiB2 crystal. Given these results, compared to graphite cathode, the sodium prefers to deposit on TiB2 surface, and the strong interaction between sodium and TiB2 promotes the early process of sodium penetration. On the other hand, the smoother landscape for Na diffusion on the TiB2 surface suggests the decreased stability of aluminum liquid, so that the current efficiency of aluminum reduction cell will decrease.

Insights into Poisoning Mechanism of Zr by First Principle Calculation on Adhesion Work and Adsorption Energy between TiB2, Al3Ti, and Al3Zr

Al-Ti-B intermediate alloys are widely used as grain refiners in aluminum alloys owing to the presence of Al3Ti and TiB2 phases. However, the existence of Zr in aluminum alloy melts often results in coarse grain size, leading to Al-Ti-B failure called Zr poisoning. There are three kinds of poisoning mechanisms related to TiB2, Al3Ti, and a combination of TiB2 and Al3Ti for Zr. First, Zr forms ZrB2 or Ti2Zr with TiB2 in Al-Ti-B to reduce the nucleation ability. Second, Zr existing in the aluminum melt with a high melting point Al3Zr then attracts Ti to reduce the dispersion of Ti as a growth inhibitor. Third, Zr reacts with Al3Ti on TiB2 surface to form Al3Zr, thereby increasing the degree of mismatch with Al and diminishing the refiner’s ability as a nucleation substrate. To gain a better understanding of the mechanism of Zr poisoning, the first principle was used in this study to calculate the adhesion works (ZrB2//Al3Ti), (Ti2Zr//Al3Ti), (Al3Zr//Al3Ti), (Al3Ti//Al), (TiB2//Al3Zr), and (Al3Zr//Al), as well as the surface energy of Al3Zr and adsorption energies of Al to Al3Ti or Al3Zr. The results demonstrated that Zr poisoning originated from the second guess. Zr element exiting in aluminum melt led to the formation of an Al3Zr (001) surface. The interfacial adhesion work of Al3Zr (001)//Al3Ti (001) was not weaker than that of TiB2//Al3Ti. As a result, Al3Zr first combined with Al3Ti to significantly decline the adsorption of Al3Ti (001) on Al, losing its role as a nucleating agent and grain coarsening. Overall, to prevent failure of the grain refiner in Zr containing aluminum melt, the adhesion work interface between the generated phase of the grain refiner and Al3Zr must remain lower to avoid the combination of the generated phase of grain refiner with Al3Zr. In sum, these findings look promising for evaluating future effects of grain refinement in Zr containing aluminum melt.

Effects of TiN content on the properties of hot pressed TiB2–SiC ceramics

This research intended to study the impacts of different contents of the TiN additive on the mechanical properties and microstructural features of the TiB2–SiC-based composites. Three different samples of TiB2-15 vol% SiC- x vol% TiN (x = 0, 3.5, and 7) were produced by hot-pressing at 2000 °C under 35 MPa for 120 min. Thanks to advancement of some reactions among the TiB2 surface oxides and the SiC reinforcement, two in-situ phases of TiC and SiO2 were produced during the sintering. Nevertheless, the TiN incorporation resulted in generating another in-situ compound (TiC0.3N0.7) in the relevant as-sintered ceramics. Moreover, introducing TiN significantly refined the microstructure of the composites, leading to higher mechanical characteristics. Finally, the highest flexural strength (781 MPa) and Vickers hardness (27.1 GPa) values were attained for the sample introduced by 7 vol% TiN.

Hot-pressing and characterization of TiB2–SiC composites with different amounts of BN additive

This research aimed to study the influence of different amounts of hBN additive on the mechanical properties and microstructure of TiB2-15 vol% SiC samples. All ceramics, containing 0, 3.5, and 7 vol% hBN, were sintered at 2000 °C using a hot-pressing route and reached their near full densities. Thanks to two different chemical reactions among the SiC reinforcement and the TiB2 surface oxides (B2O3 and TiO2), the in-situ phases of SiO2 and TiC were generated over the sintering process. The intergranular mode was identified as the predominant fracture type in all three composite samples. The hBN additive could contribute to grain refining of composites so that the sample containing 7 vol% hBN reached the finest microstructure. Finally, the highest Vickers hardness of 25.4 HV0.5 kg and flexural strength of 776 MPa were attained for the TiB2–SiC and TiB2–SiC-7 vol% hBN samples, respectively.

The nucleation sequence of α-Al on TiB2 particles in Al-Cu alloys

The refinement of Al alloys by TiB2 has been extensively investigated for decades, both in industry and academia. In order to achieve higher grain refinement potency, it is known that other solutes must be added alongside TiB2, thus tailoring the heterogeneous nucleation at the interface between the TiB2 particles and the Al matrix. Here, we report results from an atomic-scale experimental investigation of the heterogeneous nucleation interface of TiB2 in Al-Cu based alloys as well as in an Al-5Ti-1B grain refiner employed in these Al-Cu based alloys. We focus on the effect of segregation of the main solute elements (Ti, Cu) to the interface of TiB2 and attempt to disentangle this effect from other factors affecting the nucleation and growth. Significant Ti segregation in the form of a Ti-rich layer, identified as an Al3Ti two-dimensional compound, was unambiguously observed on the basal plane of TiB2 particles in the Al-5Ti-1B grain refiner, in agreement with prior literature. In the Al-Cu alloy system, prevalent Ti segregation was also observed on the basal plane of TiB2, accompanied in cases when the Cu concentration is high by the presence of a further atomic-scale Cu-rich layer. Based on these observations, a possible nucleation sequence for the Al-Cu based alloys with the addition of Al-5Ti-1B grain refiner is proposed whereby Al nucleation occurs on an Ti-rich layer on the surface of TiB2, which can then be preserved from a subsequent peritectic transformation by a surrounding eutectic reaction that forms a Cu-rich layer when sufficient Cu is present. This proposed nucleation sequence should help to link the absence or presence of Al3Ti layers on the basal plane of TiB2 to processing conditions in post-solidification studies. Although it is difficult to know with absolute certainty if a TiB2 particle observed in the post-solidification microstructure was active or inactive for heterogeneous nucleation during the solidification process, this experimental study highlights the possible role of Cu segregation on grain refinement of Al-Cu alloys by the Al-5Ti-1B grain refiner.

Mechanism for Si Poisoning of Al-Ti-B Grain Refiners in Al Alloys

Al-5Ti-1B is the most widely used grain refiner for Al alloys. However, it is not effective for grain refining Al alloys containing more than 3 wt pct Si. This adverse effect of Si is referred to as Si poisoning. In spite of extensive experimental and theoretical investigations in the past decades, the exact mechanism for Si poisoning is still not clear. In this work, the state-of-the-art electron microscopy was performed to investigate the mechanism for Si poisoning. Our experimental results suggest that Si segregates preferably to the TiB2/Al-Si melt interface and the pre-existing Al3Ti 2-dimensional compound (2DC) layer on TiB2 surface dissolves into the Al-Si melt. Based on the experimental results, we have postulated a new mechanism for Si poisoning: interfacial segregation of Si leads to enrichment of Si at the TiB2/Al-Si melt interface, and this in turn makes the pre-existing Al3Ti 2DC on the TiB2 surface unstable and dissolve gradually in the melt, resulting in a loss of TiB2 nucleation potency and hence a decreased total number of potent TiB2 particles available for heterogeneous nucleation and grain initiation and consequently an increased grain size. This mechanism for Si poisoning can explain consistently the experimentally observed phenomenon reported in the literature.