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Grain growth plays a pivotal role in determining the macroscopic properties of several polycrystalline materials. It is predominantly controlled by grain boundary and triple junction mobility, and several atomistic and mesoscale models have been developed to study this phenomenon. In particular, multi-order parameter phase-field and vertex dynamics models have been extensively used to understand the grain evolution dynamics and the effect of triple junction drag on the growth kinetics. In the current work, we present a novel sequentially-coupled phase-field and vertex dynamics model for both isotropic and anisotropic grain growth simulations. The proposed approach, which uses a backpropagation neural network, image processing and mathematical techniques for accurate grain boundary curvature detection, takes the advantages of both the models and provides an efficient way to switch from phase-field to vertex dynamics model depending on the growth controlling mechanism. Our results suggest that phase-field generated microstructures can replace Voronoi tessellation as the input to vertex dynamics, with a reduction in computational expenses and the ability to simulate more realistic and complex microstructures.

The topological transitions of the grain boundary network during grain growth under uniform grain boundary energies are believed to be known. However, this is not true for more realistic materials with varying grain boundary energies that, in principle, allow many different grain boundary configurations. A grain growth simulation for such a material therefore requires a procedure to enumerate all possible topological transitions and select the most energetically favorable one. Such a procedure is developed and implemented here for a microstructure represented by a volumetric finite element mesh. The method is implemented as a C++ library called VDlib based on SCOREC, an open source massively parallelizable library for finite element simulations with adaptive meshing. To test the implementation, a Voronoi tessellated microstructure composed of one hundred grains is generated and evolved under constant grain boundary properties until half of the grains remained. The evolution of the individual grains is compared to what is expected from the MacPherson-Srolovitz relation and is found to be in good match.As with all numerical techniques, it is important to identify systematic sources of error. Such studies for grain growth simulations are either missing from or not uniform across the literature. To address this issue and enable comparison across grain growth simulations, a set of benchmark cases, one for each boundary type of surfaces, lines, and points with analytical solutions are identified. These are used to compare a recently developed discrete-interface method for microstructure evolution to a state of the art diffuse interface (multiphase field method). In each case, the discrete method is found to meet or outperform the multiphase field method in terms of accuracy for comparable levels of refinement, demonstrating its potential efficacy as a numerical approach for microstructure evolution.Last, a novel invariant of ideal grain growth process is defined that ideally can be used to calculate the number of topological entities in a microstructure and only changes as the topology changes. VDlib is used to test the invariant on a simulated microstructure composed of a periodic arrangement of Kelvin cells. A convergence study reveals that the invariant converges to the ideal case with increasing levels of refinement, however predicting the number of entities without bounds on number of entities proves unpractical. Two error measures based on this novel invariant are proposed which can potentially be used to quantitavely measure how much simulations deviate from ideal grain growth process and experimental microstructures.

Microstructural evolution and strengthening mechanisms of Mg-3Sn-1Ca based alloys with additions of different amounts of AlN nano-particles were investigated. It was found that with increasing the amount of AlN nano-particles the grain size decreases obviously. The existence of AlN nano-particles could refine the primary crystal phases CaMgSn, which provided more heterogeneous nucleation sites for the formation of magnesium. Moreover, such nano-particles could also restrict the grain growth during solidification. After adding AlN nano-particles, both the tensile properties at room temperature and high temperature 250 °C and the hardness are largely improved. The improvement of strength is attributed to grain refinement and second phase refinement.

The extrudability, microstructural characteristics, and tensile properties of the Mg–5Bi–3Al (BA53) alloy are investigated herein by comparing them with those of a commercial Mg–8Al–0.5 Zn (AZ80) alloy. When AZ80 is extruded at 400 °C, severe hot cracking occurs at exit speeds of 4.5 m/min or more. In contrast, BA53 is successfully extruded without any surface cracking at 400 °C and at high exit speeds of 21–40 m/min. When extruded at 3 m/min (AZ80–3) and 40 m/min (BA53–40), both AZ80 and BA53 exhibited completely recrystallized microstructures with a 〈10–10〉 basal texture. However, BA53–40 has a coarser grain structure owing to grain growth promoted by the high temperature in the deformation zone. AZ80–3 contains a continuous network of Mg17Al12 particles along the grain boundaries, which form via static precipitation during natural air-cooling after the material exits the extrusion die. BA53–40 contains coarse Mg3Bi2 particles aligned parallel to the extrusion direction along with numerous uniformly distributed fine Mg3Bi2 particles. AZ80–3 has higher tensile strength than BA53–40 because the relatively finer grains and larger number of solute atoms in AZ80–3 result in stronger grain-boundary and solid-solution hardening effects, respectively. Although BA53 is extruded at a high temperature and extrusion speed of 400 °C and 40 m/min, respectively, the extruded material has a high tensile yield strength of 188 MPa. This can be primarily attributed to the large particle hardening effect resulting from the numerous fine Mg3Bi2 particles.

Here we report a novel strategy to consolidate layered alumina demonstrating the directional electrical and thermal conductivity. The material was produced via incorporation of alumina nanofibers (20 ± 2 nm in diameter) decorated by several layers of graphene wrapped around longitudinal axes of the fibers. The graphenated fibers, obtained with the help of one-step catalyst-free CVD process, offered inhibition of grain growth combined with electrical conductivity to the sandwiched composites, which were consolidated by spark plasma sintering. Impact of the concentration of the fillers together with thickness of the conductive graphene-containing layer on thermal properties was studied. A graphene-containing interlayer with 50 μm thickness, sandwiched between two monolithic 10 mm layers of alumina shows ~30% enhancement in isotropic thermal conductivity of monolithic alumina.

The thermal stability and mechanical properties of a gradient-nanograined structure (GNS) CoCrNi medium entropy alloy (MEA) processed by ultrasonic surface rolling were studied by using isothermal/isochronal annealing tests combined with quasi-in-situ electron backscatter diffraction (EBSD) characterization and Vickers micro-hardness (HV) measurements. A layer by layer high-throughput investigation method was used to quantitatively study the grain growth kinetics and grain boundary evolution with different initial grain sizes, which could effectively save specimen and time costs. The grain nucleation and growth, as well as shrink and disappearance process through Σ3 coincidence site lattice boundary migration with slightly lattice rotation during annealing were directly revealed. The layer by layer grain growth kinetics and calculated activation energy indicate that the thermal stability of nano-grained top surface layer is relatively higher than that of nano-twined subsurface layer for the gradient CoCrNi MEA processed by ultrasonic surface rolling. Further analysis show that the grain boundary relaxation and dynamic recrystallization of the topmost nano-grains led to the decrease of grain boundary energy, thus improving their thermal stability. The present work provided theoretical basis for the application of CoCrNi MEA at high temperatures. Moreover, the high-throughput method on the investigation of grain stability by using gradient structure can be easily extended to other materials and it is of great significance for understanding the microstructural evolution of gradient materials.

The grain growth kinetics and densification mechanism of (TiZrHfVNbTa)C high-entropy carbide ceramic are investigated in this work. A single phase carbide with a rock-salt structure is formed until 2300 °C, below which an apparent aggregation of V, Zr and Hf exists. It is associated with the slow diffusion rate of V element as well as the relatively poor solubility of VC in HfC (as well as ZrC). The grain growth mechanism gradually changes from surface diffusion to volume diffusion and then grain boundary diffusion with increasing sintering temperature. This is attributed to the variation of activation energy of grain growth. The densification mechanism is principally dominated by the mass transport through lattice diffusion with the activation energy of 839 ± 53 kJ/mol. Through the design of two-step sintering, it is verified that the solid solution formation can effectively promote the densification process.

In this study, B4C–SiC–rGO composites with different SiC contents were prepared by spark plasma sintering at 1800 °C for 5 min under a uniaxial pressure of 50 MPa. The effects of SiC on the microstructures and mechanical properties of the B4C–SiC–rGO composites were investigated. The optimal values for flexural strength (545.25 ± 23 MPa) and fracture toughness (5.72 ± 0.13 MPa·m1/2) were obtained simultaneously when 15 wt.% SiC was added to 5 wt.%–GO reinforced B4C composites (BS15G5). It was found that SiC and rGO inhibited the grain growth of B4C and improved the mechanical properties of the B4C–SiC–rGO composites. The clear and narrow grain boundaries of rGO–B4C and rGO–SiC, as well as the semi-coherent B4C–SiC interface, indicated strong interface compatibility. The twin structures of SiC and B4C observed in the composites improved their fracture toughness. Crack deflection and crack bridging caused by the SiC grains as well as rGO bridging and rGO pull-out were observed on the crack propagation path.

This work explores the incorporation of graphene-based two-dimensional nanostructures as moisture barriers to delay hydrothermal ageing of yttria-stabilized zirconia and strengthen its use in biomedical applications. Two sets of highly dense zirconia composites incorporating multilayered graphene with very different lateral dimensions, few layer graphene and exfoliated graphene nanoplatelets, were prepared. The effect of the addition of graphene nanostructures on zirconia ageing was investigated by conducting accelerated hydrothermal degradation experiments in an autoclave. An improved resistance to low-temperature degradation and a high tolerance to damage were achieved in the composites compared to those of monolithic zirconia. The incorporation of 1 vol% multilayered graphene was very effective in restricting the hydrothermal degradation. In particular, the composite incorporating exfoliated graphene nanosheets exhibited outstanding resistance to ageing because of their fine dispersion throughout the matrix, which effectively seemed to restrict grain growth and slow the propagation of the transformation front to the ceramic bulk.

Magnetic materials with desired architectures and high performance have become increasingly important for applications. In this work, NiZn ferrites with mesh structures have been successfully fabricated from preliminary precursors by three-dimensional (3D) printing accompanied by the solid-state reaction. The influence of compositions on structural and magnetic properties of NiZn ferrites were systematically investigated. It was obvious that the saturation magnetization was significantly enhanced to 77 emu/g for NiZn ferrites with a zinc concentration of 0.4, which was a much higher value compared to Ni ferrites. This is attributed not only to the well-known cationic distribution, but also to the grain growth with extraordinarily high crystallinity. Consequently, NiZn ferrites with intricate mesh structures were utilized for magnetic separation, and the distributions of magnetic flux density were simulated. Overall, the fabrication of NiZn ferrites by 3D printing is attractive for scaled-up applications, and also paves the promising avenue for magnetic separation.

The large grain boundary volume fraction of nanocrystalline metals can bestow desirable mechanical behaviors, but the same grain boundaries that impart these beneficial properties challenge their application. Excess surface energy due to the substantial interfacial volume drives grain growth, and thus a loss of the benefits gained by nanocrystalline grain sizes. Also, not all grain boundaries are equal, where material behaviors can fluctuate dramatically depending on the grain boundary structure and composition. New pathways to fundamentally alter the grain boundary structure are needed to stabilize the nanocrystalline grain size and leverage the full potential of nanocrystalline metals. “Complexion” is a term used to describe the phase-like behavior of grain boundaries, where grain boundaries, similar to bulk phases, can undergo discrete transitions in structure and composition based on external factors such as temperature. Amorphous intergranular films, a type of complexion, are of particular interest due to their ability to enhance both mechanical and radiation damage tolerance due to the excess free volume present in these structures. In this thesis, we seek to expand the current materials toolbox of alloys that can form amorphous intergranular films, and then investigate the impact of these unique damage tolerant features in applications where they can be leveraged. First, we propose a set of materials selection rules aimed at predicting the formation of amorphous intergranular films, and then apply these rules to discover new alloys that can form these features. In doing this, we also discover a counterintuitive, ultra-high temperature grain size stabilization regime driven by the formation of amorphous intergranular films. Next, we investigate the impact of these features in applications where damage tolerance is critical, particularly fatigue and radiation. We find that incorporation of amorphous intergranular films throughout the grain boundary network of nanocrystalline alloys increases plasticity preceding a fatigue crack, and dramatically improves radiation tolerance. In summary, through an array of sputtered, electroplated and ball-milled Cu and Ni-based alloys, specialized heat treatments, and electron microscopy techniques, we find that amorphous intergranular films can potentially be found in a large number of alloys, and can dramatically improve nanocrystalline alloy behaviors.

We investigated the effects of Sn addition on the microstructural characteristics and mechanical properties of an extruded Mg–Bi binary alloy by comparing Mg–5Bi (B5) and Mg–5Bi–4Sn (BT54). Both the extruded alloys exhibit a partially recrystallized grain structure with a strong extrusion fiber texture and numerous Mg3Bi2 precipitates. However, the addition of Sn significantly decreases the average grain size of the extruded alloy from 123.9 to 75.2 μm. The Sn solute atoms inhibit the activity of dislocation slip, which reduces the internal strain energy accumulated in the dynamically recrystallized (DRXed) grains during extrusion. Consequently, this reduced strain energy leads to the decrease in the DRXed grain size owing to weakened grain growth during natural air-cooling. The extruded BT54 alloy exhibits higher tensile strength and ductility than the extruded B5 alloy. The improvement in the strength by the Sn addition is attributed to the combined effects of grain refinement, Sn solute atoms, and increased dislocation density. The formation of {10–11} and {10–11}-{10–12} twins during tension is suppressed by the grain refinement, thereby improving the tensile elongation considerably.

SrFBiS2 is a quaternary n-type semiconductor with rock-salt-type BiS2 and fluorite-type SrF layers alternately stacked along the c axis. The tunability of the crystal and electronic structures as well as the intrinsically low thermal conductivity make this compound a promising parent material for thermoelectric applications. In the current work, we show that alloying of Se and S in SrFBiS2 reduces the optical band gap with the second conduction band serving as an electron-transport medium, simultaneously increasing the electron concentration and effective mass. In addition, the raw material Bi2Se3 is shown to act as liquid adjuvant during the annealing process, favoring preferred-orientation grain growth and forming strengthen microstructural texturing in bulk samples after hot-pressed sintering. Highly ordered lamellar grains are stacked perpendicular to the pressure direction, leading to enhanced mobility along this direction. The synthetic effect results in a maximum power factor of 5.58 μW cm-1 K-2 at 523 K for SrFBiSSe and a peak zT = 0.34 at 773 K, enhancements of 180% compared with those of pristine SrFBiS2.

A series of (Ti0.5Nb0.5)C-x wt.% SiC (x = 0, 5, 10, 20) composites were prepared by spark plasma sintering. Dense microstructures with well‐dispersed SiC particles were obtained for all composites. With the increment of SiC content, the Vickers hardness, Young's modulus and fracture toughness increase monotonically. An optimized flexural strength of 706 MPa was achieved in (Ti0.5Nb0.5)C-5 wt.%SiC composite. (Ti0.5Nb0.5)C-20 wt%SiC composite exhibits the highest fracture toughness of 6.8 MPa m1/2. The crack deflections and the suppression of grain growth were the main strengthening and toughening mechanisms. Besides, (Ti0.5Nb0.5)C-20 wt%SiC composite exhibit the highest thermal conductivity of 45 W/m·K at 800 °C.

Amosic-3 SiC/SiC composites were irradiated at 300 °C using 6 MeV Si ions to peak doses of 13 and 55 displacements per atom (dpa). The loss of amorphous carbon packets and the growth of SiC grains were simultaneously observed in Amosic-3 SiC fibers, using a combination of transmission electron microscopy (TEM) and Raman spectroscopy. A mechanism based on the grain growth theory was proposed to expound the relationship between the loss of carbon packets and the growth of SiC grains. Small and curved SiC grains can absorb surrounding carbon packets to grow themselves; at some point, these grains further grow at the expense of adjacent small SiC grains until their grain boundary became straight. TEM images were found to support the above mechanism.

Duplex-structured Mg-7Li-2Al-1.5Sn alloys with high strength were fabricated and their strengthening mechanism was investigated. The Mg-7Li-2Al-1.5Sn alloys were prepared by casting and extruded at the temperature of 533 K with an extrusion ratio of 25:1. The microstructure and mechanical properties of Mg-7Li-2Al-1.5Sn alloys were systematically investigated by OM, XRD, SEM, TEM, and tensile tests. The results show that Mg-7Li-2Al-1.5Sn alloys are mainly composed of α-Mg, β-Li, LiMgAl2, Mg2Sn and Li2MgSn phases. The yield strength (YS), ultimate tensile strength (UTS) and elongation (EL) of the extruded alloy at room temperature reach 250 MPa, 324 MPa and 11.9%, respectively. A lot of Sn-rich precipitates (Mg2Sn and Li2MgSn) are precipitated during extrusion with an average size of ∼14 nm, which is beneficial to the grain refinement. Dynamic recrystallization occurs during hot deformation and the nano-precipitates effectively refine the dynamic recrystallized (DRXed) grains. Besides, the residual dislocations existed in DRXed and un-DRXed grains result in the dislocation strengthening in the extruded alloy. Mg-7Li-2Al-1.5Sn alloys possess excellent high-temperature mechanical properties with the YS, UTS and EL of 200 MPa, 237 MPa and 26.7% at 423 K, respectively. Sn-rich precipitates with good thermal stability can effectively prevent grain growth, which is good for the improvement of the high-temperature performance of Mg-Li-Al-Sn alloy.

Lead-free ceramics with both high piezoelectric response and good temperature stability are urgently demanded for electromechanical conversion devices. Unfortunately, owing to coexistence of polymorphic phases near room temperature (RT), enhanced piezoelectric properties were usually achieved with occurrence of strong temperature dependence in modified BaTiO3 (BT)-based ceramics. In this work, we demonstrate that tailoring grain orientations of tetragonal BT-based ceramics can effectively produce substantially enhanced and thermally stabilized piezoelectric response. Both c- and c-oriented tetragonal (Ba0.85Ca0.15) (Zr0.05Ti0.95)O3 (BCZT) ceramics with texture degrees F>90% were synthesized via templated grain growth. Interestingly, the ceramics textured along the c polar axis show much higher microscopic and macroscopic piezoelectric properties than those with nonpolar c texture, indicating an “extender” ferroelectricity nature. Compared with randomly oriented samples, the c-oriented ceramics exhibit simultaneously ∼1.6 times higher piezoelectric strain d33∗ (∼760 pm/V), 4.4 times higher piezoelectric figure of merit d33×g33 (8.8 × 10−12 m2/N), and better temperature stability (strain variation ≤5% between RT and 110 °C). Such thermally stabilized strain response can be mainly attributed to wide temperature range of tetragonal phase and stable domain structure. This work provides a promising route for further developing lead-free piezoceramics with high and temperature-insensitive performance, which can greatly broaden their application areas.

In the present research, the dynamic recrystallization (DRX) behavior of a newly-developed Mg–Al–Zn–RE alloy with abundant second-phase particles during hot extrusion is investigated by coupling finite element (FE) and cellular automaton (CA) models. A two-dimensional CA model is developed to quantitatively and topologically evaluate the DRX process during deformation with constant forming conditions. Considering the fact that second-phase particles with various sizes extensively exist in the studied Mg–Al–Zn–RE magnesium alloy, models of DRX nucleation and grain growth velocity are modified. The coefficients of the modified CA model are calibrated by isothermal compression experiments of the magnesium alloy. Subsequently, the CA model is coupled with FE analysis to investigate the DRX behavior during the hot extrusions of the Mg–Al–Zn–RE alloy. The DRX behavior of the magnesium alloy at different stages and positions of extruded plates is simulated by the established model. Finer grains near the edge than in the inner of the plates result from higher strain and strain rate. The influence of extrusion conditions on microstructural evolution is explored. Under the employed forming conditions, average grain size decreases 28–62 times from as-cast and solution-treated to as-extruded state due to grain refinement by DRX. With increasing initial billet temperature or extrusion speed, average grain size increases. The finest grains are obtained at the initial billet temperature of 623 K and the extrusion speed of 7.83 mm/s. Low initial billet temperature or high extrusion speed benefits homogeneous grain distribution. The simulated results are in good agreement with experimental ones.

The effects of T4, T5, and T6 treatment on the microstructure and mechanical properties of the extruded Mg-4.3Gd-3.2Y-1.2Zn-0.5Zr (wt.%) alloy with a relatively low RE content (7.5 wt.%) were investigated. T4 treatment at 450–500 °C induces a gradual grain growth of α-Mg but an obvious transition of texture component from ⊥ED to ∥ED. Interdendritic LPSO phases are highly stable against annealing while intragranular ones experience dissolution and re-precipitation. After peak-ageing at 200 °C, the elongation of as-extruded and T4 samples is just slightly reduced or even increased due to the weak ageing hardening response. T5 sample exhibits an attractive combination of strength and ductility, with a tensile yield strength (TYS) of 303 MPa and elongation of 20.0%. The Hall–Petch relation for the alloys with or without ageing treatment has been estimated. Grain boundary strengthening rather than precipitation strengthening has the dominant contribution to TYS, and a modified equation is developed to predict grain boundary strengthening values for Mg-Gd-Y-Zn-Zr alloys which contain different Schmid factors for basal slip.

Thermal barrier coating (TBC) materials perform an increasingly important role in the thermal or chemical protection of hot components in a gas turbine. In this study, a novel high entropy hafnate (Y0.2Gd0.2Dy0.2Er0.2Yb0.2)2Hf2O7 was synthesized by solution combustion method and investigated as a potential TBC layer. The as-synthesized (Y0.2Gd0.2Dy0.2Er0.2Yb0.2)2Hf2O7 possesses a pure single disordered fluorite phase with a highly homogeneous distribution of rare earth (RE) cations, exhibiting prominent phase stability and excellent chemical compatibility with Al2O3 even at 1300 °C. Moreover, (Y0.2Gd0.2Dy0.2Er0.2Yb0.2)2Hf2O7 demonstrates a more sluggish grain growth rate than Y2Hf2O7. The thermal conductivity of (Y0.2Gd0.2Dy0.2Er0.2Yb0.2)2Hf2O7 (0.73-0.93 W m−1 K−1) is smaller than those of components RE2Hf2O7 and many high entropy TBC materials. Beside, the calculated thermal expansion coefficient (TEC) of (Y0.2Gd0.2Dy0.2Er0.2Yb0.2)2Hf2O7 (10.68 × 10−6/K, 1100 °C) is smaller than that of yttria-stabilized zirconia (YSZ). Based on the results of this work, (Y0.2Gd0.2Dy0.2Er0.2Yb0.2)2Hf2O7 is suitable for the next generation TBC materials with outstanding properties.

The Sr-substituted CaMgSi2O6 ceramics (CaMg1−xSrxSi2O6, x = 0, 0.05, 0.1, 0.2, 0.3, 0.4 and 0.5) were synthesised via solid-state reaction method. The CaMg0.9Sr0.1Si2O6 sample was dense at 1250 °C, and outstanding performance was achieved: er = 7.85, Q×f = 77,916 GHz (13.65 GHz), and τf = −42.89 ppm/°C. The effects of Mg2+ substituted by Sr2+ on the phase composition, microstructure, lattice vibration and microwave dielectric properties of CaMgSi2O6 ceramics were investigated. The doped Sr2+ with solubility between 0.05 and 0.1 mol promoted the densification and grain growth of ceramics and led to the formation of second phase and red shift of X-ray diffraction peaks and Raman active modes. Density and porosity played an important role in er. The Q×f value was optimised with homogeneous grain size distribution, high relative density and low full width at half maximum. The τf value was closely related to the second phase and changed nonlinearly with the x value.

Summary Cermets embrace the high hardness of a ceramic and the good ductility of a metal. Therefore, they potentially meet the ever-increasing demands of new materials for harsh environments in energy generation and conversion. However, cermet sintering faces many challenges, such as metal volatilization, grain coarsening, and poor wettability of the metal with the ceramic, which affect the microstructure and thus mechanical properties. Herein, we apply an ultrafast high-temperature sintering (UHS) method to sinter cermets at a high temperature in only seconds. The high temperature leads to good wettability of the metal with the ceramic, while the short sintering time limits the loss of metal components and accurately controls the grain growth. We demonstrate the UHS method using a model cermet composed of a Ni-based superalloy and aluminum oxide, where we obtain dense microstructure, minimal nickel loss, and uniform grain size. Owing to these merits, good mechanical properties and oxidation resistance are achieved.

Cold-rolled metastable β-type Ti–38Nb-0.2O alloy was subjected to annealing treatment to obtain different precipitates and grain sizes. The influence of annealing on microstructure and mechanical properties was investigated. The alloy annealed at 673 ​K or 773 ​K exhibited a single-stage yielding with high strength and low uniform elongation, due to the residual work hardening and the precipitation of ω or α phases. The alloy annealed at above 873 ​K exhibited an obvious double yielding behavior resulting from the stress-induced martensitic transformation. The grain growth kinetics of single β phase alloy is sensitive to temperature, and it is suggested that the existence of oxygen decreases the grain growth exponent and increases the required activation energy for grain growth. The critical stress for slip decreased monotonously with the increase of grain size, following the classic Hall-Petch relationship. However, the critical stress for martensitic transformation decreased to a minimum and then increased again, as the grain size increased. The results are worth for design of the heat-treatment parameters of the Ti–38Nb-0.2O alloy for engineering applications.

The investigate about the effect of the microstructure of Ti6Al4V alloy on its cavitation erosion and corrosion properties in marine can provide the key basis for the application. On the basis of as-received Ti6Al4V(TC4) alloy, FC-TC4 and AC-TC4 alloys were prepared by heat treatment with the cooling method of a furnace and atmospheric environment, respectively. Then the microstructure evolutions of three samples were scrutinized and the effect of microstructure on their cavitation erosion and corrosion resistance was explored. The results showed that more recrystallized grains formed as well as its content of α grains and high-angle grain boundaries increased in AC-TC4 alloy. To FC-TC4 alloy, there was obvious grain growth apart from recrystallization. Moreover, many nanotwins of TiV and TiAl3 were formed separately in FC-TC4 and AC-TC4 alloys due to the dislocation migration during heat treatment. The microstructure evolution led the hardness and elastic modulus of AC-TC4 alloy were the best, followed by FC-TC4 alloy, that of TC4 were the worst. Similarly, passivating ability of AC-TC4 alloy was the best among three samples because of its microstructure. Although cracks extended along the grain boundaries under the action of continual cavitation erosion, the passivation film formed by TiO2 and Al2O3 would enhance their resistance to further corrosion and cavitation erosion in artificial seawater.

We demonstrate the first successful attempt to partially substitute Cu into the Mn3AgN-antiperovskite system to form Mn3Ag(1-x)Cu(x)N thin films with an ultra-low temperature coefficient of resistance (TCR) for fabrication of ultra-precise passive components. Films were grown by reactive magnetron sputtering on alumina and glass substrates and were found to be amorphous in nature with highly negative TCR of -233 to -351 ppm/°C in their as-grown state. Increasing Cu alloying from x=0 to 1, resulted in increased sheet resistance, a negative shift of TCR and a change of grain morphology from spherical to elongated. Post-deposition heat treatment at 300-375 °C, resulted in a positive shift of TCR and an ultra-low TCR of -4.66 ppm/°C for films with x=0.6. The heat treatment induces grain growth, surface roughness and the formation of a manganese oxide upper surface layer up until temperatures of 350 °C, after which surface oxidation begins to dominate. The growth rate of the surface layer is controlled by the Cu concentration and heat treatment temperature, which both play a central role in the development of these novel ultra-low TCR Mn3Ag(1-x)Cu(x)N thin film structures.

Ultra-high temperature ceramics (UHTCs) present great opportunities for hypersonic applications, but densification via conventional sintering is challenging and often requires sintering aids, limiting UHT applications. Field-assisted sintering technology (FAST) can produce dense, mechanically-robust components without the need for sintering aids. In this work, we have developed an optimized set of FAST processing conditions without sintering aids for various compositions in the (Hf,Ta)C ternary system. The novel processing approach yields high-density ceramics with minimal grain growth. It was found that 50 vol% HfC (∼55 mol%) demonstrated record-breaking nanohardness (41.45 ± 1.37 GPa), Vickers microhardness (30.2 ± 3.1 GPa), and elastic (indentation) modulus (590.12 ± 10.64 GPa). These peak mechanical properties arose from the balance of two underling structure-property relationships: solid solution strengthening and the Hall-Petch effect. The interplay of these compositionally-linked phenomena yields an optimal regime of superior mechanical properties. Combining this interplay with optimized FAST parameters, superior ternary HfC-TaC ceramics can be realized for next-generation hypersonic applications.

International audience; Zinc oxide optical ceramics containing 0 – 2 wt% ytterbium are prepared byuniaxial hot pressing of commercial oxides at 1150 and 1180 °C. The ceramics have themain crystalline phase of hexagonal wurtzite-type ZnO. Ytterbium ions do not enter theZnO crystals but form a cubic sesquioxide phase of Yb2O3 located at the ZnO grainboundaries. Yb acts as an inhibitor for the ZnO grain growth. The ceramics exhibittransmittance up to 60% in the visible. Their transmission in the infrared is determinedby the free charge carrier absorption. The Yb3+ ions are found in C2 and C3i sites in Yb2O3crystals. Under X-ray excitation, the ceramics exhibit intense luminescence bands in theblue (near-band-edge emission) and green (defect emission) whose positions, intensitiesand decay times depend on the Yb content. Yb2O3 causes a redistribution of luminescenceintensity in favor of the near-band-edge emission and fastens the emission decay.

The effect of grain size was not fully explored in composites containing ferroelectric and magnetic phases including the independent properties of each phase. Aiming to study the dependence of ferroic and multiferroic properties with average grain size in composites this work reports preparation method and electric, magnetic and magnetoelectric properties of particulate composites with distinct combination of grain size of each phase. A new sintering method was applied to prepare a group of samples with distinct configuration of grain size distribution. With this method it was possible to densify ceramics avoiding grain growth using spark plasma sintering. It was observed that the all ferroic and multiferroic properties were correlated with the setup of grain size of each phase in the composite, by example the dependence of magnetic properties with the electric phase grain size. Besides it was noticed that is possible to improve magnetoelectric coefficient by changing grain size of both phase, even inducing the self-biased effect by decreasing the grain size of the ferroelectric phase.

One of the strategies for improving alumina ceramics' strength and optical properties is to eliminate grain growth during sintering. In this work, a casting process from water suspension was applied to prepare bulk alumina specimens doped with zirconia inclusions in the amount up to 0.5 wt.%. The effect of pure monoclinic (m-ZrO2) or cubic zirconia (ZrO2 stabilised with 8 mol.% Y2O3) on the sintering and alumina grain growth up to 1600 °C was studied. The results demonstrate that even a negligible addition of zirconia particles (0.2 wt.%) reduces grain growth after sintering up to 1400 °C. At the sintering temperature of 1600 °C, significant grain growth was observed for both pure and doped alumina ceramics. However, the careful analysis of the grain size distribution revealed narrower distribution for zirconia-doped alumina compared to pure alumina ceramic. The Vickers hardness decreases with the increasing sintering temperature due to grains' growth independently on the load applied. The indentation fracture toughness of samples sintered at 1400 °C does not significantly depend on the presence of inclusions added in amounts up to 0.5 wt.%.

Nowadays, thermal condition and solute field are considered as the potential dominant factors controlling competitive grain growth during directional solidification process. However, the controlling modes and critical conditions of competitive grain growth have been drastically debated over the past two decades. In this work, thermal condition and solute field are combined to study the competitive grain growth in the converging case by experimental observation and numerical simulation of bicrystal samples. We find the competitive grain growth is controlled by the cooperative effect of thermal condition and solute field, and the controlling modes are related to the bicrystal misorientation between favorably and unfavorably oriented grains. When the unfavorably oriented grain is low misoriented, unfavorably oriented grain dominates grain selection, and the competitive grain growth performs as solute field domination. However, with the increase of unfavorably oriented grain's misorientation, the grain selection converts into favorably oriented grain domination, and the competitive grain growth changes to thermal condition domination. To explain these abnormal transformation phenomena, we propose a misorientation dependent thermal condition-solute field cooperative domination model and identify the critical conditions by a critical misorientation (θcm). According to dynamic equation of dendrite growth, we calculate the critical misorientation θcm to prove this model. The theoretical calculation results agree well with the experimental results.

The influence of a microwave hybrid heat treatment (MHH) on the surface and in-depth mineralogical transformation of pre-sintered 3Y-PSZ was investigated. 3Y-PSZ samples were prepared by slip casting and sintered by conventional firing (1270 °C). Then, different MHH treatments from 5 to 15 min. at 1200 °C were applied to obtain a fully stabilized 3Y-TZP. The monoclinic fraction depth profiles in the first micrometres (up to 5) of thickness were investigated by means of the grazing incident X-ray diffraction technique (GIXRD). A good sintering degree with practically nil closed porosity and grain growth was achieved after MHH of 15 min. MHH increases the tetragonal phase content both in the surface and in-depth, reducing completely the monoclinic phase shell typically found after conventional sintering. A new parabolic model is proposed for the convoluted monoclinic fraction depth profile, which through the value of its horizontal asymptote allows the determination of the monoclinic shell thickness.

A study of the (1-x)BiFeO3-(x)Ba1/2Na1/2TiO2.75 (BFO-BNT) solid solutions obtained using the solid state reaction method, for different molar relative concentration of Ba1/2Na1/2TiO2.75 in the 0.0 ≤ x ≤ 0.12 composition range, is presented. The crystal structure and the dielectric and ferroelectric properties are studied in detail. Results of the Rietveld refinement of the X-ray diffraction data demonstrate that the system is single phase with R3c symmetry up to x = 0.09 while for x = 0.12, a small quantity of a secondary phase with P4mm symmetry appears. Scanning electron microscopy demonstrates that BNT presence promotes grain growth resulting in larger grains. Raman spectroscopy shows that, with increasing x, some of the A and E Raman modes slightly reduce their intensity while shifting in frequency, evincing the structural changes caused by the Ba, Na, and Ti incorporation on the BFO lattice. The X-ray photoelectron spectroscopy study confirms the successful substitution and gradual structural distortion in the samples. The improvement in dielectric properties with increasing BNT concentration can be attributed to stable dipole moment formation. Compared with pure BFO ceramics, doped BFO samples exhibit remarkably enhanced ferroelectric properties.

The present study addressed the change in the microstructure of Al–2.5 wt% Fe binary alloy produced using laser powder bed fusion (L-PBF) technique by thermal exposure at 300 °C, and the associated mechanical and thermal properties were systematically examined as well. Multi-semi-cylindrical patterns corresponding to melt pools in the microstructure were macroscopically observed for the as-manufactured sample. No change in the melt-pool morphology was observed after thermal exposure for 1000 h. Inside the melt pools, a large number of the nanoscale metastable Al6Fe phase particles were uniformly distributed inside columnar grains of the α-Al matrix containing concentrated solute Fe in supersaturation. The sequential formation and coarsening of stable θ-Al13Fe4 phases were observed upon exposure to a 300 °C environment, but a considerable amount of nano-sized metastable Al6Fe phases remained even after 1000 h. Furthermore, the thermal exposure continuously reduced the concentration of solute Fe atoms in the α-Al matrix. No significant grain growth was found in α-Al matrix after 1000 h owing to the pinning effect of the dispersed fine particles on grain boundary migration. These results demonstrate a sluggish change in microstructural morphologies of the Al–2.5 wt% Fe alloy. The quantified microstructural parameters addressed dominant strengthening contributions by the solid solution of Fe element and Orowan strengthening mechanism by fine Al–Fe intermetallics in the L-PBF-produced alloy. The high strength level was sustained even after being exposed to 300 °C for long periods. The superior balance of mechanical properties and thermal conductivity can be achieved in the experimental alloys by taking advantage of the various microstructural parameters related to the Al–Fe intermetallic phases and α-Al matrix.

Cold spray (CS) coating process is emerging spray coating technology, used in different sector owing to its unique features. This technique has the capability to form dense deposits with very low oxygen content, no grain growth, high deposition efficiency, no oxidation, high deposition rate, solid-state process (low temperature), ultra thick coating, high bond strength, low porosity and phase changes. In addition, this technique provides numerous technological benefits over thermal spray technique because it utilizes kinetic energy (K.E.) rather than thermal energy (T.E.) for deposition. Further, range of materials including metals, ceramics, composites and even polymers can be sprayed by cold spray coating techniques. Due to these attractive properties of cold spray, this technique finds wide application in different sectors. Hence the aim of this paper is to give an overview of cold spray techniques and to summarize the various coatings deposited by distinct researchers in distinct sectors.

The effects of sintering additives (Boron and SiC) and solid solution (TiB2) on the densification of HfB2-based ceramics, processed by pressureless sintering, were investigated for potential applications as advanced control rod materials. The addition of B was found to improve the densification properties of HfB2 through removal of the oxygen contamination yet incurred unfavourable grain growth. SiC was able to further enhance the densification through a grain pinning effort, improving the microstructure. In combination with oxygen impurity removal by the addition of B, the solid solution of Ti into HfB2 contributed to further improving the densification by reducing the particle size of HfB2 powder, further promoting diffusion during sintering. The grain size of SiC phase was also reduced through the refinement of HfB2 powder via solid solution. The deployment of B and SiC sintering additives, alongside TiB2 solid solution, during pressureless sintering was found to be beneficial in the preparation of high density HfB2-based ceramics, increasing microstructure density and enhancing mechanical properties.

Ytterbium (Yb) containing magnesium alloys have aroused extensive interest due to their excellent mechanical properties after thermomechanical processing and heat treatment. Unfortunately, the sole effect of Yb addition on the microstructure and mechanical properties of pure Mg matrix remains uncertain to date. In this work, the effects of Yb concentration on the texture development and tensile properties of pure Mg matrix during hot extrusion and the subsequent annealing were systematically investigated. The results revealed that the constitutional supercooling induced by Yb addition refined the as-cast microstructure but exerted a negligible effect on the original columnar grain morphology. When extruded at 300 °C, the dynamic recrystallization (DRX) process was considerably retarded. The in-grain misorientation axes (IGMA) analysis combined with TEM observation indicated that non-basal slips operated with increasing Yb concentration. Specifically, the prismatic 〈a〉 slip should be robustly activated in Mg–1.0 Yb extrudate, promoting the formation of the texture with {10–10} plane normal to the extrusion direction (ED), while for the Mg–2.0 Yb counterpart, the increased activity of pyramidal 〈c + a〉 slip and the relaxation of basal/ dislocations generated an ED-tilted texture component. The preferential grain growth dominated the subsequent annealing texture development at 400 °C when a comparable grain size was achieved. An obvious ED-tilted texture intensity with the peak around 〈–12–13〉 was observed in Mg–2.0 Yb alloy, which was primarily caused by grains with the basal orientation vanished and with the non-basal orientations intensified due to a higher concentration of Yb solute. Favored by the grain refinement, the Mg–2.0 Yb extrudate exhibited a high tensile yield strength of 304 ± 3.5 MPa, while the subsequently annealed counterpart presented a favorable elongation to failure of 14.8 ± 1.2%, which mainly due to the homogeneous grain structure, weak ED-tilted texture, and dissolution of coarse phases after high-temperature annealing.

In this work, an aluminum-doped Ti3SiC2-20 wt.% Ti5Si3 composite was prepared through reactive spark plasma sintering (SPS) from a powder mixture of Ti, SiC, C, and Al. The microstructure evolution of the formed oxide scales onto this composite were investigated in the temperature range of 1000–1400 °C in air. Up to 1200 °C, the exposed surfaces were highly protected by a continuous and dense α-Al2O3 layer with a good adherence that acts as a barrier diffusion. Above this temperature, the breakaway oxidation occurs, leading to an oxidation speed-up, which results in a thick oxide scale formation with low oxidation protective capacity. Scanning electron microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS), X-Ray Diffraction (XRD), and Raman spectroscopy analysis of the oxide scales at different temperatures revealed that Al2TiO5 grain growth is the main deterioration factor of the protective α-Al2O3 layer. The oxidation kinetics of this composite at 1100 °C is comparable with that of the best MAX phases in terms of oxidation resistance, with a parabolic rate constant (Kp) of 3.08 × 10−10 kg2/m4s.

We reported the fracture behavior of multimodal AA7075-B4C composites with three types of coarse grains (CG, AA7075 powder-1 μm, AA7075 powder-10 μm, and as-cryomilled AA7075 powder) synthesized via cryomilling and plasma activated sintering. Our results indicated that the type (particle size) and weight fraction could affect the distribution of CGs, which in turn significantly affect the fracture behavior of multi-modal composites. Multiple voids were found in the vicinity of AA7075 powder-1 μm, leading to a limited plasticity even with a lower yield strength. AA7075 powder-10 μm (25%) could distribute homogeneously in the composites, resulting in a high yield strength (712 ± 24 MPa) with the plasticity of ~7.7%. The localized grain growth occurred during consolidation with addition of as-cryomilled AA7075 powder. Noted that once the continuous distribution of CGs were detected in the composites, the yield strength decrease dramatically. The CGs could enhance the plasticity by retarding the propagation of micro-cracks during deformation.

Using boron and SiC as additives, the pressureless densification of HfB2 powders synthesized by borothermal reduction with water-washing process was studied. The water-washing HfB2 powders showed good sinterability (relative density: 96.2%), due to the small grain size with the in-situ boron impurities, whereas the microstructure of the sintered sample was coarsened with average grain size of 10.19 ± 4.12 μm. After adding 5 wt% boron, the relative density was improved (98.7%) and the grain size was reduced (4.68 ± 1.17 μm). In order to further inhibit the grain growth, enhance densification and mechanical properties, 20 vol% SiC was added. Finally, fully dense HfB2-based ceramics (relative density: 99.9%) with small grain size of 3.45 ± 0.88 μm were fabricated by the combination of boron and SiC. The Vickers’ hardness and fracture toughness of HfB2 with additions of boron and SiC were 18.27 ± 1.95 GPa and 3.33 ± 0.16 MPa m1/2, respectively.

The morphology and orientation evolution of Cu6Sn5 grains formed on (001)Cu and (011)Cu single crystal substrates under temperature gradient (TG) were investigated. The initial orientated prism-type Cu6Sn5 grains transformed to non-orientated scallop-type after isothermal reflow. However, the Cu6Sn5 grains with strong texture were revealed on cold end single crystal Cu substrates by imposing TG. The Cu6Sn5 grains on (001)Cu grew along their c-axis parallel to the substrate and finally merged into one grain to form a fully IMC joint, while those on (011)Cu presented a strong texture and merged into a few dominant Cu6Sn5 grains showing about 30° angle with the substrate. The merging between neighboring Cu6Sn5 grain pair was attributed to the rapid grain growth and grain boundary migration. Accordingly, a model was put forward to describe the merging process. The different morphology and orientation evolutions of the Cu6Sn5 grains on single crystal and polycrystal Cu substrates were revealed based on crystallographic relationship and Cu flux. The method for controlling the morphology and orientation of Cu6Sn5 grains is really benefitial to solve the reliability problems caused by anisotropy in 3D packaging.

High-performance ZnO-V2O5 based varistors doped with 0.1 mol.% In2O3 have been prepared via conventional sintering method at 850–925 °C. Experimental results show that indium doping helps to improve the nonlinear exponent (α), breakdown field (Eb) of the developed varistors. Added In2O3 restricts the grain growth in the ZnO ceramics, which in turn, causes an increase in the breakdown potential of the varistor samples. High nonlinearity (α > 100) has been obtained consistently in the developed varistors at all the sintering temperatures (850–900 °C). Doped In3+ ions appear as a donor at the grain boundaries, leading to an increase in the Schottky barrier height. The comparatively high leakage current density in ZnO–V2O5 based varistors (usually >100 μA cm−2) could be restricted within 62–83 μA cm−2 in these samples. The sample sintered at 875 °C resulted in a dense (98% TD) fine-grained (1.1 ± 0.1 μm) varistor showing optimum electrical properties with Eb value of 14.2 kV cm−1, α value of 153, and JL equals to 62 μA cm−2.

The sintering behavior of 0.36BiScO3-0.64PbTiO3 (0.36BS-0.64 PT) ceramics was studied to investigate the effect of grain growth by the sillenite Bi12PbO19 (BP) phase on their piezoelectric properties for application in high-temperature piezoelectric devices. The BP phase formed during calcination at temperatures