Nanoscale Grain Research Articles

Transforming Nanocrystals into Superhard Boron Carbide Nanostructures.

Boron carbide (B4+δC) possesses a large potential as a structural material owing to its lightness, refractory character, and outstanding mechanical properties. However, its large-scale industrialization is set back by its tendency to amorphize when subjected to an external stress. In the present work, we design a path toward nanostructured boron carbide with greatly enhanced hardness and resistance to amorphization. The reaction pathway consists of triggering an isomorphic transformation of covalent nanocrystals of Na1-xB5-xC1+x (x = 0.18) produced in molten salts. The resulting 10 nm B4.1C nanocrystals exhibit a 4-fold decrease of size compared to previous works. Solid-state 11B and 13C NMR coupled to density functional theory (DFT) reveal that the boron carbide nanocrystals are made of a complex mixture of atomic configurations, which are located at the covalent structural chains between B11C icosahedral building units. These nanocrystals are combined with a spark plasma-sintering-derived method operated at high pressure. This yields full densification while maintaining the particle size. The nanoscaled grains and high density of grain boundaries provide the resulting nanostructured bodies with significantly enhanced hardness and resistance to amorphization, thus delivering a superhard material.

Cross-scale early damage mechanism of VHCF based on the initiation and evolution of fine granular area in selective laser melting Ti-6Al-4 V alloy

Due to the initiation of fine granular area inside the material and the formation of nanoscale grains, it is difficult to conduct in-situ observation and high-scale characterization. This is the main reason why the formation mechanism of fine granular area in very high cycle fatigue has been unknown and controversial. Therefore, on the basis of fracture analysis method to invert the fine granular area formation, we further put forward an experimental proposal whether a microcrack in the fine granular area formation stage can be prepared to observe the critical event of early damage evolution. Here, we selected selective laser melting Ti-6Al-4 V alloy with inherent defects as the model material to obtain significant defect-initiating fine granular areas, found two secondary microcracks after dissecting along the defects, and then carried out multiscale characterization and quantitative analysis of main cracks and secondary cracks. We found that the fine grains originate from the severe plastic deformation in local of crack tip plastic zone and lowered the cracking threshold by the grain boundary sliding between the hard-oriented grains, which results in grain refinement, cavitation and cracking. This work systematically describes damage evolution mechanism, which has guiding significance for the reliability evaluation and fatigue resistance design of materials.

Achieving high energy storage density and charge-discharge performance via high-energy ball milling combined with two-step sintering

In this study, the microstructure, ferroelectricity, energy storage density, and charge-discharge characteristics of 0.95(K0.5Na0.5)NbO3-0.05Ba(Zn1/3Nb2/3) (0.95KNN-0.05BZN) ceramic, fabricated by combining two-step sintering with high-energy ball milling, were investigated. The two-step sintering technique enabled a wide sintering temperature range of 1020–1120 °C for the ceramic. Accordingly, under optimal sintering conditions, the ceramic exhibited high relative density (98.1 %) and nanoscale grain size (<90 nm). The samples also displayed excellent pulse power performance at room temperature with a high recoverable energy storage density (Wrec) of 3.1 J/cm3, along with the following charge-discharge parameters: current density (CD), power density (PD), and discharge time (t0.9) reached 1821.7 cm−3, 227.7 MW/cm3, and 36.8 ns, respectively. The ceramic also obtained robust frequency stability (10–1000 Hz), good temperature stability (30–130 °C), and outstanding fatigue resistance (105 cycles), indicating its potential application in improved pulse capacitor materials.

Novel biphasic high-entropy ceramic aerogels and their fiber composites with low thermal conductivity, high thermal stability and significant thermal insulation property

A novel biphasic high-entropy (Y0.2Ho0.2Tm0.2Yb0.2Lu0.2)2SiO5/(Y0.2Ho0.2Tm0.2Yb0.2Lu0.2)2Zr2O7 ceramic aerogel (BP-HEZSA) was prepared using formamide/sol-gel, supercritical CO2 drying and combined with a heat treatment process. The results indicate that BP-HEZSA exhibits a biphasic high-entropy structure at 1200 °C, and it maintained its stability at 1400 °C while demonstrating a homogeneous distribution of elements. BP-HEZSA exhibited nanoscale fine grains (with a size range of 18.82 nm to 27.62 nm), high BET specific surface area (16.24 m2·g−1), and good pore structure, all of which were controlled by the thermal treatment temperature. BP-HEZSA had low room temperature thermal conductivity and high structural and phase thermal stability at high temperatures. Furthermore, the applicability of the aerogel was improved by compounding with aluminosilicate fibers. The composites exhibited low density and low thermal conductivities (0.04–0.14 W·m−1K−1). Butane blowtorch testing confirmed superior thermal insulation in the composites, meeting demands for high-temperature resistance in the advanced nanostructured materials.

Energy storage properties, transmittance and hardness of Er doped K0.5Na0.5NbO3-based ceramics

Transparent energy storage ceramics can balance energy storage characteristic and optical characteristic, and are expected to be used in areas such as transparent pulse capacitors. However, excellent energy storage performance and dramatic light transmittance are difficult to achieve simultaneously, limiting their subsequent development in the actual applications. The paper proposes a way to improve the transparent energy storage properties of KNN-based samples by regulating domain and grain size. By introducing an appropriate amount of Er2O3, fine cube grains and nanodomains are constructed. When doping 0.20 mol% Er2O3, the ceramics exhibited excellent recoverable energy storage density Wrec ∼ 6.2 J/cm3, superior energy-storage efficiency η ∼ 71.3 %, large dielectric breakdown strength Eb ∼ 670 kV/cm, ultrahigh hardness value of 6.9 GPa, and a maximum transmittance T ∼72 % at 880 nm. Dense microstructure, nanoscale grains, symmetrical lattice structure and relaxor behavior are the main reasons for obtaining high energy storage and transparency properties.

Effect of bias voltage on the structure and properties of CoCrFeNi high entropy alloy thin films prepared by magnetron sputtering

In this study, CoCrFeNi high entropy alloy thin films were prepared on 304 stainless steel and single crystal Si substrates using the direct-current magnetron sputtering process under different bias voltages. The effects of these bias voltages on the film micrographs, phase structure, surface roughness, corrosion resistance, and mechanical characteristics of thin films were studied by a scanning electron microscope, transmission electron microscope, X-ray diffractometer, atomic force microscope, electrochemical workstation and nanoindentation tester, respectively. The results indicated that all films exhibited equiaxed nano-scale grains and developed a columnar structure. Fe, Co, Cr, and Ni were uniformly dispersed in all films and all films displayed face centered cubic solid solution (FCC). At a bias voltage of 0 V, the CoCrFeNi HEAs film formed a FCC single phase. With increasing bias voltage, the crystallinity decreased and then increased, reaching the worst crystalline state at bias voltage of −200 V due to the fact that the structure of the film was composed of FCC nanocrystalline and amorphous biphasic structure. Meanwhile, the surfaces of the film prepared at bias voltage of −200 V exhibited the densest structures and the least roughness, with grain sizes reaching a minimum value of approximately 10 nm. In addition, columnar grains of the film prepared at a bias voltage of −200 V displayed the densest state and had the least size, thus this film displayed the best corrosion resistance of a minimum value 1.51 × 10−7 A/cm2, a maximum hardness of approximately 11.18 ± 0.2 GPa and a maximum elastic modulus of approximately 193 ± 2.8 GPa. The passivation films of CoCrFeNi HEAs films were mainly composed of Cr2O3.

Simultaneously enhanced toughness and hardness of nanocrystalline SiC sintered under high pressure

The inherent brittleness of silicon carbide (SiC) poses a significant challenge in concurrently enhancing its fracture toughness and hardness. While nanostructuring has effectively increased SiC hardness through the Hall−Petch effect, its impact on improving fracture toughness remains less explored. Consequently, there is an urgent need for strategies that can simultaneously enhance both hardness and fracture toughness. This study investigates the effects of microstructural modulation on nanocrystalline SiC bulks sintered under high pressure high temperature. By employing multidimensional modifications encompassing grain size, grain boundary, and lattice defect density, we successfully improved the mechanical properties of SiC ceramics. Notably, SiC bulks sintered at 25 GPa and 1500 °C, characterized by nanoscale grains, strong intergranular bonding, and abundant lattice defects within individual grains, exhibited an impressive combination of high hardness (32.2−33.3 GPa) and remarkable fracture toughness (4.8−4.9 MPa·m0.5). Detailed transmission electron microscopy analysis revealed multiple toughening mechanisms intimately linked to the microstructure.

Synthesis of an Ultra-high Hardness Nanostructured AlSi10Mg Alloy via A Hybrid Laser Powder Bed Fusion/High-Pressure Torsion Approach

The hybrid combination of laser powder bed fusion (L-PBF) and high-pressure torsion, respectively an additive manufacturing (AM) and severe plastic deformation (SPD) has recently emerged as an important field of study due to the ability to produce novel nanostructured metals/alloys with simultaneous enhancements in mechanical (hardness, yield and tensile strengths) and functional (corrosion and wear) performances. Pioneering studies on the hybrid L-PBF/HPT approach was focused on 316L stainless steel (316L SS) and AlCuMg alloys due to their widespread engineering applications. AlSi10Mg, an alloy widely used in the automotive industry is expected to benefit from this hybrid combination but has not been investigated before. Thus, this study aims to investigate the microstructural features and hardness of the nanostructured AlSi10Mg attained by the hybrid L-PBF/HPT technique via transmission electron microscopy (TEM) and Vickers microhardness (HV) measurements, respectively. The results showed novel microstructures, including nano-scale grain sizes (&lt; 200 nm), nano-sized Mg2Si precipitates (~200 nm), and dense dislocation networks. The average hardness value was determined as 230 ± 20 HV, homogeneously distributed throughout the disk surface as indicated by the low deviation (error bar). The microstructures of L-PBF/HPT-synthesised AlSi10Mg are different and significantly refined than conventionally cast AlSi10Mg, which contributed to the two- to four-fold hardness (typical HV values of as-cast AlSi10Mg ranges from 60 – 68 HV) increase via grain boundary and dislocation strengthening mechanisms.

Nanostructured Titanium with Ultrafine‐Grained Structure as Advanced Engineering Material for Biomedical Application

Titanium and its alloys are popular materials for medical application, particularly in implant devices, where high mechanical properties and osseointegration are critical factors for successful implantation. In this work, the progress in the studies of nanostructured commercially pure Grade 4 titanium (nanoTi) is demonstrated, in which an ultrafine‐grained structure with nanoscale grain size is formed using severe plastic deformation processing. Nanostructured Grade 4 Ti has a very high strength, and its physical nature and strengthening mechanisms are analyzed herein. NanoTi proved also to have very high osseointegration during in vivo experiments. At the same time, the highest biofunctionality is demonstrated by the etched nanoTi samples with pronounced surface roughness, the latter being revealed from precise roughness measurements. The present study provided convincing evidence of accelerated bone formation on nanoTi, which is very promising for manufacture of dental and maxillofacial implants.

Oxide precipitation strengthening in Cu alloy modified with trace amounts of Nd during dynamic impact

In this study, three kinds of Cu-Nd alloys were prepared. The effect of Nd content on the dynamic impact behavior of the Cu-Nd alloys and its strengthening mechanism were studied. The results show that during the dynamic impact process, nano-scale oxide particles are precipitated in the alloy matrix, and a large number of cracks are formed along the grain boundaries, and the formation of shear bands and thermal grooves are also observed in local areas. With the increase of Nd content, the compressive strength of the alloys increases significantly, the grain size is refined, the number of twins increases, the grooves are much deeper and the number of the precipitated nano-oxide particles also increase significantly. In addition, due to the refinement of the grain size, the crack length is reduced. The increase of the alloy strength is mainly attributed to the nano-scale oxide precipitation strengthening and grain refinement strengthening.

An ultra high-endurance memristor using back-end-of-line amorphous SiC

Integrating resistive memory or neuromorphic memristors into mainstream silicon technology can be substantially facilitated if the memories are built in the back-end-of-line (BEOL) and stacked directly above the logic circuitries. Here we report a promising memristor employing a plasma-enhanced chemical vapour deposition (PECVD) bilayer of amorphous SiC/Si as device layer and Cu as an active electrode. Its endurance exceeds one billion cycles with an ON/OFF ratio of ca. two orders of magnitude. Resistance drift is observed in the first 200 million cycles, after which the devices settle with a coefficient of variation of ca. 10% for both the low and high resistance states. Ohmic conduction in the low resistance state is attributed to the formation of Cu conductive filaments inside the bilayer structure, where the nanoscale grain boundaries in the Si layer provide the pre-defined pathway for Cu ion migration. Rupture of the conductive filament leads to current conduction dominated by reverse bias Schottky emission. Multistate switching is achieved by precisely controlling the pulse conditions for potential neuromorphic computing applications. The PECVD deposition method employed here has been frequently used to deposit typical BEOL SiOC low-k interlayer dielectrics. This makes it a unique memristor system with great potential for integration.

Overview of corrosion behaviour of ceramic materials in molten salt environments

This study reviewed previous studies between the years 2015 and 2021 on how ceramic materials degraded in the presence of molten salt environments. The processes of corrosion resistance of various ceramic compositions subjected to various molten salt compositions and temperatures were also scrutinized. The results offer important new insights into the variables affecting ceramics' corrosion behaviour and the production of corrosion products. The reported result reveals that the ceramic material with the composition (Sm0.5Sc0.5)2Zr2O7 performed better than that of Sm2Zr2O7 in terms of hot corrosion resistance in molten salt (V2O5 + Na2SO4). It has also been reported that corrosion behaviour is influenced by particle size. Notably, zirconia (n-YSZ) with nanoscale grain sizes was more susceptible to hot corrosion, which was explained by increased specific surface areas. On the other hand, sintering and additives have been found to enhance corrosion resistance. The Y-Y2Si2O7 ceramic's resistance to corrosion in (V2O5 + Na2SO4) molten salt was enhanced by the addition of alumina. The results of these investigations help us understand how corrosion works and what influences ceramic materials' susceptibility to deterioration in molten salt media. This information can direct the creation of more corrosive-resistant ceramic materials for use in high-temperature environments or molten salt-based energy systems, among other corrosive uses.

Evaluation of microstructure and mechanical characteristics of 2024 aluminum alloys with μ-Al2O3 additives produced via hydrostatic cyclic expansion extrusion with back-pressure

Grain refinement techniques are commonly used to produce enhanced materials with micro- and nanoscale grain sizes. This study investigates the severe plastic deformation of 2024 aluminum alloys reinforced with microparticles of alumina (Al2O3) using a new method based on hydrostatic cyclic expansion extrusion. Cuboid samples of aluminum 2024 were subjected to severe deformation at 20 °C through the pressing of a steel punch from the top and the application of back-pressure to the bottom part of the workpiece after the addition of the reinforcing powder. The mold design was created to facilitate both modes of operation. The experimental results were used to examine deformation behavior and flow patterns in the samples. Furthermore, the mechanical properties and microstructure of the processed samples were analyzed to determine the effects of the applied strain.

Effect of the sintering parameters on the structure and mechanical properties of zirconia-based ceramics

The paper presents the results of an investigations of the sintered zirconia ceramics that have been stabilized with Y2O3 and CeO2. The initial powders were synthesized via decomposition of the fluoride salts, which determined morphological features and dimensions of the particles. The specific electroconsolidation process, performed using the modified spark plasma sintering device, allowed for the retention of the nanoscale grain sizes and related properties of the sintered ceramic composites. It was found that the as-obtained materials with cerium oxide exhibited high bending strength of 609 MPa, by ca. 33 % higher than that of yttria-stabilized ones (410 MPa). In turn, the best combination of hardness and fracture toughness, K1С = 5.8 МPа·m1/2 аnd Нv = 14.8 GPа, respectively, exhibited ZrO2+3 wt% Y2O3. This result can be attributed to the chemical composition and morphology of the powders, which in turn is influenced by the synthesis conditions and calcination time and temperatures, as well as to the sintering parameters. In particular, yttria-stabilized zirconia showed higher sensitivity to the variations of the sintering temperatures and holding times.

Effect of gradient nanostructures on the damage mechanism of aermet100 steel at various strain rates

The introduction of nano-scale grains, which form a gradient nanostructure (GNS) within the material, usually increases the service life of metallic materials. In this study, the uniaxial tensile mechanical properties of AerMet100 steel in initial and GNS states under different strain rate conditions were compared, and the microanalysis of tensile fracture was performed. It is found that GNS has little effect on the mechanical properties of AerMet100 but has an effect on its damage mode. The initial AerMet100 was basically ductile fracture, and GNS increased the brittle fracture mode in AerMet100.

Effect of dopant loading and calcination conditions on structural and optical properties of ZrO2 nanopowders doped with copper and yttrium

Undoped, Cu and/or Y doped ZrO2 nanopowders were synthesized with Zr, Y, and Cu nitrates using a co-precipitation approach. Their structural and optical properties were examined regarding dopant content (0.1–8.0 mol.% of CuO and 3–15 mol.% of Y2O3) and calcination conditions (400 °C–1000 °C and, 1,2 or 5 h) through Raman scattering, XRD, TEM, EDS, AES, EPR, UV–vis and FTIR diffused reflectance methods. The results showed that both Cu and Y dopants promoted the appearance of additional oxygen vacancies in ZrO2 host, while the formation of tetragonal and cubic ZrO2 phases was primarily influenced by the Y content, regardless of Cu loading. The bandgap of most of the powders was observed within the 5.45–5.65 eV spectral range, while for those with high Y content it exceeded 5.8 eV. The (Cu,Y)-ZrO2 powders with 0.2 mol.% CuO and 3 mol.% Y2O3 calcined at 600 °C for 2 h demonstrated nanoscaled tetragonal grains (8–12 nm) and a significant surface area covered with dispersed CuxO species. For higher calcination temperatures, the formation of CuZr 2+ EPR centers, accompanied by tetragonal-to-monoclinic phase transformation, was found. For fitting of experimental FTIR reflection spectra, theoretical models with one, five, and seven oscillators were constructed for cubic, tetragonal, and monoclinic ZrO2 phases, respectively. Comparing experimental and theoretical spectra, the parameters of various phonons were determined. It was found that the distinct position of the high-frequency FTIR reflection minimum is a unique feature for each crystalline phase. It was centered at 700–720 cm−1, 790–800 cm−1, and 820–840 cm−1 for cubic, tetragonal, and monoclinic phases, respectively, showing minimal dependence on phonon damping coefficients. Based on the complementary nature of results obtained from structural and optical methods, an approach for monitoring powder properties and predicting catalytic activity can be proposed for ZrO2–based nanopowders.

Approaching 1 GPa Ultra‐High Tensile Strength in a Nanostructured Al–Zn–Mg–Cu–Zr–Sc Alloy Prepared by Severe Plastic Deformation

Alloy composition and heat treatment processes have limited possibility to enhance ultra‐high strength of aluminum alloys, which restricts their widespread application in lightweight equipment. Consequently, high‐density dislocations and grain refinement are suggested to strengthen ultra‐high strength aluminum alloys. Herein, a novel nanostructured Al–Zn–Mg–Cu–Zr–Sc (AZMCZS) alloy with homogeneous microstructure is prepared through the synergistic processing of hot extrusion and high‐pressure torsion. Additionally, the microstructures and strengthening mechanisms of the nanostructured Al alloy are analyzed. It is observed that the ultimate tensile strength of the nanostructured Al alloy reaches nearly 1 GPa, and the elongation of the alloy is 1.9%. The nanostructured Al alloy mainly consists of nanoscale grains (≈117.7 nm), high‐density dislocations (2.4 × 1015 m−2), nano‐sized precipitates (the size of 20–51 nm), and solute atom clusters (≈3 nm). The multiple strengthening mechanisms of the nanostructured Al alloy are revealed in terms of grain refinement, dislocations, precipitates, and solute atom clusters. Grain refinement and dislocation strengthening show superior outcomes and are considered to be the predominant strengthening mechanisms. These findings demonstrate that this nanostructural architecture offers a new way to design super‐strength metals and alloys by effectively controlling the processing regime of severe plastic deformation.

Optimizing the cell compatibility and mechanical properties in TiZrNbTa medium-entropy alloy/β-Ti composites through phase transformation

Medium-entropy alloys (MEAs) typically exhibit outstanding mechanical properties, but their high Young's modulus results in restricted clinical applications. Mismatched Young's modulus between implant materials and human bones can lead to “stress shielding” effects, leading to implant failure. In contrast, β-Ti alloys demonstrate a lower Young's modulus compared to MEAs, albeit with lower strength. In the present study, based on the bimodal grain size distribution (BGSD) strategy, a series of high-performance TiZrNbTa/Ti composites are obtained by combining TiZrNbTa MEA powders with nano-scale grain sizes and commercially pure Ti (CP-Ti) powders with micro-scale grain sizes. Concurrently, Zr, Nb, and Ta that are β-Ti stabilizer elements diffuse into Ti, inducing an isomorphous transformation in Ti from the high Young's modulus α-Ti phase to the low Young's modulus β-Ti phase at room temperature, optimizing the mechanical biocompatibility. The TiZrNbTa/β-Ti composite demonstrates a yield strength of 1490 ± 83 MPa, ductility of 20.7 % ± 2.9 %, and Young's modulus of 87.6 ± 1.6 GPa. Notably, the yield strength of the TiZrNbTa/β-Ti composite surpasses that of sintered CP-Ti by 2.6-fold, and its ductility outperforms TiZrNbTa MEA by 2.3-fold. The Young's modulus of the TiZrNbTa/β-Ti composite is reduced by 28 % and 36 % compared to sintered CP-Ti and TiZrNbTa MEA, respectively. Additionally, it demonstrates superior biocompatibility compared to CP-Ti plate, sintered CP-Ti, and TiZrNbTa MEA. With a good combination of mechanical properties and biocompatibility, the TiZrNbTa/β-Ti composite exhibits significant potential for clinical applications as metallic biomaterials. Statement of significanceThis work combines TiZrNbTa MEA with nano-grains and commercially pure Ti with micro-grains to fabricate a TiZrNbTa/β-Ti composite with bimodal grain-size, which achieves a yield strength of 1490 ± 83 MPa and a ductility of 20.7 % ± 2.9 %. Adhering to the ISO 10993-5 standard, the TiZrNbTa/β-Ti composite qualifies as a non-cytotoxic material, achieving a Class 0 cytotoxicity rating and demonstrating outstanding biocompatibility akin to commercially pure Ti. Drawing on element diffusion, Zr, Nb, and Ta serve not only as solvent atoms to achieve solid-solution strengthening but also as stabilizers for the transformation of the β-Ti crystal structure. This work offers a novel avenue for designing advanced biomedical Ti alloys with elevated strength and plasticity alongside a reduced Young's modulus.

Room-Temperature Synthesis and Low Thermal Conductivity in Nanocrystalline Ag3CuS2.

Noble-metal-based chalcogenide materials recently gained massive attention in the field of thermoelectrics. In most cases, materials are synthesized using (i) high-temperature solid-state reactions or (ii) soft chemical methods where temperature requirements are lower than those of solid-state reactions (generally below 400 °C). Herein, we present a simple, surfactant-free, room-temperature, and energy-efficient synthesis of Ag3CuS2 nanocrystals. The present synthesis technique is scalable and capable of gram-scale production. A spark plasma sintering (SPS) pressed sample exhibits ultralow thermal conductivity (∼0.31 W/mK at room temperature). We found that Ag3CuS2 exhibits low sound velocity, as well as a non-Debye-like behavior based on a low-temperature heat capacity measurement. A high degree of anharmonicity of bonding, soft vibrations modes, and nanoscale grain boundary scattering in Ag3CuS2 lead to ultralow thermal conductivity, which can be important for thermoelectrics, optoelectronics, and thermal barrier coating applications.

Microstructural evolution and its influence on the wear resistance of a laser directed energy deposited Ni-based single crystal superalloy

The wear behavior of Ni-based single crystal (NBSC) superalloy SRR99 fabricated by laser-directed energy deposition (LDED) is investigated and compared with that of its cast counterpart. While γ’ precipitate size in the latter is > 400 nm, that in the former is an order of magnitude lower. Dry sliding wear tests reveal that the wear rate and coefficient of friction of the LDED alloy are 75 % and 20 % lower than that of its cast counterpart, respectively. Detailed transmission electron microscopy investigation of the wear-tested cast alloy indicates that there is orientation change and formation of nanoscale grains only at the top layer of the worn surface, whereas regions below undergo moderate plastic deformation via dislocation slip. In contrast, the sub-surface of the worn LDED alloy has a graded microstructure, with a composite of NiO/γ-Ni on the top, γ’ free nano-grains in the middle, and a highly deformed nanoscale layer at the bottom. The improved wear behavior of the LDED alloy is attributed to its higher dislocation density, finer γ’ precipitates, and the formation of this graded microstructure. Finally, a detailed description of mechanisms that lead to the formation of this unique graded microstructure is provided.