Multi-phase Nanocrystalline Research Articles

Ultra-high hard and fracture-resistant multi-phase nanocrystalline AlCrFeMoNbNi multi-principal element alloy

Multi-phase nanocrystalline AlCrFeMoNbNi multi-principal element alloy was developed using mechanical alloying followed by spark plasma sintering. Mechanical alloying resulted in a two-phase microstructure (BCC and FCC), whereas subsequent sintering resulted in a multi-phase microstructure consisting of FCC, Rhombohedral and B2 phases. Extremely high hardness values in the range of 35.79–16.05 GPa were measured as a function of indentation load between 25 and 5000 g. A highly pronounced indentation size effect was observed. From classic Nix-Gao analysis, a load independent hardness of 13.82 GPa was estimated. The high hardness arises from a combination of finer grain size and solid solution strengthening (FCC phase) and multi-phase structure consisting of B2 and Rhombohedral phases. The major phase being FCC (83%) offers nearly 85% of the total flow stress realized for this MPEA and the remaining 15% of the flow stress must be arising from both the B2 and Rhombohedral phases. It is to be noted that the phase fractions of B2 and Rhombohedral phases are 10% and 7% respectively. Indentation fracture toughness, evaluated using the cracks developed during Vickers indentation, yielded encouraging numbers in the range 12.28–13.85 MPam. The excellent combination of ultra-high hardness and indentation fracture toughness of this AlCrFeMoNbNi MPEA surpasses the reported data on several related harder materials, including silicides, nitrides, carbides, etc.

Interaction of Si Atom with the (001) Surface of TiN, AlN and TaN Compounds

Nowadays, the application of multicomponent coatings with multiphase nanocrystalline structure is the most promising direction in the search for wear-resistant protective coatings with a full set of necessary operational properties. Nanocrystalline multicomponent coatings based on the Ti-Al-Ta-Si-N system have a high hardness combined with thermal stability and oxidation resistance. Silicon atoms are weakly soluble in the TiN, Ti1−xAlxN, and TaN crystalline phases of the Ti-Al-Ta-Si-N system and interact preferentially with N atoms, forming the amorphous Si3N4 phase. In this context, it is important to first study the peculiarities of the interaction of Si atoms with the simplest structural units of the Ti-Al-Ta-Si-N system, such as TiN, AlN, and TaN compounds with the NaCl structure. This work is devoted to the study of the interaction of a Si atom with the (001) surface of AlN, TiN, and TaN compounds with the NaCl structure using ab initio calculations. This provides information for a deep understanding of the initial stages of the formation of different crystallites of the considered composite. It was established that the adsorption of silicon on the (001) surface of AlN, TiN, and TaN significantly increases the relaxation of the surface layers and leads to an increase in the corrugation observed on the clean surfaces. The largest corrugation is observed on the surface of the TaN compound. The most energetically favorable adsorption positions of Si atoms were found to be the position of Si above the N atom on the TiN and TaN surfaces and the quadruple coordinated position on the AlN surface. The valence electron density distribution and the crystal orbital Hamiltonian population were studied to identify the type of Si atom bonding with the (001) surface of AlN, TiN, and TaN compounds. It was found that silicon forms predominantly covalent bonds with the nearest metal and nitrogen atoms, except for the quadruple coordinated position on the surface of TiN and TaN, where there is a high degree of ionic bonding of silicon with surface atoms.

Extraordinary superplasticity at low homologous temperature and high strain rate enabled by a multiphase nanocrystalline network

Superplasticity is a highly sought-after property of components manufactured with complex geometries in metal forming processes. However, superplasticity usually occurs at high temperatures and/or low strain rates, which entails high energy consumption, long processing time, and severe surface oxidation. Herein, we have developed a multiphase nanocrystalline network (MPNN) in a Ti6Al4V5Cu model alloy, where the grain boundary β phases promote the sliding and rotation of ultrafine α grains, while the nanosized Ti2Cu particles pin down the α/β boundaries to maintain the thermostability of the nanostructure. Results show that the onset temperature for superplasticity of the model alloy is 250 °C lower than that of the Ti6Al4V alloy at the strain rate of 10−4s − 1. Remarkably, superplasticity was also observed at an extremely high strain rate of 1 s − 1 at 750 °C, which is 2–4 orders of magnitude larger than conventional superplastic metals. The present work is of great significance in developing more economical and efficient superplastic deformation processes.

Inoculation mechanism and strengthening effect of V-based multi-element multiphase inoculant on high chromium cast iron

High chromium cast iron (HCCI) has been widely used as an antiwear material in the industry. Adding inoculants is an effective way to improve the microstructure and mechanical properties of HCCI. In this paper, a new multi-element V-Fe-Ti-Nb-C-Zr-B ribbon inoculant was prepared by the melt-spinning method, and the effect of inoculant addition on the microstructure and mechanical properties of HCCI was studied. The results show that the inoculant consists of V(Fe, M) matrix, V0.89Ti0.11C0.50-, V2C-, VC1−x-, and V3B2-type multiphase nanocrystalline. Adding inoculant to molten HCCI alters the morphology and size of M7C3 carbide, and the possible inoculation mechanism to the as-cast alloy is: the V(Fe, M) matrix melts first, and then the exposed nanograins are dispersed in the melt; The atoms on the particle surface diffuse into the melt, reducing the nanoparticle size; During pouring, the primary M7C3 carbide, austenite, and the eutectic M7C3 carbide nucleate and grow with these nanoparticles as the cores. After inoculation, the grain size of the as-cast and heat-treated HCCI alloy decreased by 39.8 % and 69.1 %, respectively. The combined effect of solution strengthening and fine grain strengthening increased the hardness of as-cast and heat-treated HCCI alloy by 4.1 % and 6.9 %, respectively. Finally, the impact toughness of the heat-treated HCCI alloy increased by 26.8 %, the friction coefficient decreased by 13.5 %, and the wear loss decreased by 21.1 %.

Temperature-dependent mechanical properties and elastocaloric effects of multiphase nanocrystalline NiTi alloys

We investigate the temperature-dependent mechanical properties and elastocaloric effects of different multiphase nanocrystalline NiTi alloys with grain sizes of 10 nm, 15 nm, 20 nm, 27 nm, 34 nm and 55 nm. These multiphase nanocrystalline NiTi alloys mainly consist of B2, B19′ and R phases and can undergo the thermally induced phase transition over a wide temperature range from 208 K to 353 K. Their superelastic deformation (loading to a strain of 4%) is contributed by combinations of the elastic deformation of B2 phase, the R → B2 phase transition, the reversible martensite reorientation and the B2 → R and B2/R → B19' phase transitions. Such complex deformation mechanism together with the microstructure evolution with temperature brings strong temperature-dependences of the mechanical properties and elastocaloric effects. As results, at the grain sizes of 15 nm ∼ 20 nm, the stress under a constant superelastic strain can show a significant V-shaped variation with temperature. At the grain size of 55 nm, the adiabatic temperature drop can even change from −8.5 K to −29 K ∼ −35.8 K as the ambient temperature rises from 293 K to 333 K. By comparison of the mechanical properties and elastocaloric effects of the different multiphase nanocrystalline NiTi alloys, it is found that the one with the grain size of 20 nm exhibits the best comprehensive cooling performances, which has a soft near-linear superelasticity (a moderate driving force), a very wide refrigeration temperature window (> 245 K) with considerable adiabatic temperature drops (∆Tmax= −17.1 K), and a good functional stability. This work provides an experimental basis for using the multiphase nanocrystalline NiTi alloys as solid-state refrigerants.

Ultra-hard, multi-phase nanocrystalline MoTa and MoTaTi based concentrated refractory alloys

Binary (MoTa) and ternary (MoTaTi) based concentrated refractory alloys are synthesized using ball milling and spark plasma sintering. A single BCC phase which was observed after high energy ball milling has transformed to multiple phases after sintering. Presence of Ti and Fe (from milling media) appears to trigger the phase transformations. An extremely high hardness in the range 16–10 GPa was measured in the load range 0.25–9.81 N and with a very distinct indentation size effect. Density-normalized hardness was found to be exceptionally high indicating their potential for high strength/density applications. This study also suggests that several elements need not be added in high concentrations to achieve high end mechanical properties.

Crystallization mechanism of Zr55Cu30Al10Ni5 metallic glass in an extended range of heating rates

The crystallization mechanisms of Zr55Cu30Al10Ni5 (Zr55) amorphous alloys from slow continuous heating to rapid laser heating are investigated. The multiphase nanocrystalline structures such as CuZr2-type, NiZr2-type and Al2Zr3-type phases with various compositions are observed at heating rates lower than 50 K/s, while the CuZr2-type/ZrCu-type eutectic dendrites are produced at heating rates higher than 50 K/s. A numerical model is established to calculate the onset crystallization temperatures as a function of the heating rates. This model indicates that in addition to rapid dendritic growth from a certain number of quenched-in nuclei, the non-Arrhenius behavior of high diffusion coefficient in the supercooled liquid region contribute to the serious crystallization during rapid heating process. The activation energy of dendritic crystallization is obviously smaller than that of multiphase nanocrystallization, which is due to a much smaller scale of diffusion distance and larger atomic diffusivity during dendritic growth.

On the origins of ultra-high hardness and strain gradient plasticity in multi-phase nanocrystalline MoNbTaTiW based refractory high-entropy alloy

In this paper, we report a remarkably high hardness in fully-dense nanocrystalline multi-phase MoNbTaTiW based refractory high-entropy alloy which was prepared by ball milling and spark plasma sintering. A single phase BCC structure was realized after 30 h of milling whereas sintering led to the decomposition of the as-milled complex lattice into a BCC and two FCC phases. Vickers microindentation performed at various loads led to the observation of pronounced indentation size effect with hardness in the range 18.87-13.89 GPa. It appears that Ti and Fe (from processing media) are responsible for the multi-phase structure realized as well as extraordinarily high absolute hardness (13.89 GPa at 500 g load) and “density-normalized hardness”. These values are the highest reported so far in the family of MoNbTaW alloys. Comprehensive analysis on strengthening mechanisms responsible for the extraordinarily high hardness observed in this alloy indicates that solid solution strengthening and grain boundary (Hall-Petch) strengthening are the dominant factors while lattice frictional stress and Taylor hardening also contribute to some extent. The strain gradient plasticity appears to follow the considerations based on the concept of geometrically necessary dislocations and the characteristic multi-phase structure of this alloy. The back stress that would develop in order to have compatible deformation during the indentation at low loads is also expected to contribute to higher hardness values observed at low indentation loads. The depth-independent hardness of 11.80 GPa and a characteristic length scale of 1.07 μm were realized from the analysis. Achieving of ultra-high hardness in this alloy suggests that careful microstructural engineering as well as optimal addition of alloying elements would be beneficial towards development of novel structural materials based on multi-principal element approach.

Modelling and Experimental Validation of the Porosity Effect on the Behaviour of Nano-Crystalline Materials

Nano-crystalline metals have attracted considerable attention over the past two decades due to their increased mechanical properties as compared to their microcrystalline counterparts. However, the behaviour of nano-crystalline metals is influenced by imperfections introduced during synthesis or heat treatment. These imperfections include pores, which are mostly located in the area of grain boundaries. To study the behaviour of multiphase nano-crystalline materials, a novel fully parametric algorithm was developed. The data required for implementing the developed numerical model were the volume fraction of the alloying elements and their basic properties as well as the density and the size of randomly distributed pores. To validate the developed algorithm, the alloy composition 75 wt% tungsten and 25 wt% copper was examined experimentally under compression tests. For the investigation, two batches of specimens were used; a batch having a coarse-grained microstructure with an average grain diameter of 150 nm and a nanocrystalline batch having a grain diameter of 100 nm, respectively. The porosity of both batches was derived to range between 9% and 10% based on X-ray diffraction analyses. The results of quasi-static compression testing revealed that the nanocrystalline W-Cu material exhibited brittle behaviour which was characterised by an elastic deformation that led to fracture without remarkable plasticity. A compressive strength of about 1100 MPa was derived which was more than double compared to conventional W-Cu samples. Finite element simulations of the behaviour of porous nano-crystalline materials were performed and compared with the respective experimental compression tests. The numerical model and experimental observations were in good agreement.

Multi-layered gradient (Zr,Nb)N coatings deposited by the vacuum-arc method

Multi-layered gradient (Zr,Nb)N coatings were deposited with vacuum-arc evaporation of pure Zr and Nb metal cathodes. The gradient of the Nb concentration through the thickness was obtained by varying the arc discharge current with a Nb cathode within 80–150 A under the constant pressure of a working Ar/N2 gas mixture and a constant current of arc discharge with a Zr cathode. These coatings were studied by scanning and transmission electron microscopes, X-ray diffraction analysis, micro- and nanoindentation, and tribological measurements. The synthesized coatings show a high hardness, relatively low Young's modulus, relatively high H/E ratio, a high degree of elastic recovery, low surface roughness, a low friction coefficient, and a low wear rate. These coatings also feature a multiphase and multi-layered nanocrystalline structure with alternation of only layers with higher and lower Nb concentration.

Breakdown performance of transformer oil in the presence of single‐phase nanocrystalline ZnO and nano‐partial substitution

In this study, newly structured nanoparticles are synthesised and incorporated in the transformer oil for enhancing the dielectric properties. A new nanocrystalline series of zinc oxide (ZnO) and ZnO partially substituted by lanthanum oxide is presented for the first time, where the precursor technique is utilised to create the nano-powders. Different partial substitutions facilitate to manipulate nanocrystalline sizes, and therefore the effect on nanofluid dielectric characteristics is attained. Another factor during the laboratory nanoparticles preparation is considered that the precursor is annealed at different temperatures. The nanocrystalline phase compositions are experimentally examined and evaluated using X-ray diffraction and transmission electron microscope. Accordingly, single-phase and multi-phase nanocrystalline powders are ascertained. The transformer oil incorporated with nanoparticles is prepared and its dielectric strength is measured using an oil breakdown voltage tester. The measured dielectric strength of transformer oil with and without nanoparticles is statistically evaluated using Weibull distribution. The most appropriate nanofluid and concentration are estimated experimentally. Toward interpreting the nanofluid dielectric enhancement, their electric field distribution is numerically evaluated at different nanoparticle concentrations using COMSOL multiphysics. This study supports the analysis and optimisation of transformer oil dielectric performance.

Formation of the oxide coating on the titanium surface by multipulse femtosecond laser irradiation

The effect of the femtosecond laser irradiation on the formation of oxide layers on the surface of a commercially pure titanium VT1-0 was studied. The methods of X-ray analysis, scanning electron and transmission electron microscopies were used to study the structural and phase state of oxide layers. As a result of the femtosecond laser irradiation, the porous multi-phase nanocrystalline oxide coating with a thickness of 50 μm is formed on the titanium surface. The coating consists of titanium oxides: TiO2 (rutile and anatase), TiO and Ti3O5.

Propagation of waves in nonlocal porous multi-phase nanocrystalline nanobeams under longitudinal magnetic field

ABSTRACTThis article investigates wave propagation behavior of a multi-phase nanocrystalline nanobeam subjected to a longitudinal magnetic field in the framework of nonlocal couple stress and surface elasticity theories. In this model, the essential measures to describe the real material structure of nanocrystalline nanobeams and the size effects were incorporated. This non-classical nanobeam model contains couple stress effect to capture grains micro-rotations. Moreover, the nonlocal elasticity theory is employed to study the nonlocal and long-range interactions between the particles. The present model can degenerate into the classical model if the nonlocal parameter, couple stress and surface effects are omitted. Hamilton’s principle is employed to derive the governing equations which are solved by applying an analytical method. The frequencies are compared with those of nonlocal and couple stress-based beams. It is showed that wave frequencies and phase velocities of a nanocrystalline nanobeam depend on the grain size, grain rotations, porosities, interface, magnetic field, surface effect and nonlocality.

Fe-based multi-phase nanocrystalline ribbons with hierarchically flowerlike structured metal oxides after modified by Orange II for CrVI absorption

Three-dimensional flowerlike nanostructured metal oxides attached on the surfaces of Fe-based multi-phase nanocrystalline ribbons (Fe-MNRs) were prepared by a simple way (through immersing the Fe-MNRs in Orange II solution). It has been found that the as-prepared Fe-MNRs with 3D flowerlike nanostructures (Fe-MNRs + FNs) exhibit good absorption property for a typical heavy metal ion (CrVI) in wastewater, while Fe-MNRs do not possess such properties. The Fe-MNRs + FNs could remove 99% CrVI ions from the solution in 40 min, and this adsorption property can be attributed to the ion exchange between CrVI and surface hydroxyl groups (O–H) of 3D flowerlike nanostructures. The present result suggests that the Fe-MNRs + FNs, prepared by facile way, possess great potentials in removing heavy metallic ions in wastewater.

Axial magnetic field effects on dynamic characteristics of embedded multiphase nanocrystalline nanobeams

This article investigates vibrational behavior of a multi-phase nanocrystalline nanobeam resting on Winkler–Pasternak foundation and subjected to a longitudinal magnetic field in the framework of nonlocal couple stress and surface elasticity theories. In this model, the essential measures to describe the real material structure of nanocrystalline nanobeams and the size effects were incorporated. This non-classical nanobeam model contains couple stress effect to capture grains micro-rotations. Moreover, the nonlocal elasticity theory is employed to study the nonlocal and long-range interactions between the particles. The present model can degenerate into the classical model if the nonlocal parameter, couple stress and surface effects are omitted. Hamilton’s principle is employed to derive the governing equations and the related boundary conditions which are solved applying differential transform method. The frequencies are compared with those of nonlocal and couple stress based beams. It is showed that vibration frequencies of a nanocrystalline nanobeam depend on the grain size, grain rotations, porosities, interface, elastic foundation, magnetic field, surface effect, nonlocality and boundary conditions.

Stability analysis of porous multi-phase nanocrystalline nonlocal beams based on a general higher-order couple-stress beam model

This article investigates buckling behavior of a multi-phase nanocrystalline nanobeam resting on Winkler-Pasternak foundation in the framework of nonlocal couple stress elasticity and a higher order refined beam model. In this model, the essential measures to describe the real material structure of nanocrystalline nanobeams and the size effects were incorporated. This non-classical nanobeam model contains couple stress effect to capture grains micro-rotations. Moreover, the nonlocal elasticity theory is employed to study the nonlocal and long-range interactions between the particles. The present model can degenerate into the classical model if the nonlocal parameter, and couple stress effects are omitted. Hamilton\'s principle is employed to derive the governing equations and the related boundary conditions which are solved applying an analytical approach. The buckling loads are compared with those of nonlocal couple stress-based beams. It is showed that buckling loads of a nanocrystalline nanobeam depend on the grain size, grain rotations, porosities, interface, elastic foundation, shear deformation, surface effect, nonlocality and boundary conditions.

Damping Vibration Behavior of Viscoelastic Porous Nanocrystalline Nanobeams Incorporating Nonlocal–Couple Stress and Surface Energy Effects

Damping vibration analysis of multi-phase viscoelastic nanocrystalline nanobeams on viscoelastic medium is carried out accounting for nano-grains and nano-voids sizes. For the first time, a contribution of nonlocal, couple stress and surface energy effects is applied for vibration analysis of nanocrystalline nanobeams. In fact, couple stress theory considers grains microrotations, while nonlocal elasticity theory considers long-range interactions between the particles. Viscoelastic medium is described as infinite parallel springs as well as shear and viscous layers. Hamilton’s principle is employed to derive the governing equations and the related boundary conditions which are solved applying differential transform method. The frequencies are compared with those of nonlocal and couple stress-based beams. It is observed that damping frequencies of a nanocrystalline nanobeam are significantly influenced by the grain size, grain rotations, porosities, interface, damping coefficient, surface energy, nonlocality and structural damping.

Post-buckling analysis of imperfect multi-phase nanocrystalline nanobeams considering nanograins and nanopores surface effects

In this paper, a size-dependent nonlinear higher order refined beam model is developed based on modified couple stress theory. Then, it is applied to investigate post-buckling behavior of multi-phase nanocrystalline silicon nanobeams with geometrical imperfection. Nanocrystalline materials (NcMs) are multi-phase composites with the contribution of nanopores, nanograins and interface phase. Because of experimental observation of strain gradients near interface phase, the nanobeam is modeled via strain gradient based couple stress theory. A micromechanical model based on Mori-Tanaka scheme is employed to incorporate the size of nanograins/nanopores and their surface energies. The post-buckling load-deflection relation is obtained by solving the governing equations having cubic nonlinearity applying Galerkin’s method needless of any iteration process. New results show the importance of porosity percentage, nanograins size, geometrical imperfection, couple stress parameter, foundation parameters and surface phase of nanograins/nanopores on nonlinear buckling behavior of NcM nanoscale beams.

Dynamic modeling and vibration analysis of double-layered multi-phase porous nanocrystalline silicon nanoplate systems

Abstract Nanocrystalline materials (NcMs) are multi-phase composites containing nanograins, nanovoids and interface. A softening mechanism due to the interface phase and nanovoids is observed in NcMs. For the first time, vibration behavior of double-layered nanocrystalline silicon nanoplates on elastic substrate is analyzed based on a two-variable refined plate model. Due to the experimentally observation of grains micro-rotation and strain gradients near interfaces, the strain gradient based couple stress theory is employed to describe the size-dependent behavior of the nanocrystalline nanoplate. A micromechanical model is employed to incorporate the effects of inclusions and their surface energies. Galerkin's method is implemented to obtain the natural frequencies of nanocrystalline nanoplate with different boundary conditions. One can see that the nanograins size, nanograins surface energy, nanovoids size, void percentage, interface region, scale parameter, foundation constants and boundary conditions have a great influence on the vibration behavior of a double-layered nanocrystalline nanoplate.

Frequency analysis of porous nano-mechanical mass sensors made of multi-phase nanocrystalline silicon materials

Nanocrystalline materials (NcMs) are multi-phase composites containing nanograins, nanovoids and interface. Dynamic behavior of nanocrystalline silicon mass sensors on an elastic substrate is analyzed based on a two-variable refined plate model. Due to the experimental observation of grains micro-rotation and strain gradients near interfaces, the strain gradient based couple stress theory is employed to describe the size-dependent behavior of the nanocrystalline sensors. A micromechanical model is employed to incorporate the effects of inclusions and their surface energies. Galerkin’s method is implemented to obtain the frequency shifts of the nanocrystalline mass sensor with different boundary conditions. One can see that the nanoparticle mass, nanograins size, nanograins surface energy, nanovoids size, void percentage, interface region, scale parameter, foundation constants and boundary conditions have a great influence on the frequency shifts of nanocrystalline mass sensors.