Microstructures, densification and mechanical properties of TiC–Al2O3–Al composite by field-activated combustion synthesis

Dense TiC–Al₂O₃–Al composite was prepared with Al, C and TiO₂ powders by means of electric field-activated combustion synthesis and infiltration of the molten metal (here Al) into the synthesized TiC–Al₂O₃ ceramic. An external electric field can effectively improve the adiabatic combustion temperature of the reactive system and overcome the thermodynamic limitation of reaction with x < 10 mol. Thereby, it can induce a self-sustaining combustion synthesis process. During the formation of Al₂O₃–TiC–Al composite, Al is molten first, and reacted with TiO₂ to form Al₂O₃, followed by the formation of TiC through the reaction between the displaced Ti and C. Highly dense TiC–Al₂O₃–Al with relative density of up to 92.5% was directly fabricated with the application of a 14 mol excess Al content and a 25 V cm⁻¹ field strength, in which TiC and Al₂O₃ particles possess fine-structured sizes of 0.2–1.0 μm, with uniform distribution in metal Al. The hardness, bending strength and fracture toughness of the synthesized TiC–Al₂O₃–Al composite are 56.5 GPa, 531 MPa and 10.96 MPa m^1/2, respectively.     

Mechanical characterization of Ni1−xZnxFe2O4 prepared by non-conventional methods

The mechanical properties like hardness, H_v and compressive strength, σ of Ni_1−xZn_xFe₂O₄ (x = 0.2, 0.3, 0.4 and 0.5) prepared by the non-conventional flash combustion and citrate-gel decomposition techniques are studied and reported. It is observed that there is an increase in hardness with zinc content as well as sintering temperature. The hardness in the order of 2.0–3.63 GPa and compressive strength in the order of 150–240 MPa are obtained for Ni–Zn ferrites prepared by these non-conventional techniques. The influence of density, porosity and microstructure on hardness and compressive strength of Ni–Zn ferrites with respect to sintering temperature was studied.     

Prediction of stiffness and strength of single-walled carbon nanotubes by molecular-mechanics based finite element approach

Molecular-mechanics based finite element approach was used to predict the tensile stiffness and strength of single-walled carbon nanotubes. Different types of nanotubes, such as Arm-Chair, Zig-Zag, and chiral type, were discussed in detail. Nanotube stiffness was predicted to be independent of both the nanotube diameter and the nanotube helicity, but Poisson ratio was dependent of the nanotube diameter. In addition to the stiffness, nanotube strength was also analyzed by molecular-mechanics based finite element approach. Modified Morse potential function was selected to model the breakage of CC chemical bond with the separation energy of 7.7 eV. Nanotube strength was predicted at 77–101 GPa with the fracture strain around 0.3. The nanotube strength was found to be moderately dependent of the nanotube helicity, but independent of the nanotube diameter.     

Grain size effects on the tensile properties and deformation mechanisms of a magnesium alloy, AZ31B, sheet

The grain size dependence of the tensile properties and the deformation mechanisms responsible for those properties are examined for Mg alloy, AZ31B, sheet. Specifically, the Hall–Petch effect and strain anisotropy (r-value) are characterized experimentally, and interpreted using polycrystal plasticity modeling. {1 0 . 2} extension twins, {1 0 . 1} contraction twins, and so-called “double-twins” are observed via microscopy and diffraction-based techniques, and the amount of twinning is found to increase with increasing grain size. For the sheet texture and tensile loading condition examined, {1 0 . 2} extension twinning is not expected, yet the polycrystal plasticity model predicts the observed behavior, including this ‘anomalous’ tensile twinning. The analysis shows that the Hall–Petch strength dependence, of the polycrystal as a whole, is primarily determined by the grain size dependence of the strength of the prismatic slip systems.     

Mechanical behaviour of diamond reinforced metals

Diamond reinforced metals with a diamond content of 55–60 vol.% were made by gas driven liquid metal infiltration. They were characterized with regard to their stiffness, strength and fracture toughness as a function of diamond particle size and matrix alloy by means of tensile and Chevron notch tests, respectively. The choice of the metal matrix, i.e. pure Al, Al–Cu, Cu–B and Ag–Si alloys was made in view of their application in thermal management where high thermal conductivity is important. For undamaged material Young's moduli, measured in unloading–reloading cycles necessary to measure static Young's modulus, of 250 GPa for Al-based and 300 GPa for Ag-based composites were obtained. The copper-based composites exhibited much lower values indicating that the small deformation necessary to measure Young's modulus induced already considerable damage. Strain to fracture of the composites was found to be a few tenth of a percent. An ultimate tensile strength of approximately 300 MPa was reached for the silver-based composites compared with roughly 150 MPa for the Al-based and below 50 MPa for the Cu-based composites. The size of the diamond particles had little influence on stiffness and strength of the composites but fracture toughness increased with increasing particle size. The differences in the mechanical behaviour of the configurations investigated can be rationalized by observations made during fractographic investigations by scanning electron microscopy. Additionally, the damage evolution in the composites was observed by the repeated determination of the specimen's stiffness during the tensile tests.     

Magneto-elastic interchange in ferromagnetic alloys

The internal friction δ, exchange integral A, magnetocrystalline anisotropic constant K_I and saturation magnetization M_s of Fe–Cr–Al and Fe–Cr–Al–Si alloys annealed at 1373 and 1473 K are measured. The energy density and volume fraction of domain walls (DWs) of these alloys are calculated based on the theories of ferromagnetism and the magnetic parameters measured. The physical process of irreversible movement of 90° DWs is suggested. The results indicate the dissipated elastic energy per unit volume due to the irreversible movements of 90° DWs is equal in value to the energy density of DWs, that is γ_w/δ_w=λ_sE/2. It is an effect of magneto-elastic interchange in ferromagnetic alloys.     

Al–Si coating fused by Al + Si powders formed on Ti–6Al–4V alloy and its oxidation resistance

An Al–Si coating was successfully produced by means of the low oxygen pressure fusing technology for improving the oxidation resistance of Ti–6Al–4V alloy. The Al and Si concentration in coating and coating thickness could be controlled by adjusting powder mixing ratio and changing the technical parameters (fusing temperature and time), respectively. At 1273 K, the weight gain of the Al–20Si coating increased with prolonging fusing time and its equation could be described as Δm² = 3.62t. After 105 h oxidation, the oxidation rate of the Al–20Si coated specimen with fusing time 100 min was about two to four times than that of the Al–10Si coated specimen with fusing time 60 min.     

First-principles study of the tensile and fracture of the Al/TiN interface

Employing the density functional theory, we investigate the tensile and fracture processes of the Al/TiN(0 0 1) interface. The simulation presents directly the strain–stress relationship, the ideal tensile strength and the process of bond breaking of the system. Through the analysis of deformation, we find that the softer Al layers deform larger than the harder TiN layers during the tensile process. And fracture occurs between the interface and the sub-interface Al layers. In addition, the results show that during the tensile process, the ripple of the interfacial TiN layer decreases gradually with the increment of the strain. Charge transfer was detected from the Al to TiN layers near the interface area during the tensile process by means of charge density and density of states analyses. The charge transfer affects the fracture process. Compared to our previous study of the Al/TiN(1 1 1) interface, the Al/TiN(0 0 1) interface has smaller work of adhesion and larger tensile strength than the Al/TiN(1 1 1) interface. Our investigation shows that the fractures of the Al/TiN(0 0 1) and (1 1 1) interface systems both happen in the Al layers near the interface.     

Crystal structure of the orthorhombic U2-Al4Mg4Si4 precipitate in the Al–Mg–Si alloy system and its relation to the β′ and β″ phases

The atomic structure of a common precipitate in the Al–Mg–Si system has been determined. It is isotypic with TiNiSi (space group Pnma) and contains four units of MgAlSi in a unit cell of size a = 0.675 nm, b = 0.405 nm, c = 0.794 nm. EDS analyses support the composition. A model was based on the atomic structure of the β′ precipitate, electron diffraction and high-resolution transmission electron microscopy (HRTEM) images. A quantum mechanical refinement of the model removed discrepancies between simulated and experimental diffraction intensities. Finally, a multi-slice least square refinement confirmed the structure. The structural relation with β″ is investigated. A similar Mg–Si plane also existing in β″ and β′, can explain most coherency relations between the precipitate phases and with matrix.     

Synthesis of fly ash particle reinforced A356 Al composites and their characterization

A356 Al–fly ash particle composites were fabricated using stir-cast technique and hot extrusion. Composites containing 6 and 12 vol.% fly ash particles were processed. Narrow size range (53–106 μm) and wide size range (0.5–400 μm) fly ash particles were used. Hardness, tensile strength, compressive strength and damping characteristics of the unreinforced alloy and composites have been measured. Bulk hardness, matrix microhardness, 0.2% proof stress of A356 Al–fly ash composites are higher compared to that of the unreinforced alloy. Additions of fly ash lead to increase in hardness, elastic modulus and 0.2% proof stress. Composites reinforced with narrow size range fly ash particle exhibit superior mechanical properties compared to composites with wide size range particles. A356 Al–fly ash MMCs were found to exhibit improved damping capacity when compared to unreinforced alloy at ambient temperature.     

Formation of Al2Cu and AlCu intermetallics in Al(Cu) alloy matrix composites by reaction sintering

We report the fabrication and characterization of a series of Al(Cu) alloy-based matrix composites. The composites were produced by sintering and rapid quenching three powder mixtures of Al and Cu with hypoeutectic, eutectic, and hypereutectic compositions. The morphology of the reinforcements formed in the Al(Cu) matrices of these composites was found to be variable. A two-phase Al₂Cu–Al(Cu) nanoeutectic, with lamellar spacing of 200–300 nm, was found in the Al(Cu) matrix of the sample having hypoeutectic composition after it was oil-quenched from 1000 °C to room temperature. While oil quenching the sample with eutectic composition, produced single Al₂Cu crystals of 2–2.5 μm size, embedded in a lamellar nanoeutectic matrix. As for the hypereutectic alloy, the matrix of the oil-quenched sample consisted mainly of Al₂Cu intermetallic, and a secondary phase of AlCu dendrites with dendrite arms spacing of 1–1.5 μm.     

新型超硬材料z-BC2N的弹性、硬度与热导率研究

采用第一性原理计算研究了超硬材料z-BC₂N的弹性各向异性性质、应力-应变关系、硬度及最小热导率性质。计算得到的晶体力学行为判据B/G为0.87, 泊松比为0.084, 普适弹性各向异性指数为0.09992。[100]晶向上最大拉伸强度达到180 GPa, 应变方向上最大剪切强度达到160 GPa, 维氏硬度值为77.07 GPa。基于Cahill模型得到的最小热导率为6.811 W/(m·K)。结果表明: z-BC₂N是脆性材料且力学稳定性良好, 有非常高的拉伸强度、剪切强度, 体弹模量为各向同性, 杨氏模量各向异性程度不大。z-BC₂N的最小热导率低于金刚石的最小热导率。     

Characterization of single crystal silicon and electroplated nickel films by uniaxial tensile test with in situ X-ray diffraction measurement

In this study, mechanical properties of micron‐thick single crystalline silicon (Si) and electroplated nickel (Ni) films at intermediate temperatures are investigated by means of X‐ray diffraction (XRD) tensile testing. The developed tensile test technique enables us to directly measure lateral (out‐of‐plane) elastic strain of microscale crystalline specimen using XRD during tensile loading, and determines Young's modulus, Poisson's ratio and tensile strength of the Si and Ni specimens. The specimens, measuring 10 μm thick, 300 μm wide and 3 mm long, are prepared through a conventional micro‐machining process, and the ultraviolet lithographie galvanoformung abformung (UV‐LIGA) process including a molding and an electroplating. The Si specimens, showing brittle fracture at room temperature (R.T.), have average Young's modulus and Poisson's ratio of 169 GPa and 0.35, respectively, in very good agreement with analytical values. The Ni specimens, showing ductile fracture, have those of 190 GPa and 0.24, lower than bulk coarse grained Ni. Young's moduli of both the Si and Ni specimens decrease with increasing temperature, but Poisson's ratios are independent of temperature. The influence of specimen size on elastic‐plastic properties of the specimens is discussed.     

A study on the tensile response and fracture in carbon nanotube-based composites using molecular mechanics

A study on the mechanical properties of polyethylene and carbon nanotube (CNT) based composites is presented using molecular mechanics simulations. The systems being investigated consist of amorphous as well as crystalline polyethylene (PE) composites with embedded single-walled CNTs. All the systems are subjected to quasi-static tensile loading, with the assumption that no cross-link chemical bonds exist between the CNT and polyethylene matrix in the case of nanocomposites. Based on the numerical simulations, we report Young’s moduli (C₃₃) of 212–215 GPa for crystalline PE, which closely match the experimental measurement. Furthermore, elastic stiffness of 3.19–3.69 GPa and tensile strength of 0.21–0.25 GPa are obtained for amorphous PE. The tensile responses are found to be highly isotropic. In the case of crystalline PE reinforced by long through CNTs, moderate improvements in the tensile strength and elastic stiffness are observed. However, the results differ from the predictions using the rule of mixtures. On the other hand, although significant increase in the overall tensile properties is observed when amorphous PE is reinforced by long through CNTs, the load transfer at the nanotube/polymer interface has negligible effect. Finally, degradations in both tensile strength and elastic stiffness are reported when amorphous PE is reinforced by embedded CNTs. The study presented indicates the importance of specific CNT and polymer configurations on the overall properties of the nanocomposite.     

Molecular dynamic simulations of uniaxial tension at nanoscale of semiconductor materials for micro-electro-mechanical systems (MEMS) applications

Molecular dynamic (MD) simulations of uniaxial tension at nanoscale were conducted on two semiconductor materials, namely, silicon (Si) and germanium (Ge) to determine their mechanical properties and investigate the nature of deformation under applied load at nanolevel. A general form of Tersoff-type, three-body potential was used for the interaction between the Si atoms and between the Ge atoms in the simulations. Both, Si and Ge were found to exhibit a linear elastic behavior followed by a nonlinear increase in stress in the plastic region up to the ultimate tensile stress (instead of catastrophic brittle fracture soon after the elastic limit, which is typical of most nominally brittle materials at macrolevel). Further loading beyond the ultimate tensile stress resulted in catastrophic failure of these materials by a ductile fracture mode, namely, slip at 45° to the loading direction. The strain at failure was found to be much higher than the corresponding values at macroscale possibly due to the higher loading rates used. Based on the simulation results, the Young's modulii of Si and Ge in the [100] direction were determined to be 130 and 103 GPa, respectively, and the ultimate strengths, 25 and 20 GPa, respectively, at 500 m s⁻¹. These results are in reasonable agreement with the experimental and simulation results reported in the literature. The effect of strain rate via the rate of loading (10–500 m s⁻¹, where 1 m s⁻¹ corresponds to 10⁻² Å ps⁻¹) on the nature of deformation and the measured properties were also investigated. As the rate of loading (or the strain rate) decreases, the stress–strain curves more or less overlap up to the ultimate strength with a slight decrease in the ultimate tensile stress but a significant decrease in the value of strain at failure or strain at ultimate tensile stress.     

Development of Si-B-O-N fibres from polyborosilazane

A polyborosilazane, which is a precursor of ceramic fibre, was synthesized from perhydropolysilazane and trimethyl borate. The polyborosilazane was dry-spun and then pyrolysed to produce amorphous Si-B-O-N fibre. The Si-B-O-N fibre retained its high tensile strength to higher temperatures (about 1600 °C). The fibre has a density of 2.4 g cm^–3, tensile strength of 2.5 GPa and an elastic modulus of 180 GPa.     

Internal stress and adhesion of amorphous Ni–Cu–P alloy on aluminum

This study investigated the effect of saccharin on the internal stress and the adhesion of amorphous Ni–Cu–P deposited on aluminum. An amorphous Ni–Cu–P deposit with slight compressive stress can be produced when one adds 8–10 g/l saccharin into the Ni–Cu–P deposition solution. The stress relief mechanism was investigated. The addition of saccharin restrains the coalescence of the islands within Ni–Cu–P nodules and reverses the internal stress of the electroless Ni–Cu–P deposit from tensile to compressive. The adhesion strength of the Si/Ti/Al/Ni–Cu–P multilayer specimen obtained with 10 g/l saccharin is around 35 to 45 MPa, and the fracture occurs at the silicon substrate after the pull test. The shear strength of the Ti/Al/Ni–Cu–P bump (100×100 μm) on Si is 132.9±12.7 g, and the fracture occurs at the Ni–Cu–P deposit after the shear test. Moreover, the inhibition of coalescence of the fine islands within Ni–Cu–P nodules increases the brightness and the hardness of the deposit.     

Mechanical properties of U-0.95 mass fraction of Ti alloy quenching and aging treatment:a first principles study

First principles plane wave pseudopotential method was executed to calculate the mechanical properties with respect to the uranium-0.95 mass fraction of titanium(U-0.95 mass fraction of Ti) alloy for quenching and aging,including the elastic modulus,the value of shear modulus to bulk modulus(G/B) and the ideal tensile strength.The further research has also been done about the crack mechanism through Griffith rupture energy.These results show that the elastic moduli are 195.1 GPa for quenching orthorhombic a phase and 201.8 GPa for aging formed Guinier-Preston(G.P) zones,while G/B values are0.67 and 0.56,respectively.With the phase change of uranium-titanium(U-Ti) alloy via the quenching treatment,the ideal tensile strength is diverse and distinct with different crystal orientations of the anisotropic α phase.Comparison of quenching and short time aging treatment,both of the strength and toughness trend to improve slightly.Further analysis about electronic density of states(DOS) in the electronic scale indicates that the strength increases continuously while toughness decreases with the aging proceeding.The equilibrium structure appears in overaging process,as a result of decomposition of metastable quenching a phase.Thereby the strength and toughness trend to decrease slightly.Finally,the ideal fracture energies of G.P zones and overaging structure are obtained within the framework of Griffith fracture theory,which are 4.67 J/m2 and 3.83 J/m2,respectively.These results theoretically demonstrate strengthening effect of quenching and aging heat treatment on U-Ti alloy.     

耐高温的SiC(Al)纤维

聚硅碳硅烷 (PSCS)与乙酰丙酮铝(Al(AcAc)₃)在一定条件反应制备了耐高温SiC(Al)纤维先驱体聚铝碳硅烷( PACS)。PACS通过熔融纺丝、预氧化处理、低温烧成、高温烧结等一系列工艺过程制备了耐高温SiC (Al)纤维 。SiC (Al) 纤维的化学组成为Si₁C_1.15O_0.026Al_0.013,主要结构是平均晶粒为95 nm的β-SiC,O和游离C含量均大大低于Nicalon纤维 ( O>10wt%,游离C>10wt%),同时含有微量的Al和少量的 α -SiC。纤维表层O含量和Si含量略高于纤维内部,表面光滑平坦,没有明显表面缺陷。 SiC (Al) 纤维的平均直径为13 μm,平均强度为2.3 GPa,1400℃氩气中处理1 h,强度保留率95%以上;1800℃氩气中处理1 h,强度保留率为71%。纤维的高温稳定性高于Nicalon纤维,低于Tyranno SA纤维。      

Fatigue behavior of a 7075-T6 aluminum alloy coated with an electroless Ni–P deposit

An investigation has been carried out in order to study the fatigue and corrosion–fatigue behavior of a 7075-T6 aluminum alloy coated with an electroless Ni–P (EN) deposit, in the as-plated condition, of approximately 38–40 μm in thickness and a high P content, of approximately 18 wt%. The results obtained, show that the EN coating can give rise to a significant improvement in the fatigue and corrosion–fatigue performance of the substrate, depending on the testing conditions. When the coated system is tested in air, it is observed that the increase in fatigue properties decreases as the alternating stress applied to the material increases. At stresses of the order of 0.4 σ_0.2% the increase in fatigue life is more than about 100%. However, as the stress increases to values in the range of 0.7 σ_0.2%, no improvement in the fatigue performance of the system is observed and the behavior is similar to that of the uncoated substrate. Under corrosion–fatigue conditions, the fatigue life is observed to increase between approximately 60% and 70%, depending on the stress applied. It is shown that fatigue cracks are associated with nodular-like defects present on the surface of the coated samples. The deleterious effect of such defects seems to be more pronounced as the alternating stress applied to the material increases. A crude estimate of the yield strength of the EN coating from tensile measurements indicates that such a parameter is in the range of 3.8 GPa, in agreement with the computation of the absolute hardness of the deposit, of about 4 GPa, by means of Meyer’s law. It is also shown that the EN deposit has a very good adhesion to the substrate even when the system is subjected to tensile stresses greater than the yield strength. Such characteristics as well as the higher mechanical properties of the EN coating in comparison with the aluminum alloy substrate and the preservation of its integrity during fatigue testing contribute to the better fatigue performance of the coated system.