Superplastic behavior of a TiAl alloy in its as-cast state

Superplastic behavior of a TiAl alloy was investigated in a temperature range between 800 and 1000°C and at a strain rate of 1×10⁻⁴ s⁻¹. The results show that the present alloy possesses very good superplasticity even in its as-cast state. A tensile elongation of 628% was obtained at 850°C. The observed superplastic behavior can be explained by the formation of a fine microstructure containing a metastable β-phase during solidification.     

Fabrication and mechanical properties of monolithic MoSi2 by spark plasma sintering

Fine MoSi₂ powders containing a small amount of Mo₅Si₃ have been prepared by self-propagating high-temperature synthesis (SHS), followed by spark plasma sintering (SPS) for 10 min at 1200-1500°C and 30 MPa. Dense MoSi₂ materials, in which the grain size is ∼7.5 μm, have been fabricated at 1300°C. They exhibit excellent mechanical properties: Vicker’s hardness H_v (10.6 GPa), fracture toughness K_IC (4.5 MPa m^1/2), and bending strength σ_b (560 MPa). The strength of 325 MPa can be retained up to 1000°C.     

The effect of carbon nanotubes microstructures on reinforcing properties of SWNTs/alumina composite

Alumina reinforced with 1 wt% single-wall carbon nanotubes (SWNTs) was fabricated by hot-pressing. The fracture toughness of SWNTs/Al₂O₃ composite reaches 6.40 ± 0.3 MPa m^1/2, which is twice as high as that of unreinforced alumina. Nanoindentation introduced controlled cracks and the damage were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SWNTs reinforcing mechanisms including CNT pullout, CNT fracture, CNT bridging and crack deflection were directly observed, and the relationship between carbon nanotubes microstructures in the matrix and mechanical properties was also discussed in detailed.     

Study on the structure and properties of fine-grained alumina fast sintered with high heating rate

Fine-grained alumina was obtained in 2 min by a new densification method based on Self-propagating High-temperature Synthesis plus Quick Pressing (SHS/QP). The sample was densified to more than 99% of theoretical density under a large mechanical pressure (100 MP) and a fast heating rate (1600 °C/min). Compared with the alumina sample obtained by spark plasma sintering (SPS) at lower heating rates (100 or 500 °C/min), almost no grain growth was found in the sample obtained in this work. The microstructure and mechanical properties were studied. Hardness value of 18.6 ± 0.4 GPa and fracture resistance value of 3.4 ± 0.3 MPa m^1/2 were measured for fine-grained alumina of this work. The densification mechanism was discussed.     

Differently mobile twin boundaries and magnetic shape memory effect in 10 M martensite of Ni–Mn–Ga

We investigated twin boundaries with sharply different mobility or twinning stress in five-layered modulated (10 M) martensite of Ni–Mn–Ga exhibiting magnetic shape memory effect or magnetically induced reorientation. Different mobility is ascribed to the different microstructures of the macrotwin planar interface. In monoclinic approximation the mobile boundaries are of Type I and Type II with complex microstructure of adjoining variants. These boundaries respond differently to magnetic field. For Type II boundary the reorientation takes place at very low field 250 Oe.     

Microstructural evolution and mechanical properties of ultrafine grained Al3Ti/Al–5.5Cu composites produced via hot pressing and subsequent friction stir processing

In situ Al₃Ti/Al–5.5Cu composites fabricated by powder metallurgy and subsequent forging were subjected to multiple pass friction stir processing (FSP) with and without active cooling. The forged sample exhibited lower strength and ductility due to the presence of coarse Al₃Ti clusters with a size range of 50–100 μm and coarse matrix grains. Four-pass FSP in air resulted in the refinement and redistribution of the Al₃Ti clusters, and the generation of micron matrix grains, thereby increasing the strength and ductility of the composites. Furthermore, coarse Al₂Cu particles dissolved and re-precipitated due to a relatively long duration of thermal exposure. Additional two pass FSP with rapid water cooling (FSP-water) dissolved most of the Al₂Cu into the matrix and retained the solutes in solution due to the short duration of thermal exposure. Meanwhile, ultrafine matrix grains with a high density of dislocations were obtained. These microstructural changes led to significant increase in strength and a decrease in ductility in the FSP-water sample. After aging, the FSP-water sample exhibited further increased yield strength and ultimate tensile strength due to the precipitation of metastable Al₂Cu phases. However, the ductility did not decrease due to the decrease of dislocation density after aging.     

Porous alumina/zirconia layered composites with unidirectional pore channels processed using a tertiary-butyl alcohol-based freeze casting

Symmetric three-layer porous alumina/zirconia composites with controlled “designer” pore structure have been prepared by a tertiary-butyl alcohol-based freeze casting and sintering at 1400–1500 °C in air. Unidirectional aligned macropore channels were developed over a long range by controlling the solidification direction of tertiary-butyl alcohol solvent; in this case, they were surrounded by more dense structured walls. In layered composite, the bottom layer consisted of small sized pore channels (∼11 μm in diameter) compared with the middle and the top layer, due to the comparatively rapid velocity of the TBA crystal growth during solidification. The compressive strength (63–376 MPa) of the sintered porous layered composite remarkably increased as the porosity decreased (64–32%).     

Development of a novel magnesium-copper based composite with improved mechanical properties

Magnesium based composite with 17.95 wt.% of copper reinforcement was fabricated using an innovative DMD technique followed by hot extrusion. Microstructural characterization of the extruded composite samples showed fairly uniform reinforcement distribution, presence of Mg-Cu based intermetallics, good Cu_P-Mg interfacial integrity, and the presence of minimal porosity. Mechanical properties characterization revealed that the presence of copper as reinforcement lead to a significant increase in hardness, elastic modulus, 0.2% yield strength and UTS of pure magnesium while the ductility was adversely affected. An attempt is made in the present study to correlate the effect of copper as reinforcement with the microstructural and mechanical properties of magnesium.     

Dispersion of iron aluminide with polyelectrolyte in water

A stable dispersion of iron aluminide (FeAl) intermetallic powders was achieved in water using ammonium salt of poly(methacrylic acid) (PMAA-NH₄). The electrosteric stabilization was identified to be the prevailing mechanism. pH value correlates inversely with the amount of PMAA-NH₄ adsorbed onto the surface of FeAl particle. The aqueous FeAl solution stabilized by PMAA-NH₄ displays a Newtonian behavior as revealed by rheological measurements.     

Mechanical and thermal properties of LaMgAl11O19

Lanthanum magnesium hexaaluminate (LaMgAl₁₁O₁₉) powders were synthesized successfully at 1300 °C for 4 h by solid-state reaction, and LaMgAl₁₁O₁₉ ceramic was prepared at 1700 °C for 6 h by pressureless sintering. Phase composition, microstructure, mechanical and thermophysical properties of LaMgAl₁₁O₁₉ ceramic were investigated. Results show that the flexural strength and fracture toughness of LaMgAl₁₁O₁₉ ceramic are 353.3 ± 12.5 MPa and 4.60 ± 0.46 MPa m^1/2. Young's Modulus and Poisson ratio is 295 GPa and 0.23, respectively. The linear thermal expansion coefficient of LaMgAl₁₁O₁₉ ceramic from 473 K to 1473 K is 9.17 × 10⁻⁶/K, and thermal conductivity at 1273 K is 2.55 W/m K.     

The preparation of silver sulfide nanoparticles in lamellar liquid crystal and application to lubrication

The silver sulfide (Ag₂S) nanoparticles were prepared by the reaction of AgNO₃ and Na₂S in the lamellar liquid crystal (LLC) formed by Triton X-100, n-C₁₀H₂₁OH and H₂O. The size of the particles is about 2-3 nm. The existence of Ag₂S nanoparticles can improve the lubrication of the lamellar liquid crystal.     

Structural-mechanical relationship of epoxy compatibilized polyamide 6/polycarbonate blends

Polyamide 6 (PA6)/polycarbonate (PC) blends compatibilized with solid epoxy resin (bisphenol type-A) were prepared by extrusion followed by injection molding. The effects of epoxy resin on the microstructure, tensile, impact and compatibility of the PA6/PC blends were investigated. The results showed that both the tensile modulus and elongation at break of PA6/PC blends were inferior as compared to their parent polymers. This resulted from incompatibility between the PA6 and PC phases. SEM observation revealed that the introduction of 0.5 part per hundred (phr) epoxy resin into the PA6/PC75/25 blend yields a finer dispersion of PC phase in PA6 matrix. The boundaries between the PC domains and PA6 matrix became obscure with the incorporation of 1 phr epoxy resin. Such an improvement in compatibility was suggested to be resulted from the formation of in situ epoxy bridged PA6-PC block copolymer in the blend during compounding. Consequently, the tensile modulus, yield strength and impact strength of the PA6/PC 75/25 blend improved considerably with increasing epoxy content.     

Elastic constants of metal/ceramic composites with lamellar microstructures: Finite element modelling and ultrasonic experiments

The elastic properties of single domains of lamellar AlSi12/Al₂O₃ composites produced by metal infiltration of freeze cast preforms have been examined. The anisotropic elastic constants determined from ultrasonic phase spectroscopy (UPS) experiments have been compared to microstructure based FE-models created with the program OOF2, micromechanical models (Mori–Tanaka and inverse Mori–Tanaka) and an analytical model for an ideal laminate. The influences of lamellae orientations and ceramic contents on the elastic constants have been investigated. Along the lamellae directions the microstructure based FE model and the inverse Mori–Tanaka model are in good agreement with experimental results. Perpendicular to the lamellae the experiment shows a stiffer than expected elastic behaviour.     

A simple route to the synthesis of BaCrO4 microstructures at room temperature

BaCrO₄ with various morphologies such as X-shaped, shuttle, rhombus was produced by using poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (P123) as a structure-directing agent at room temperature. The effects of experimental parameters on the morphology of BaCrO₄ were investigated. This synthetic route was also extended to the synthesis of BaWO₄ and Pb₂CrO₅. The samples were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and Fourier transform-infrared spectroscopy (FT-IR). The photoluminescence (PL) of the sample was investigated.     

Preparation of graphene nanosheet/alumina composites by spark plasma sintering

Graphene nanosheet/alumina composite has been prepared by spark plasma sintering. A homogeneous distribution of nanosheets in an alumina matrix could be obtained by the electrostatic attraction between graphite oxide and alumina particles and their subsequent reduction. The introduction of graphene nanosheet leads to refinement of grain size of alumina after hot pressing. The experimental results have shown that the fracture toughness and conductivity of the graphene nanosheet/alumina composite are about 53% and 13 orders of magnitude higher than those of unreinforced alumina material, respectively.     

Microstructural characterization of a rapidly solidified Al-Sr-Ti alloy

The microstructure of the melt-spun Al-7Sr-3Ti alloy has been characterized using X-ray diffraction, transmission electron microscopy and differential scanning calorimetry. The results show the microstructure of the melt-spun Al-7Sr-3Ti alloy is composed of the equilibrium α-Al, Al₄Sr, Al₃Ti and metastable Al₂₃Ti₉, different from that of the ingot-like alloy comprising α-Al, Al₄Sr and Al₃Ti. Moreover, the amount of Al₃Ti is much less than that of Al₄Sr and metastable Al₂₃Ti₉ in the melt-spun alloy. The melting temperature of primary phases and enthalpies of fusion for the melt-spun Al-7Sr-3Ti alloy are lower than those for the ingot-like alloy. The formation mechanism of the microstructure of the melt-spun alloy has also been discussed.     

Microstructure and mechanical properties of small amounts of In2O3 reinforced Pb(ZrxTi1−x)O3 ceramics

Piezoelectric Pb(Zr_xTi_1−x)O₃ (PZT) ceramics with small amount (0.5-2.0 wt.%) of In₂O₃ are prepared by conventional sintering method. Based on X-ray diffraction analysis, the tetragonality of PZT matrix decreases with In₂O₃ content, indicating that In₂O₃ diffuses into PZT matrix. The microstructure of PZT matrix is significantly refined by doping small amounts of In₂O₃. The grain size reduction and the matrix grain boundary reinforcement are the probable mechanism responsible for the high strength and hardness in the PZT/In₂O₃ materials. The enhancement in Young’s modulus is attributed to In³⁺ substitution. The decreased tetragonality with In₂O₃ addition results in less crack energy absorption by domain switching and, hence, causes the small reduction in fracture toughness.     

Shape deformation and texture development of consolidated Ca α-SiAlON ceramics prepared by hot-forging

Consolidated Ca α-SiAlON ceramics with gradually varying microstructure and property was prepared by hot-forging. The shape deformation as well as texture development of the sample during forging was studied in detail. It was found that the forging process promoted the growth of elongated α-SiAlON grains and enhanced their preferred orientation with the longitude perpendicular to the pressing force. The strong texture offered the hot-forged sample an increased fracture toughness of 7.9 MPa m^1/2, which was primarily attributed to the pull-out and debonding behaviors of elongated grains.     

Elastic modulus of rat whiskers—A key biomaterial in the rat whisker sensory system

Nanoindentation tests were carried out to measure the elastic modulus of rat whisker, which is used as a high-acuity sensor for exploring the world and discriminating object distance, size and surface texture. The measured load–depth curves show that the biomaterial of rat whisker exhibits obvious visco-elastic characteristic, such as load relaxation and displacement creeping. The measured indentation modulus of rat whiskers decreases with indentation depth at the same location. The mean value of the measured indentation moduli of 24 major whiskers varies from 0.33 GPa to 4.92 GPa. The elastic modulus is independent of the freshness of the whisker sample. It is also found that the length and diameter of whiskers have no direct relationship to the indentation modulus at the base. Based on the measured elastic modulus, the bending stiffness at the base of a whisker is predicted in the range of 0.58–134.79 mN mm² and the rotational stiffness at the middle of a whisker is within 0.06–10.93 mN mm/rad. These results can be used to study the mechanical behavior of the rat whisker sensor system.     

Damping characterization of Mg-SiC composites using an integrated suspended beam method and new circle-fit approach

In the present study, a new methodology of using free-free beam method coupled with circle-fit approach is used to determine damping of Mg-SiC composites. This technique is based on classical vibration theory, by which the geometry and material properties of the metal matrix composites are related to resonant frequency and structural damping of the test specimen. Using the fact that the ratio of the vibration response to the applied force fits to a circle in the Argand plane for each resonant frequency of the test specimen, the damping factor and natural frequency is predicted accurately for the test specimen. An attempt is made to rationalize the increase in damping capability of the composites when compared against the monolithic specimen in terms of increase in dislocation density and presence of plastic zone at the particulate-matrix interface.