Fuchi Wang

Effect of temperature on mechanical behavior of AZ31 magnesium alloy

Strain rate sensitivity and tension/compression asymmetry of AZ31 magnesium alloy at different temperatures and strain rates were investigated. Both of mechanical behaviors are temperature dependent. Strain rate sensitivity increases with increasing temperature. Thermally activated slip is the source of strain rate sensitivity. At the temperature below or near 373 K, strain rate sensitivity is very little. Tension/compression asymmetry in yielding decreases with increasing temperature. Twinning is the reason of tension/compression asymmetry. At the temperature above or near 573 K, the material shows little tension/compression asymmetry of the flow stress.

Plastic Deformation Mechanisms of AZ31 Magnesium Alloy under High Strain Rate Compression

The twinning and slip activities of AZ31 magnesium alloy sheet at a strain rate of 1200 s^(-1) were investigated. Dynamically mechanical properties of various oriented samples were measured using Split Hopkinson Pressure Bar (SHPB). Optical microscope observations reveal that the dominant deformation mechanism is twinning for 90°oriented sample, and is slip for 45° and 0° oriented samples. TEM analysis for samples at a strain of 0.3% shows that the main deformation mechanisms for 90°, 45° and 0° oriented sample are {10(average)12} ＜10(average)11＞ and {10(average)11}＜10(average)12＞ twinning, basal slip and non basal slip, respectively. The main features of the true stress-true strain curves can be explained based on deformation mechanism analysis.

Formation and corrosion behavior of Fe-based amorphous metallic coatings prepared by detonation gun spraying

Amorphous metallic coatings with a composition of Fe48Cr15Mo14C15B6Y2 were prepared by detonation gun spraying process. Microstructural studies show that the coatings present a densely layered structure typical of thermally sprayed deposits with the porosity below 2%. Both crystallization and oxidation occurred obviously during spraying process, so that the amorphous fraction of the coatings decreased to 54% compared with fully amorphous alloy ribbons of the same component. Corrosion behavior of the amorphous coatings was investigated by electrochemical measurement. The results show that the coatings exhibit extremely wide passive region and low passive current density in 3.5% NaCl (mass fraction) and 1 mol/L HCl solutions, which illustrates excellent ability to resist localized corrosion.

Microstructure and mechanical properties of Mg-Gd-Y-Zr alloy processed by equal channel angular pressing

Abstract Microstructure evolution and texture development and their effects on mechanical properties of a Mg-Gd-Y-Zr alloy during equal channel angular pressing (ECAP) were investigated. It is found that the microstructure is still inhomogeneous after four passes, and two zones, namely the fine grain zone (FGZ) and the coarse grain zone (CGZ) are formed. The grain refinement occurs mainly by particle-stimulated nucleation (PSN) mechanism, which led to a more random texture after four passes of ECAP. In the ECAP-processed alloy, the strength did not increase while the ductility was enhanced dramatically compared with the as-received condition. The change of ductility of this alloy was discussed in terms of texture and second phase particles.

Synergistic strengthening effect of nanocrystalline copper reinforced with carbon nanotubes.

In this study, a novel multi-walled carbon nanotubes reinforced nanocrystalline copper matrix composite with super high strength and moderate plasticity was synthesized. We successfully overcome the agglomeration problem of the carbon nanotubes and the grain growth problem of the nanocrystalline copper matrix by combined use of the electroless deposition and spark plasma sintering methods. The yield strength of the composite reach up to 692 MPa, which is increased by 2 and 5 times comparing with those of the nanocrystalline and coarse copper, respectively. Simultaneously, the plasticity of the composite was also significantly increased in contrast with that of the nanocrystalline copper. The increase of the density of the carbon nanotubes after coating, the isolation effect caused by the copper coating, and the improvement of the compatibility between the reinforcements and matrix as well as the effective control of the grain growth of the copper matrix all contribute to improving the mechanical properties of the composite. In addition, a new strengthening mechanism, i.e., the series-connection effect of the nanocrystalline copper grains introduced by carbon nanotubes, is proposed to further explain the mechanical behavior of the nanocomposite.

Dynamic fracture of Ti-6Al-4V alloy in Taylor impact test

Abstract The dynamic fracture behaviors of Ti-6Al-4V alloy at high strain rate loading were investigated systemically through Taylor impact test, over the range of impact velocities from 145 m/s to 306 m/s. The critical impact velocity of fracture ranges from 217 m/s to 236 m/s. Smooth surfaces and ductile dimple areas were observed on the fracture surfaces. As the impact velocity reached 260 m/s, the serious melting regions were also observed on the fracture surfaces. Self-organization of cracks emerges when the impact velocity reaches 260 m/s, while some special cracks whose “tips” are not sharp but arc and smooth, and without any evidence of deformation or adiabatic shear band were also observed on the impact end surfaces. Examination of the sections of these special cracks reveals that the cracks expand along the two maximum shear stress directions respectively, and finally intersect as a tridimensional “stagger ridge” structure.

Theoretical and Experimental Studies on the Crystal Structure, Electronic Structure and Optical Properties of SmTaO4

The crystal structure, electronic structure and optical properties of SmTaO4 were identified through an experimental method and first principles calculation. X-ray powder diffraction (XRD) and a spectrophotometer were used to characterize the crystal structure, reflectivity and band gap of this material; furthermore, the electronic structure and optical properties were investigated according to three exchange-correlation potentials, LDA, GGA and GGA + U. Results show that the SmTaO4 calcined at 1400 °C with the solid-state reaction method is in monoclinic phase in the space group I2/a. In addition, the calculated lattice parameters are consistent with the experimental values. The electron transitions among the O 2p states, Sm 4f states and Ta 5d states play a key role in the dielectric function, refractive index, absorption coefficient and reflectivity of SmTaO4. The calculation of first principles provides considerable insight into the relationship between the electronic structure and optical properties of this material.

Microstructure characteristics and mechanical properties of TiB/Ti-1.5Fe-2.25Mo composites synthesized in situ using SPS process

Abstract TiB/Ti-1.5Fe-2.25Mo composites were synthesized in situ using the spark plasma sintering (SPS) method at temperatures of 850-1150 °C. The effect of the sintering temperature on microstructure and mechanical properties of the composites was investigated. The results indicate that the aspect ratio of the in situ synthesized TiB whiskers in Ti alloy matrix decreases rapidly with an increase in sintering temperature. However, both the relative density of the sintered specimens and the volume content of TiB whiskers in composites increase with increasing sintering temperature. Thus, the bending strength of the composites synthesized using SPS process increases slowly with increasing the sintering temperature from 850 to 1150 °C. TiB/Ti-1.5Fe-2.25Mo composite synthesized at 1150 °C using SPS method exhibits the highest bending strength of 1596 MPa due to the formation of fine TiB whiskers in Ti alloy matrix and the dense microstructure of the composite.

Effect of Si content on microstructure and properties of Si/Al composites

Abstract Si/Al composites with different Si contents for electronic packaging were prepared by spark plasma sintering (SPS) technique. Properties of the composites were investigated, including density, thermal conductivity, coefficient of thermal expansion and flexural strength. The effects of the Si content on microstructure and thermal and mechanical properties of the composites were studied. The results show that the Si/Al composites consist of Si and Al components and Al uniformly distributes among Si grains. The relative density of the Si/Al composites gradually increases with the decrease of Si content and reaches 98.0% when the Si content is 50%. The thermal conductivity, the coefficient of thermal expansion and the flexural strength of the composite all decrease with the increase of the Si content, and an optimal matching of them is obtained when the Si content is 60% (volume fraction).

Adiabatic shear fracture in Ti-6Al-4V alloy

Abstract Separated specimens of Ti-6Al-4V alloy were dynamically loaded at a strain rate of 3 900 s −1 using a split Hopkinson pressure bar (SHPB) apparatus. The fracture features of the separated specimens were investigated by a scanning electron microscope. The results show that adiabatic shear failure occurs in the tested specimens, and two typical areas (dimple and smooth areas) with different features are alternatively distributed on the whole fracture surface. The dimple areas originate from voids generation and coalescence, exhibiting ductile fracture characteristics. Simultaneously, ultrafine grains (UFGs) and microcracks among grains are observed on the smooth areas, indicating that the emergence of UFG areas is caused by the propagation of microcracks along grain boundaries and exhibits brittle fracture characteristics. Fracture occurring in adiabatic shear bands is not uniform and ultimate rupture is resulted from ductile and brittle fracture modes.