Jianshe Lian

Effects of microstructural variables on the deformation behaviour of dual-phase steel

Abstract An expression for the stress of martensite in dual-phase steel was developed, which shows the interdependence of the stress of martensite and strain hardening in the ferrite matrix and the contribution of microstructural variables (the volume fraction of martensite fm, ferrite grain size df, and martensite particle size dm). The onset of plastic deformation of martensite in dual-phase steel was predicted to depend on its yield strength and the microstructural variables, and this was verified by the modified Crussard-Jaoul analysis. It was found that for this dual-phase steel, refining the grain size and increasing fm increase the flow stress and raise the strain hardening rate at low strains, but little affect the strain hardening rate at high strains. The effect of the ferrite grain size on the flow stress of this dual-phase steel was found to obey the Hall-Petch relation, i.e. σ = σ 0 e + K e d f − 1 2 , where the Hall-Petch intersection σ0e and slope Ke are functions of strain, fm and dm. The effects of the plastic deformation of martensite and the microstructural variables on the strain hardening rate and the Hall-Petch behaviour were analysed in terms of the densities of statistically stored dislocations and geometrically necessary dislocations using the previously developed theoretical model.

A black phosphate coating for C1008 steel

This paper describes an effective metal finishing technology for obtaining black phosphate coatings on steel. The black conversion coating obtained on steel (C1008, AISI) and cast iron (No.35, ASTM) was dense and uniform with a thickness up to 18 μm. The coating shows better corrosion resistance; lubricating ability and higher efficiency of light-absorption than the traditional phosphate coatings. The black phosphate-forming mechanism was investigated. It is shown that sodium molybdate added to the black phosphate treatment bath can refine the microstructure. Corrosion tests showed that the corrosion rate was lower than that of traditional phosphating. The black phosphate coating, when employed as the pretreatment layer before laser heat-treatment, can remarkably improve the efficiency of light-absorption during laser heat-treatment. This phosphate technology had been employed successfully on the production line in the automobile industry.

Experimental and modelling investigations on strain rate sensitivity of an electrodeposited 20 nm grain sized Ni

Two experimental techniques of nanoindentation and tensile testing were used at room temperature to investigate the strain rate sensitivity of an electrodeposited Ni with a mean grain size (d) of 20 nm, respectively. It was found that the nanocrystalline (nc) Ni possessed a higher strain rate sensitivity exponent (m) during nanoindentation than during tensile testing. Furthermore, a higher m was accompanied by a smaller activation volume (V ). It is believed that the higher stress concentration could activate a shorter dislocation line length (L), which should be responsible for the higher m value during the nanoindentation. Based on a model of dislocation nucleation or bowing-out mechanism, the relationship between m and d for Ni and its alloys was investigated. In the end, a simple and straightforward equation relating m to d was proposed in aid of a simple assumption associating L with d, which implied that the enhanced m in nc Ni and its alloys with d> ∼ 6 nm should be due to the reduction of the dislocation line length. (Some figures in this article are in colour only in the electronic version)

Strain rate sensitivity of a nanocrystalline Cu synthesized by electric brush plating

A method for synthesizing bulk nanocrystalline Cu by an electric brush-plating technique is reported. This brush-plated nanocrystalline Cu has a fine (26nm) and quite uniform grain structure and predominant high-angle grain boundaries. A pronounced strain rate sensitivity of the stress with an m of 0.104 and the Coble creep and a subsequent transition to the power-law creep were observed in room temperature tensile and creep tests. The dominant grain boundary deformation due to the truly nanocrystalline structure of this nanocrystalline Cu is responsible for the observed strain rate sensitivity.

Enhancing photocatalytic activity of disorder-engineered C/TiO2 and TiO2 nanoparticles

We demonstrate a simple and green synthetic pathway to prepare TiO2–carbon nanoparticles (C/TiO2 NPs) by the sol–gel method, abandoning additional carbon sources but utilizing the organic group in the Ti precursor. Then the C/TiO2 NPs were decarburized under an air atmosphere at 500 °C for 2 h to form the reduced TiO2 nanoparticles. XRD, Raman spectrum, HRTEM and electron energy loss spectrum (EELS) analyses showed that the C/TiO2 NPs were composites of core-shell structured TiO2 and amorphous carbon; and both C/TiO2 and reduced TiO2 NPs contained a large number of oxygen vacancies, which led to structural disorders in them. These structural disorders induced the valence band tails to enhance visible light absorption and to tailor the bandgap structures of the two modified TiO2 samples to match the hydrogen and oxygen production energy levels. As a result, the two structure-disordered C/TiO2 and reduced TiO2 nanocrystals showed excellent solar-driven photocatalytic activities: the C/TiO2 performed best on the photodegradation of phenol and methyl blue, while the reduced TiO2 displayed an excellent hydrogen generation rate, 10 times higher than that of the reference TiO2 by photo-splitting water.

Effect of grain size on corrosion behavior of electrodeposited bulk nanocrystalline Ni

Nanocrystalline (NC) and coarse-grained Ni with different grain sizes (from 16 nm to 2 μm) were fabricated by direct current electrodeposition. Effect of grain size on the electrochemical corrosion behavior of these Ni deposits in different corrosion media was characterized by using potentiodynamic polarization test, electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS) and immersion corrosion test. Results show that in the NaOH or NaCl solution, the NC Ni exhibits improved corrosion resistance with the decrease of grain size. But in H2SO4 solution, the higher grain boundary density accelerates corrosion due to no passive process and the corrosion resistance of NC Ni decreases with refining grain size. The distinct experimental results of NC Ni in corrosion behavior can be reasonably explained by the positive or negative effect of high-density grain boundaries in different corrosion media.

An analytical study of the influence of thermal residual stresses on the elastic and yield behaviors of short fiber-reinforced metal matrix composites

Abstract An analytical model was developed to study the influence of thermal residual stresses on the elastic and yield behaviors of aligned short fiber-reinforced metal matrix composites. Three-dimensional solutions of elastic stress field in the matrix of a circular cylindrical unit cell were first obtained based on the theory of elasticity. On this basis, an alternative analysis of the stress transfers between the matrix and the fiber was given by using the shear lag analysis, which provides the expressions for the stress distributions in the matrix and the fiber. The thermal residual stresses and the distributions of the stresses under tensile and compressive loadings as a function of the material parameters, such as fiber volume fraction, fiber aspect ratio and matrix yield stress, were calculated. Furthermore, simple expressions for the composite elastic modulus and yield strength under tensile and compressive loadings were derived. These expressions were used to study the influence of the thermal residual stresses and the material parameters on the elastic moduli and yield strengths under tensile and compressive loadings. Finally, the model predictions were compared with the experimental results on the SiC w /Al–Li T6 composite and several SiC w /Al T6 composites in literature and also with the other theoretical models.

Strain rate sensitivity of face-centered-cubic nanocrystalline materials based on dislocation deformation

The relationship between strain rate sensitivity and activation volume for face-centered-cubic metals is proposed based on the bow-out model of single dislocation from its source, which gives reasonable prediction of the enhanced strain rate sensitivity that occurs in nanostructured and ultrafine grained Ni and Cu.

Synthesis of a Thin-Layer MnO2 Nanosheet-Coated Fe3O4 Nanocomposite as a Magnetically Separable Photocatalyst

A facile hydrothermal method combined with a mild ultrasonic means has been developed for the fabrication of a magnetically recyclable thin-layer MnO2 nanosheet-coated Fe3O4 nanocomposite. The photocatalytic studies suggest that the MnO2/Fe3O4 nanocomposite shows excellent photocatalytic efficiency and stability simultaneously for the degradation of methylene blue under UV-vis light irradiation. Moreover, its good acid resistance and stable recyclability are very important for its future practical application as a photocatalyst. Magnetic measurements verify that the MnO2/Fe3O4 nanocomposite possesses a ferromagnetic nature, which can be effectively separated for reuse by simply applying an external magnetic field after the photocatalytic reaction. This novel composite material may have potential applications in water treatment, degradation of dye pollutants, and environmental cleaning.

Influence of preparation methods on photoluminescence properties of ZnO films on quartz glass

The influence of preparation methods on the photoluminescence properties of ZnO film was studied. Two methods were applied to fabricate ZnO films in a conventional pulsed laser deposition apparatus. One is high temperature (500−700 ℃) oxidation of the metallic zinc film that is obtained by pulsed laser deposition. The other is pulse laser ablation of Zn target in oxygen atmosphere at low temperature (100−250 ℃). The photoluminescence property was detected by PL spectrum. The room temperature PL spectra of the ZnO films obtained by oxidation method show single violet luminescence emission centered at 424 nm (or 2.90 eV) without any accompanied deep-level emission and UV emission. The violet emission is attributed to interstitial zinc in the films. Nanostructure ZnO film with c-axis (002) orientation is obtained by pulsed laser deposition. The ZnO film deposited at 200 ℃ shows single strong ultraviolet emission. The excellent UV emission is attributed to the good crystalline quality of the film and low intrinsic defects at such low temperature.