Advanced Materials Research Vols. 97-101

Abstract: A fluidized beds method used in the industry for deacidification of the fumed silica has several drawbacks. The present work was intended to develop a simple, rapid and cost-effective method for the deacidification of fumed silica with high specific surface area using microwave technique. The effect of microwave irradiation on the body of the fumed silica has been investigated. The experimental results show that the microwave irradiation technique has been effective in deacidification and suppressing the aggregation process of fumed silica. This new approach may be useful to accelerate the deacidification process and avoid the aggregation growth.

Abstract: Using femtosecond laser pump-probe technique, the transient change in nickel film reflectivity after excitation by femtosecond laser pulse was measured, which indicated the thermalization process of the nickel film. A novel mechanism of the themalization process of the nickel film was proposed to interpret experimental results. It is found that the trend of the transient reflectivity change and the temperature change of nickel film excitated by femtosecond laser pulse are attributed to the special energy gap of the nickel film.

Abstract: Color-optical microscopy, energy spectrum analysis, hardness measuring, tensile and corrosion testing were conducted to investigate characteristics of microstructure, mechanical properties and corrosion resistance of S32750 super-duplex stainless steel aged at 850～920°C. The results indicate that with the increase of aging temperature and aging time the content of σ phase increases, while the ferrite content decreases. The forming and growing of σ phase obtained during aging causes an increase in hardness and a reduction in ductility of the aged steel. Moreover, increasing aging time the corrosion resistance reduces, owing to a new formed austenite occurs.

Abstract: A three-phase constitutive model for TiNiNb shape memory alloys (SMAs) is proposed based on the fact that TiNiNb SMAs are dynamically composed of austenite, martensite and -Nb phases. In the considered ranges of stress and temperature, the behaviors of austenite, martensite and -Nb phases are assumed to be elastoplastic, and the behavior of an SMA is regarded as the dynamic combination of the individual behavior of each phase. Then a macroscopic constitutive description for TiNiNb SMAs is obtained by the conventional theory of plasticity, the theory of mixture, the theory of inclusion, and the description of phase transition by Tanaka. The method for determination of the material parameters is given. This constitutive model can describe the main characteristics of SMAs, such as ferrcelasticity, pseudoelasticity and shape memory effect.

Abstract: The effect of specimen’s mean radius to average wall-thickness ratio rs/t, has been studied using the Split Hopkinson Torsional Bar. The rs/t ratio was varied between 6.5 and 39 keeping the outer diameter and the gauge length of the specimen constant. Higher strain rates and strains are observed for specimens with larger specimen radius to wall-thickness ratio rs/t. However no monotonic trend of the shear stress with rs/t variation is found. A sharp decrease in the torsional stiffness of the specimen is observed with increasing the specimen’s rs/t ratio. The rs/t ratio can also be used as a useful dimension to enhance the strain rate in tubular specimens.

Abstract: Advanced high strength steels are increasingly utilized to realize the lightweight of automotive body for economical and ecological considerations. The fatigue durability of notched components is one of the significant evaluation parameters for reasonable material selection. Strain-controlled fatigue experiments of low-alloy TRIP steel and DP steel with 590MPa grade were performed at room temperature in this study. Experimental results indicate that both fatigue life and cyclic stress amplitude of TRIP steel are superior to those of DP steel at the same strain amplitude. Furthermore, local strain-life models of two steels were determined with linear regression method to predict the fatigue life of notched body structures with finite element method. It can be concluded that TRIP steel can provide more excellent potential than DP steel for the lightweight design of notched automotive structures from the viewpoint of fatigue resistance.

Abstract: Mechanical properties and microstructural evolution of an Al-Cu-Mg-Ag alloy during thermal exposure at elevated temperature were characterized by means of tensile tests, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observation. The tensile test results suggested that tensile strength (σBbB) and yield strength (σBsB) reduced with both increasing exposure time and temperature. When exposed at 150°C, the Al-Cu-Mg-Ag alloy exhibits good structural stability due to the splendid thermal stability of the Ω phase. SEM results showed that the specimen was fractured in a combined manner of intergranular and transgranular fracture. TEM results suggested the reduction of mechanical properties of samples was attributed to the coarsening of Ω phase and the precipitation of θ phase during thermal exposure.

Abstract: The liquid structure of two lead-free solder Molten alloys, Sn-0.5Cu and Sn-1.8Cu (wt.%), has been examined using X-ray diffraction method. The main peak for liquid structure of Sn-0.5Cu is similar to that of pure Sn. A pre-peak has been found in the low Q part on the structure factor S(Q) of Sn-1.8Cu tested under 320°C, but it disappeared finally when the testing temperature reached 350°C. The both viscosity was measured using a torsional oscillation viscometer. It was found that the anomalous variations of viscosity had a direct relation with the transition of the liquid structure, which is consonant with the results of high temperature X-ray diffraction. The microstructure of the solder matrixes as well as interfacial reaction between liquid solders and Cu substrates was also studied. The results show that particle-like Cu6Sn5 intermetallic compounds (IMCs) emerge in Sn-1.8Cu solder matrix. The IMC layer at Sn-1.8Cu/Cu joint is thicker than that at Sn-0.5Cu/Cu interface. The correlative effect of liquid structure on phase evolution in the solder joints is analyzed.

Abstract: Three-point bending specimens of MgAl are loaded by MTS material testing equipment and Hopkinson pressure bar. Strength ratio of MgAl subjected to static compression loading is gained. The dynamic response of three-point bending specimen subjected to impact loading is simulated by ABAQUS finite element software. The stress distribution around crack tip is like shape of butterfly. It indicates that stress decreases quickly away from crack tip. Strain gauges are affixed at the swings of butterfly in the experiments and can experience higher strain signal. So the choice of strain gauge position is reasonable. Based on finite displacement method and the least multiplication value theory, dynamic stress intensity factors at different loading velocities are obtained. According to crack initiation time, dynamic initiation toughness is computed. The results indicate that dynamic initiation toughness of MgAl varies at different loading rates and increases with ball velocity.

Abstract: A comparison between crack growth rate (da/dN) vs. effective stress intensity range factor (ΔKeff) curve behavior and microscopic and macroscopic fracture surface of commercial Ti-6Al-4V alloy are presented. Three different regimes are correlated with characteristics measured on the fracture surfaces. Three regions can be observed in which part I is rough and darker than others parts known as pre-transition, part II is smooth and light known as transition region and part III is a little darker than part II known as post-transition region. In the present investigation the correlation of fatigue crack growth rate for Ti-6Al-4V and microstructure of fracture surface has been presented.