Kaifeng Zhang

Superplastic Properties of Al2O3/Ni-Mn Nanocomposite Fabricated by Electrodeposition

Ni-Mn nanocomposite reinforced by Al 2 O 3 particles was fabricated by pulse electrodeposition. The average grain size is 60 nm and the content of Mn and Al 2 O 3 particles is 0.3 and 0.6 wt pct, respectively. The superplastic deformation behavior was further studied at the temperatures ranging from 673 to 873 K. A maximum elongation of 530% is obtained in the tension test at a temperature of 773 K and at a strain rate of 1.67×10 −3 s −1 . The curves of the flow stress vs strain rate have features similar to the trend of conventional superplastic materials. The test temperature (773 K) equals to 0.35 T m , which means the material obtains low temperature superplasticity. The microstructures of the composite were examined and grain growth was observed during deformation.

Superplastic deformation behaviour of electrodeposited nanocrystalline nickel

Abstract In this paper, the superplasticity of an electrodeposited nanocrystalline nickel with a grain size of 65 nm was examined under different strain rates and temperatures. A maximum elongation of 550% was obtained at a relatively low temperature of 450°C and a strain rate of 1·67 × 10−3 s−1. The strain rate sensitivity index is found to be >0·5 demonstrating its good superplasticity. The fracture surfaces and the deformed microstructures reveal that significant grain growth occurs during deformation, and it is found that the addition of SiC particles can effectively improve the superplasticity of the material. Experimental results further illustrate that the deformed microstructure is dependent on the strain rate, and the surface morphology of the material is relating to its oxidation phenomenon during deformation.

Superplastic Forming of Silicon Nitride at Low Temperature

Si3N4 ceramic bodies were prepared by liquid phase sintering (LPS) with the amorphous nano-sized Si3N4 powders. Nano-sized Al2O3 and Y2O3 powders were introduced as additives. XRD analysis showed that the sintered body consists of β-Si3N4 and Si2N2O which confirms that phase change temperature of β-Si3N4 is lower than traditional Si3N4. SEM examination showed that the grain size of sintered body is smaller than 300 nm. Superplastic forming can be undertaken at the low temperature of 1550°C in a nitrogen atmosphere when the forming velocity is less than 0.5 mm/min. The formed parts rupture when the forming velocity is 1 mm/min or the forming temperature is 1500°C. Only a few defects are observed in the blank before forming, but many cavity groups are present in the formed workpiece.

Superplasticity and Sinter-Forging of Fine-Grained Si3N4-Si2N2O Composite

The Si3N4- Si2N2O composites are fabricated with amorphous nano-sized silicon nitride powders by the liquid phase sintering(LPS) method in this article. XRD analysis shows that the sintered body consists of β-Si3N4 and Si2N2O. SEM experiment conforms that the average grain size of sintered body is less than 300nm. The superplastic deep-drawing forming can be proceed at a low temperature of 1550°C with a forming velocity of 0.2mm/min. There are only a few small sintered defects before forming, but there are a lot of cavity groups after forming. Cavitation failure occurs by nucleation, growth and interlinkage of cavities. The complex-shape gears can be formed by a sinter-forging technology when the sintering temperature is 1600°C and the superplastic forging temperature is 1550°C.

Densification and Mechanical Properties of B4C Based Composites Sintered by Reaction Hot-Pressing

Boron carbide (B4C) possesses unique physical and thermal properties. In this paper, B4C based composites toughened by TiB2 were fabricated by in-situ reaction sintering with the original microcrystalline powders B4C, TiO2 and glucose. The influences of sintering temperature and content of TiO2 on the sintering behavior and mechanical properties were investigated. (TiB2, Al2O3)/B4C and (TiB2,SiC)/B4C composites with almost fully dense were fabricated by using additives　of Al2O3 and Si powders and sintering at 1950°C and 1900°C, the fracture toughness of composites reach to 7.09 and 6.35 MPa•m1/2 respectively. The analysis of microstructure shows that the main toughen mechanism is the crack deflection due to the existence of residual stress.

Microstructure Evolution and Fracture Behavior in Superplastic Deformation of Hot-Rolled AZ31 Mg Alloy Sheet

Fine-grained AZ31 magnesium alloy sheets were prepared through hot rolling process. The superplastic properties of hot-rolled AZ31 Mg alloy was examined by uniaxial tensile tests at a temperature range 250~450 and strain rate range 0.7×10 -3 ~1.4×10 -1 s -1 . Optical and scanning electronic microscope (SEM) were used to observe the microstructure evolution and fracture behavior in superplastic deformation of AZ31 Mg alloy and the values of deformation activation energy at various temperatures were calculated. It is demonstrated that, the hot-rolled AZ31 alloy begins to exhibit superplasticity from 300 and a maximum elongation of 362.5% is obtained at 400 and 0.7×10 -3 s -1 . In the temperature range 300~400 , the dominant super plastic deformation mechanism is grain boundary slide (GBS) controlled by grain boundary diffusion and the effects of temperatures on the fracture behavior of AZ31 Mg alloy are embodied by the transition of deformation mechanism from intracrystalline slip to GBS.

The Superplastic Forming Technology of Ti-6Al-4V Titanium Alloy Bellows

A new forming technology for bellows, which uses a combined superplastic forming (SPF) method by applying gas pressure and an axial compressive load, is developed. It can be used to fabricate large diameter U type titanium alloys bellows. Compared with conventional methods, the method has the advantage of low cost, high dimension accuracy and a high rate of finished products. During the SPF process, the tubular blank is restrained in a multi-layer die. The final dimensions of the bellows conform to corresponding dies. In order to obtain optimum thickness distribution of the components, the load route is divided into three stages free bulging, clamping and calibrating. Numerical simulations of this bellows SPF process are performed using a rigid visco-plasticity shell finite element code that was developed by the authors. Simulation results show great agreement with the experiments.

Influence of electric current on superplastic deformation mechanism of 5083 aluminium alloy

5083 aluminium alloy superplastic forming adopted resistance heating can not only improve efficiency and cut energy but also generate electroplastic/electrosuperplastic effect to make the material deformation possess lower flow stress and higher plasticity. By analysing the influence of current on dislocation slipping, grain boundary migration and dynamic recrystallisation, it is found that the electron wind force can enhance the mobility of dislocations; meanwhile, the current also can reduce the activation heat of dislocation motion by joule heating effect. What is more, the grain size of resistance heating forming sample is significantly smaller than furnace heating, and the cavities in the sample become small and dispersive, so the resistance heating forming specimen possesses better performance.

Superplastic Extrusion of Ultra Fine-Grained 3Y-TZP/Al2O3 Ceramic for Turbine Disk

Superplastic extrusion of axial forcing and radial flowing under different conditions was adopted to form a turbo-rotor, using hot-press sintered compact of zirconia (3Y)-toughened alumina composite that was prepared at 1450 °C for 1 hour with relative density of higher than 96%. Subsequent superplastic extrusion was attempted at temperature of 1500 °C-1650 °C. The results indicate that 3Y-ZrO2 plays an important role as a second-phase pinning agent and inhibits grain growth. The material shows good deformability and potential of near-net-shape forming. Comparing with undeformed sample, extruded sample was observed apparent coarsening in grain size and the remaining mechanical properties after deformation are not improved, irrespective of improved density. The dominating deformation mechanism is grain sliding and rotation accommodated with zirconia coordinated deformation.

Superplasticity and Microstructure Evolution of Electrodeposited Nanocrystalline Nickel

Nanocrystalline pure nickel (nc-Ni) was produced by pulse electrodeposition and its superplastic properties at and above room temperature were investigated. The electrodeposited nickel has a narrow grain size distribution with a mean grain size of 70nm. Uniaxial tensile tests at room temperature showed that nc-Ni has a limited plasticity but high tensile strength up to 1GPa at strain rates between 10-5 and 10-2s-1. However, when the temperature increased to 420 and higher, test specimens showed uniform deformation and the elongation value was larger than 200%. A maximum elongation value of 380% was observed at 450°C and a strain rate of 1.67x10-3s-1, SEM and TEM were used to examine the microstructures of the as-deposited and deformed specimens. The results indicated that fracture was caused by intergranular cracking and most cracks were originated from the brittle oxide formed during the tensile test. Grain coarsening was observed in the deformed specimen. The role of temperature and strain on grain growth was evaluated by comparing the microstructure of deformed samples with that of samples statically annealed. Deformation mechanism was discussed based upon the deformed microstructure and strain rate jump tests.