Superplasticity in Advanced Materials - ICSAM 2006

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Authors: Jun Ting Luo, Qing Zhang, Hong Bo Li
Abstract: Nano-structured Si2N2O-Sialon composites were prepared by liquid phase sintering method with the amorphous nano-sized Si3N4 and nano-sized AlN powders. Nano-sized Al2O3 and Y2O3 powders synthesized by polyacrylamide gel method were introduced as additives. SEM examination shows the grain size of sintered body to be less than 80nm. Superplastic extrusion can be undertaken at 1550°C with a high velocity of 0.5mm/min and a big extrusion ratio of 3.57. There are a lot ring-shaped structure just like “annual ring” of trees in radial fracture and clear flow trace in axial fracture, all which is very similar to deformed metal materials and approve good superplastic deformation capability of Si2N2O-Sialon composites.
Authors: F. Wang, Kai Feng Zhang, Wen Bo Han
Abstract: 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.
Authors: F. Wang, Kai Feng Zhang, Wen Bo Han
Abstract: Superplastic forming processing of ceramic materials at high temperatures and high pressure puts forward very high requests to moulds and lubricants, at which conditions seeking for appropriate lubricants is one of important preconditions in the superplastic forming of ceramic materials. Using ring compressive testing, lubricities of hexagonal boron nitride in superplastic compressive forming processing of 3Y-TZP/Al2O3 ceramic composite at elevated temperatures were investigated. Theoretical curves of friction factor were drawn according to upper bound method. Ring compressive tests were adopted at temperature of 1400 °C-1600 °C. The results indicate that when BN is used, friction factor is about 0.4, meanwhile friction factor don’t have great changes with higher temperature and increasing strain rate. X-ray diffraction shows that under high temperature and high pressure no boron nitride converts to cubic phase, which remains at room temperature.
Authors: Yorinobu Takigawa, Hiroaki Takadama, Kenji Higashi
Abstract: Superplastic joining of 3mol% Yttria stabilized tetragonal zirconia polycrystal (3Y-TZP) is examined using 5wt%TiO2-doped 3Y-TZP as an insert material, in order to decrease the joining temperature and stress and to realize the local deformation near the joining surface. The joining tests are conducted by uniaxial compression in a temperature range of 1523 to 1723K. The joined specimen is characterized by a scanning electron microscopy (SEM), showing a clean interface with no cavitation. Four point bending tests are conducted to evaluate the joining strength, resulting an average flexural strength of 960MPa in the specimen joined at 1573K for sample compressed at 10MPa for 30 min, which is 80% of the strength of 3Y-TZP matrix. The temperature of 1573K is 100K lower than that obtained in sample without the insert material. Fracture occurs not at joining surface but at matrix in bending test. Residual stress is suggested to be introduced during joining process.
Authors: K.C. Chan, C.L. Wang, Guo Feng Wang, Kai Feng Zhang
Abstract: In this paper, the superplastic deformation behaviour of a pulse electrodeposited Ni-SiC nanocomposite was further studied at temperatures ranging from 330 to 530oC. It was found that optimum temperature of the composite was 450oC, and the shape of its flow stress vs strain rate curves is similar to that of conventional superplastic materials. The microstructures of the composite before and after deformation were examined using SEM and TEM. Dislocations and deformation twinning were observed in the deformed samples, and there was significant grain growth during deformation. The deformation mechanisms were also discussed in the paper based on the experimental findings.
Authors: Guo Qing Chen, Shao Hua Sui, X.D. Wang, Wen Bo Han
Abstract: In this paper constrained extrusion of the zirconia dispersed alumina nanocomposite under superplastic conditions was conducted. The mechanical properties of deformed material were studied and its results were compared with those of the initial materials. The microstructure evolution during superplastic deformation was also analyzed. The results demonstrated that after superplastic extrusion the flexural strength, relative density, Vickers hardness as well as fracture toughness of the material increased noticeably. The flexural strength of the deformed composite even retained at a high value of 310MPa at 800°C. The fracture toughness of the material increased from 6.92 MPa·m1/2 to 8.87 MPa·m1/2 after deformation. After superplastic extrusion due to grain boundary sliding and the compressive stress state, the internal porosities in as-sintered materials were eliminated. During extrusion with grain coarsening the effect of t-ZrO2 to m-ZrO2 transformation toughening increased because more zirconia grains reached the critical dimension. Although grain coarsening may cause the decrease of the fracture toughness in some extent, the phase transformation toughening and strengthening dominated. As a result, the mechanical properties of the deformed material were improved.
Authors: Guo Feng Wang, D.Z. Wu, C.W. Wang, Wen Bo Han
Abstract: In order to investigate the superplasticity of ceramic/ceramic laminated composite under tensile stress, the superplastic deep-drawing process was simulated by FEM. The results shown that, the strain and stress conditions of laminated composite, made by ceramic with different superplastic formability, are better than that of monolithic ceramic. Consequently, the material may exhibit high superplasticity. To testify this, some experiments were carried out. Tape casting and hot-press sintering are used to fabricate Al2O3/3Y-TZP laminated composite. As-received material is deep-drawing at high temperature to research its superplasticity. The results shown that, when suitable strain rate and forming temperature were used, the as-received material has excellent superplasticity.
Authors: S. Ding, Kai Feng Zhang, Guo Feng Wang
Abstract: 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.
Authors: S. Ding, Kai Feng Zhang, Guo Feng Wang
Abstract: Superplastic deformation behavior of nanocrystalline nickel was investigated under equibiaxial tension at different strain rates and forming temperatures. The nickel sheets with a thickness of 0.1 mm were prepared by pulse electrodeposition process. The average grain size of the as-deposited nickel was 70 nm and equiaxed. To determine the optimum processing parameters relevant to micro deep drawing, uniaxial tensile tests were carried out at temperatures ranging from 370°C to 500°C and strain rates ranging from 10-4 to 10-3s-1. In the selected temperature and strain rate ranges, the elongation value is larger than 200%, which indicates good superplastic formability of the electrodeposited nickel. Equibiaxial forming was subsequently performed at 370°C and 450°C, using a punch with a diameter of 1mm. The effects of forming temperature, punch rates on deep drawing process were experimentally investigated. The results indicated that the nickel specimens can be readily drawn at 450°C and punch rates ranging from 0.1mm• min-1 to 5mm• min-1. TEM and SEM were also used to examine microstructures of the as-deposited nickel sheet and deformed nickel specimens. The observations showed that significant grain growth occurs even at low superplastic forming temperatures. Microstructure was found to depend on the stress state and level of deformation.
Authors: Oleg M. Smirnov, V.K. Portnoy, M.A. Tsepin, N.L. Lissunets
Abstract: There are presented original experimental results of studying rheological behavior of Vitralloy 1 (Zr41Ti14Cu12.5Ni10Be22.5), which shows in the certain strain rate range stable linear viscous (Newtonian) flow. Comparative analysis of these results and the results related to other superplastic alloys as well as to crystallizing metallic melts has been carried out.

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