Materials Science Forum Vols. 475-479

Abstract: Superplastic behavior of the Zr65Al10Ni10Cu15 glass metallic alloy produced by powdermetallurgy method was examined in the supercooled liquid region. Stress-strain and stress-strain rate relationships showed that Newtonian viscous flow governed the plastic flow until strain hardening took place. The large strain hardening was proved to a result of occurrence of crystallization during deformation.

Abstract: 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.

Abstract: Superplastic behavior and cavitation were investigated for WC-15 mass % Co cemented carbides with the WC grain sizes of 0.7 µm (A) and 5.2 µm (B), WC-10 mass % Co cemented carbide with the WC grain size of 1.5 µm (C) and WC-5 mass % Co cemented carbides with the WC grain sizes of 0.5 µm (D) and 2.5 µm (E) by tensile tests at 1473 K. WC contiguity were 0.51, 0.31, 0.27, 0.56 and 0.49, respectively. The large elongations about 200 % were obtained for the B and the C having smaller values of WC contiguity compared to the other cemented carbides. The values of cavity volume fraction for them were less for the other cemented carbides, furthermore, cavities formed at WC/WC interfaces. Therefore, it is noted that the distribution of the Co phase is important for superplasticity of the cemented carbides.

Abstract: The microstructural evolution during superplastic deformation of the extruded stoichiometric NiAl polycrystals were systemically investigated in various conditions of temperature, strain rate and strain by means of optical microscopy (OM) and transmission electron microscopy (TEM). Consequently, The deformation microstructures corresponding to the large tensile elongation consisted of subgrains, low angle grains as well as high angle grains, which indicated that continuous dynamic recrystallization (CDRX) process was operating during superplastic deformation.

Abstract: Sandglass extrusion is an ultrafine grain size method. Due to the repetitive and multiple extrusions, large strain will be accumulated and ultafine grain size can be obtained. There are some factors that can affect the experimental result of sandglass extrusion. Among these factors, free space in mould cavity is very important, which can affect the forming of the fold during the extrusion. In this paper, the effects of free space in mould cavity on sandglass extrusion have been discussed and theory analysis and experimental results have been reported.

Abstract: The influence of temperature, strain rate, strain on mechanical parameters and microstructure evolution in isothermal deformation of GH4169 alloy were investigated. Lower resistance is obtained under higher temperature and or lower strain rate. However, coarser microstructure also is gained under such condition. The tensile property at room temperature and elevated temperature, permanent properties at elevated temperature of the GH4169 alloy with the treatment of “direct aging after isothermal deformation” are better than those of the same alloy with the treatment of solid-solution+direct aging after isothermal deformation. Isothermal forming was undertaken for a 1/4 size simulated part of a kind of turbo disk of an aviation engine. Its external is fine and the microstructure of the material was fine and homogeneous for both straightly after forming and after direct aging.

Abstract: The superplastic deformation behavior of a fine-grained 7075 Al has been investigated to clarify the issue of threshold stress. A series of mechanical tests has been conducted at various temperatures for the specimens with various grain sizes. The quantitative constitutive parameters have been determined from load relaxation test by applying the internal variable theory of structural superplaticity (SSP) proposed by Chang et al. The GBS flow could be formulated as a viscosity-type equation, characterized by the Newtonian exponent of 1.0. The unresolved issue of threshold stress is clarified and identified as a critical stress required for the GBS. The micro-mechanical roll of grain size refinement has also been manifested in terms of proposed constitutive parameters.

Abstract: In general, the mechanical behavior of superplastic spatially extended crystalline systems (SP-SECS) is characterized by a sigmoidal relationship between the applied stress and the steady state strain rate. The sigmoidal curve is defined by three regions: low stress - region I, intermediate stress or superplastic - region II and high stress - region III. The region I is known as the region where the threshold stress exists, but there are controversies on their existence. In this way, some experimental results reported in the past are analyzed. Earlier investigations have reported the apparent activation energy for creep in SP-SECS as a function of the applied stress, where it is exhibited a marked dependence of impurities or precipitates concentration. In addition, recent experimental evidences have revealed that the unified interpretation of creep, plasticity and superplasticity is the deductive rule [1]. In order to describe the interaction between precipitates at grain boundaries and dislocations during deformation processes a phenomenological expression for the threshold stress at the Grain Boundary in SP-SECS is described in this work.

Abstract: A Zn-Al eutectoid alloy known as a fine-grained superplastic material is tested both in tension and compression under superplastic conditions, and the influence of the stress state on superplastic deformation behavior, such as flow stress, is investigated. In the compression test, deformation is interrupted and turning is performed to remove a barreled portion so that the gauge section becomes uniform, and then the deformation is continued. Microstructural change during the deformation is also investigated. As a result, flow stress in the compression becomes higher than that in tension even after the barreled portion is removed. After deformation, grain growth is observed both in the tension and compression, and growth rate in the compression is larger than in the tension.

Abstract: A new application of superplasticity was proposed in the manufacturing process of metal foams. Preform sheets were manufactured using superplastic 5083 aluminum alloy sheets through accumulative roll-bonding (ARB) process. Microcellular aluminum foam plates with 50% porosity were produced through solid-state foaming under the superplastic condition. The cell shape was oblate spheroid, which is effective to reduce the thermal conductivity. The present aluminum foam plates have a potential as an excellent heat insulator.