Materials Science Forum Vols. 551-552

Abstract: In this work the superplastic behaviour of a hot rolled AZ31 magnesium alloy sheet under a biaxial tension test with the blow forming technique is presented and reported. The specimen dome height and its thickness distribution, during and after the test, have been used as characterizing parameters. A numerical FE model of the test has been developed in order to easily characterize the material and to directly analyze experimental results. The influence of the rolling cycle on the microstructure and consequently on the material behaviour has been also analyzed. A synergic use of experimental results and of the numerical model has been done for finding material constants in different situations. The material flow parameters have been found and results are presented.

Abstract: In order to describe scale effects in rigid-viscosity-plastic deformation at micro scale, a strain rate gradient model with couple stress is proposed. In the constitutive equation, couple stress, strain rate gradient are introduced on the assumption that the potential energy depends upon the scalar invariants of the strain rate tensor and strain rate gradient tensor, at the same times, the influences of length scale of body, grain size and temperature are also considered. A FEM-based program based on this strain rate gradient model is used to simulate the process of high temperature gas pressure forming, and the value of length scale 2 l is evaluated by simulation pilot calculation. It is indicated that rigid-viscosity-plastic deformation can be more accurately simulated by the present strain gradient rate model as long as the material parameters in constitutive equation are selected suitably.

Abstract: The grain boundary surface is the excess energy of the grain boundary as the lattice on one side of the grain is translated relative to the lattice on the other side of the grain. The maximum in the slope of the grain boundary surface determines the ideal shear strength for the grain boundary sliding. We presented the ideal shear strength for the grain boundary sliding in aluminum Σ3(11 2)[110] tilt grain boundary from the first-principles calculations. The ideal shear strength for the grain boundary sliding was much smaller than the ideal shear strength of a perfect single crystal.

Abstract: In the present work, we studied the nanostructural behavior of a denominated alloy Clad- Zinag, which is composed of a single layer of Zinag coated on both surfaces by an aluminum film. The alloy was then deformed by two different processes, one in which a superplastic conformed technique was applied and the second was by way of tensile testing. For both techniques, the alloy was examined and analyzed by viewing the materials topography with an atomic force microscope (AFM). From these techniques we were able to observe both processes of plastic deformation and the presence of marks which correspond to bands of grain sliding. This mechanism is a result of superplastic deformation, and influences the grain size of aluminum. Nevertheless, we must clarify that the name of mesoscopic band of slides, is based on a study according to Vinogradov.

Abstract: By means of superplastic tensile test above the Ac1 temperature (γ→α transformation temperature), superplastic deformation activation energy of ultrafine-grained commercial die steel CrWMn is investigated on the basis of the Arrhenius theory equation, exp( / ) 1 ε& = Aσ m −Q RT , which indicates the resistance of the superplastic deformation. According to the Arrhenius equation, the activation energy is estimated from the log σt vs 1/T relationship at a constant of sensitivity index of strain rate. The results show that the strain rate sensitivity index is a constant and rather high at the conditions of superplastic deformation for the CrWMn steel, the activation energy for superplastic deformation of steel CrWMn above the critical temperature is 187KJ/mol, and the superplastic deformation activation energy is approached to the grain boundary diffusion activation energy of γ-Fe. This indicates that the grain boundary sliding (GBS) in superplastic deformation of CrWMn steel is controlled by grain boundary diffusion. The characters of superplastic deformation of the steel above the critical temperature, on the other hand, are also analyzed in this paper.

Abstract: Scale effects in the high temperature gas pressure forming of electrodeposited fine-grained copper thin sheets were investigated by a series of tests at various forming temperatures and die apertures. The average as-deposited copper grain size was 5 μm. The geometrical parameters of the bugling die system and the thickness of copper sheet varied in proportion. Different radius hemisphere parts from 0.5mm to 5mm were obtained at a strain rate of 5.0×10−4 s−1, which was controlled by pressure forces curves determined in terms of a finite element method (FEM) based on constitutive equation proposed by Backoften in 1964. The experimental relative bulging height (RBH) values were measured, and compared with that predicted by the same finite element method (FEM). It was found that the experimental values of large scale parts approach to simulated values, whereas the experimental values of small scale parts were quite different from simulated values. In order to explain these phenomena, a grain-rotation-weakened mechanism was proposed.

Abstract: The early view of superplasticity was that it was a phenomenon that could only be exhibited by fine grained, two phase alloys. This effectively ruled out most alloys that possessed attractive service properties. The first material to demonstrate good superplastic properties from a virtually single phase microstructure was the Al-6%Cu-0.5%Zr, AA 2004 but this was followed by superplastic versions of AA7475, AA8090 and AA5083. Superplasticity was also demonstrated in magnesium based alloys at an early stage. More recently different grain control additions, such as scandium or erbium have been investigated and it has also been demonstrated that, in certain circumstances, aluminium simply with the addition of a grain controlling element can exhibit good superplastic behaviour. While conventional wisdom teaches that large fabricating strains are required to confer good superplastic properties in the sheet product, recent results with both aluminium and magnesium alloys cast doubt on this belief. Although, for many years, strip casting has appeared to provide an attractive semi-fabricating route for superplastic sheet problems with centre line segregation in alloys with a wide freezing range have precluded its use. It has been demonstrated that recent developments in strip casting enable production of alloys with as wide a freezing range as AA5182 to be cast with a fine, equiaxed grain structure across the strip thickness. The paper will review the state of these various developments and their implications for the manufacture of superplastic sheet materials.

Abstract: High-temperature deformation behavior and microstructural evolution process of ELI Ti-6Al-4V alloy having martensite microstructure were investigated with the variation of strain, strain rate and temperature. A series of hot compression tests was carried out isothermally for martensite microstructure at the true strain range of 0.6 to 1.4, strain rate range of 10-3 s-1 to 1 s-1 and temperature range of 700 oC to 950 oC. The processing maps for martensite microstructures were constructed on the basis of dynamic materials model (DMM). At the strain rate higher than 10-2 s-1 and the temperature lower than 750 oC regions of flow instability such as adiabatic shear band and micro-cracking were observed. Also, after imposing an effective strain of ≈ 1.4, deformed microstructure showed the significant kinking/bending behavior of lamellae resulting in the dynamic globularization associated with the fragmentation of beta-phase. The effects of strain, strain rate and temperature for dynamic globularization were discussed based on the microstructure and efficiency of power dissipation.

Abstract: In this work, studies were carried out to investigate the superplasticity of a commercially pure (CP) titanium alloy during high temperature deformation. Uniaxial tensile tests were carried out at 600, 750 and 800°C with an initial strain rate from 10-1s-1 to 10-4s-1. It was found that the alloy do not show good superplasticity due fast grain growth at high temperature and cavity. The effects of temperature on the grain growth and cavity phenomena as well as the dynamic recrystallization of the alloy were studied and a ‘two-step-method’ was introduced to increase the superplasticity of the alloy.

Abstract: Superplastic compression of the Ti-6.62Al-5.14Sn-1.82Zr alloy were carried out at a Thermecmaster-Z simulator at deformation temperatures of 960°C, 980°C, 1000°C and 1020°C, strain rate of 0.001 s-1, 0.01 s-1 and 0.1 s-1, and height reduction of 50%, 60% and 70%. The α phase decreases with the increasing of deformation temperature, and the grain size of α phase has a slight variation with the deformation temperature. The strain rate affects both the morphologies and the grain size of α phase, and the optimal strain rate makes the grains be fine. The optimal height reduction also makes the α phase be fine and well distributed on the matrix of Ti-6.62Al-5.14Sn-1.82Zr alloy after superplastic compression.