Materials Science Forum Vols. 838-839

Abstract: In the framework connected with the unification of physics, the activation energy for super plastic flow in advanced materials has been obtained by applying the new quantum mechanics and relativistic model proposed by Muñoz-Andrade. This new model allows the direct evaluation of the activation energy for super plastic flow at instantaneous thermo-mechanical material forming conditions. Also, in order to establish the phenomenology and mechanics of super plastic flow, the dependence on strain rate and phase velocity de Broglie is obtained, for the reason that the nature wavelength of the cellular dislocations is essential in the association with coupled mechanisms during super plastic flow, such as grain boundary sliding, cooperative grain boundary sliding and self-accommodation process. In conclusion, cellular dislocation dynamics is a nature mechanism during super plastic flow in advanced materials. The results obtained in this work are in a closed agreement with results reported previously.

Abstract: The equation σ=Kέ^m, where σ is the applied stress, έ is the strain rate, K and m are material constants that depend on stress / strain rate, temperature and grain size is often used to describe structural superplasticity. The general shape of the logσ-logέ curve is sigmoidal. Based on limited data, it was suggested by us earlier that a universal σ-έ curve could exist in a properly normalized space. έ and m are normalized with respect to έ_opt and m_max, the strain rate at which m is a maximum and the maximum m value respectively. Here a multi-dimensional relationship involving σ/σ_opt-έ/έ_opt-m/m_max-ΔF₀/kT-η/η_opt is developed; σ_opt corresponds to έ_opt, ΔF₀ is the free energy of activation for the rate controlling mechanism, k the Boltzmann constant, T the absolute test temperature, η the (apparent) viscosity of the superplastic alloy and η_opt is the viscosity of the same alloy for m=1 in a dimensionless σ-έ space. Using data concerning many systems, the phenomenology of structural superplasticity in all classes of materials is shown to be unique.

Abstract: The paper studies deformation mechanisms of nanocrystalline (NC) pure Al and its binary alloys with various distributions of an alloying element which can be Co or Mg via molecular dynamics simulations. It is revealed that a shear deformation of the pure Al is associated with the grain boundary sliding (GBS) and their simultaneous migration. Mg atoms in grain boundaries (GBs) of an Al-Mg alloy lead to GBS which does not accompany with a grain growth, while the deformation process of the corresponding alloy with a random distribution of Mg is close to that for the pure Al. Unlike Mg, GB segregations of Co atoms detain both GBS and GB migration and result in high strength of an Al-Co alloy. On the contrary, the strength of the alloy with the Co atoms distributed randomly is very low due to the structure amorphisation leading to the ease of plastic flow.

Abstract: This paper is a brief review of the concept of superplasticity, which has been extensively used to explain the superplastic behavior of yttria-tetragonal zirconia polycrystals. The diverse theories develop to account for the origin of this quantity are outlined. In addition to that, a modern approach to this concept is reported. This new contribution can give rise to a revision of the until-now established concept of invariant microstructure associated to superplasticity.

Abstract: Two-dimensional grain movements were microscopically observed in high-temperature shear deformation of an oxide-dispersion-strengthened ferritic steel with an elongated and aligned grain structure that was sheared in a direction perpendicular to the grain long axis. The microstructure was analyzed using electron back-scattered diffraction and electron channeling contrast imaging techniques before and after the shear deformation. Clear grain switching events, which are assumed to occur via grain-boundary sliding (GBS), were observed and the switching mechanism was characteristic of the core–mantle superplasticity model proposed by Gifkins; dislocation densities got much higher in narrow areas near the grain boundaries (mantles) than the grain interiors (cores). The mantle regions typically appeared in protruding portions of grains that was likely resistant to GBS, and low-angle boundaries were found to emerge at the core–mantle boundaries via slipping of dislocations within the mantle regions.

Abstract: This study investigated strain-rate sensitivity (SRS) in an as-extruded AZ31 magnesium (Mg) alloy with grain size of about 10 mm. Although the alloy shows negligible SRS at strain rates of >10^-5 s^-1 at room temperature, the exponent increased by one order from 0.008 to 0.06 with decrease of the strain rate down to 10^-8 s^-1. The activation volume (V) was evaluated as approximately 100b³ at high strain rates and as about 15b³ at low strain rates (where b is the Burgers vector). In addition, deformation twin was observed only at high strain rates. Because the twin nucleates at the grain boundary, stress concentration is necessary to be accommodated by dislocation absorption into the grain boundary at low strain rates. Extrinsic grain boundary dislocations move and engender grain boundary sliding (GBS) with low thermal assistance. Therefore, GBS enhances and engenders SRS in AZ31 Mg alloy at room temperature.

Abstract: Friction stirring is a fundamental process in the friction stir welding (FSW), and moreover, high strain rate deformation in elevated temperature to lead to extremely high ductility and fine grain size. In the present study, friction stirring process has been successfully modelled as a high temperature deformation depending on strain rate and temperature, assuming shear deformation of material in stir zone and generation of frictional heat by rotating tool. Axial load and torque during the process were estimated based on the model, and compared with the experimental data at two kinds of combination ratio in FSW of aluminum and Al-Mg alloy. It was, consequently, confirmed that the model could evaluate flow stress and strain rate from the experimental load and torque.

Abstract: Friction stir processing (FSP) is one of the severe plastic deformation (SPD) processes. It has been reported that SPD-processed Al with various purities attained a minimum grain size when Zener-Hollomon parameter is larger than 10¹⁶ s^-1. The minimum grain size is different by purity level and alloying elements. We investigated the influence of Fe solute atoms on grain refinement of high-purity Al on the condition that Zener-Hollomon parameter was larger than 10¹⁶ s^-1. FSP was conducted on Al-0.01%Fe, which was fabricated by using 5N Al (99.999% purity). FSP-ed Al-0.01%Fe exhibits the minimum grain size of 1.4 μm, although high-purity aluminums with more than 99.998% exhibits much larger minimum grain sizes of 30-40 μm. Only 101 at.ppm Fe played a critical role in the grain refinement of pure aluminums.

Abstract: Although superplasticity has intensively been studied for half century, few observations have been reported for pure metals due to fast grain growth at temperatures required for superplasticity. With developing of nanocrystalline materials, there was a hope that superplasticity could be obtained in a number of pure metals. Indeed, low temperature superplasticity in pure nickel was reported in pioneering work in 1999, later superplastic feature of nanonickel was attributed to sulfur presence in grain boundaries. Recently, it was concluded that superplasticity it is not related to the presence of sulfur at grain boundaries or a liquid phase at grain boundaries. Thereby, the phenomenon of superplasticity in pure metals is still far away for our understanding and it requires future work. This report is devoted to reassessment of superplastic behavior of nanonickel and it provides new results on enhanced plasticity of pure nickel processed by HPT consolidation of rapid quenched ribbons.

Abstract: Superplasticity of supplied 5A06 aluminum alloy is reviewed in this paper. Supplied 5A06 aluminum alloy is researched on superplasticity by the methods of same strain rate high temperature uniaxial tensile tests at temperature range375°C-500°Cand strain rate range 2.5×10-4s-1~1.0×10-2s-1. Microstructure and fracture of tensile samples are analyzed and discussed, deduce that grain boundary sliding (GBS) is the predominant deformation mechanism. Superplastic formability of the alloy is evaluated by gas bulging test at elevated temperatures. Gas bulging test demonstrates the deformation process parameters for the best superplastic formability is 400°Cand 0.005s-1 ,suggesting good application prospect for this aluminum alloy.