Materials Science Forum Vol. 735

Abstract: The Earth deforms dominantly by solid-state creep. Diffusion creep is known to be important. It is less clear whether mechanisms in which grain boundary sliding is accompanied by other processes (dislocation activity), and/or are associated with stress exponents closer to 2 than to 1 are important. Since the mechanisms of superplasticity are themselves not fully resolved, we cannot say for sure whether the Earth deforms superplastically. Models for diffusion creep are relevant for the Earth and possibly for superplastic materials. Modelling shows that large strains may not necessarily obliterate initial textures because grain rotations, although they occur, slow down as microstructures evolve. Modelling also predicts major strength anisotropy induced by grain shape alignment. Models for two-phase diffusion creep can be constructed for when the second phase is inert (insoluble). If both phases are soluble and can participate in diffusion, the basic theory for single phase diffusion creep cannot be applied and new insight is required.

Abstract: In order to attain high-strain-rate superplasticity (HSRS) in ceramics, flow behavior was examined with ZrO₂ reference sample. The results suggest that the enhancement of the accommodation processes of grain boundary sliding (GBS) is important in addition to the careful controlling the microstructural factors, such as stable fine grain structure, reducing residual pores and so on. The spinel particles dispersion can simultaneously provide the following positive factors to ZrO₂: i) suppressed grain growth due to pinning effect of spinel particles, enhanced accommodation due to ii) accelerated relaxation of stress concentrations exerted by GBS through dislocation motion and iii) accelerated lattice diffusion caused by the dissolution of aluminum and magnesium into ZrO₂ from the spinel particles. The positive factors due to spinel dispersion make it possible to attain HSRS in ZrO₂ ceramics.

Abstract: HCP metals show new dislocation creep at temperatures below 0.3 Tm with stresses below σ0.2, while FCC metals show it above σ0.2. In the former, grain boundaries absorb the dislocations through slip-induced grain-boundary sliding, while in the latter dislocations are accommodated by cross slip at cell walls. The difference comes from the difference in the crystal symmetry. In UFG-Al at low temperatures, it is anticipated that grains without cell structure lead creep deformation similar to CG HCP metals rather than CG Al. UFG Al specimens were fabricated by ARB method. They showed remarkable creep behavior at less than σ0.2 similary to CG HCP metals. It posseses stress exponent of about three, grain-size exponent of almost zero, and very low apparent activation energy of 20 kJ/mol, and also grain boundary sliding behavior is obserbed by AFM.

Abstract: Highly-textured, rolled AZ31 sheet material shows a significant drop in the plastic anisotropy (r-value; r=ew/et) in tension between 25°C and 200°C. This behavior was initially explained as a result of the increased activity of non-basal slip with increased temperature. Other authors suggested, however, that the mechanism responsible for this phenomenon was the activation of grain boundary sliding (GBS). Here, in-situ tensile tests have been carried out in an SEM at various temperatures in order to obtain further evidence of the role of GBS during moderate to high temperature deformation of Mg alloys, which remains highly controversial.

Abstract: An earlier proposal is generalized to explain superplasticity in different classes of materials and grain size ranges. A definition of “superplasticity” as due to a unique physical mechanism, rather than in terms of extreme elongations and/ or strain rate sensitivity index, m, being more than or equal to 0.30 emerges.

Abstract: The conventional consensus has it that the magnitude of the strain rate sensitivity observed in superplastic materials is linked with grain boundary sliding. The grain boundary sliding mechanism is thought to theoretically produce a strain rate sensitivity exponent of 0.5, which is in good agreement with experimental data. The present paper argues that a rate sensitivity of 0.5 can be generated by dislocation slip under certain temperature and strain rate regimes that overlap with conditions representative of superplasticity. A physically based slip model that links the relevant microstructural parameters to the macroscopic strain rate is proposed.

Abstract: Superplastic-like forming takes advantages of both deep drawing and bulge forming. The use of non-superplastic grade materials enables it to be more compatible with existing forming process with less material and time cost. Here, a non-isothermal heating system was adopted to selectively heat up selected localized areas to form the workpiece more efficiently. Electron backscattered diffraction (EBSD) was then used to investigate the wide range of grains in the formed samples resulting from elevated-temperature and large-strain deformation. The crystallographic textures of the material before and after deformation were observed for comparison. Very little recrystallization was found in the midst of the deformed grains. Considerable amount of elongated grains with high angle boundaries were produced during deformation. Many subgrain boundaries have developed within the big grains due to dynamic recovery.

Abstract: Thermomechanical processing to enable superplasticity in AA5083 materials includes cold working followed by heating prior to hot blow forming. Upon heating for forming at 450°C, a B-type ({110}) rolling texture is replaced by a near-random texture with a weak superimposed cube orientation parallel to the sheet normal. The presence of refined grains 7 – 8μm in size reflects the predominance of particle-stimulated nucleation of recrystallization prior to forming. The subsequent evolution of microstructure, texture and cavitation behaviour during biaxial deformation in the solute drag creep (SDC) and grain boundary sliding (GBS) regimes will be presented.

Abstract: The method was developed for quantitative estimation of inputs to plastic deformation of crystallographic and non-crystallographic modes of slip by data of X-ray texture measurements. The texture analysis allows to split material into fractions, deformed by predominant operation of crystallographic and non-crystallographic mechanisms, differing in final orientations of grains. Whereas texture maxima in pole figures correspond to grains, deformed by means of crystallographic slip and having predictable final orientations, texture minima are formed by grains, whose deformation does not submit to crystallographic regularities and therefore their orientations are deflected from stable positions or even prove to be arbitrary. These effects are demonstrated as applied to semi-products from Zr-based alloys, subjected to the deformation treatment at temperatures of the (α+β)-region of Zr-Nb phase diagram.