Materials Science Forum Vols. 633-634

Abstract: This work studies the deformation mechanisms active in two pure hexagonal close packed metals, beryllium (Be) and zirconium (Zr), during equal channel angular extrusion processing. An experimental-theoretical approach is employed to assess their relative contributions through measurement and calculation of texture evolution. A new multi-scale constitutive model, incorporating thermally activated dislocation density based hardening, is shown to effectively predict texture evolution as a function of processing route, number of passes (up to four), initial texture, pressing rate, and processing temperature. Texture predictions are shown to be in very good agreement with experimental measurements. Also, it is found that the two most active deformation modes in Be are basal slip and prismatic slip, where the predominant one is interestingly found to depend on die angle. Deformation in Zr during the first pass is predicted to be accommodated not only by its easiest mode, prismatic slip, but by basal slip and tensile twinning.

Abstract: The deformation mechanisms of various kinds of single crystals and bicrystals during the process of equal channel angular pressing (ECAP) have been paid more attention world wide. This paper reviews the recent progresses in the understanding of the deformation mechanisms of single crystals and bicrystals subjected to one-pass ECAP, and discusses the effect of initial crystallographic orientation and grain boundary on the microstructural evolution of these crystals. Based on those experimental results and analysis, it is suggested that in addition to the shear deformation along the intersection plane (IP) of ECAP die, the shear along the normal of IP also plays an important role in affecting the microstructural evolution and deformation mechanisms of these single crystals and bicrystals.

Abstract: The effect of the combination of natural aging and severe plastic deformation (SPD) produced by Equal-Channel Angular Pressing (ECAP) on the microstructure, the strength, as well as the ductility of age-hardenable AlZnMg alloys was investigated. A strategy is proposed for the processing of these “difficult-to-work” alloys at room temperature. Several advantages such as strengthening, precipitation-accelerating and ductility-improving effects of ECAP at room temperature are also shown and discussed in this work.

Abstract: It is generally accepted that grain refinement by the mechanisms of dislocation interaction, deformation twinning and/or stress-induced martensitic transformation is of relatively low efficiency. Rapid production of nanostructured metallic materials by conventional processing technologies remains a challenge. A new mechanism of fast grain refinement, through highly localized plastic deformation, was recently found in a -type biomedical titanium alloy (Ti2448). This mechanism leads to rapid grain refinement to tens nanometers and even amorphous transition during conventional cold processing. Since such grain refinement induces little strengthening, this process was previously termed soft nanostructuring. Here we review the research into this new way of nanostructuring and discuss the mechanism of grain refinement as well as dispersion strengthening of Ti2448 alloy by the precipitation of a second phase from the nano-sized  matrix.

Abstract: We review the current development status of Mo-Si-B alloys consisting of Mo solid solution and the intermetallic phases Mo3Si and Mo5SiB2 which could take advantage of the beneficial oxidation resistance of the silicide phases and of the outstanding mechanical properties of molybdenum. For adequate low temperature toughness a continuous Mo solid solution matrix should be established in the microstructure. Besides, wrought processing of such alloys at elevated temperatures requires the presence of an ultra-fine grained (UFG) microstructure. Both the prerequisites can be fulfilled using mechanical alloying (MA) as the crucial processing step which even yields nanostructured supersaturated powders after milling. However, values for the ductile-to-brittle transition temperature (DBTT) close to room temperature are unlikely due to grain boundary embrittlement by Si segregation. The possibility of reducing this segregation tendency by various micro-alloying additions will be demonstrated. Finally, the high temperature deformation behaviour of these UFG materials will be comparatively assessed against state-of-the-art Nickelbase single-crystalline superalloys.

Abstract: This paper has examined some recent findings concerning the processing of fully dense hetero-nanostructured materials (i.e. consisting of nano, ultrafine and micrometric grains) which can be produced by using the interplay between heavy deformation and recrystallization. By plastic deformation of bulk materials, an improved strength/ductility balance can be obtained directly by imparting high strain deformation (by ECAE) until the occurrence of recrystallization. Using a powder metallurgy route, the strong potential of electric field assisted sintering (ECAS) for producing multi-scale microstructures when a milled powder is used is also demonstrated. In this case, in addition to modify the classic processing parameters (time/temperature of ECAS), altering the nature of the milled powder - by Y2O3 addition during the milling stage - is also a good way to delay the onset of recrystallization and, thereby, increase the fraction of ultrafine grains.

Abstract: The present paper deals with the review of earlier studies and original investigation of microcrystalline (MC) and nanostructured (NS) nickel based INCONEL® alloy 718 processed by severe plastic deformation (SPD) via multiple isothermal forging (MIF). The alloy with a mean grain size of 1 µm - 80 nm has been studied in terms of its thermal stability, superplastic and mechanical properties. It was established that the NS state with the 80 nm grain size can be considered as thermally stable up to the temperature 600°C (0.56Tm). The increase of annealing temperature beyond 600°C causes static recrystallization. Investigations of mechanical properties at room temperature have shown that the decrease of a mean grain size provides the enhancement of strength and reduction of plasticity. Thus, the alloy with a grain size of 80 nm has shown the ultimate strength - 1920 MPa and ductility - 4.8%. Mechanical properties of the NS state of the alloy after annealing are discussed. It has been established that alloy 718 with a grain size of 80 nm displays superplasticity at 600°C and a strain rate of 1.510-4s-1. The values of relative elongation and strain rate sensitivity coefficient m are 350% and 0.37, respectively.

Abstract: A bulky SUS316 austenitic stainless steel (SUS316) was multi-directionally forged (MDFed) at 77 K and 300 K up to a cumulative strain of = 6 at maximum. With increasing cumulative strain, the grains were subdivided by mechanical twinning and martensitic transformation. Especially, mechanical twins accelerated grain fragmentation by subdivision of the initial grains and by intersection of the previously formed twins during MDF. The intersection of twins caused finally evolution of packet grains, which were composed of lamellar structured twins. The packet size and the lamellar twin spacing decreased down to 35 nm and 15 nm by MDF to  = 6 at 77 K. The average grain size achieved was estimated to be about 10 nm. Twinning appeared more frequently and uniformly at 77 K than at 300 K. Tensile test at 300 K revealed ultimate tensile strength of 2.1 GPa and fracture strain of about 0.2. The fracture strain, however, appeared to be constant over ∑Δε = 2.4 independent of cumulative strain. The observed excellent balance of strength and ductility of the nano-grained SUS316 is discussed in relation with the effects of twins on grain fragmentation and mechanical properties.

Abstract: The paper presents data on structure refinement and electropulse current effects on deformability, strength and ductility of shape memory TiNi alloy, processed different severe plastic deformation techniques including equal channel angular pressing and electroplastic rolling. It was found that deformability and elongation to failure decreases with grain refinement but they can be enhanced by electropulse current.

Abstract: The paper is devoted to research of an influence of average grains size on scalar dislocation density, fraction of geometrically necessary dislocations, internal stresses and bending- torsion of crystal lattice. Polycrystals of submicrocrystalline copper produced by torsion under hydrostatic pressure were investigated by TEM method.