Microstructure and Evolution of Mechanically-Induced Ultrafine Grain in Surface Layer of Al-Alloy Subjected to USSP

Experiments were conducted to investigate the ultrafine-grained (UFG) microstructures in the surface layer of an aluminum alloy 7075 heavily worked by ultrasonic shot peening. Conventional and high-resolution electron microscopy was performed at various depths of the deformed layer. Results showed that UFG structures were introdued into the surface layer of 62 μm thick. With increasing strain, the various microstructural features, e.g., the dislocation emission source, elongated microbands, dislocation cells, dislocation cell blocks, equiaxed submicro-, and nano-crystal grains etc., were successively produced. The grain subdivision into the subgrains was found to be the main mechanism responsible for grain refinement. The simultaneous evolution of high boundary misorientations was ascribed to the subgrain boundary rotation for accommodating further strains. Formed microstructures were highly nonequilibratory.  2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

Microstructural characterization of ultrafine-grain interstitial-free steel by X-ray diffraction line profile analysis

This paper highlights the microstructural features of commercially available interstitial free (IF) steel specimens deformed by equal channel angular pressing (ECAP) up to four passes following the route A. The microstructure of the samples was studied by different techniques of X-ray diffraction peak profile analysis as a function of strain (epsilon). It was found that the crystallite size is reduced substantially already at epsilon=2.3 and it does not change significantly during further deformation. At the same time, the dislocation density increases gradually up to epsilon=4.6. The dislocation densities estimated from X-ray diffraction study are found to correlate very well with the experimentally obtained yield strength of the samples.

Ultrafine grain formation in a Ti-6Al-4V alloy by thermomechanical processing of a martensitic microstructure

In the current study, ultrafine equiaxed grains with a size of 150 to 800 nm were successfully produced in a Ti-6Al-4V alloy through thermomechanical processing of a martensitic starting microstructure. This was achieved through a novel mechanism of grain refinement consisting of several concurrent processes. This involves the development of substructure in the lath interiors at an early stage of deformation, which progressed into small high-angle segments with increasing strain. Consequently, the microstructure was gradually transformed to an equiaxed ultrafine grained structure, mostly surrounded by high-angle grain boundaries, through continuous dynamic recrystallization. Simultaneously, the supersaturated martensite was decomposed during deformation, leading to the progressive formation of beta phase, mainly nucleated on the intervariant lath boundaries.

Processing and properties of ultrafine-grain aluminium alloy 6111 sheet

The equal channel angular processing (ECAP) technique has been applied to an automotive aluminium alloy sheet (A6111). The technique utilizes a machine that was specially designed for this purpose at Monash University. It was determined that ECAP is able to refine the grain size of the sheet, diminish the detrimental as-rolled texture components in the sheet and retain an acceptable level of bi-axial ductility such as is required during the automotive forming process. Experiments were carried out on annealed, as-received sheets that were subjected to either one or two passes through the ECAP machine. For the second ECAP pass, the sheet could be processed in the same orientation as the first pass (route A) or it could be rotated 180° about the direction of feeding (route C). It was determined that route A produced marginally improved properties compared to sheet processed via route C, and that due to the frictional heating generated during the second pass, a significant amount of recovery occurred in the sheet such that an improved combination of texture and formability resulted after two passes compared to the same sheet exposed to only a single pass.

Texture evolution and grain refinement of ultrafine-grained copper during micro-extrusion

Samples of oxygen-free high conductivity (OFHC) coarse-grained (CG) and ultrafine-grained (UFG) copper were micro-extruded to an equivalent strain of 2.8 in one pass at room temperature. Samples of the OFHC copper were annealed at 650C for 2 h to produce CG copper. Some samples were subsequently processed by equal channel angular pressing of eight passes, route Bc, at room temperature to produce the UFG material. Crystallographic texture and misorientation distributions were obtained locally from EBSD mappings at different radial positions after micro-extrusion. To model the strain path during micro-extrusion, the analytic flow line model of Altan etal. [J Mater. Process. Tech. 33 (1992) p.263] was used and also validated by finite element calculations. Modelling was carried out using the viscoplastic self-consistent (VPSC) model and a recently developed grain refinement model. The results showed large texture variations along the cross-section of the extruded sample for both UFG and CG copper. These cyclic drawing textures in UFG copper were simulated in good agreement with experiments using the presented modelling framework.

Bimodal grain size distributions in UFG materials produced by SPD - their evolution and effect on the fatigue and monotonic strength properties

In ultrafine-grained (UFG) materials produced by severe plastic deformation (SPD) techniques such as ECAP (equal channel angular pressing), bimodal grain size distributions have been observed under different circumstances, for example shortly after ECAP, after rest or anneal and/or after mild cyclic deformation at rather low homologous temperature. It has been shown that the mechanical monotonic and fatigue properties of some UFG materials can be modified (sometimes enhanced) by introducing a bimodal grain size distribution by a mild annealing treatment which leads, in some cases, to a good combination of strength and ductility. Here, the conditions under which bimodal grain size distributions evolve by (adiabatic) heating during ECAP and during subsequent annealing or cyclic deformation will be explored, and the effects on the mechanical properties, as studied by the authors and as reported so far in the literature, will be reviewed and discussed. In particular, the role of temperature rise during ECAP will be considered in some detail.

The contribution of grain boundary sliding in tensile deformation of an ultrafine-grained aluminum alloy having high strength and high ductility

An as-cast Al-7 % Si alloy was processed by high-pressure torsion (HPT) for up to 10 turns at temperatures of 298 or 445 K. The HPT-processed samples had ultrafine-grained structures and they were tested in tension at room temperature at various strain rates in the range from 1.0 x 10(-4) to 1.0 x 10(-2) s(-1). The contributions of grain boundary sliding (GBS) to the total strain were measured directly using atomic force microscopy. Samples simultaneously showing both high strength and high ductility contained the highest fractions of high-angle grain boundaries (HAGB) and exhibited the highest contributions from GBS, whereas samples showing high strength but low ductility gave negligible values for the sliding contributions. It is concluded that high strength and high ductility require both an ultrafine grain size and a high fraction of HAGB.

Review: Overcoming the paradox of strength and ductility in ultrafine-grained materials at low temperatures

Ultrafine-grained (UFG) materials with grain sizes in the submicrometer or nanometer range may be prepared through the application of severe plastic deformation (SPD) to bulk coarse-grained solids. These materials generally exhibit high strength but only very limited ductility in low-temperature testing, thereby giving rise to the so-called paradox of strength and ductility. This paradox is examined and a new quantitative diagram is presented which permits the easy insertion of experimental data. It is shown that relatively simple procedures are available for achieving both high strength and high ductility in UFG materials including processing the material to a very high strain and/or applying a very short-term anneal immediately after the SPD processing. Significant evidence is now available demonstrating the occurrence of grain boundary sliding in these materials at low temperatures, where this is attributed to the presence of non-equilibrium grain boundaries and the occurrence of enhanced diffusion along these boundaries.

Cyclic Deformation Of Nanocrystalline And Ultrafine-Grained Nickel

The cyclic deformation behavior Of ultrafine-grained (UFG) Ni samples synthesized by the electrodeposition method was studied. Different from those made by severely plastic deformation, the UFG samples used in this study are characterized by large-angle grain boundaries. Behaviors from nanocrystalline Ni and coarse-grained Ni samples were compared with that Of Ultrafine-grained Ni. With in situ neutron diffraction. unusual evolutions of residual lattice strains as well as cyclic hardening and softening behavior were demonstrated during the cyclic deformation. The microstructural changes investigated by TEM are discussed with respect to the unusual lattice strain and cyclic hardening/softening. (C) 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The development of ultrafine-grained hot rolling products using advanced thermomechanical processing

The aim of the work is development of industry guidance concerning production of ultrafine-grained (UFG) High Strength Low Alloy (HSLA) steels using strain-induced dynamic phase transformations during advanced thermomechanical processing. In the first part of the work, the effect of processing parameters on the grain refinement was studied. Based on the obtained results, a multiscale computer model was developed in the second part of the work that was subsequently used to predict the mechanical response of studied structures. As an overall outcome, a process window was established for the production of UFG steels that can be adopted in existing hot rolling mills. © 2014 Elsevier B.V.

Enhanced mechanical response of an ultrafine grained Ti-6Al-4V alloy produced through warm symmetric and asymmetric rolling

An equiaxed ultrafine-grained (UFG) microstructure was successfully produced in a Ti-6Al-4V alloy with an average grain size of 110-230. nm through symmetric and asymmetric warm rolling of a martensitic starting microstructure. The UFG material displayed a combination of ultrahigh strength and ductility at room temperature. Compared with the conventional symmetric rolling, the asymmetric rolling process led to a more pronounced effect of microstructure refinement and a higher tensile ductility. The optimum mechanical response was obtained though the asymmetric rolling at 70% reduction, offering an ultimate tensile strength of 1365. MPa and a total elongation of ~23%. Apart from the magnitude of grain refinement, the inclination of basal texture component from the normal towards the rolling direction during asymmetric rolling and possible strain induced β to martensite transformation may concurrently contribute to a remarkable tensile strength-ductility balance.

Second Phase Precipitation in Ultrafine-grained Ferrite Steel

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Evolution of sub-micron grain size and weak texture in magnesium alloy Mg-3Al-0.4Mn by a modified multi-axial forging process

This is the first successful attempt to produce simultaneously ultrafine grain size and weak texture in a single-phase magnesium alloy Mg-3Al-0.4Mn through an optimal choice of processing parameters in a modified multi-axial forging (MAF) process. An average grain size of similar to 0.4 mu m and a weak texture could be achieved. This has led to an increase in the strength as well as room-temperature ductility (55%). The plot of the yield loci shows a decrease in anisotropy after MAF. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Formation of ultrafine grained microstructures in steel through strain induced transformation during single pass hot rolling

In the present study, wedge-shape samples were used to study the effect of strain induced transformation on the formation of ultrafine grained structures in steel by single pass rolling. The results showed two different transition strains for bainite formation and ultrafine ferrite (UFF) formation in the surface layer of strip at reductions of 40% and 70%, respectively, in a plain carbon steel. The bainitic microstructure formed by strain induced bainitic transformation during single pass rolling was also very fine. The evolution of UFF formation in the surface layer showed that ferrite coarsening is significantly reduced through strain induced transformation combined with rapid cooling in comparison with the centre of the strip. In the surface, the ferrite coarsening mostly occurred for intragranular nucleated grains (IG) rather than grain boundary (GB) ferrite grains. The results suggest that normal grain growth occurred during overall transformation in the GB ferrite grains. In the centre of the strip, there was significantly more coarsening of ferrite grains nucleated on the prior austenite grain boundaries.