Papers by Keyword: Ultra-Fine Grain (UFG)

Abstract: Billets of aluminum and aluminum alloys have been deformed at room temperature using a die having equal channels of 10 mm diameter intersecting at an inner angle of 120° and outer arc of 60° by equal channel angular pressing (ECAP) to ultra fine grain (UFG) size level, adopting route Bc. Mechanical properties were evaluated by tensile testing and microhardness measurement. Effects of alloying elements on strengthening were explored. The strengths increase rapidly at first few passes and then reach to a saturation level. The improvement in strength at initial passes of ECAP is due to work hardening and subgrain or dislocation cell formation. However, strengthening at large number of passes is due to the grain refinement alone. The rate of strengthening as a function equivalent strain decreases to a minimum. The strengthening level of bulk UFG alloys is about 3.5 to 4.5 times to that of starting materials. The major cause of strengthening is grain refinement apart from solute strengthening. Among Mg, Zn and Ag alloying elements, the strengthening effect is highest for Mg and lowest for Ag. Ductility is regained without affecting the strength after sufficient number of passes when microstructure becomes equiaxed and ultra-fine in size. However, ductility of UFG Al alloys is lower than that of their coarse grained counterpart.

Abstract: The evolution process of ultrafine grains during hot severe plastic deformation (SPD) was studied in several aluminum alloys. The structural changes can be characterized by the evolution of deformation bands such as microshear bands (MSBs) at moderate strains. The process of strain-induced grain formation can be categorized into the three stages irrespective of deformation mode and temperature: i.e. i) an incubation period for new grain evolution in low strain; ii) a grain fragmentation by frequent development of MSBs and subsequently new grains in medium strain, and iii) a full development of fine grains in large strain. Temperature effect on the new grain formation in aluminum alloys is also analysed in detail and the mechanism operating is discussed.

Abstract: The Armco iron is one of the purest commercial iron with very low levels of carbon, oxygen and nitrogen. In order to improve the mechanical properties, it is worth applying severe plastic deformation to obtain ultrafine-grained bulk materials, with grain size lower than 1 μm. In this study, samples of Armco iron were subjected to a technique of severe plastic deformation named Accumulative Roll Bonding (ARB). This method consists in rolling to 50% two sheets pack of which the stacked surfaces were initially cleaned. Then, the rolled strip is sectioned in two halves, cleaned and stacked again and the procedure of roll-bonding repeated. Practically, the process can be repeated without limits. The important parameter of ARB is the number of cycles and then the consequent number of layers of the final sample. By means of the Electron Backscattered Diffraction (EBSD) technique, the evolution of both microstructure and texture as regard to the number of ARB cycles was studied. The analysis of mean grains size and high angle grain boundaries (HAGB) fraction as a function of the number of cycles showed an early formation of a subgrained structure with low angle boundaries and then the evolution of the microstructure towards an ultrafine-grained structure with an increase of HAGB.

Abstract: A commercial AZ61 magnesium (Mg) alloy composed of coarse initial grains was multi-directionally forged (MDFed) under decreasing temperature conditions from 673 K to 463 K up to a cumulative strain of ΣΔε = 6.4 at a true strain rate of 3 × 10^-3 s^-1. A pass strain of Δε = 0.8 was employed. The average grain size decreased gradually with an increase in the cumulative strain. After straining to ΣΔε = 6.4 (i.e., after 8 passes of MDF), equiaxed ultrafine grains (UFGs) with an average size of approximately 0.8 μm were uniformly produced. These grains were relatively coarse as compared with MDFed Mg alloys having initially finer grains. The hardness of the AZ61 Mg alloy increased monotonically up to 910 MPa with decreasing grain size. The Hall-Petch relationship held within this experimental condition.

Abstract: Microstructure, mechanical properties and electrical conductivity in Cu-0.73%Cr alloy after HPT process and the subsequent aging treatment have been investigated. Ultrafine grained structure with the grain size ~150 nm has been achieved after the HPT and the subsequent aging treatment. Ultrafine grains with some growth twins were preserved in the overaged state, showing high thermal stability. The peak microhardness and tensile strength of Cu-0.73%Cr alloy after the HPT was found at 480 °C for 2 hours. Electrical conductivity shows an increase trend in the different aging states.

Abstract: The present study reports the microstructure evolution and mechanical behaviour of severe plastically deformed pure Cu under cryogenic conditions. The samples were severely deformed by cryo rolling upto 50%, 75% and 95% deformation. Microstructure evolution of cryo rolled samples has been characterized by using optical, TEM and EBSD technique. The rolled samples were heat treated at various temperatures so as to control the recrystallization in the severely deformed samples. The effect of recrystallization on the mechanical behaviour was investigated in detail by tensile testing. The EBSD analysis performed on 95% rolled + heat treated samples showed that partial recystallized microstructure demonstrate an optimum combination of strength and ductility in cryo rolled Cu.

Abstract: This investigation uses electron backscatter diffraction (EBSD) to study the development of microtexture with increasing deformation in an AlMgSi alloy having an initial grain size of about 150 µm subjected to high pressure torsion (HPT) up to a total of 5 turns. An homogeneous microstructure was achieved throughout the disc sample at high strains with the formation of ultra-fine grains. Observations based on orientation distribution function (ODF) calculation reveals the presence of the torsion texture components often reported in the literature for f.c.c. materials. In particular, the C {001}<110> component was found to be dominant. Furthermore, no significant change in the texture sharpness was observed by increasing the strain.

Abstract: The Armco iron is one of the purest commercial iron with very low levels of carbon, oxygen and nitrogen. In order to improve the mechanical properties, it is worth applying severe plastic deformation to obtain ultrafine-grained bulk materials, with grain size <1µm. In this study, samples of Armco iron were subjected to a technique of severe plastic deformation named Accumulative Roll Bonding (ARB). The important parameter of ARB is the number of cycles and then the von Mises equivalent strain. By means of the Electron BackScattered Diffraction (EBSD) technique, the texture evolution with the number of cycles was studied. The microhardness was also measured in function of the equivalent strain. Finally, the mean grain size and the fraction of high angle grain boundaries were determined as a function of the number of cycles.

Abstract: Grain refinement of aluminum deformed by equal channel angular pressing is strongly dependent on the amount of strain. The refinement process at low to high strain level involves elongation of the existing grains by shear deformation, their subdivision into bands and subgrain formation within bands, intersection of the bands during subsequent passes and finally conversion of the subgrains to grains by continuous dynamic recrystallization process. At room temperature the conversion of subgrains to grains takes place by progressive lattice rotation.

Abstract: The present study aims to understand the evolution of microstructure leading to nano/ultrafine grain formation during cyclic thermal process. A commercial grade of AISI 304L austenitic SS was cold rolled which resulted in a creation of a dual microstructure having strain induced martensite (43%) and heavily deformed retained austenite. The dual phase microstructure was subjected to cyclic thermal annealing process at 825 °C. The events occurring in; a) retained austenite and b) reverted austenite formed by phase reversion of strain induced martensite, during annealing treatment, were studied by the Electron backscattered diffraction (EBSD). The study revealed recrystallisation process of the two austenite grains, which resulted into ultrafine grain formation during cyclic thermal process.