Materials Science Forum Vols. 702-703

Abstract: The severe plastic deformation (SPD) of metallic materials has become attractive in recent years. Accumulative roll bonding (ARB) is a SPD process that can impose deformation to a sheet metal, as it develops fine grains of typically submicrometer or even the nanometer level in polycrystalline materials. The purpose of the present study is investigation of microstructural evolution and micro hardness behavior of the commercial purity Aluminium (AA1050) severely deformed by the ARB process. The microstructural evolution during 13 cycles of ARB process was investigated. With increasing ARB cycles the grains size reduced in nanometer range. Furthermore, Micro-Vickers hardness measurement was carried out throughout thickness of the ARB processed sheets.

Abstract: The plastic anisotropy was studied on aluminium sheets with layers of different purity (A: 5N and B: 2N+) produced by accumulative roll bonding (ARB). Both material layers show a contrasting recrystallization behavior where A and B are discontinuously and continuously recrystallized, respectively. Global textures were measured by neutron diffraction. The mechanical anisotropy was measured by tensile testing after different numbers of ARB cycles. The planar anisotropy decreases with the number of ARB cycles while the normal anisotropy reaches a plateau after 4 cycles. Simulations of the Lankford parameters were carried out with the help of the viscoplastic self-consistent scheme (based on the global texture) and compared with the experimental data. Deviations of the simulated values from those of experiment are discussed with regard to through-thickness texture and material heterogeneities.

Abstract: An Ultrafine grain (UFG) microstructure is developed on the sheet of Al-2.4wt%Cu-0.3wt%Si alloy after three passes of accumulative roll bonding (ARB) process. The detailed of the microstructural features and phases at different strain condition has been studied by transmission electron microscopy (TEM). Observation indicates at the possibility of dynamic recovery and recrystallisation during the ARB processing itself. The material becomes ultrafine grains after three passes of ARB itself with the formation of dynamically recrystallised grains all over the sample. TEM evidence is presented in support of this proposal.

Abstract: Lamellar bands are the primary structural features in accumulative roll bonding (ARB) of sheet metals. The structural refinement in ARB sheets occur by forming a dense distribution of lamellar band boundaries. The lamellar band boundaries initiate as low angle interfaces, parallel to the existing lamellar band boundaries, irrespective of the crystallographic orientations of the parent lamellar bands. From an extensive investigation it was found that the transverse directions across the lamellar band boundaries are rotated by an angle equal to their misorientations. Such a phenomenon is not sustained when the boundaries turn to high angle.

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 texture of polycrystalline Ni₅₀Mn₂₉Ga₂₁ alloys fabricated by high pressure torsion (HPT) was investigated with high-energy synchrotron radiation. HPT was performed at temperatures between 873K and 1173K under a hydrostatic pressure of 400 MPa. During HPT above 973K the initial cyclic fibre texture changes to a strong cube and a weak F component. Below 973K a strong rotated cube and weak F and C components develop. Additionally, electron backscatter diffraction reveals that samples deformed at low temperature do not completely transform to martensite giving rise to residual austenite.

Abstract: Texture of the ARB processed commercial purity aluminium was investigated in order to explore the stable component in rolling texture more than 95 % of rolling reduction in FCC metal sheets having high stacking fault energy. Weak location dependence along normal direction was observed in the sheet up to ARB 8 cycle, whereas the texture could be regarded to be uniform after ARB 8 cycle. The main orientation was Copper component, whereas the minor orientations were Brass and S orientations in the ARB processed pure aluminium sheets rolled up to around 99.9 % reduction.

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: The present study explores a way to improve predictions of the mechanical anisotropy of textured polycrystalline aggregates. The underlying hypothesis is that grain-shape-dependent backstresses developed during the elastic-plastic transition influence the selection of active slip systems inside individual grains. Recently, a model was developed and applied successfully to electro-deposited pure iron with a columnar grain structure \cite{Delannay2011}. In the present study, we first suggest another definition of the boundary separation distance experienced by individual slip systems. Then, the model is adapted from the case of spheroidal grains, considered initially, to the more general situation of ellipsoidal grains. A combined effect of grain size, grain shape and texture on plastic anisotropy at yielding is illustrated in case of a rolled IF steel sheet.

Abstract: Grains in f.c.c. polycrystals that accommodate imposed deformation purely byslip processes develop a multi-scale dislocation substructure that evolves with deformation.When the polycrystal is subjected to rolling deformation or to channel-die compression, oneof the elements of this substructure, called cell block boundaries, are widely reported to alignparallel to the transverse direction and close to the macroscopic plane of maximum shear. Thisobservation is explained based on standard rate-independent crystal plasticity augmented bythree hypotheses.