Papers by Author: Niels Hansen

Abstract: Polycrystalline Ni (99.5 %) has been deformed to an ultra-high strain of ε_vM=100 (ε_vM, von Mises strain) by high pressure torsion (HPT) at room temperature. The deformed sample is nanostructured with an average boundary spacing of 90 nm, a high density of dislocations of >10¹⁵m^-2 and a large fraction of high angle boundaries (>15^o) 68% as determined by transmission electron microscopy and 80% as determined by electron backscatter diffraction. The thermal behavior of this nanostructued sample has been investigated by isochronal annealing for 1h at temperatures from 100 to 600°C, and the evolution of the structural parameters (boundary spacing, average boundary misorientation angle and the fraction of high angle boundaries), crystallographic texture and hardness have been determined. Based on microstructural parameters the stored energy in the deformed state has been estimated to be 24 MPa. The isochronal annealing leads to a hardness drop in three stages: a relatively small decrease at low temperatures (recovery) followed by a rapid decrease at intermediate temperatures (discontinuous recrystallization) and a slow decrease at high temperatures (grain growth). Due to the presence of a small amount of impurity elements, the recovery and recrystallization are strongly retarded in comparison with Ni of high purity (99.967%). This finding emphasizes the importance of alloying in delaying the process of recovery and recrystallization, which enables a tailoring of the microstructure and properties through an optimized annealing treatment.

Abstract: A model is suggested to analyze recovery kinetics of heavily deformed aluminum. The model is based on the hardness of isothermal annealed samples before recrystallization takes place, and it can be extrapolated to longer annealing times to factor out the recrystallization component of the hardness for conditions where recovery and recrystallization overlap. The model is applied to the isothermal recovery at temperatures between 140 and 220°C of commercial purity aluminum deformed to true strain 5.5. EBSD measurements have been carried out to detect the onset of discontinuous recrystallization. Furthermore, comparison between the present model and a similar recently developed recovery model is made, and the result is discussed.

Abstract: t has been demonstrated in previous work that a two-step annealing treatment, including a low-temperature, long-time annealing and a subsequent high-temperature annealing, is a promising route to control the microstructure of a heavily deformed metal. In the present study, structural parameters are quantified such as boundary spacing, misorientation angle and dislocation density for 99.99% aluminium deformed by accumulative roll-bonding to a strain of 4.8. Two different annealing processes have been applied; (i) one-step annealing for 0.5 h at 100-400°C and (ii) two-step annealing for 6 h at 175°C followed by 0.5 h annealing at 200-600°C, where the former treatment leads to discontinuous recrystallization and the latter to uniform structural coarsening. This behavior has been analyzed in terms of the relative change during annealing of energy stored as elastic energy in the dislocation structure and as boundary energy in the high-angle boundaries.

Abstract: Deformation structures and annealing behaviour have been analysed in the centre layer of two AA1050 samples cold-rolled to von Mises strains of 3.6 and 6.4. During annealing at 270-300°C structural coarsening and discontinuous recrystallization occurred in both samples. In the coarsened microstructure, the fraction of high angle boundaries was slightly lower than that in the as-rolled conditions. Recrystallization textures of both samples contained significant fractions of the rolling texture components. The fraction of the retained rolling texture was however greater in the strain-6.4 sample. The {001}<310> and {110}<566> components were also pronounced in this sample. The size of recrystallized grains having orientations of the rolling texture was considerably smaller than the size of grains having other crystallographic orientations. This may be attributed to orientation pinning that hinders growth of grains with orientations of the rolling texture.

Abstract: Metals deformed to high and ultrahigh strains are characterized by a nanoscale microstructure, a large fraction of high angle boundaries and a high dislocation density. Another characteristic of such a microstructure is a large stored energy that combines elastic energy due to dislocations and boundary energy. Parameters of the deformed microstructure significantly affect annealing processes such as recovery and recrystallization. For example, the recovery rate can be significantly increased after high strain deformation and restoration may occur as either discontinuous recrystallization or structural coarsening. A characterization and analysis of deformed and annealed microstructures presented in this work covers Al, Ni, Cu and Fe heavily deformed by rolling, accumulative roll bonding (ARB), equal channel angular extrusion (ECAE) and high pressure torsion (HPT). The important effect of recovery on subsequent restoration processes is discussed along with the effect of heterogeneities both on the local scale and on the sample scale.

Abstract: Plastic deformation leads to a structural refinement by introducing low angle dislocation boundaries and high angle boundaries in the initial coarse grains. To understand the mechanisms for the structural refinement and to establish the structure-strength relationship requires a precise characterization of key structural parameters, namely the boundary spacing and boundary misorientation angle. This study gives the results of such a characterization of pure Ni subjected to high pressure torsion (HPT) up to a strain of 300. The structural analysis was carried out by transmission electron microscopy in the longitudinal sample section in which the detailed structural features can be resolved. It is found that the microstructure in the HPT Ni samples is dominated by a lamellar structure. The spacing of the lamellar boundaries decreases and their misorientation angle increases with strain following a power law up to strain of 12, above which saturation is reached at a strain of about 34. The distribution of lamellar spacings normalized by their respective average values at each strain show an identical form. This scaling behavior is discussed also with reference to other metals and processing routes.

Abstract: Annealing-induced hardening and deformation-induced softening behavior has recently been found in nanostructured aluminum (fcc) produced by severe plastic deformation. It has also been demonstrated that annealing led to a decrease in ductility while deformation led to an increase in ductility. These mechanical responses are totally opposite to those in conventional coarse-grained samples. The present study explores the effect of post-process annealing or deformation on mechanical properties of nanostructured interstitial free (IF) steel (bcc). Accumulative roll-bonding was used to produce the nanostructured IF steel. The deformation structure was characterized by a lamellar boundary structure with a mean spacing of about 200 nm, consisting of high-angle boundaries, low-angle dislocation boundaries and dislocations in the volume between the boundaries. When the deformed sample was annealed at 400oC for 0.5 h, the yield stress and ultimate tensile strength increased and the elongation to failure decreased markedly. In contrast, when the annealed treatment was followed by a light rolling deformation of 15 % thickness reduction, the strength decreased and the elongation to failure increased. These results are consistent with those observed in the aluminum samples. Structural observations by transmission electron microscopy indicated that a removal of dislocations between the boundaries leads to a lack of dislocation sources, resulting in a higher stress to activate alternative dislocation sources. It was suggested that deformation rather than annealing could be a new route to improve the ductility of nanostructured metals and that a moderate light deformation gives a good balance of strength and ductility.

Abstract: The strength of a deformed metal depends on the content of high angle boundaries, low angle dislocation boundaries and the dislocations between the boundaries. High angle boundaries contribute by Hall-Petch strengthening, whereas for the low angle dislocation boundaries and dislocations between boundaries the strengthening is proportional to the square root of the dislocation density. Based on an assumption of additivity of these contributions, the flow stresses of metals deformed by cold rolling have been calculated successfully. In the present investigation pure Ni (99.9%) has been deformed by high pressure torsion (HPT) to von Mises strains of 0.9, 1.7, 8.7 and 12. The strength of the HPT Ni has been determined by Vickers microhardness (HV) measurements and the microstructural parameters have been determined by transmission electron microscope (TEM) in the longitudinal section. HPT has been compared with deformation by cold rolling and torsion based on the structural evolution with strain and the stress-structure relationship. Based on an assumption of a linear additivity of boundary strengthening and dislocation strengthening, good agreement has been found between the calculated and the experimental flow stress.

Abstract: Deformation structures produced by high pressure torsion (HPT) and accumulative roll-bonding (ARB) were characterized by transmission electron microscopy and electron backscatter diffraction, and the mechanical properties of the ARB samples were determined by uniaxial tensile testing. The structural evolution during HPT in high purity nickel has been examined and an extended lamellar boundary structure was observed at high strains. For ARB samples deformed to high strains, an almost similar structural morphology has been observed in both interstitial free steel and in commercial purity aluminum, whereas a relatively equiaxed structural morphology was observed in high purity aluminum samples. In all samples, both deformed by HPT and ARB, the deformation structures were composed of a large fraction of high-angle boundaries, together with low-angle boundaries and isolated dislocations between the boundaries. Common characteristics have been identified in the mechanical behavior of the ARB samples, namely a very high strength, a small uniform elongation and a relatively large post-uniform elongation after necking. For HPT and ARB the structural morphology and structural parameters are compared, and for the ARB samples structure-property relationships are also discussed.

Abstract: The isochronal annealing behavior of nanostructured commercial purity aluminium (AA1100 and AA1200) following either cold – rolling or accumulative roll bonding up to an ultra high strain of εvM = 6.2 (99.5% reduction in thickness) has been studied via hardness testing and by a microstructural investigation. A large effect of rolling strain is observed on the recovery at temperatures below approx. 200 °C. At higher temperatures an assessment of the changes in hardness and microstructure leads to a characterization of the annealing process as one of conventional (discontinuous) recrystallization.