Papers by Author: X. Huang

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.

Abstract: Copper sheet samples composed of nanometer scale lamellar twins was produced by electrodeposition. The coherent lamellar twin boundaries were within 20˚ of being parallel to the sheet plane in more than 60% of the grains. The electrodeposited sample was cold rolled to 30 and 85% reductions in thickness and the structural evolution during cold rolling was examined by transmission electron microscopy (TEM) and high resolution TEM (HRTEM). Extensive activity of partial dislocations along twin boundaries and of perfect dislocations within twins (in particular in coarse twins >100nm) were identified. Moreover, it was found that shear banding occurred, which locally destroyed the lamellar twin structure. A dislocation structure developed within the shear bands, and such a structure evolved with strain and gradually replaced the lamellar twin structure. After 85% deformation, a large volume fraction of the lamellar twin structure was replaced by a lamellar dislocation structure characteristic of high strain rolling where the lamellar dislocation boundaries are almost parallel to the rolling plane. It was also found that the structural scales are coarser in the lamellar dislocation structure than in the initial lamellar twin structure.

Abstract: TEM, Kikuchi diffraction analyses, EBSD, neutron diffraction and hardness measurements have been applied in a study of commercial purity aluminum (AA1200) cold rolled to strains 2 and 4 and afterwards recovered by a heat treatment for 2h at temperatures up to 220 °C. The deformation microstructure is a lamellar structure delineated by dislocation boundaries and high angle boundaries ( ) parallel to the rolling plane. The macrotexture is a typical rolling texture which is composed of individual texture components present as micrometer- and submicrometre-sized volumes. In the lamellar structure, correlations have been established between microstructural parameters and the local texture, showing for example that the density of high angle boundaries and the stored energy vary locally. The local variations affect the annealing behaviors in a way that some regions coarsen faster than others, leading to a recovered structure which is heterogeneous.

Abstract: High purity aluminum (99.99% purity) was severely deformed by accumulative roll-bonding (ARB) to a thickness reduction of 98.4%. Quantitative microstructural characterization of the deformed sample was carried out by electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). EBSD scans at various locations from the sample surface to the mid-thickness revealed a fairly uniform and equiaxed structure, although a small fraction of an elongated structure parallel to rolling direction (RD) was also observed. Misorientation angle distributions for grain boundaries of which misorientation angle was larger than 2° were evaluated by EBSD, showing that more than 70% of the boundaries were high-angle ones (>15°). More detailed structural features, such as low-angle boundaries (<2°) and dislocations between boundaries were characterized by TEM. The TEM results indicated that about 17% of the boundaries have misorientations <2° and that the fraction of high-angle boundaries is about 52%. An estimated yield strength based on the structural parameters determined by TEM was in good agreement with the measured value.

Abstract: Structural refinement in interstitial free (IF) steels has been obtained by three different methods: (i) deformation by cold or warm rolling, (ii) martensitic transformation and (iii) a combination of a martensitic transformation and plastic deformation. For all these processes, the refinement is discussed in terms of grain subdivision by high angle boundaries and dislocation boundaries on length scales from the micrometer level to the nanometer dimension. The characteristics of the subdividing boundaries are discussed, leading to the formulation of strength-structural relationship for IF steel in the deformed state.

Abstract: The microstructural evolution during annealing below the recrystallization temperature of a commercial purity aluminum (99wt.% purity) cold rolled to a true strain of 2 has been investigated by transmission electron microscopy concentrating on microstructural and orientational aspects. The deformation microstructure was a typical lamellar structure with extended lamellar boundaries, GNBs (geometrical necessary boundaries), and short interconnecting boundaries, IDBs (incidental dislocation boundaries). The microstructure was divided into regions representing typical rolling texture orientations and regions of other orientations. During annealing the structure coarsened towards an equiaxed structure and it was observed that this coarsening was significantly slower in regions of rolling texture orientations than in regions of other orientations. This difference was discussed based on the characteristics of the deformation structure.

Abstract: With the 3DXRD microscope it is now possible to follow in-situ the 3D shape of a recrystallizing grain in the bulk of a deformed microstructure. This opens up the possibility of making direct measurements of boundary migration through a deformed matrix. This paper focuses on recrystalliztion in a deformed Aluminum single crystal with initial orientation (112)<111> (Cu).