M.Z. Quadir

Processing of Multi-Layered Al-Al(Sc) Hybrid Sheet by Accumulative Roll Bonding

A high-purity Al alloy and a supersaturated Al-0.3wt.% Sc alloy were accumulative roll bonded (ARB) at 200 °C to generate 0.5 mm gauge sheet consisting of 32 alternating layers of Al and Al(Sc). The material was subsequently annealed for 6h at 350 °C. The deformation and annealed microstructures were investigated using transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). The deformation microstructure composed primarily of lamellar bands of unequal fineness with shear bands and deformation bands being additional substructural features in the Al(Sc) layers. The high strain deformation generated Al layers containing lamellar boundaries separated by a large fraction of high angle grain boundaries, thereby creating the ideal microstructure for continuous recrystallization. Annealing of the as-deformed material generated a hybrid microstructure consisting of alternating layers consisting of ~20 0m grains produced by continuous recrystallization (Al layers) and a lightly recovered substructure (Al(Sc) layers); the latter were highly resistant to recrystallization due to precipitation of nanosized Al3Sc particles during annealing.

The effect of initial microstructure and processing temperature on microstructure and texture in multilayered Al/Al(Sc) ARB sheets

Abstract A commercial purity Al alloy and an Al-0.3Sc (wt.%) alloy, the latter in either the supersaturated or artificially aged condition, were accumulative roll bonded at either 200 or 350°C to high strain to generate sheet materials consisting of 32 or 64 alternating layers of Al and Al(Sc). The microstructure and texture of the processed materials were investigated mainly using electron backscattered diffraction scanning electron microscopy and transmission electron microscopy. The deformation microstructure and texture of these two alloy combinations were strongly influenced by both the initial heat treatment condition of the Al(Sc) alloy whereby large-scale shear bands were generated during rolling when a dispersion of fine Al3Sc particles is present in the Al(Sc) layers. The effect of initial microstructure and processing temperature affected the subsequent recrystallization microstructure and texture of the Al/Al(Sc) composite during annealing at 350°C. Here, the Al(Sc) layers remain unrecrystallized in all materials with the Al layers undergoing continuous and discontinuous recrystallization after low and high temperature ARB, respectively. The lack of recrystallization in the Al(Sc) layers generated an alternating recrystallized/recovered microstructure in all materials.

The Mode of Deformation in a Cold-Swaged Multifunctional Ti-Nb-Ta-Zr-O Alloy

Multifunctional titanium alloys, termed Gum Metal™, are β-phase Ti alloys first developed in 2003. These alloys exhibit many interesting properties including, for example, low rate of work-hardening and superplasticity during cold deformation. The original report described a new plastic deformation mechanism not involving major dislocation activity to explain such deformation behavior. In the current study, a comparable Ti-36.8Nb-2.7Zr-2.0Ta-0.44O (wt pct) alloy to the original investigators was produced by powder sintering, hot forging, solution treatment, and cold swaging with the aim at investigating the microstructural development during swaging. XRD and TEM showed that the forged/solution-treated alloy was β-phase with a small amount of ω-phase. After cold swaging by up to 96 pct area reduction, TEM/HRTEM revealed the existence of dislocations, deformation twins, ω-phase, nanodisturbances, and lattice bending, with EBSD showing the grains to be highly elongated in the swaging direction, fragmented, and distorted. Most notably, swaging also generated a strong 〈110〉 fiber texture, even after moderate strains. The foregoing structural analysis provides substantial evidence that dislocations are present in the alloy after cold swaging. The major support of dislocation glide processes acting as the dominant plastic deformation mode in the swaged alloy is the strong 〈110〉 fiber texture that develops, which is a characteristic feature of all cold-drawn/swaged body centered cubic metals and alloys.

Subgrain Boundary Identification in 3D EBSD Data through Fast Multiscale Clustering

Complete and accurate characterization of subgrain microstructural features permits study of the relationships among loading, microstructure and properties in plastically deformed metals. 3D electron backscatter diffraction data can produce reconstructed crystallographic volumes, however low angle subgrain boundaries cannot be determined simply with point-to-point misorientation thresholding because many are gradual transitions in orientation. We demonstrate a novel 3D implementation of the data segmentation technique Fast Multiscale Clustering, which uses a quaternion representation of orientation and a corresponding distance metric. Examples of the 3D segmentation of microbands and morphological analysis from the results are presented for die-compressed nickel single crystal and uniaxially loaded commercially pure aluminum.

On the Complexities of Microband Interactions in a Cold-Deformed Goss-Oriented Ni Single Crystal

In this investigation, the three-dimensional nature of microband boundaries was investigated in a Goss-oriented Nickel single crystal. The sample was 30 pct deformed by plane strain compression and then characterized by several advanced techniques including three-dimensional electron backscatter diffraction (3D-EBSD) and transmission electron microscopy (TEM). In the longitudinal section of the sample, microbands were found to exhibit their classical strip-like appearance. However, the microband interfaces contain numerous curved features despite the average inclination of the interfaces closely matching the potential {111} slip planes. The microband boundary irregularities are argued to be the outcome of microband interactions, which are also linked to the orientation spread of deformation structures. Based on the detailed TEM investigation of the dislocation structures associated with intersecting microbands, it is argued that these interactions result in the disintegration and/or dissolution of microband boundary segments. A correlation between microband and cell boundaries was also found.

Mechanical Properties of an Accumulative Roll Bonded Aluminium Alloy Composite

A multilayered sheet composite of commercial purity Al and Al-0.3%Sc alloys was produced by accumulative roll bonding. The final sheet material consisted of 64 ultra fine grained layers, each of ~7.8mm in thickness. The as-deformed material was annealed at temperatures ranging from 250 to 350°C to study the changes in microstructure and their associated influence on mechanical properties. The as-deformed structures largely comprised of high angle grain boundaries in the Al layers and low angle grain boundaries in the Al(Sc) layers. During annealing, the structures in the Al(Sc) layers remained unaltered, whereas the Al layers recrystallized rapidly to the full layer thickness. The mechanical properties of the Al-Al(Sc) composite were measured and found to be unique in strength and ductility with annealing temperature having a significant influence on these properties.

Uncovering the true nature of deformation microstructures using 3D analysis methods

Three-dimensional electron backscatter diffraction (3D EBSD) has emerged as a powerful technique for generating 3D crystallographic information in reasonably large volumes of a microstructure. The technique uses a focused ion beam (FIB) as a high precision serial sectioning device for generating consecutive ion milled surfaces of a material, with each milled surface subsequently mapped by EBSD. The successive EBSD maps are combined using a suitable post-processing method to generate a crystallographic volume of the microstructure. The first part of this paper shows the usefulness of 3D EBSD for understanding the origin of various structural features associated with the plastic deformation of metals. The second part describes a new method for automatically identifying the various types of low and high angle boundaries found in deformed and annealed metals, particularly those associated with grains exhibiting subtle and gradual variations in orientation. We have adapted a 2D image segmentation technique, fast multiscale clustering, to 3D EBSD data using a novel variance function to accommodate quaternion data. This adaptation is capable of segmenting based on subtle and gradual variation as well as on sharp boundaries within the data. We demonstrate the excellent capabilities of this technique with application to 3D EBSD data sets generated from a range of cold rolled and annealed metals described in the paper.

Diffusion and precipitation phenomena across layer interfaces in a roll bonding composite of Al(Si) and Al(Cu)

Layered composite structures can be generated in metallic sheets by roll bonding of dissimilar metals/alloys. In this investigation, heat treatable (Al(Cu)) and non-heat treatable (Al(Si)) aluminium alloys are roll bonded in sheet form. Large hardness differences between layers poses significant bonding challenges in the form of multiple necking within the hard Al(Cu) layers. For successful processing, it is vital to choose the candidate materials in a state of marginal hardness differences during rolling, but being capable of altering properties through subsequent heat treatments. Atomic diffusion takes place during heat treatment of the composite sheet and results in gradual hardness variation across sheet thickness. The Al(Cu) layers contribute to strength, whereas the Al(Si) layers provide protection from corrosion/wear-related degradation in the newly developed hybrid sheet. The overall mechanical properties of the heat-treated composite fall between the base alloys. The bonding interfaces are noted as the potential spots for initiating failure.