Papers by Author: Wan Qiang Xu

Abstract: A focused ion beam (FIB) coupled with high resolution electron backscatter diffraction (EBSD) has emerged as a useful tool for generating crystallographic information in reasonably large volumes of microstructure. In principle, data generation is reasonably straightforward whereby the FIB is used as a high precision serial sectioning device for generating consecutive milled surfaces suitable for mapping by EBSD. The successive EBSD maps generated by serial sectioning are combined using various post-processing methods to generate crystallographic volumes of the microstructure. This paper provides an overview of the use of 3D-EBSD in the study of various phenomena associated with thermomechanical processing of both crystalline and semi-crystalline alloys and includes investigations on the crystallographic nature of microbands, void formation at particles, phase redistribution during plastic forming, and nucleation of recrystallization within various regions of the deformation microstructure.

Abstract: A focused ion beam (FIB) coupled with high resolution electron backscatter diffraction (EBSD) has emerged as a useful tool for generating crystallographic information in reasonably large volumes of microstructure. In principle, data generation is reasonably straightforward whereby the FIB is used as a high precision serial sectioning device for generating consecutive milled surfaces suitable for mapping by EBSD. However, there are several challenges facing the technique including the need for accurate reconstruction of the EBSD slice data and the development of methods for representing the myriad microstructural features of interest including, for example, orientation gradients arising from plastic deformation through to the structure of grains and their interfaces in both single-phase and multi-phase materials. This paper provides an overview of the use of 3D-EBSD in the study of texture development in alloys during deformation and annealing and includes an update on current research on the crystallographic nature of microbands in some body centred and face centred cubic alloys and the nucleation and growth of grains in an extra low carbon steel.

Abstract: The effect of initial microstructure (acicular ferrite (AF), polygonal ferrite (PF) and strip cast (SC)) on the recrystallization behaviour of low carbon (LC) steel was investigated. Steel strip samples (0.05 wt.% C) of 2 mm in thickness were heat treated to produce an AF and PF microstructure from coarse austenite. The AF, PF and a similar chemistry SC sample manufactured from a twin roll caster were cold rolled to 50, 70 and 90% reduction, and annealed for various times in the temperature range 580-680 °C. The evolution of microstructure during recrystallization was studied by optical microscopy and electron backscatter diffraction (EBSD) in the SEM. The initial microstructure was found to have a substantial influence on the recrystallization behavior. PF recrystallized more rapidly than AF with SC showing extremely sluggish recrystallization behaviour. The recrystallizing grains in these initial microstructures have a lognormal distribution and the recrystallized number density (grains/mm2) decreased during annealing, with the initial microstructures affecting the degree of this decrease in number density.

Abstract: A typical dual-beam platform combines a focused ion beam (FIB) microscope with a field emission gun scanning electron microscope (FEGSEM). Using FIB-FEGSEM, it is possible to sequentially mill away > ~ 50 nm sections of a material by FIB and characterize, at high resolution, the crystallographic features of each new surface by electron backscatter diffraction (EBSD). The successive images can be combined to generate 3D crystallographic maps of the microstructure. A useful technique is described for FIB milling that allows the reliable reconstruction of 3D microstructures using EBSD. This serial sectioning technique was used to investigate the recrystallization behaviour of a particle-containing nickel alloy, which revealed a number of features of the recrystallizing grains that are not clearly evident in 2D EBSD micrographs such as clear evidence of particle stimulated nucleation (PSN) and twin formation and growth during PSN.

Abstract: An extra low carbon steel was cold rolled to 85% reduction and annealed at 680 °C to generate a microstructure containing ~2 % recrystallized grains. A partly recrystallized volume was analyzed using 3-D FIB-EBSD tomography. The results show that nucleation and subsequent growth of recrystallizing grains is more complex processes than that revealed using 2-D metallographic techniques. In the present steel, it was found that subgrains were found to be the origin of nucleation and these grains exhibit an internal structure similar to the surrounding deformation substructure. However, a certain subgrain keeps expanding to a stage where some part or parts of the boundary reach(es) and consume(s) a high stored energy deformation zone(s) to form (a) local dislocation free zone(s) having an orientation similar to the subgrain. After this stage, the residual dislocations in the original subgrain are annihilated and nuclei enter a well-defined growth stage. The overall growth of recrystallization nuclei was found to be controlled by the variation in both the stored energy and orientation of the surrounding deformation substructure that results in heterogeneous growth by so-called orientation pinning.

Abstract: A typical dual-beam platform combines a focussed ion beam (FIB) microscope with a field emission gun scanning electron microscope (FEGSEM). Using this platform, it is possible to sequentially mill off > ~ 50 nm slices of a material by FIB and characterise, at high resolution, the crystallographic features of each new surface by electron backscatter diffraction (EBSD). The successive images can be combined to generate 3-D crystallographic maps of the microstructure. This paper describes various aspects of 3-D FIB tomography in the context of understanding the microstructural evolution of metals during deformation and annealing. The first part of the paper describes the influence of both metal type and milling parameters on the quality of EBSD patterns generated from a surface prepared by FIB milling. Single crystals of some face centred cubic metals were examined under varying FIB milling parameters to optimise EBSD pattern quality. It was found that pattern quality improves with increasing atomic number with the FIB milling parameters needed to be adjusted accordingly. The second part of the paper describes a useful technique for FIB milling for the reliable reconstruction of 3-D microstructures using EBSD. There is an initial procedure involving extensive milling to generate a protruding rectangular-shaped volume at the free surface. Serial sectioning is subsequently carried out on this volume. The technique was used to investigate the recrystallization behaviour of a particle-containing nickel sample, which revealed a number of features of the recrystallizing grains that are not clearly evident in 2-D EBSD micrographs.

Abstract: The effect of initial microstructure on the recrystallization behaviour and texture development of low carbon (LC) steel was investigated. Steel strip samples (0.05 wt.% C) of 2 mm in thickness were heat treated to produce a microstructure consisting predominantly of either polygonal ferrite or acicular ferrite. Samples were cold rolled 50, 70 and 90% reduction then annealed for various times in the temperature range 580-640 oC. The microstructures and textures produced by deformation and annealing were studied by optical microscopy and electron backscatter diffraction in the SEM. The initial microstructure was found to have a substantial influence on the rate of recrystallization and final texture. It was found that polygonal ferrite recrystallizes more rapidly than acicular ferrite with the former generating the strongest <111>//ND recrystallization texture. The results are examined within the framework of improving the formability of LC steel produced by direct strip casting whereby controlled thermal and mechanical processing prior to cold rolling and annealing can generate the same types of initial microstructures, as studied in this work.