Martina C Avalos

EBSD characterization of an IF steel processed by Accumulative Roll Bonding

The objective of this work is to study the texture and microstructure evolution of an IF steel deformed by Accumulative Roll Bonding (ARB) using Electron Backscatter Diffraction. Texture changes occur with increasing number of ARB cycles. For the early cycles, the main components are the α and γ fiber components characteristic of steels. With increasing the number of ARB cycles a tendency towards a random texture is obtained. In the initial state, the mean grain size is 30 μm and after 5 cycles it decreases to 1.2 μm. For the first ARB cycles, the fraction of high angle grain boundary is low but it increases with the number of cycles to about 80% for 5 cycles. The Kernel Average Misorientation (KAM) has no appreciable changes with the number of ARB cycles for all the texture components.

Electron backscatter diffraction study of orientation gradients at the grain boundaries of a polycrystalline steel sheet deformed along different loading paths

In a polycrystal, the heterogeneity of plastic deformation in a particular grain is greatly enhanced by adjacent grains that constrain the grains local behavior, often imposing orientation gradients. This work aims to characterize and quantify the local orientation gradients near grain boundaries (GBs). Electron backscatter diffraction (EBSD) measurements were made on a 0.67 mm thick aluminium-killed drawing quality (AKDQ) steel sheet subjected to different loading paths that are typical of forming operations. A statistical analysis shows that a considerable fraction of the analyzed GB profiles can be described by an orientation profile with a constant slope near the GB. In order to quantify this behavior, as well as the degree of localization, two new parameters, based on the local orientation gradient assessed by EBSD, are proposed: BET (boundary effective thickness) and GAS (gradient average severity). These parameters should be considered together, the BET as an effective thickness of the GB zone where the orientation gradient takes place and the GAS as a measure of the magnitude or severity of the orientation gradient. Additionally, the GAS parameter shows a strong correlation with the accumulated macroscopic strain for the investigated deformation levels and loading paths, while the BET profile clearly reveals the influence of the GB on the misorientation profiles. Tension and biaxial stretching results lead to a BET value between 1.5 and 2 µm. Finally, it is shown that the local misorientation in the GB zone, on both sides of the GB line, is disperse and it does not correlate simply with misorientation or even the slip-transfer geometry across the GB. Moreover, the observed average local misorientation dispersions in GB zones are different for each loading condition.

Assessing the Power of Electron Back Scattering Diffraction Characterization of Deformed F-138 Steel from the View Point of Crystal Diffraction

Abstract Microstructural characterization by combined EBSD and X-ray diffraction analysis is shown for an FCC material deformed by rolling and further heat treatment on air. The wide variety of x-rays sources combined with EBSD allows some correlations between microstructural parameters to be withdrawn by carefully analyzing x-ray peak broadening. The combination of both techniques provides a deeper understanding of anisotropic accumulation of dislocation arrays on a deformed F 138 stainless steel. The anisotropy results in a smaller peak breadth for crystal orientations with [110] planes contained on the rolling plane, further explained as resulting from a lower storage of dislocations. After heat treatment the effect is reverted and the formerly less misoriented orientations become more uniformly oriented revealing a faster dislocation migration and annihilation and/or trapping.

Anisotropic and Heterogeneous Development of Microstructures. Combining Laboratory/Synchrotron X-rays and EBSD on a few SPD Metallic Systems

The onset of Severe Plastic Deformation (SPD) regime is quite instructive on the possible origins of the nano-microstructures developed in metals and alloys. It is known that grain fragmentation and dislocation accumulation, among other defects, proceed at different paces depending fundamentally on grain orientations and active deformation mechanisms. There have been many attempts to characterize nano-microstructure anisotropy, leading all of them to sometimes contradictory conclusions. Moreover, the characterizations rely on different measurements techniques and pos-processing approaches, which can be observing different manifestations of the same phenomena. On the current presentation we show a few experimental and computer pos-processing and simulation approaches, applied to some SPD/alloy systems. Williamson-Hall and Convolutional Multiple Whole Profile (CMWP) techniques will be applied to peak broadening analysis on experimental results stemming from laboratory Cu Ka X-rays, and synchrotron radiation from LNLS (Laboratorio Nacional de Luz Sincrotron, Campinas, Brazil) and Petra III line (HEMS station, at DESY, Hamburg, Germany). Taking advantage of the EBSD capability of giving information on orientational and topological characteristics of grain boundaries, microstructures, grain sizes, etc., we also performed investigations on dislocation density and Geometrically Necessary Dislocation Boundaries (GNDB) and their correlation with texture components. Orientation dependent nano-microstructures and domain sizes are shown on the scheme of generalized pole figures and discussions provide some hints on nano-microstructure anisotropy.

Surface Microstructure Modification in Square Extruded Al-Nb Powder Composites by Shot Peening

75%Al-25%Nb powder composites, fabricated by square shape cold extrusion, were subject to shot peening treatment with full coverage. Shot peening results in a high number of intense local deformations, with a surface roughness in our case of about l6gm. Due to the high local deformation down to nano-scale surface grain refinement and strain accumulation was generated. Previous texture characterization was performed by neutron diffraction and laboratory X-rays (Cu Kα radiation). The first method took advantage of the high penetration power and averaging capabilities and the second method was further used taking advantage of the low penetration to characterize surface microstructure modification. Peak broadening, before and after shot peening, was analyzed by MAUD software and domain sizes and microstrains were calculated for both phases. Simultaneous EBSD and EDS scans, on 30 nm step sizes, were performed on a FESEM Quanta 200 + TSL-EDAX, showing the highly heterogeneous microstructure developed because of shot peening. Protrusions, due to particle impacts, are clearly seen on EBSD maps. Results mainly revealed that, for Al phase, domain sizes decrease, while microstrains and dislocation densities consistently increase after the materials have been subjected to SP. For Nb phase the visible effect of SP is an increment of microstrains, and related dislocation densities, but keeping the domain sizes almost constant.