Physics-based Simulation Models for EBSD: Advances and Challenges

EBSD has evolved into an effective tool for microstructure investigations in the scanning electron microscope. The purpose of this contribution is to give an overview of various simulation approaches for EBSD Kikuchi patterns and to discuss some of the underlying physical mechanisms

Synchrotron Sub-μ X-ray Tomography of Kirkendall Porosity in a Diffusion Couple of Nickel-Base Superalloy and Nickel after Annealing at 1250 \ub0C

Kirkendall porosity that forms during interdiffusion in a diffusion couple of nickel-base superalloy CMSX-10 with pure nickel is investigated. The diffusion experiments are conducted at a temperature of 1250 \ub0C, where the strengthening γ′-phase is partially dissolved. The porosity is studied by X-ray sub-μ tomography with a spatial resolution of about 0.353 μm3 at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. It is found that depending on the distance from the interface the Kirkendall pores take different shapes: octahedral, rounded pyramidal, drop shaped, dendritic, pear shaped, and joint shapes. Such a variety of pore morphologies indicates a complex multistage process of porosity nucleation and growth under vacancy supersaturation of different degrees. The experimental findings are interpreted on the basis of the results of diffusion modeling. It is shown that the kinetics of porosity growth is essentially influenced by the dissolution of the γ′-phase

Elucidation of orientation relations between Fe-Cr alloys and corrosion products after high temperature SO2 corrosion

The early stages of corrosion of Fe-Cr-model alloys (2 and 9 % Cr) were investigated after exposure at 650 degrees C in 0.5 % SO2 containing gas by electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM). The impact of the grain orientation of the base alloy on the orientation relations of the corrosion products is presented. After 2 min - 5 min exposure the formation of a multi-layered corrosion zone was discovered. A clear orientation relationship between ferrite and the (Fe,Cr)(3)O-4 spinel could be demonstrated. The obtained results show the importance of the grain orientation on oxidation resistance

EBSD orientation analysis based on experimental Kikuchi reference patterns

Orientation determination does not necessarily require complete knowledge of the local atomic arrangement in a crystalline phase. We present a method for microstructural phase discrimination and orientation analysis of phases for which there is only limited crystallographic information available. In this method, experimental Kikuchi diffraction patterns are utilized to generate a self-consistent master reference for use in the technique of Electron Backscatter Diffraction (EBSD). The experimentally derived master data serves as an application-specific reference in EBSD pattern matching approaches. As application examples, we map the locally varying orientations in samples of icosahedral quasicrystals observed in a Ti40Zr40Ni20 alloy, and we analyse AlNiCo decagonal quasicrystals

Tetragonality mapping of martensite in a high-carbon steel by EBSD

The locally varying tetragonality in martensite grains of a high-carbon steel (1.2 mass percent C) was resolved by electron backscatter diffraction (EBSD) with a spatial resolution in the order of 100 nm. Compared to spatially integrating X-ray diffraction, which yielded an average tetragonality of c/a = 1.05, the EBSD measurements in the scanning electron microscope allowed to image a local variation of the lattice parameter ratio c/a in the range of 1.02 <c/a <1.07. The local variation of tetragonality is confirmed by two different EBSD data analysis approaches based on the fitting of simulated to experimental EBSD patterns. The resulting EBSD-based tetragonality maps are pointing to a complex interaction of carbon concentration and local lattice distortions during the formation process of martensitic structures

Creep behavior of a γ′-strengthened Co-base alloy with zero γ/γ′-lattice misfit at 800 °C, 196 MPa

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.Deformation and structural behavior of an experimental γ′-strengthened Co-base alloy during creep at 800 °C and 196 MPa have been investigated. The characteristic features of this alloy are zero γ/γ′-lattice misfit and a fine γ/γ′-microstructure. In the initial condition, the γ′-precipitates in this alloy are small (size of about 100 nm), have polyhedral morphology, and are separated by the very narrow γ-channels (width of about 10 nm). The tests performed up to about 1% creep strain (about 500 h creep time) gave creep curves with a slow constant strain rate and without an apparent transient creep, typical for superalloys with nonzero misfit. In this initial stage of creep, entering of the narrow γ-channels by dislocations is blocked by a strong Orowan force. The micromechanism of creep was identified as an octahedral glide of 〈011〉 superdislocations simultaneously in two phases, γ and γ′. The γ/γ′-microstructure with zero misfit shows no rafting but rapidly coarsens isotropically. It is concluded that zero misfit is beneficial at the initial stages of the creep but is unfavourable for long-term creep because of the continuous microstructural coarsening