Investigation of heavily deformed and dual phase materials by means of Transmission Kikuchi Diffraction

Abstract

The metal industry is continuously searching for new materials with improved mechanical properties. Nanocrystalline, nanostructured and ultrafine-grained materials offer properties that are vastly different from and often superior to those of the conventional microcrystalline materials [1]. Grain size refinement by severe plastic deformation (SPD) is a processing technique that introduces nanoscale structures into the material, including dislocation substructures, nanotwins, and nanoscale precipitates, all of which can further improve the material’s mechanical strength[1]. Among the SPD techniques, High Pressure Torsion (HPT) process is one of the most powerful techniques to obtain ultrafine-grained (UFG) materials, leading to non-homogeneous deformation with large strain gradient[2]. A test specimen, often a disc, is placed between two anvils. Once the pressure is applied, one anvil is rotated with respect to the other. Due to friction in the contact surfaces between the specimen and the anvils, the specimen is deformed by shear force. The main volume of the specimen is strained under hydrostatic compression, which will repress any fracture in the work piece.[3] It enables the grain refinement of bulk materials until a saturation region is reached where no further microstructural refinement can be observed[4]. The production of nanoscale structures by HPT is nowadays very well-known and the problem has shifted from their production to their characterization[5]. Electron Backscatter Diffraction (EBSD) is commonly used to characterize UFG metals and alloys, with grain sizes down to sub-micron scale. However, the spatial resolution of the EBSD technique, even in the latest field emission gun (FEG) scanning electron microscope (SEM), is limited to about 20 nm for dense materials, and 50 nm for lighter materials. In addition, these values represent the resolution parallel to the sample tilt axis, but for EBSD the sample should be tilt to a high angle, tipically 70º from horizontal and the spatial resolution is approximately three times worse down the tilted suface. Clearly conventional EBSD in SEM can not be applied as a routine characterization tool for nanostructured materials.[5] The transmission electron microscope (TEM) is generally considered today the tool of choice for the microstructural analysis of materials at the nanoscale[7]. It has the necessary spatial resolution, electron diffraction analysis enables the measurement of crystallographic orientations on the nanometre scale and recent developments in automated electron diffraction systems utilizing precision techniques are promising for enabling rapid collection of orientation maps on truly nanocrystalline materials. However, TEM analyses require significant technical expertise and are relatively difficult to perform[1]. Bright or dark field images can be difficult to interpret in terms of grain size and, although automated diffraction techniques do exist in the TEM, they generally suffer in terms of speed or angular resolution when compared to EBSD[5]. A great advance would be to marry the routine quantitative capability of automated EBSD with the spatial resolution of TEM[7]. In the last 2 years there is a significant interest in the development of an alternative electron diffraction technique[1]. A new approach to SEM-based diffraction has emerged, namely using an electron transparent sample in a SEM coupled with conventional EBSD hardware and software. This technique, referred to as transmission EBSD (t-EBSD: Keller and Geiss, 2012) or SEM transmission Kikuchi difracction (TKD: Trimby, 2012) enables spatial resolutions better than 10 nm, and it is ideal for routine EBSD characterization of both nanostructured and highly deformed samples[8]. The aim of this Master Thesis is to determine the optimum settings for the different operating parameters of the electron microscope in order to get the maximum possible resolution and be able to observe samples severely deformed by HPT with small grain size, by transmission Kikuchi diffraction in SEM. And subsequently compare the results between both normal EBSD and TKD techniques