Metallographic Specimen Preparation for Electron Backscattered Diffraction

Electron backscattered diffraction (EBSD) is performed with the scanning electron microscope (SEM) to provide a wide range of analytical data; e.g., crystallographic orientation studies, phase identification and grain size measurements. The quality of the diffraction pattern, which influences the confidence of the indexing of the diffraction pattern, depends upon removal of damage in the lattice due to specimen preparation. It has been claimed that removal of this damage can only be obtained using electrolytic polishing or ion-beam polishing. However, the use of modern mechanical preparation methods, equipment and consumables does yield excellent quality diffraction patterns. The experiments discussed here covered a wide variety of metals and alloys prepared mechanically using three to five steps, based on straightforward methods that generally require less than about twenty-five minutes

Photogrammetric measurement of 3D freeform millimetre-sized objects with micro features: an experimental validation of the close-range camera calibration model for narrow angles of view

The measurement of millimetre and micro-scale features is performed by high-cost systems based on technologies with narrow working ranges to accurately control the position of the sensors. Photogrammetry would lower the costs of 3D inspection of micro-features and would be applicable to the inspection of non-removable micro parts of large objects too. Unfortunately, the behaviour of photogrammetry is not known when photogrammetry is applied to micro-features. In this paper, the authors address these issues towards the application of digital closerange photogrammetry (DCRP) to the micro-scale, taking into account that in literature there are research papers stating that an angle of view (AOV) around 10° is the lower limit to the application of the traditional pinhole close-range calibration model (CRCM), which is the basis of DCRP. At first a general calibration procedure is introduced, with the aid of an open-source software library, to calibrate narrow AOV cameras with the CRCM. Subsequently the procedure is validated using a reflex camera with a 60mm macro lens, equipped with extension tubes (20 and 32mm) achieving magnification of up to 2 times approximately, to verify literature findings with experimental photogrammetric 3D measurements of millimetresized objects with micro-features. The limitation experienced by the laser printing technology, used to produce the bi-dimensional pattern on common paper, has been overcome using an accurate pattern manufactured with a photolithographic process. The results of the experimental activity prove that the CRCM is valid for AOVs down to 3.4° and that DCRP results are comparable with the results of existing and more expensive commercial techniques.Percoco, G.; Sánchez Salmerón, AJ. (2015). Photogrammetric measurement of 3D freeform millimetre-sized objects with micro features: an experimental validation of the close-range camera calibration model for narrow angles of view. Measurement Science and Technology. 26(9):1-9. doi:10.1088/0957-0233/26/9/095203S19269Mitchell, H. L., Kniest, H. T., & Won‐Jin, O. (1999). Digital Photogrammetry and Microscope Photographs. The Photogrammetric Record, 16(94), 695-704. doi:10.1111/0031-868x.00148Chen, Z., Liao, H., & Zhang, X. (2014). Telecentric stereo micro-vision system: Calibration method and experiments. Optics and Lasers in Engineering, 57, 82-92. doi:10.1016/j.optlaseng.2014.01.021Stamatopoulos, C., & Fraser, C. S. (2011). Calibration of long focal length cameras in close range photogrammetry. The Photogrammetric Record, 26(135), 339-360. doi:10.1111/j.1477-9730.2011.00648.xYang, X., & Fang, S. (2014). Effect of field of view on the accuracy of camera calibration. Optik, 125(2), 844-849. doi:10.1016/j.ijleo.2013.07.089Strobl, K. H., Sepp, W., & Hirzinger, G. (2009). On the issue of camera calibration with narrow angular field of view. 2009 IEEE/RSJ International Conference on Intelligent Robots and Systems. doi:10.1109/iros.2009.5354776Ricolfe-Viala, C., & Sanchez-Salmeron, A.-J. (2010). Lens distortion models evaluation. Applied Optics, 49(30), 5914. doi:10.1364/ao.49.005914Ricolfe-Viala, C., Sanchez-Salmeron, A.-J., & Valera, A. (2013). Efficient Lens Distortion Correction for Decoupling in Calibration of Wide Angle Lens Cameras. IEEE Sensors Journal, 13(2), 854-863. doi:10.1109/jsen.2012.2229704Zhang, Z. (2000). A flexible new technique for camera calibration. IEEE Transactions on Pattern Analysis and Machine Intelligence, 22(11), 1330-1334. doi:10.1109/34.888718Percoco, G., Lavecchia, F., & Salmerón, A. J. S. (2015). Preliminary Study on the 3D Digitization of Millimeter Scale Products by Means of Photogrammetry. Procedia CIRP, 33, 257-262. doi:10.1016/j.procir.2015.06.046Ricolfe-Viala, C., & Sanchez-Salmeron, A.-J. (2011). Camera calibration under optimal conditions. Optics Express, 19(11), 10769. doi:10.1364/oe.19.010769Guidi, G. (2013). Metrological characterization of 3D imaging devices. Videometrics, Range Imaging, and Applications XII; and Automated Visual Inspection. doi:10.1117/12.2021037Herráez, J., Martínez-Llario, J., Coll, E., Rodríguez, J., & Martin, M. T. (2013). Design and calibration of a 3D modeling system by videogrammetry. Measurement Science and Technology, 24(3), 035001. doi:10.1088/0957-0233/24/3/03500

Spatially resolved acoustic spectroscopy for rapid imaging of material microstructure and grain orientation

Measuring the grain structure of aerospace materials is very important to understand their mechanical properties and in-service performance. Spatially resolved acoustic spectroscopy is an acoustic technique utilizing surface acoustic waves to map the grain structure of a material. When combined with measurements in multiple acoustic propagation directions, the grain orientation can be obtained by fitting the velocity surface to a model. The new instrument presented here can take thousands of acoustic velocity measurements per second. The spatial and velocity resolution can be adjusted by simple modification to the system; this is discussed in detail by comparison of theoretical expectations with experimental data

Hot nanoindentation in inert environments

An instrument capable of performing nanoindentation at temperatures up to 500 °C in inert atmospheres, including partial vacuum and gas near atmospheric pressures, is described. Technical issues associated with the technique (such as drift and noise) and the instrument (such as tip erosion and radiative heating of the transducer) are identified and addressed. Based on these considerations, preferred operation conditions are identified for testing on various materials. As a proof-of-concept demonstration, the hardness and elastic modulus of three materials are measured: fused silica (nonoxidizing), aluminum, and copper (both oxidizing). In all cases, the properties match reasonably well with published data acquired by more conventional test methods.United States. Office of Naval Research (Contract No. N00014-08-1-0312)Massachusetts Institute of Technology. Institute for Soldier Nanotechnologie

Indentation Hardness Measurements at Macro-, Micro-, and Nanoscale: A Critical Overview

The Brinell, Vickers, Meyer, Rockwell, Shore, IHRD, Knoop, Buchholz, and nanoindentation methods used to measure the indentation hardness of materials at different scales are compared, and main issues and misconceptions in the understanding of these methods are comprehensively reviewed and discussed. Basic equations and parameters employed to calculate hardness are clearly explained, and the different international standards for each method are summarized. The limits for each scale are explored, and the different forms to calculate hardness in each method are compared and established. The influence of elasticity and plasticity of the material in each measurement method is reviewed, and the impact of the surface deformation around the indenter on hardness values is examined. The difficulties for practical conversions of hardness values measured by different methods are explained. Finally, main issues in the hardness interpretation at different scales are carefully discussed, like the influence of grain size in polycrystalline materials, indentation size effects at micro-and nanoscale, and the effect of the substrate when calculating thin films hardness. The paper improves the understanding of what hardness means and what hardness measurements imply at different scales.Funding Agencies|Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University ((Faculty Grant SFO Mat LiU) [2009 00971]</p

Effect of Austenite Deformation on the Microstructure Evolution and Grain Refinement Under Accelerated Cooling Conditions

Although there has been much research regarding the effect of austenite deformation on accelerated cooled microstructures in microalloyed steels, there is still a lack of accurate data on boundary densities and effective grain sizes. Previous results observed from optical micrographs are not accurate enough, because, for displacive transformation products, a substantial part of the boundaries have disorientation angles below 15 deg. Therefore, in this research, a niobium microalloyed steel was used and electron backscattering diffraction mappings were performed on all of the transformed microstructures to obtain accurate results on boundary densities and grain refinement. It was found that with strain rising from 0 to 0.5, a transition from bainitic ferrite to acicular ferrite occurs and the effective grain size reduces from 5.7 to 3.1 μm. When further increasing strain from 0.5 to 0.7, dynamic recrystallization was triggered and postdynamic softening occurred during the accelerated cooling, leading to an inhomogeneous and coarse transformed microstructure. In the entire strain range, the density changes of boundaries with different disorientation angles are distinct, due to different boundary formation mechanisms. Finally, the controversial influence of austenite deformation on effective grain size of low-temperature transformation products was argued to be related to the differences in transformation conditions and final microstructures