1、Designation: E2627 10E2627 13Standard Practice forDetermining Average Grain Size Using Electron BackscatterDiffraction (EBSD) in Fully Recrystallized PolycrystallineMaterials1This standard is issued under the fixed designation E2627; the number immediately following the designation indicates the yea
2、r oforiginal adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice is used to determine grain size from meas
3、urements of grain areas from automated electron backscatterdiffraction (EBSD) scans of polycrystalline materials.1.2 The intent of this practice is to standardize operation of an automated EBSD instrument to measure ASTM G directly fromcrystal orientation. The guidelines and caveats of E112 apply he
4、re, but the focus of this standard is on EBSD practice.1.3 This practice is only applicable to fully recrystallized materials.1.4 This practice is applicable to any crystalline material which produces EBSD patterns of sufficient quality that a highpercentage of the patterns can be reliably indexed u
5、sing automated indexing software.1.5 The practice is applicable to any type of grain structure or grain size distribution.1.6 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.1.7 The values stated in inch-pound units are to be
6、regarded as standard. The values given in parentheses are mathematicalconversions to SI units that are provided for information only and are not considered standard.This standard does not purport toaddress all of the safety concerns, if any, associated with its use. It is the responsibility of the u
7、ser of this standard to establishappropriate safety and health practices and determine the applicability of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2E7 Terminology Relating to MetallographyE112 Test Methods for Determining Average Grain SizeE177 Practice for Use
8、 of the Terms Precision and Bias in ASTM Test MethodsE691 Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test MethodE766 Practice for Calibrating the Magnification of a Scanning Electron MicroscopeE1181 Test Methods for Characterizing Duplex Grain SizesE1382 Test Me
9、thods for Determining Average Grain Size Using Semiautomatic and Automatic Image Analysis3. Terminology3.1 Definitions:3.1.1 cleanupPost processing applied to EBSD scan data to reassign extraneous points in the scan grid to neighboring points.The extraneous points are assumed to arise from non-index
10、ed or misindexed EBSD patterns.3.1.2 (crystallographic) orientationThe rotation required to bring the principle axes of a crystal into coincidence with theprinciple axes assigned to a specimen. For example, in a rolled material with cubic crystal symmetry, it is the set of rotationsrequired to bring
11、 the 100, 010 and 001 axes of the crystal into coincidence with the rolling, transverse and normal directionsof the specimen. Orientations may be described in terms of various sets of angles, a matrix of direction cosines or a rotation vector.1 This practice is under the jurisdiction of ASTM Committ
12、ee E04 on Metallography and is the direct responsibility of Subcommittee E04.11 on X-Ray and ElectronMetallography.Current edition approved April 1, 2010Nov. 1, 2013. Published May 2010December 2013. DOI10.1520/E2627.Originally approved in 2010. Last previous edition approvedin 2010 as E262710. DOI:
13、 10.1520/E262713.2 For referencedASTM standards, visit theASTM website, www.astm.org, or contactASTM Customer Service at serviceastm.org. For Annual Book of ASTM Standardsvolume information, refer to the standards Document Summary page on the ASTM website.This document is not an ASTM standard and is
14、 intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Becauseit may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the
15、current versionof the standard as published by ASTM is to be considered the official document.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States13.1.3 electron backscatter diffraction (EBSD).A crystalline specimen is placed in a scanning
16、 electron microscope (SEM) ata high tilt angle (70). When a stationary electron beam is positioned on a grain, the electrons are scattered in a small volume(typically 30nm in the tilt direction, 10nm in the transverse direction and 20 nm in depth for a field emission gun SEM andapproximately an orde
17、r of magnitude larger in the lateral directions for a tungsten filament SEM). Electrons that satisfy Braggslaw are diffracted back out of the specimen. The diffracted electrons strike a phosphor screen (or alternatively a YAG crystal)placed in the chamber. The colliding electrons fluoresce the phosp
18、hor and produce a pattern. The pattern is composed of a set ofintersecting bands (Kikuchi lines). These bands are indicative of the arrangement of crystal lattice planes in the diffracting crystalvolume. Assuming the material is of known crystal structure, the orientation of the crystal within the d
19、iffracting volume can bedetermined.3.1.4 EBSD patternAn EBSD pattern is composed of a set of intersecting bands. The geometrical arrangement of these bandsis indicative of the crystallographic orientation of the crystal lattice within the diffraction volume.3.1.5 EBSD scanUnder computer control, the
20、 beam of the SEM is moved to a point on the specimen, an EBSD patterncaptured and indexed to determine the crystallographic orientation at the beam location. This process is repeated for a set of pointslying on a regular grid.3.1.6 grainIn EBSD, grains have a specific meaning. They are defined as a
21、group of similarly oriented neighboring points onthe scan grid. The group is surrounded by a perimeter where misorientation across that perimeter exceeds a specified tolerancevalue.3.1.7 indexingThe process of identifying the crystallographic orientation of the crystal lattice associated with an EBS
22、D patterngenerated by the interaction of the electron beam with that lattice.3.1.8 misorientation The set of rotations (Euler angles) required to bring one crystal lattice into coincidence with a secondcrystal lattice.3.1.9 misorientation toleranceIf the angular difference between two neighboring pi
23、xels is less than this tolerance value thenthey are assumed to belong to the same grain.3.1.10 orientation MapEach point in the scan grid is assigned a color according to its orientation. This forms an imageshowing the microstructure.3.1.11 step size () The distance between neighboring points on the
24、 scan grid.4. Summary of Practice4.1 An EBSD scan is performed on a specimen, post-processing routines are applied to the scan data, and the individual pointsof the scan are grouped into grains according to their orientation. Average grain size is determined from the field average of grainareas base
25、d on the number of points in the EBSD map and the step size.5. Significance and Use5.1 This practice provides a way to estimate the average grain size of polycrystalline materials. It is based on EBSDmeasurements of crystallographic orientation which are inherently quantitative in nature. This metho
26、d has specific advantage overtraditional optical grain size measurements in some materials, where it is difficult to find appropriate metallographic preparationprocedures to adequately delineate grain boundaries.6. Apparatus6.1 An electron backscatter diffraction (EBSD) system mounted on a Scanning
27、Electron Microscope (SEM) is used. The EBSDsystem is constituted by a low-light sensitive video camera (typically a charge-coupled device or CCD camera).The camera imagesa medium for detecting the diffracted electrons such as a phosphor screen or YAG crystal. EBSD patterns formed on the detectingmed
28、ium are imaged using the camera and transmitted to a computer.6.2 Software capable of reliably indexing an EBSD pattern to determine the crystallographic orientation from the EBSD patternis needed. The computer and resident software should be capable of rapid collection of orientation data from EBSD
29、 patterns.6.3 Electronics and software to control the beam in the SEM (or the stage, or both) are required to collect orientation data atpoints on a regular scan grid.7. Hazards7.1 There are no hazards specific to this test method. However, SEM operators should be familiar with safe SEM operatingpro
30、cedures to prevent exposure to X rays and coming in contact with the high voltages inherent to SEMs. Care should also beexercised in preparing specimens for EBSD as is the case for specimen preparation for light and electron microscopy.8. Sampling and Test Specimens8.1 Specimens should be selected t
31、o represent average conditions within a heat lot, treatment lot, or product, or to assessvariations anticipated across or along a product or component, depending on the nature of the material being tested and the purposeof the study. Sampling location and frequency can be based upon agreements betwe
32、en the manufacturers and the users.E2627 1328.2 Specimens should not be taken from areas affected by shearing, burning, or other processes that will alter the grain structure.8.3 The surface to be polished should be large enough in area to permit measurement of at least five fields at the desiredmag
33、nification.9. Specimen Preparation9.1 It is important to follow good metallographic preparation procedures for successful EBSD work. EBSD is a surface sensitivetechnique. The surface should be free from deformation and have minimal topography. Careful mechanical polishing orelectropolishing, or both
34、 should be performed on specimen surfaces. However, as compared with preparing specimens for grainsize measurements based on optical microscopy, the surface should not be etched or treated to produce relief to delineate grainboundaries. This would be disadvantageous for obtaining EBSD patterns that
35、can be indexed. The grain boundaries are delineatedby processing of the orientation measurement data.10. Preparation of Apparatus10.1 Good practices should be used in the operation of both the SEM and EBSD systems. Particular attention should be givento geometric alignment of the specimen surface wi
36、th the assumed measurement plane (typically 70 from the horizontal).Misalignments can easily arise from sample preparation or mounting of the sample. It is critical to mitigate such misalignment,as they affect both the accuracy of the orientation measurements as well as the accuracy of the beam posi
37、tion, particularly in thedirection of tilt. Accurate measurements of orientation and position are critical for accurate characterization of grain size.11. Calibration and Standardization11.1 The EBSD instrumentation should be properly calibrated according to specific manufacturer instructions, inclu
38、ding theEBSD imaging system magnification calibration per Practice E766 (SEM magnification calibration).12. Procedure12.1 The first step is to perform an EBSD scan. Some a-priori knowledge of the estimated grain size is helpful in selecting anappropriate scan area and step size. A map should be cons
39、tructed from the scan data to insure that sensible results are obtained.In particular, the map should be inspected to see if any grains appear speckled. If the automated EBSD software is having difficultyindexing the EBSD patterns, the resulting orientation maps will appear “speckled”. Indexing prob
40、lems arise when the patterns aretoo weak for the band detection algorithms to accurately detect the bands in the patterns, if the system is poorly calibrated, if thecrystal structure data is incorrect, or if the configuration parameters for the indexing algorithms are not optimized. The “speckling”a
41、rises because incorrect orientations are obtained from individual patterns. Operators should following good practice asrecommended by their EBSD vendors to mitigate such problems.12.2 Once the scan data have been collected, a cleanup routine should be applied to the data to assimilate any non-indexe
42、d ormisindexed points into the surrounding neighborhood grains. The number of points modified by the cleanup process should bemonitored. No more than 10% of the points should be modified in the process.12.3 Points of similar orientation should be grouped together to form grains. A misorientation tol
43、erance value of 5 isrecommended to define the orientation similarity, although other misorientation values could be specified in producer/purchaseragreements.After grain grouping, each point in the scan should be associated with a grain. Each grain group is identified as a grain.At least 50 whole gr
44、ains should be observable in each scan area.12.4 Set up an EBSD scan so that the average grain contains about 500 points. Count the number of scan points contained withineach whole grain, Pi, within the scanned field, for a number of fields (n), until at least 500 grains have been measured. Excludeg
45、rains with point counts less than 100. For each grain with Pi exceeding 100 calculate the corresponding area, Ai. For scan datameasured on a regular square grid, the area of a grain, Ai, is equal to the number of points in the grain multiplied by the squareof the step size:Ai 5Pi 2 (1)For data colle
46、cted on a regular hexagonal grid, the area of a grain, Ai, is equal to the number of points in the grain multipliedby the square of the step size, 2 multiplied by the square root of three divided by two:Ai 5=32 Pi2 (2)Store the areas of each grain in memory or record to a file. Calculate the mean ar
47、ea, , for N grains measured in true areaunits (m2 or mm2):12.5 A histogram of the frequency of the grain areas can be constructed to determine or illustrate the uniformity of the grainareas and to detect and analyze duplex grain size conditions. The analytical method is described in Test Methods E11
48、81,AppendixX2.A 5 1n (i5lNAi (3)E2627 133A 5 1n (i5lNAi (3)12.6 Calculate the standard deviation, s, of the measured grain areas, Ai:s 5F 1N 21 (i5lN Ai 2A!2G12 (4)s 5F 1N 21 (i5lN Ai 2A!2G12 (4)13. Interpretation of Results13.1 Follow the methodology set out in Section 14 of Test Methods E1382 for
49、grain area measurements, namely: after thedesired number of grains, N, have been measured, calculate the mean value of the measurement as described in 13.4 and itsdeviation as described 13.5. The ASTM grain size number, G, is then calculated according to the following:G 523.3223log A!22.995 A in mm2 (5)G 523.3223logA 106!#22.995 A in m2G 523.3223log A!22.995 A in mm2 (5)G 523.3223logA 106!#22.995 A in m2Round off the value of G to the nearest tenth unit.13.2 Determine the 95 % confidence intervals, 95%CI, of each measureme
