National Physical Laboratory

Microstructural Metrology

3a3
Grain size distribution 
Average grain size and grain size distribution are key microstructural parameters which, along together with density, phase volume fraction and porosity, essentially define a material’s potential properties and performance. The drive towards finer and finer grain sizes in both ceramic and hardmetals requires attention to the preparation techniques used for the section to be examined, as well as the observation method and the subsequent counting methods. 

The most common method for average grain size determination is the linear intercept method, in which the number of intersections across lines of given length drawn on the field of observation is counted, and divided by the line length; while the determination of grain size distribution requires individual line intercept lengths to be measured. 

Issues of counting statistics are important, and often it is necessary to count several hundred intercepts before a stable convergent result is obtained, but if the sample is heterogeneous then additional considerations are raised.  Small grains may dominate the statistics and so area fractions may need to be used to give a true reflection of the microstructure, depending on what features control the sample properties.

The use of electron backscattering diffraction (EBSD) is increasing the possibilities for examination of grain size data through its enhanced resolution and the automated measurement of grain areas and linear intercepts it enables, but it also requires research to understand how results may be affected by the operating parameters chosen and how they may be compared with other methods.

For more information: Ken Mingard

Last Updated: 25 Apr 2012
Created: 25 Jul 2007

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