Correct grain size is crucial to material microstructure and in obtaining correct physical and mechanical characteristics for a wide range of materials such as metals, plastics, mineral and composites in the engineering, construction, medical device, semiconductor/electronic and many other sectors. Fine grained steels, for example, provide strength while coarse grained steels are more easily machined. The analysis of grain size is used as a quality control tool to ensure that materials are manufactured to specification and are fit for purpose. Grain size analysis is also used diagnostically to understand material failures in research and development settings and in on-going quality control. The process of grain analysis in quality control must be performed to established standards available from organizations such as ASTM and JIS.
Microscopic analysis of grain size is usually performed on a sample cross section. Grain boundaries can be revealed by manipulating light, color and contrast during imaging and / or by pre-etching the sample. Even in samples with a uniform grain size, apparent grain size and shape may vary depending on where each grain has been cut (oblique cutting, for example, results in a grain with an elongated appearance). Microscopic examination is usually accompanied by image analysis with appropriate software (such as NIS Elements Metallo module) that automatically measures the number and size distribution in a given area to calculate the grain size. Several measurement methods are commonly used, such as 'planimetric', 'linear', 'circular' or 'Abrams'. Automated imaging and image analysis can greatly accelerate grain analysis in quality control applications.
Important microscopy techniques in grain size analysis (depending on the material examined) include brightfield, darkfield polarized light, epi-fluorescence, reflected light microscopy and interference contrast techniques.
Terms commonly used in grain size analysis:
• Planimetric grain sizing
• Linear grain sizing
• Circular grain sizing
• Jeffries planimetric method,
• Triple-point count method
• Heyn intercept method
• Phase contrast
• Differential interference contrast
• Reflected light microscopy
• Polarized light microscopy
• Metallurgical microscope
• Image analysis software
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