A comparison of grain imaging and measurement using horizontal orientation and colour orientation contrast imaging,...

The problems associated with the definition of a grain, grain size measurement, and the issues associated with making one- and two-dimensional measurements on a three-dimensional structure are discussed. The relatively new scanning electron microscope (SEM)-based techniques of colour orientation contrast imaging (COCI) and automated electron backscatter pattern (EBSP) are explained and examples given. Comparisons with conventional (horizontal) orientation contrast imaging (HOCI) in the SEM are made. A direct comparison is made between conventional metallographic methods and these new techniques on precisely the same region of an interstitial free iron specimen. Both optical imaging and HOCI were found to miss a large number of grain boundaries (7 and 12%, respectively), and to create boundaries ( approximately 2%). COCI was found to be reliable, with only 3% of boundaries missed. EBSP was taken to be the standard against which the others were compared, as it unambiguously measured changes in crystallographic orientation. Errors in the number of grain boundaries that are detected have a considerable effect on grain size measurements, e.g. mean linear intercept, and a follow-on effect on the modelling of mechanical properties. New methods for increasing the acquisition speed of orientation maps are discussed, along with examples. The combination of COCI (for grain location) and EBSP (for orientation measurement) is promising, but requires improvements in either imaging or image analysis to be totally reliable.