Quantitative image analysis for evaluation of eutectic fractions in as cast microstructures

The measurement of eutectic fraction in as cast microstructures is prone to systematic error. This is caused by part of the eutectic consisting of the same phase as the primary phase that crystallises on to the primary phase during the formation of the interdendritic eutectic. Due to a lack of contrast, it is not possible to separate this part of the eutectic from the primary phase through direct image analysis. The present study describes two image analysis techniques that account for this part of the eutectic and which thereby allow precise determination of the eutectic fraction. The techniques developed have been successfully applied to measure eutectic fractions in a variety of alloys, e. g. Al–Cu alloys with fine regular eutectics, and Al–Si and Al–Fe–Si alloys with coarse irregular eutectics. Widely varying microstructures have been produced using different solidification conditions in order to test the accuracy of the techniques. Experimental results are compared with predictions of a microsegregation model. The applicability and advantages of the two techniques are discussed with the help of practical examples.