Is this a bulk alumina substrate surface? Process details of the origin of the sample are important to interpret the image. Are we sure of the purity? The word phase implies that the difference in contrast is due to two different crystalline structures, which isn't necessarily the case. Perhaps one region is amorphous and another particulate crystals.

The SEM image is from In-Lens detector, which unless I'm mistaken is a Secondary Electron detector (SE). If this is true then it will not be possible to say anything about the phases in the image as SE gives topographical information. Most likely, depending on the processing and whether the surface was etched, the bright and dark regions just indicate rough and smooth surfaces. If you need to get phase information make sure you image in Backscatter Mode.

Krish response highlights my point that we can't assume phase difference is reason for contrast.i mentioned particulate crystals because their high surface area will yield brighter contrast due to greater secondary electron emission. You can negative bias the ETH grid voltage to get backscatter contrast instead of SE topographic contrast. Better yet is to use immersion modee to collect the back scattered electrons normal to the surface which will reveal crystal phase difference if there is any. I think what we're seeing is difference in mesoscale microstructure, perhaps some greater oxidation of the higher surface area rough regions also contribues...again, with out process or sample prep info this question is in a vacuum, no pun intended

Hi Cameron and Kris

Thank you for your nice reply. I really appreciate that.

The image is from bulk alumina. It is 99.99% pure. (i don't have details about processing of alumina)

I agree with your discussion. I was also not able make any conclusion as well based on the above image and my all other images i have at "in-lens" mode (this is different then secondary detector).

I did some XRD analysis on the specimen for clarification. It is giving a combination of alpha and gamma peak. The alpha peak is very prominent which may indicate dark area is alpha since alpha phase in alumina is compact. Some of the small peaks matched with gamma alumina. Considering gamma phase in alumina is porous , i may interpret the porous zone in image is gamma phase?

Its correct that In-lens detector is different than SE detector (ETD), but still in-lens gets signal from secondary electron, meaning that it is just another type of SE detector. In-lens detector gives you information only about topography (surface). Since your sample is almost pure Alumina, backscatter mode would give you very little or no contrast. Backscatter mode cannot detect polymorphs (different crystal structures) of the same compound. Since you know the relative fraction of phases from XRD, you can check if the area of porous zone in the image matches that with the amount of gamma phase, and you need to do it over multiple images to confirm this hypothesis.

Krishna: in lens detector certainly provided BSE signal. The electrons closest to the source axis angle have highest energy. The detector bias can repel low energy SE and this is exactly how to get a clean BSE signal from in lend detector

I think that your image is formed from secondary electrons that do not evidence compositional contrast (as the backscattered electrons) but only topographic contrast. So the different brightness in your image is due to morphological differences.