Hi Mustafa, I suggest you try to use an Image Processing and Analysis Software, ex. Leica or others free, you can measure mains linear dimensions and get some morphometric statistic results. If you are not studding surface morphology, one point very important is to make polished transversal section in epóxi resin like: Struers ou Buehler products. After, you could cover with carbon layer and proceed with materialographic analysis in Software.

With such good images, interpretation of the results is very simple:

Gas pressure has a significant influence on the particle size and particle shape: the higher the pressure, the smaller and rounder are the particles of the powder (in the figure, 3 is a typing error, it must not be 5 bar but 25 bar).

The dependence of the mean particle size of gas pressure is not linear, but rather complex: at 5 bar - approx. 500 μm; at 10 bar - approx. 400 μm; at 25 bar - approx. 100 μm and at 35 bar - approx. 50 μm. This tells us that 5 and 10 bar - are too low process pressures for the atomization of the melt. At 35 bar particles are almost round and there are many particles < 20 μm. This is exactly what is meant by metal atomisation.

The structures of individual particles, produced with different gas pressures, differ little from each other.

 Thank you very much for your interest and suggestions. But these answers are not adequate for my question. Especially, I ask about the indentations and the shapes look like a covered stretch film which are seen on the 15000x magnified micrographs. What are the causes of those surface shapes and how does increasing gas pressure value affect powder surface?

Now I'm understanding your question. I think Valdim answered.