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I couldnt get a good explanation differentiating phase and grain.From what I understood, a collection of unit cells arranged in the same orientation is called grain.A collection of grains with similar structure is called phase.Is this interpretation right?

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A phase can be defined as as a homogeneous portion of a system that has uniform physical and chemical characteristics, so every pure material.

grain is defined as an individual crystal in a polycrystalline metal or ceramic.Let's clear the confusion with an example:

enter image description here

Here there is SEM photo of a pearlite (multiphase). The white area represents iron and the black lines represent carbon. The grains are also visible in this microscopic image. As you can see, each grain consists of two phase nl. iron and carbon, but the orientation of lamellar carbon is not the same in each grain. Your first interpretation of grain concept is almost correct, but be carful with the word 'structure' in the definition of phase.

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Sam's response is good, but just to confirm your ideas:

"a collection of unit cells arranged in the same orientation is called grain" - Yes.

"A collection of grains with similar structure is called phase" - Yes.

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