Why is the austenite to martensite volume expansion calculated from "tetragonal" austenite and not FCC?

Why do we calculate austenite to martensite volume expansion using "tetragonal" austenite (a*(a/sqrt(2))*(a/sqrt(2)) and not the austenite lattice parameter cubed for the FCC system? And, is it...

12 December 2016 7,221 3 View