I am based in Europe, and I am about to start my PhD. Europe refers to the set of countries I may want to work in: Germany, Switzerland, France, Austria.
Is it formally necessary that I earn it in field X if I want to have an academic career in field X?
By academic career I mean taking up a professorship, or in any case, a research position in some university.
Assuming that my research falls significantly under field Y as well, what disadvantages may happen in trying to have an academic career in Y?
What about X = mechanical engineering (focus computational mechanics) and Y = mathematics (focus numerical methods) ?