I would rather metal foams than others.As far as I know there are lots of applications for this area and could be linked to other fields such as biomaterials. In brief, you would have more choices to make.

I recommend the ductile iron based on the career opportunity in the industry.

Step one

1)industry or academia as your career path?

2) if academia search the topics for number of publications and citations and the choose that that appears most promising.

3) if industry then you need to look at applications. Foams have very little applications, cast iron you have heavy equipment, automotive and for oxide bifilma that is light metals and automotive and electrical heat management as the leading applications

3) choose also carefully the supervisor as this person is to guide you and will if good propel your career in a good way.

Thanks Mr. Sarum Boonmee and Mr. Anders Jarfors . This question was for two years ago and I chose to work on bi-films characteristics.