Should I opt for B.Tech in metallurgical and materials engineering in NIT or prepare for IAS exam while pursuing B.Sc from a local college?

Both fields denoted by you are opposite to each other. If your aim is to join IAS than forget everything. Just complete your graduation and start preparing for UPSC. It will require a lot of focused energy. So you can go ahead with B.Sc. In the meantime while the course of your graduation develop a

If you want to prepare for IAS then you can go for it after completing your B.Tech.