Have I taken a wrong decision of taking material science and technology at IIT BHU?

No, the Material Science and Technology Department at IIT BHU is highly reputed and better than other institutes. You have many advantages with an IITan tag, and many opportunities will open up for you. They are:

1. If you have a strong interest in this field, then try to engage in stream projects
2. Try to pursue your MS abroad
3. Material SC has a broad area of research opportunities, you can become a research scholar
4. If you felt Material Science and Technology is not your cup of tea, then you can switch to any other field including coding, and start a career in the CS field
5. IIT BHU offers effective placement support, which will help you to secure a job in a reputed company