In its most simple form, foam metal is made by bubbling gas through molten metal to form a frothy mixture which then sets as a lightweight matrix. This leaves a material that offers a lighter alternative to conventional metals, while still maintaining a comparable strength.
Afsaneh Rabiei, a professor of mechanical and aerospace engineering at North Carolina State University, last year produced a foam metal shield that could block X-rays, various forms of gamma rays and neutron radiation, giving it potential as a lightweight alternative to the bulky radiation shielding currently available.
Read more at http://www.gizmag.com/metal-foam-bullets/42731/.