1. A process, comprising:providing a first material having a higher atomic number and a second material having a lower atomic number; and
diffusion bonding the first material and the second material together to form a Z-grade material having:
a first layer of the first material;
a second layer of the second material; and
a third layer, between the first and second layers, consisting of an alloy of the first and second materials, wherein a density of material in the third layer increases along a gradient from the first layer to the second layer, and wherein the third layer is at least 0.5 mil in thickness.