The introduction of more effective ceramic composite materials allows for a larger width of these metal layers within the armour shell: given a certain protection level provided by the composite matrix, it can be thinner. Because these metal layers are denser than the rest of the composite array, increasing their thickness requires reducing the armour thickness in non-critical areas of the vehicle. They typically form an inner layer placed below the much more expensive matrix, to prevent extensive damage to it should the metal layer strongly deform but not defeat a penetrator. They can also be used as the backing plate for the matrix itself, but this compromises the modularity and thus tactical adaptability of the armour system: ceramic and metal modules can then no longer be replaced independently. Furthermore, due to their extreme hardness, they deform insufficiently and would reflect too much of the impact energy, and in a too wide cone, to the ceramic tile, damaging it even further. Metals used include a tungsten alloy for the Challenger 2 or, in the case of the M1A1HA (Heavy Armor) and later American tank variants, a depleted uranium alloy. Some companies offer titanium carbide modules.