Callaway Golf’s materials and manufacturing advances have reduced the weight required to create the club head.
That allows their designers to ‘place’ weight to improve a particular performance characteristic of a club.
In the EPIC Irons Tungsten is used to change the location of the CoG in each iron. 

Most golfers struggle to hit a #4 Iron any further than a #5 Iron
and really battle with a #3 Iron. Tungsten lowers the CoG
in the longer clubs, making it easier to launch #5, #4
and #3 Irons higher and further. 

Most golfers have no trouble getting shorter Irons airborne
on a good trajectory. In fact the last thing most need is a
higher ball flight. So in the shorter irons the CoG is a little
higher to create more control over distance.  

When we conduct a Fitting and Swing
Assessment, we’ll also discuss with you
where replacing an Iron with a Hybrid
is the best decision.