Barney Frank and Luis Gutierrez have introduced [1] the "Coinage Materials Modernization Act (H.R. 3330). It authorizes the change in composition of coins.

"The Treasury Department estimates that changing the composition of pennies and nickels will save the government over $100 million a year. By making similar changes to the half dollar, quarter and dime, the government can save as much as $400 million annually."
...
it costs the government 1.7 cents to make a penny and 10 cents to make a nickel.

They don't know what they will use next, they just know the current materials cost too much. Guess the savings will not be realized for quite some time.