1974 Aluminum Cent
to find a replacement for the current copper cents.  This was
a result of rising copper prices.  The mint experimented with
many different alloys and materials, including aluminum.  
The use of aluminum would save the U.S. Mint millions of
dollars in metal costs and minting costs.  Aluminum coins
cost less to make.  

1,579,324 aluminum cents were struck in 1974.  Of these all
but less than a dozen were destroyed.  These were given to
high ranking Congressmen and other officials.  One resides
in The Smithsonian Institution's collection.  The other was
found by a U.S. Capitol Officer who found the coin dropped
by a unnamed U.S. Congressman.  This coin was graded by
IGC  as AU-58, and is known as the Toven Example.   It is
unknown whether more still exist.  All examples are
considered illegal to own and are subject to confiscation by
the Secret Service.  

The 1974 pieces are sometimes listed as "patterns".  
However, they should be considered "cancelled" or
"rejected" U.S. issues.  

Why didn't the U.S. Mint continue minting Aluminum Cents?

There are several reasons.  First, the price of copper
dropped making copper cents affordable for the Mint to
make.   Second, the coins would not work and possibly jam
vending machines.  And third, aluminum would not show up
on X-Rays in the event a coin was swallowed by a child.

In 1982 the mint transitioned from copper cents to copper
plated zinc cents.