Are all 1974 pennies aluminum

All circulating 1974 U.S. cents are made of bronze, not aluminum. The U.S. has never made a circulating coin out of aluminum.

You're probably thinking of the experimental cents struck in aluminum to test it as a possible replacement for copper, but they weren't put into circulation. Also, they were silvery in color and weighed less than 1/3 as much as a standard cent.

A few of them were shown to various members of Congress to get their opinions about such a substantial modification to an iconic coin. Rumors persist that a few of them were "palmed" by a couple of congressmen but no one has ever seen the coins.

The only known ones are in museums and as such are almost impossible to value because there is no open market for them. If one were to surface at auction it could conceivably sell for as much as the famous 1943 copper cents, which would put them in excess of $35,000.