PAINFUL DECEPTIONS -- ILLUSTRATED SCREENPLAY & SCREENCAP GALLERY
According to the Urban Legend's article, the answer is: "Any pieces of wreckage large enough to be identifiable ...
burned up in the intense fire."
However, aluminum and steel do not burn easily. You can prove this to yourself with a gas stove.
The flames in the stove are at the maximum temperature possible for hydrocarbons burning in the atmosphere.
Therefore, if you cannot catch aluminum on fire in the flames of a stove, then aluminum ain't likely to catch on fire in the flames of an airplane crash.