The vaporific effect is a flash fire resulting from the impact of high-velocity projectiles with metallic objects.
Particles heated from the force of impact can burn in the presence of air (oxidizer) or water vapor.
An explosion can result from the mixture of metal-dust and air, the resulting dust explosion causing significant overpressure within metallic enclosures (aircraft, vehicles, metallic enclosures, etc.).
[1] This effect is often referenced in movies, such as when a single bullet makes a helicopter explode.
This was greatly diminished in a nitrogen atmosphere (compared to air), thus it was found that the "oxidation of metal fragments is a major factor."