Jack Williamson 
Lifeburst [EPUB ebook] 

Somewhere, eons past, the seekers had begun as weapons – cyborg war machines. Nuclear explosions and powerful lasers meant little to them, for each was larger than ten battleships and subsisted on a diet of heavy metals, preferably radioactive. Soon the seekers began their own Lifeburst, flying to nearby stars, evolving, eating, destroying…Our solar system was next.