strike down vs overturn

strike down is not a noun while overturn is a noun.

Word	Noun	Adjective	Verb	Adverb
strike down	No	No	Yes	No
overturn	Yes	No	Yes	No

As verbs, overturn is a hyponym of strike down; that is, overturn is a word with a more specific, narrower meaning than strike down:

strike down (noun)	overturn (noun)
	the act of upsetting something
	an improbable and unexpected victory

strike down (verb)	overturn (verb)
cause to fall by or as if by delivering a blow	change radically
cause to die, especially suddenly	rule against
declare null and void; make ineffective	cancel officially
	turn from an upright or normal position
	cause to overturn from an upright or normal position
	cause the downfall of; of rulers