fixity vs unchangeability

fixity and unchangeability both are nouns.

Word	Noun	Adjective	Verb	Adverb
fixity	Yes	No	No	No
unchangeability	Yes	No	No	No

As nouns, unchangeability is a hypernym of fixity; that is, unchangeability is a word with a broader meaning than fixity:

fixity: the quality of being incapable of mutation
unchangeability: the quality of being unchangeable; having a marked tendency to remain unchanged

Other hypernyms of fixity include changelessness, unchangeableness, unchangingness.

fixity (noun)	unchangeability (noun)
the quality of being fixed in place as by some firm attachment	the quality of being unchangeable; having a marked tendency to remain unchanged
the quality of being incapable of mutation