fixedness vs unchangeability

fixedness and unchangeability both are nouns.

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

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

fixedness: the quality of being fixed and unchangeable
unchangeability: the quality of being unchangeable; having a marked tendency to remain unchanged

Other hypernyms of fixedness include changelessness, unchangeableness, unchangingness.

fixedness (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 fixed and unchangeable
remaining in place