Iron Duke vs Duke of Wellington

Iron Duke and Duke of Wellington both are nouns.

Word	Noun	Adjective	Verb	Adverb
Iron Duke	Yes	No	No	No
Duke of Wellington	Yes	No	No	No

As nouns, Iron Duke and Duke of Wellington are synonyms defined as:

Iron Duke and Duke of Wellington: British general and statesman; he defeated Napoleon at Waterloo; subsequently served as Prime Minister (1769-1852)

Other synonyms of Iron Duke include Arthur Wellesley, First Duke of Wellington, Wellington.

Iron Duke (noun)	Duke of Wellington (noun)
British general and statesman; he defeated Napoleon at Waterloo; subsequently served as Prime Minister (1769-1852)	British general and statesman; he defeated Napoleon at Waterloo; subsequently served as Prime Minister (1769-1852)