Avengers: Once an Avenger...

One aspect of the Avengers that was established fairly early on into its publication was a constant changing its roster, with characters coming and going at a rate that the team introduced in issue #1 had completely changed by issue #16. While characters like Iron Man and Thor had their own series, a number of the Avengers debuted as secondary characters or villains in other titles.

See: Avengers: Earth’s Mightiest Heroes and Avengers: The Great Betrayal

Goliath/Yellowjacket and the Wasp
Avengers #26-#76

Unlike the rest of the characters in this collection, I'm including these two here to account for the fact that the Ant-Man and Giant-Man features in Tales to Astonish (along with their appearances in Avengers #1-#16) ended about halfway through the decade. With that said, Hank and Janet returned to the Avengers not long after they left and remained on the team through the rest of the decade.

Hawkeye and Black Widow
Tales of Suspense #52
Tales of Suspense #53
Tales of Suspense #57
Tales of Suspense #60
Tales of Suspense #64
Avengers #16-#76

Black Widow and Hawkeye initially debuted as villains in the Iron Man feature of Tales of Suspense, with Widow starring solo in issues #52-#53, before teaming up with Hawkeye for a trio of issues (#57, #60 and #64).

Hawkeye becomes a member of the team in issue #16 and stays into the 1970's. Black Widow doesn't become an official member of the team until the next decade, but her appearances through the 1960's happen in Avengers due to her association with Hawkeye. Her appearances can be found in most of the issues between #29-#47 along with #57, #63-#64 and #76.

Quicksilver and Scarlet Witch
X-Men #4
X-Men #5
X-Men #6
X-Men #7
Strange Tales #128
X-Men #11
Avengers #16-#49
X-Men #43
X-Men #44
X-Men #45
Avengers #53
Amazing Spider-Man #71
X-Men #59
X-Men #60
Avengers #75-#76

Hercules
Journey into Mystery Annual #1
Journey into Mystery #124
Journey into Mystery #125
Thor #126
Tales to Astonish #79
Thor #128
Thor #129
Thor #130
Thor #131
Avengers #38-#50

Black Panther
Fantastic Four #52
Fantastic Four #53
Fantastic Four #54
Fantastic Four #56
Fantastic Four Annual #5
Tales of Suspense #97
Tales of Suspense #98
Tales of Suspense #99
Captain America #100
Avengers #51-#63
Daredevil #52
Avengers #64-#76

Vision
Avengers #57-#76

Black Knight
Tales to Astonish #52
Avengers #6
Tales of Suspense #59
Avengers #15-#16
Tales of Suspense #73

Avengers #47-#48
Avengers #54-#55
Marvel Super-Heroes #17
Doctor Strange #178
Avengers #61-#62
Avengers #70-#71

Unlike the other villains/rivals that would go on to become Avengers in this collection, the Black Knight identity has been used for both heroic and villainous purposes, but with different people underneath. I'll be including the appearances of both of them here for context.