In general, academic journals require authors to sign a copyright transfer to the journal publisher. Therefore you should not post the published copies of such articles on the open web (including Researchgate or another website).
However, in most cases, the author does retain some rights over his or her work.
Increasingly, this includes the right to "self-archive" a pre-publication copy of one's article by posting it to:
There are two main types of pre-publication copies: the original manuscript, which is the version of the paper the author submits for review; and the accepted manuscript, which is the version of the paper that includes any revisions made in the review process.
This list may help you conduct a fair use analysis before using copyrighted materials.