Replace author with custom field in feed

try this:

add_filter( 'the_author', 'feed_author' );
function feed_author($name) {
    if( is_feed() && !is_admin()) {
        global $post;
        $author = get_the_terms($post->ID,'article_author');
        if ( $author )
            $name = the_terms($post->ID,'article_author','',' & ');
        return $name;  
    }
}