Models in general and class diagrams as defined by the UML in particular play an important role throughout all steps of modern software development. However, the UML contains several modeling concepts which are not defined precisely and hence are used either rarely or in varying ways and with different semantics. In this article, an extension of n-ary associations as one of those concepts is presented including their refinement by specialization, subsetting and redefinition. DHHTGraphs are introduced as one realization of this extension treating associations and links as first-class objects. In particular, a metamodel and a possible implementation realizing association end refinement by association specialization are presented.