Items preserved from the memorial that grew in Copley Square in the days after the Boston Marathon bombings will be displayed Tuesday at a free, public exhibit at Northeastern University, six months after the April 15 attacks. The exhibit will be open from 1 to 8 p.m. at Snell Library, 360 Huntington Ave. It will feature the memorial items on loan from the City of Boston Archives, as well as highlights from the Our Marathon project, a crowd-sourced digital archive hosted online by Northeastern at www.marathon.neu.edu.