Arconic said Monday that Klaus Kleinfeld has agreed to step down as CEO and chairman after the company’s board of directors discovered that he sent a letter to its largest shareholder without telling them. Arconic said in a statement that German-born Kleinfeld “showed poor judgment.” A spokeswoman at Arconic declined to say what was in the letter. Arconic Inc. makes aluminum, titanium or nickel parts for planes, cars and electronics. The New York-based company was spun off from aluminum company Alcoa last year. Arconic said Kleinfeld’s letter was sent to a senior official at Elliott Management, an activist investor that has been pushing the company to replace Kleinfeld. Elliott Management has an 11.6 percent stake in Arconic. Board member David Hess will serve as interim CEO as the company looks for a permanent replacement. Patricia Russo was named interim chair of the board.