Arconic Inc. Chief Executive Officer Klaus Kleinfeld stepped down after showing what the board of directors called “poor judgment” in sending a letter to Elliott Management Corp. amid a proxy fight.

David Hess, a board member, has been appointed interim CEO, the company said in a statement Monday. Lead director Patricia Russo will take over as interim board chair.

“Mr. Kleinfeld stepped down as Chair and CEO by mutual agreement after the Board learned that, without consultation with or authorization by the Board, he had sent a letter directly to a senior officer of Elliott Management that the Board determined showed poor judgment,” Arconic said in the statement.

The resignation disrupts what had been shaping up to be the biggest and most aggressive U.S. proxy battle this season. Elliott had been seeking to oust Kleinfeld and replace four directors at the aluminum parts maker’s May 16 annual meeting.

By Jack Kaskey