In a surprise move, Klaus Kleinfeld has stepped down as chair and CEO of Arconic and resigned from the company's board after he displayed "poor judgment," Arconic announced on Monday.
Shares of the aluminum, titanium, and nickel products manufacturer were initially 9 percent higher following the news. The stock closed more than 3 percent higher on Monday.
Arconic said Kleinfeld's departure was not in response to the proxy fight Paul Singer's Elliott Management launched or to the activist investor's criticisms of Arconic and lobbying for it to replace Kleinfeld.
"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," according to the release.
Later, the activist firm said "the letter read as a threat to intimidate or extort a senior officer of Elliott Management based on completely false insinuations, a threat that we took seriously and about which we immediately and privately informed the Board."
Elliott said, however, that the problem ultimately lies with the board, which Elliott said "repeatedly excused, endorsed, and participated in Dr. Kleinfeld's poor leadership and attempts to entrench himself and his allies on the Board."
The activist firm said it intends to continue its pursuit of "fundamental Board-level change as vigorously as ever."
Current board member David Hess has been appointed as the interim CEO, and Patricia Russo has been appointed the board's interim chair.
Kleinfeld oversaw the split of Arconic and Alcoa.
Now that Kleinfeld has stepped down, Arconic pointed out that Elliott's main goal has been realized and said "it is Elliott Management's decision whether to continue to burden Arconic and its shareholders with its highly disruptive and distracting proxy fight, or to support Arconic in facilitating an effective CEO search and a strong transition."
Arconic and Elliott did not immediately respond to CNBC's requests for comment.
Watch: More upside for Arconic