UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-K (Mark One) ☒ Annual report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Title of each class Trading Symbol Name of each exchange on which registered Common stock, par value $0.10 ATI New York Stock Exchange For the fiscal year ended December 31, 2023 OR ☐ Transition report pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 For the transition period from to Commission file number 1- ...