The U.S. Treasury Department said the sale of Magnachip Semiconductor Corp. to a Chinese private equity firm poses “risks to national security,” as Chinese investments in critical technologies meet with enhanced U.S. scrutiny. Magnachip, a South Korean producer for display and power chips, sold its controlling stake in late March to Chinese private equity firm Wise Road Capital in an all-cash deal worth about $1.4 billion. The Committee on Foreign Investment in the United States (CFIUS), the U.S. interagency panel overseen by the Treasury Department that scrutinizes foreign deals for national security implications, sent a letter to Magnachip on Aug....