But just as it undertakes two ambitious expansions — a gleaming new headquarters in Kendall Square and major shift in business strategy — Akamai is now under threat by one of the world's biggest, most powerful hedge funds, which has a long record of bringing its corporate targets to heel. Activist investor Elliott Management Corp. last week disclosed it has control of about 6.5 percent of Akamai's shares and will be pushing for changes to its business practices, possibly even a sale of the $11 billion Cambridge company. Famed in investment circles for once seizing an Argentine Navy vessel to force that nation to pay up on its bonds, Elliott's previous activism has shifted the course of other big Massachusetts companies, including EMC, Iron Mountain, and athenahealth.