Structural changes in a bulk metallic glass subjected to high-strain plastic deformation was investigated. A zirconium-based bulk metallic glass was friction stir processed at different tool rotational speeds. The alloy retained its fully amorphous structure at lower speed. At higher tool rotational speed there was partial nano-crystallization with nearly three times increase in surface hardness. Changes in the glass transition temperature, relaxation and crystallization enthalpies were analyzed to explain the physics of high-strain deformation in amorphous metals. © 2014 Elsevier B.V.