Abstract: A method for sealing the internal peripheral surface of a tube, such as a heat exchanger tube. A plug is provided which includes a plug body and at least one annular wing formed on an outer periphery of the plug body. The plug is chilled to a temperature below a transition temperature of the plug, then the annular wing or wings are bent backward such that the outer diameter is decreased to a diameter which is less than an inner diameter of the tube to be sealed. The chilled plug is then inserted into an open end of the tube, until a back portion of the plug is flush with the open end of the tube. As the ambient temperature heats the plugs, the plug returns to an undeformed state such that the annular wings return to a state of increased diameter, thereby sealing the tube. In order to ensure that the plug returns to this undeformed state, the plug is formed of a material having at least two allotropic states.