be accompanied by locking means which hold it in the position to which it has been brought by being heated in the example shown in fig3 a pawl 34 is mounted on a pin 36 carried by the end fitting 26 a spring not shown keeps the pawl pressed against the other end fitting 28 which has a step 38 against which the pawl engages at the end of the movement a generally adjustable end of stroke abutment 40 can be mounted on the end fitting 26 to determine a limit position for the end fitting 28 and to prevent relative movement or vibration heating can then be switched off when it is desired to make the actuator reversible resilient return means are provided that are capable of exerting a torque that is not less than the torque required for re cocking the bar into its initial state the re cocking torque is smaller than the drive torque that the bar made of memory alloy is capable of delivering when it is heated heat energy being transformed into mechanical energy the re cocking means can be constituted for example by a spring interconnecting the two ends of the bar or interconnecting the two end fittings under such circumstances the torque that can be exerted by the bar must be sufficient to overcome both friction forces and the opposing force of the resilient re cocking means the characteristics of an actuator of the invention are particularly advantageous for deploying solar panels on a satellite.