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A closed loop servo-controlled hydraulic hot-torsion testing machine is described that is capable of applying a maximum of 100 revolutions of twist at velocities up to 15 revolutions/s with a maximum torque of 110 N·m. When a test specimen measuring 25 by 6 mm is used, these factors correspond to a maximum surface strain of 80, a shear strain rate of 12 s−1, and a shear stress of 1800 MPa. The motor displacement is programmed with the aid of a digital function generator and either torque or rotary displacement may be used for feedback control. The test pieces are held between a hydraulic motor and a fixed torque cell by superalloy loading members mounted in self-aligning chucks. They are heated by a radiant furnace and are enclosed in a transparent quartz tube that is flushed with argon during testing. By flooding the tube with water at the completion of a test, the specimen may be quenched to permit the study of the microstructures developed during hot deformation as well as the effect of various holding times on the deformation structures. The capability of the machine is illustrated by results obtained on specimens of Waspaloy® tested at temperatures between 20 and 1093°C at strain rates in the range 0.03 to 7 s−1. The flow curves generated during these tests and the relationship between stress, strain rate, and temperature are discussed.