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The influence of Mo alloying on the thermal stability of grain size, dislocation density and hardness of ultrafine-grained (UFG) Ni alloys was studied. The UFG microstructure in alloys with low (â¼0.3 at.%) and high (â¼5 at.%) Mo contents was achieved by high-pressure torsion (HPT) performed for 20 turns at room temperature. The thermal stability of the two alloys was studied by calorimetry. A Curie-transition from ferromagnetic to paramagnetic state was not found for the Niâ5% Mo alloy due to the high Mo content. It was found that heating at a rate of 40 K/min up to â¼850 K resulted in a complete recovery and recrystallization of the UFG microstructure in the alloy with 0.3% Mo. The same annealing for Niâ5% Mo led only to a moderate reduction of the dislocation density and the grain size remained in the UFG regime. Therefore, the higher Mo content yielded a much better thermal stability of the Ni alloy. The influence of the change of the microstructure during annealing on the hardness is discussed. Keywords: High-pressure torsion, NiâMo alloys, Dislocations, Grain size, Hardness, Thermal stability