Solid solution softening of polycrystalline molybdenum-hafnium alloys

In the present study the solid solution softening of molybdenum-hafnium alloys (0.15-3.0 at.%) was investigated at homologous temperatures ranging from 0.027 to 0.153 (77-443 K). The hardness measurements revealed a significant softening effect of alloys with a hafnium content below 0.63 at.%. In order to determine the distribution of the alloying element and the impurities, a detailed investigation of the microstructure and the chemistry was carried out. With atom probe tomography it could be shown that interstitials were not removed from random solution in the molybdenum matrix during alloying with hafnium. This indicated that solid solution softening in the Mo-Hf system is attributed to an intrinsic mechanism. Furthermore, with atom probe investigations it can be shown that the dissolved hafnium increases the oxygen solubility in molybdenum.