CREEP-BEHAVIOR OF MOLYBDENUM AND A MOLYBDENUM-HAFNIUM CARBIDE ALLOY FROM 1600 TO 2100-K

The creep behavior of arc-melted molybdenum-0.5% hafnium carbide and c ommercial purity molybdenum was evaluated at temperatures from 1600 to 2100 K while subjected to stresses of 10-60 MPa in a vacuum below 1.3 x 10(-6) Pa (1.0 x 10(-8) Torr). The effects of temperature and stres s on the steady-state creep rate of these materials were examined. The stress exponent and activation energy for creep deformation were dete rmined. The stress exponents for molybdenum-0.5% hafnium carbide and m olybdenum were 4.1 and 2.27. The activation energies for molybdenum-0. 5% hafnium carbide and molybdenum were determined to be 104 and 66 kca l mol-1, respectively. The creep strength of molybdenum-0.5% hafnium c arbon at a creep rate of 10(-6) was determined as a function of temper ature and compared with that of molybdenum. Hafnium carbide particles were found to be effective in strengthening molybdenum at high tempera tures. The steady-state creep rate of molybdenum-0.5% hafnium carbide was approximately two orders of magnitude lower than molybdenum and th e creep strength of molybdenum-0.5% hafnium carbide was about two time s greater than that of molybdenum over the entire temperature range. T he microstructures of post-test molybdenum-0.5% hafnium carbide specim ens were examined with a transmission electron microscope. The creep s trength of molybdenum-0.5% hafnium carbide was correlated with its mic rostructures that developed during high temperature deformation. The r esults illustrate that the great creep resistance of this alloy was as sociated with the presence of HfC particles which retarded the movemen ts of dislocations, resulting in a dispersion strengthening.