Maximizing creep deformation resistance

Abstract

This work attempted to maximize the creep deformation resistance of a precipitation hardening fcc iron alloy by using the accumulated knowledge relating creep to microstructure rather than by trial and error. It was decided that three structural criteria must be met and that multiple mechanical thermal treatment (mmtt) was the treatment most likely to meet them. The criteria and treatment results were: (a) a dislocation density not less than 1·5 × 10⁷ mm/mm³; that produced was 1·8 × 10⁸ mm/mm³; (b) a large fraction of the precipitate particles should lie on dislocations in order to anchor them; the result was 68%, and moreover precise examination with the 1 MV electron microscope showed that network growth occurred only at the rate allowed by particle coarsening; and (c) the dislocation network should be as uniform as possible; from about 40 measurements it was found that the standard deviation of the dislocation density, expressed as a percentage of the mean density, was 4·5 compared with 22 for ordinary cold work. Tests showed mmtt samples to have substantially greater creep deformation resistance than samples treated in other ways. The value of maximizing a structure sensitive property even by an expensive treatment like mmtt is that the results of commercial treatments can be compared with the upper bound so determined to see if there is worthwhile room for improvement. The ductility dropped sharply after mmtt, probably because the inherent cohesion of the particular alloy used is weak in relation to the high deformation resistance developed.