STRAIN ENHANCED GROWTH OF PRECIPITATES CONTRIBUTING TO THE SUBSTRUCTURE STABILITY OF HIGH STRENGTH HEAT RESISTANT MATERIALS

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Advanced heat resistant steels usually contain fine subgrains and densely populated dislocations, which are both introduced during martensitic transformation. Their excellent high temperature strength is ascribable mainly to the substructure and precipitation strengthening. It is important to note that the thermal stability of substructure is controlled by the coarsening rate of precipitates which can retard the dislocation motion and the migration of subboudaries. A recent study on T91 suggests that the rate of coarsening is dependent on creep strain. The present study was undertaken to examine whether or not the coarsening rate of precipitates is strain dependent, and what mechanisms is likely to occur. Within the project term, three items were examined ; (1) analysis of existing data on T91, (2) experimental study and analysis on Al-Cu and magnesium alloys, and (3) numerical simulation of dislocation motion with solute atmosphere. From the studies on Al-Cu and magnesium alloys, confirmation was made that the coarsening rate of precipitates was strain dependent, and the effect of strain on coarsening became gentle when strain rates were higher. Based on the assumption that the dislocation motion with solute atmosphere enhanced the solute transport and lead to the enhancement of coarsening rate, a model was proposed. The model explained the experimental results satisfactorily.