Important Factors Affecting the Stability of Substructure in Heat-resistant Materials

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In the project term, we confirmed that the growth of subgrain during creep (i.e., degradation of substructure) and the Ostwald ripening observed in a heat resistant steel, Al-Cu alloys and a commercial magnesium alloy were all strain dependent notwithstanding that attention was not paid to the effect of strain on the structural degradation so far. Furthermore, the critical stress necessary for subgrain boundary to escape from pinning of precipitates was examined. The results showed that the subrain size remained unchanged in a stress range below the critical stress while it begins to increase in a stress range above the critical stress. On the theoretical basis, the critical stress is expected to be independent of testing temperature, however, the measured critical stress was temperature-dependent. In order to clear the reason for this temperature dependence, the average size and inter-particle spacing of precipitates were measured as a function of testing temperature. The experimental … More results showed that the inter-particle spacing increased with increase in testing temperature because particle coarsening occurred during heating to testing temperature before stress application. The analysis showed that the temperature dependence of critical stress was only apparent. The magnitude of critical stress was almost equal to the theoretical one calculated from the model proposed in Mater.Trans.44(2003),239-246.Since the structural degradation of T91 steel is associated with migration of subboundary, we can view that the structural degradation was promoted by dislocation motion (plastic strain). Actually, the origin of structural degradation of this type is seemingly different from the strain enhanced Ostwald ripening, however, we can view that the origin of both phenomena is the same because the growth of precipitates which hinder the sub-boundary migration can also be assisted by the motion of subboundary.Generally, many heat-resistant materials contain solute atoms with a relatively large size misfit. This is because solutes of this type are effective to strengthen their matrix, however, these solutes are liable to introduce the solute transport assisted by dislocation motion, and eventually lead to strain-enhanced growth of precipitates. So far, solutes with a relatively large size misfit were preferably added to heat-resistant materials to strengthen their matrix, however, these elements are susceptible to strain enhanced coarsening of precipitates. Strain enhanced structural degradation was not taken into account so far in alloy designing, however, it is important to re-consider the alloying elements taking into account the scavenging effect which causes the strain enhanced coarsening of precipitates. Less