Effects of precipitation on serrated yielding in Inconel 718

The influence of precipitate distribution on the occurrence of dynamic strain aging has been investigated in Inconel 718. In process annealed and solution treated materials, serrated yielding has been observed in the temperature range 400 K - 900 K. Three different types of serrations, identified as A, B and C serrations were observed depending upon the temperature, strain rate, and test mode (i.e., tension or compression). Types A and B serrations were observed in the lower temperature range of 400K - similar to 750K while type C serrations were observed in the temperature range similar to 750 K - 900 K. An average activation energy, calculated using five different analysis techniques, of 117 +/- 29 kJ/mol was determined for types A and B serrated flow while the average activation energy ranged from 185 +/- 42 kJ/mol to 206 +/- 39 kJ/mol for type C serrated flow. Annealing the solution treated materials at 1033 K/8 hours resulted in a contraction of the regime associated with serrated flow and in a change in serration type from types A and B to type C. Continued annealing for 110 hours results in a complete disappearance of serrated yielding. Comparison of the results with literature data suggest that the rate controlling mechanisms for dynamic strain aging are the migration of interstitial solutes for type A and B serrations and the migration of substitutional solutes for type C serrations. The disappearance of serrated flow at longer aging times is attributed to solute depletion due to precipitation.