Nanoscale study of ageing and irradiation induced precipitates in the DIN 1.4970 alloy

This work presents a thorough investigation of what happens to the DIN 1.4970 Ti-stabilized austenitic steel alloy when it is subjected to ageing heat treatments and heavy ion irradiation. Of particular interest was the fine TiC nanoprecipitate population, and how these precipitates interacted with the rest of the microstructure. A multitude of nanoscale characterization techniques, including TEM and APT, were combined to yield quantitative descriptions of the microstructure.

First, the influence of heat treatment parameters on the microstructure and mechanical properties was investigated. The composition, structure, size and number densities of precipitates on multiple length scales were analyzed and quantified. Ageing temperature and time have only a modest effect on the TiC nanoprecipitate population, and it appears nucleation is limited primarily by the dislocation density. Coarsening is very slow, so heat treatments in a wide range of conditions yield similar populations. However, low temperature ageing (~600 ºC) promotes the growth of M23C6 phase on grain boundaries which may compete with TiC for carbon. High temperature ageing ≥800 ºC results in partial ecrystallization, which is accompanied by dissolution of the nanoprecipitates and a severe loss of hardness and strength. To minimize these effects, a heat treatment of 800 ºC/ 2 h was selected for the ion irradiations.

Heat-treated and pristine materials were irradiated with Fe ions at three different temperatures as a first simulation of the reactor environment. The main points of interest were investigating the stability regime of the TiC precipitates, and their role in the further microstructural development that occurs during irradiation. The TiC nanoprecipitates were found to become unstable somewhere between 300 ºC and 450 ºC after a dose of 40 dpa. When TiC is not stable, the heat-treated and pristine materials evolve to a similar final state with Ni and Si segregation to dislocations. At irradiation temperatures where TiC is stable, it forms during irradiation in the pristine material.
The population of irradiation induced TiC precipitates is finer than the ageing induced population. The precipitates are coated with Ni and Si due to RIS, and this effect may be severe enough to nucleate G-phase. Generally, a higher volume fraction of G-phase was found when the number density of TiC was lower, indicating that the precipitates are important point defect sinks. The limitations of heavy ion irradiation for simulating real neutron irradiations were discussed at length, and different models from the literature were employed to make predictions on the material behavior in a real reactor environment.

Since G-phase is usually associated with microstructural instability and effects such as swelling, the highest possible number density of TiC nanoprecipitates is desireable. For this purpose, prior heat treatments were found to be counterproductive, because they deplete the solid solution of Ti and C. At low irradiation temperatures, TiC dissolves in the heat treated material. Therefore, in all cases, prior heat treatment is ill advised for the application of the material in the reactor. The means by which the material could be further improved is therefore limited to optimizing the initial cold work level and the Ti and C content in solid solution.

We could not make strong conclusions about how the different irradiation conditions in a fast reactor will influence the stability regime of TiC. Dual ion beam irradiation with heavy ions and helium or ion irradiation at lower dose rates may be employed to investigate this further. It would also be desirable to investigate whether the TiC precipitates remain stable up to higher doses, such as 150-200 dpa.

Finally, for the low irradiation temperatures of MYRRHA, irradiation hardening and embrittlement may be more critical than swelling. This work demonstrates that very little can be done to avoid this effect.