DIffusion-PLasticity coupling during selective OXidation of metal alloys – DIPLOX

Two methods are used to obtain the diffusion profiles: the radiotracer techniques and secondary ion mass spectrometry (SIMS).
The work program is structured and organized as described in the following:
1/ Cross-checking of grain boundary (GB) diffusion measurements. The Cr diffusion in ultra-fine grained (UFG) Ni with various grain sizes is studied and compared using both the radiotracer technique and SIMS. The equivalence of the SIMS and radiotracer approaches for GB diffusion measurements will be addressed.

2/ Cr bulk and grain boundary diffusion without pre-strain and applied stress. Single crystals and well-annealed high purity Ni-Cr polycrystals are used. The results obtained will provide the basis for comparative analysis of the effect of pre-straining or applied stress on diffusion.

3/ Effect of pre-straining on bulk and grain boundary diffusion between 350 and 750 °C. In this temperature range, the dislocation microstructure induced by pre-straining is susceptible to evolve during diffusion annealing, especially at the higher temperatures. Therefore, special attention will be paid to the evolution of the dislocation microstructure and its influence on bulk and grain boundary diffusion using TEM. The experiments will be carried out using single crystals and polycrystals to be able to easily distinguish the contribution of grain boundaries to the diffusion process by a further comparative study.

4/ Bulk and grain boundary diffusion in materials undergoing plastic deformation to quantify the effect of plastic deformation on diffusion enhancement in single-crystals and polycrystalline Ni-Cr alloys.

5/ Application to selective oxidation modeling. Oxidation experiments and calculations will then be performed using the Ni-Cr polycrystalline alloys used previously in conditions where pure chromia forms. Previously measured diffusion coefficients will be used as input data for modeling the selective oxidation using the EKINOX code.

Methodology
A methodology for the specimen preparation before diffusion treatment was defined to standardize it between the three partners to avoid the formation of a cold-worked layer at the surface.

Cross-checking of GB diffusion measurements
Diffusion treatments were performed at several temperatures on the UFG Ni. Diffusion profiles were acquired by the radiotracer technique and by SIMS analyses. The former technique allows deep diffusion profiles over a large surface to be acquired while the latter gives access to shallower diffusion depth over a much smaller area.
A comparison of results provided by both techniques indicates lower values of diffusion coefficients obtained by SIMS. Such a difference may rely on different concentrations of diffusing elements. A pure Cr layer serves as a diffusion source for the SIMS experiment while tiny amounts of radioactive Cr are used for radiotracer measurements. For the intermediate temperature range, GB diffusion is strongly affected by GB segregation. Different Cr concentrations used in SIMS and radiotracer measurement may result in different Cr GB segregation levels resulting then in slower diffusion as studied by SIMS.

Bulk and grain boundary diffusion
Diffusion profiles of Cr in Ni single-crystals were acquired by SIMS between 500°C and 860°C. To validate the proposed sample preparation methodology, diffusion of Cr in Ni was studied at 860 °C for durations of 5 and 168 hours to investigate the effect of different diffusion depths on the obtained diffusion coefficients. Both values were similar and in agreement with the literature data suggesting that these applied experimental procedures were reliable.
The results obtained in the temperature range 600-500 °C are two orders of magnitude higher than the literature data obtained at high temperatures and extrapolated to low temperatures. Additional analyses are being carried out to get more insight into the physical reasons for such a difference.

- Finalization of the analyses regarding the comparison between SIMS and radiotracers measurements

- On-going measurements of the bulk and GB diffusion coefficients in ni-Cr alloys without any pres-straining and applied stress

- Study of the effect of pre-straining and plasticity on bulk and GB diffusion in Ni-Cr alloys.

International conference « Diffusion in solids and liquids » in Athens (June 2019):
- Organization of the special session « Diffusion processes under straining »

Two oral communications:
- C. Duhamel et al. « Plasticity-enhanced diffusion of Cr in Ni »
- L. Martinelli et al. « Experimental study of Cr tracer diffusion in Ni(-Cr) and application to NiCr oxidation«

The aim of the project is to study the effect of plastic deformation on bulk and grain boundary diffusion in model binary and ternary Ni-Cr-based alloys (Ni-Cr, Ni-Cr-Fe) in a temperature range including low temperatures where the bulk diffusion cannot be easily measured (350-750 °C). Diffusion coefficients of chromium, nickel and iron will be measured by a combination of two methods to get reliable data: secondary ion mass spectrometry (SIMS) using stable isotopes (France) and the radiotracer technique (Germany). The experiments will be carried out at various temperatures (i) without strain or pre-strain; (ii) on pre-strained samples in order to study the effect of the dislocation network and its evolution during diffusion experiment and, (iii) on specimens during creep to study the influence of dislocation motion. Finally, the measured diffusion coefficients will be integrated in an oxidation kinetics code in order to take into account the effect of plastic deformation in the Cr selective oxidation phenomenon.

The following partners will be involved in the project: ARMINES acting through the Centre des Matériaux (CDM) common research centre of ARMINES and MINES ParisTech (coordinator, France), CEA (France) and University of Münster (Germany).