Average rating: | Rated 4 of 5. |
Level of importance: | Rated 4 of 5. |
Level of validity: | Rated 4 of 5. |
Level of completeness: | Rated 4 of 5. |
Level of comprehensibility: | Rated 3 of 5. |
Competing interests: | None |
Reviewed for PREreview (https://doi.org/10.5281/zenodo.7371559)
Overview
The paper presents interesting experimental and molecular dynamics (MD) simulation studies on nanoindention of a face centred cubic Fe-Ni-Cr stainless steel 310S. 310S is high in both Cr (~25wt%) and Ni (~20wt%) compared to other austenitic stainless-steel grades and finds application in areas where high temperature environmental degradation is a concern. A strength of the work is that the high quality of both the nanoindentation experiments and the MD simulations.
The nanoindentation experiments were undertaken with a Berkovich tip with loads in the range 0.25 to 10 mN, so that shallow indents below 200 nm depth resulted. Data for even the smallest <50nm, 0.25nM) indents appeared to be of good quality indicating careful experimentation on difficult measurements. Results from repeat tests are given to indicate noise levels and scatter in material response.
The MD simulations of the 310S alloy were undertaken using LAMMPS with an embedded-atom method (EAM) potential and models with Fe, Ni and Cr atoms in an initially randomised substitutional solid solution. In line with other literature a fixed layer furthest from the indented surface, and a thermostatic layer allowing for heat dissipation were included with the model which was initially equilibrated at 300 K. Given that repeat simulations were conducted for multiple orientations the models size was kept as large as reasonable possible and consisted of a total of 8.5-9 million atoms.
The most significant challenge for the work is in making a strong connection between the experiments and simulations when computational resource prohibits using a larger model, while experimental uncertainties are more marked for smaller indents. The dilemma is perhaps made most evident by comparing the 10 nm tip radius and 5 nm maximum indent depth used in the simulations with the smallest experimental indents of a little under 50 nm. A more fundamental barrier to direct comparison is that the simulations are for tip radius of 10 nm, while the experiments are for much larger value (~100 nm seems likely from load-displacement data though the actual value is not quoted). The larger tip radius in the experiments provides access to much larger indentation strains than is possible in the simulations, while larger strain gradients are in place for the simulations. Finally, there is a large difference in loading rate (and therefore deformation rate).
Reading the preprint provoked the following comments and questions some of which might be useful to the authors.
Main Points
Minor Points