- freely available
Quantum Beam Sci. 2020, 4(1), 1; https://doi.org/10.3390/qubs4010001
- The carbon content in the graded materials should be well controlled, which is made difficult by the high temperature required for the inter-diffusion heat treatment.
- The spatial extension of the composition gradient should be sufficient as compared to the typical size of a synchrotron beam (∼200 m) so that the sample volume probed for one diffraction pattern can be considered approximately homogeneous in composition. Thus, many alloy compositions can be measured in a single experiment. Besides, it would also ensure that the composition gradients are sufficiently weak to not interfere with the phase transformation itself, since it will have an almost constant composition at the scale of the transformation (a few microns). Such an extension of several mm being difficult to achieve by inter-diffusion alone, a further step of plastic deformation is added to the process.
- The grain size within the sample gradient should be sufficiently small to ensure quantitative and statistic characterization of the phase transformation from the HEXRD data. This requirement is facilitated by the cold plastic deformation step, which results in recrystallization upon heating at the beginning of the in-situ heat treatment. However, this condition has only been partly achieved in the present study, which points towards some possible improvements in future work.
2. Preparation of Compositionally Graded Alloys
3. High Energy X-Ray Diffraction Experiments
4.1. Effect of Mn on Ferrite Growth
4.2. Effect of Mn and Mo on Ferrite Growth Kinetics
5. Artifacts and Possible Solutions
- The first problem was the coarse grain size of the graded samples, which resulted in discontinuous 2D diffraction patterns and subsequent shouldered peaks, making the Rietveld adjustment more difficult and resulting in errors on the calculated phase fractions. One solution to overcome this problem is to use a newly developed furnace with a rotating sample holder. This configuration will lead to an increased number of analyzed grains and an average pattern over the rotation angle, which should in turn translate into more continuous Debye–Scherrer rings. This furnace was already used on a synchrotron beam line in other studies and showed good results [40,41].
- The second problem encountered in developing the methodology was decarburization during phase transformation experiments. As mentioned above, a layer of ferrite was observed on samples surface at the end of HEXRD experiments. This artifact resulted in errors over the measured ferrite fraction. The furnace mentioned above also features superior atmosphere control and should solve this issue as well.
- The last and most important complication was related to the through-thickness gradient of composition generated during cold rolling, due to a deformation disparity between the surfaces and the center of samples, which prevented measuring the transformation at constant composition for a given X-ray beam position. The through-thickness composition profile was measured by EPMA, and only datasets where there was no through-thickness gradient are presented here. A significant number of gradient samples had to be discarded based on this criterion. The many low-reduction passes are suspected to have amplified the through-thickness inhomogeneity of deformation normally expected from rolling. However, we are not able at this point to precisely determine the conditions on the alloy’s rheology, sample geometry, and rolling schedule that would minimize this effect. One of the solutions would be to use higher reductions to promote a more homogeneous deformation. It is thought that this could be achieved with a minimal number of high-reduction hot rolling passes. Alternatively, the plastic deformation step could be skipped altogether and the long composition gradients generated using thermal treatments only. To this end, diffusion should be carried out in the delta ferrite range where the diffusion of substitutional elements is 100 times faster than in austenite. However, this is possible only in low carbon samples. Thus, millimeter scale composition gradients would be formed first and the required amount of carbon would be reintroduced by re-carburizing the samples. Prolonged high temperature treatments without deformation make grain size control particularly challenging with this method. In this case, cyclic austenitization and quench can be used to refine the microstructure .
Conflicts of Interest
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|Composition wt.%||C||Si||Mn||Mo||Cr||Ni||Al||Other Alloying Elements|
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