Data on the microstructure and deformation of Fe50Mn25Cr15Co10Nx (x=0∼1.6) supporting the modifications of partial-dislocation-induced defects (PDIDs) and strength/ductility enhancement in metastable high entropy alloys

The data presented in this article are related to a research paper on the modification of deformed nanostructure and mechanical performance of metastable high entropy alloys (HEAs) [1]. Fe50Mn25Cr15Co10 alloys with and without nitrogen were synthesized in a vacuum induction furnace using pure metals of 99.99% purity and FeCrN2 as nitrogen source. The nitrogen content was determined by Leco O/N-836 determinator for nitrogen-doped alloys. Transmission electron microscopy (TEM) were carried at 200 kV equipped with energy dispersive spectroscopy (EDS). Tensile testing was performed at room temperature. The strain rate jump tests were conducted by changing the strain rate between 10−3 and 10−2 s−1 to measure the strain rate sensitivity. The nanostructural evolutions by deformation including extended stacking faults (ESFs), ε-martensite and twins were examined using EBSD and TEM for the annealed samples and those strained to different strain levels. The role of partial dislocations on the formation of various PDIDs were analysed and the energies stored as deformed nanostructure (ESDN) after the PDID band formation were used to predict the evolution of various nanostructure with strain. The data and approach would provide a useful insight into the nanostructural evolution in metastable high entropy alloys.

the energies stored as deformed nanostructure (ESDN) after the PDID band formation were used to predict the evolution of various nanostructure with strain. The data and approach would provide a useful insight into the nanostructural evolution in metastable high entropy alloys.
© 2021 The Author(s). Published by Elsevier Inc. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ ) Table   Subject Metals and Alloys Specific subject area Microstructure and mechanical properties of high entropy alloys. Type of data can be used for developing a strategy for modifying the phase metastability and the mechanical performance of high entropy alloys. • The dataset on the relationship between the deformation homogeneity caused by high strain rate sensitivity and the nanostructural evolution in nitrogen-free and nitrogen doped alloys can be useful for researchers on alloy design and nanostructural analyses of metastable HEAs. • The data of the energy storage on the PDID nanostrucure based on the energy criteria and the modification of the PDIDs would provide a useful basis for designing high entropy alloys and high entropy steels with the strength/ductility enhancement.

Data Description
Han et al. [2] reported that nitrogen addition in CoCrFeMnNi HEA enhanced the mechanical properties due to the increase of lattice frictional forces and the maintaining planar slip and twinning. Nitrogen doping is known to stabilize the face centered cubic (fcc) phase [3] and therefore modify the metastability of high entropy alloys [4 , 5 , 6] . Fig. 1 shows TEM micrographs of annealed Fe 50 Mn 25 Cr 15 Co 10 (a), Fe 50 Mn 25 Cr 15 Co 10 N 1.0 (b) and Fe 50 Mn 25 Cr 15 Co 10 N 1.6 (c) high entropy alloys. Considerable fraction of thick thermally induced ε-martensite bands was formed in nitrogen-free Fe 50 Mn 25 Cr 15 Co 10 alloys ( Fig. 1 (a)) and the fraction of annealing twin increased appreciably with increase of nitrogen content in Fe 50 Mn 25 Cr 15 Co 10 N 1.6 (c). The absence of thermally induced ε-martensite bands in annealed nitrogen-doped alloys is consistent with the XRD data and EBSD data of the research article [reference 1]. The increase of annealing twins with increase of nitrogen content as shown in Fig. 1 (b) and 1(c) is also supported by EBSD phase map ( Fig. 1 (d1) and Fig. 1 (e1) of Reference 1). The large fraction of the thick thermally induced εmartensite in Fig. 1 (a) is attributed to the stability of hexagonal close packed (hcp) ε-martensite in nitrogen free Fe 50 Mn 25 Cr 15 Co 10 alloy. The stability of the phases is dependent on the thermodynamic parameters [1 , 7 , 8] .
The strain rate jump tests were conducted by changing the strain rate between 10 −3 s −1 and 10 −2 s −1 to measure the strain rate sensitivity. Fig. 2 exhibits stress-strain curves with the strain rate periodically changed from 10 −3 s −1 to 10 −2 s −1 and then back to 10 −3 s −1 at room temperature. The strain rate sensitivity m was calculated based on the change of the flow stress obtained from the strain rate jump tests shown in Fig. S2 using the following equation [9 , 10] : where σ is the flow stress and ε is the strain rate. Repetitive strain rate jumps between 10 −3 S −1 and 10 −2 S −1 were carried out. The strain rate sensitivity values calculated from the strain rate jump tests ( Fig. 2 ) are summarized in Table 1 . The strain hardening rates obtained from the     Table 2 . Interestingly, the strain rate sensitivity (SRS) increased significantly with increase of nitrogen content. The SRS at low strains of Fe 50 Mn 25 Cr 15 Co 10 N 1.6 ( ∼0.024) was close to those of Cantor alloy [11] , but it decreased more slowly than in Cantor alloy and the value ( ∼0.013) remained higher than Cantor at the strain of 0.3. The strain rate sensitivity of flow stress was found to decrease with strain as in equiatomic CoCrFeMnNi [10] . The enhanced ductility in Fe 50 Mn 25 Cr 15 Co 10 N 1.6 can be attributed partly to the enhanced strain rate sensitivity. Fig. 3 exhibits bright field and dark field TEM images of nitrogen-free Fe 50 Mn 25 Cr 15 Co 10 deformed to 20%. The formation of formation α'-martensite (body centered tetragonal (bct)) phase was confimed in the region of intersecting ε-martensite bands at the strain of 0.2 in Fe 50 Mn 25 Cr 15 Co 10 . The dark field image (b) obtained from a spot from α'-martensite of the diffraction pattern (inset in (a)) proves that the highlighted phase in Fig. 3 (b) is bct α'martensite. The presence of larger α'-martensite was confirmed with the clear diffraction pattern    Fig S5(a). No diffraction spots from ε-martensite (hcp) were observed, suggesting they are ESFs [1] . The stacking faults in Fig. 5 were more widely extended than those observed in low stacking fault energy Cu-Al alloys [13 , 14] and other HEAs [6 , 15] . Propagating ESFs in Fig. 5 were observed to be impinged on the grain boundaries, and the enlarged view ( Fig. S5(b)) from the region enclosed by a red dotted square in Fig. 5 (a) shows the ESFs impinged on and nucleated from grain boundaries.
In Fig. 6 [1] . The reduction of stacking irregularities would be more effective for more closely spaced ESFs and this was suggested to promote the structure with closely spaced ESFs and increase the chance for the transition from closely spaced ESFs to deformation-induced ε-martensite and deformation twins with increase of strain [1] . The energies stored as deformed nanostructure (ESDN) for ESFs, ε-martensite or deformation twins in a grain with the size of 5 μm are summarized in Table 3 for comparison with those for grain size of 12 μm in Table 2 of Reference [1] . The differences of ESDN between ε-martensite and deformation twins in all three alloys for grain size of 5 μm were found to be greater than those of their counterparts with the grain size of 12 μm, suggesting the difficulty of deformation twinning with decrease of grain size. The ratios of grain boundary strain energy (GBSE) to partial dislocation induced defect energy (PDIDE) for deformation twinning of alloys with the grain size of 5 μm were also found to be far greater than their counterparts with the grain size of 12 μm (Compare the data in Table 3 to those of reference [1] ).

Experimental Design, Materials and Methods
Fe 50 Mn 25 Cr 15 Co 10 Nx ( x = 0, 1.0, 1.6 at.%) alloys were cast in a vacuum induction furnace using pure metals of 99.99% purity and FeCrN 2 as nitrogen source for nitrogen-doped alloys. Ingots Table 3 Energies stored as deformed nanostructure (ESDN) after the formation of PDID bands including bands of ESFs, ε-martensite or deformation twins in a grain with the size of 5 μm. were homogenized at 1323 K for 24 hrs in Ar atmosphere. Homogenized ingots were machined to cakes with the thickness of 15 mm (for nitrogen doped alloys) or 6 mm (for nitrogen-free alloys) and cold-rolled to sheets with 1 mm thickness and annealed at 1173 K for 1 h in an Ar atmosphere and air-cooled. Nitrogen-free alloys were found to be less ductile because of massive thermally and mechanically induced ε-martensite during rolling and the total cold rolling reduction was strictly controlled (47% cold rolling reduction compared to 92% reduction in nitrogen containing alloys) to avoid rolling cracks. The amount of FeCrN 2 addition was adjusted to obtain the targeted nitrogen composition and the nitrogen content was determined to be 0. The strain rate jump tests were conducted by changing the strain rate between 10 −3 and 10 −2 s −1 to measure the strain rate sensitivity. The structural evolutions by deformation were investigated using EBSD and TEM for the strained and fractured tensile sample with different strain levels. The externally applied energy during quasi-static tensile testing was assumed to be converted to and stored as SFE, interface energy, twin boundary energy, grain boundary strain energy and/or phase transformation free energy of ε-martensite transformation (Eqs. (1), (2) and (3)). The energies stored as deformed nanostructure (ESDN) after the formation of PDID bands including bands of ESFs, ε-martensite or deformation twins was used to predict the nanostructural development based on the assumption that type of most dominant PDIDs were determined by the critical ESDN value required to form ESFs, ε-martensite or deformation twins.

Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships which have or could be perceived to have influenced the work reported in this article.