Pancake-like antiphase domains in perfect nitrogen-expanded austenite by TEM characterization

Elongated superstructure diffraction spots were observed in perfect nitrogen-expanded austenite without stress or plastic strain, which was prepared by plasma-based low-energy nitrogen-ion implanting Fe-Cr-Ni austenitic alloy foils at low temperature of 380 °C. The Fe4N-like ordered nitrogen-expanded austenite (γ′N) antiphase domains formed in the disordered nitrogen-expanded austenite (γ N) matrix. Only the high Cr-content alloys presented elongated diffraction spots. The reconstruction of the diffraction spots strength distribution shows an elongated egg-shape in reciprocal space. This elongation corresponded to a pancake-like antiphase domains distribution in matrix, which was confirmed in dark field images as a lamellar structural feature. The Cr-N short-range ordering (SRO) and anisotropic elastic module rather than the constrained strain in the γ N phase layer determine the formation of γ′N antiphase domains.


Introduction
Nitrogen-expanded austenite is characterized as a metastable, precipitate-free and superhard phase formed on the Fe-Cr-Ni austenitic stainless alloy and steel by low-temperature nitriding [1].The nitrided layer provides improved wear, corrosion and fatigue resistance.The micro-and nanostructure of the nitrogen-expanded austenite is complex owing to interstitially nitrogen supersaturation in the austenite matrix at the moderate nitriding temperature [2].Recent research has advanced the understanding of the microstructure of the nitrogen-expanded austenite.The nitrided layer, generally identified as expanded austenite consists of a duplex structure.In the outer surface zone, with the highest N content, a long-range ordering (LRO) among the N atoms occurs in addition to the short-range ordering (SRO) of N and Cr atoms; and in the subsurface zone, with the lower N content, only the SRO of N and Cr atoms is encountered.These zones are dubbed γ′ N and γ N , respectively [3,4].The Fe 4 N-like LRO γ′ N phase is obtained as small ordered domains separated by antiphase boundaries.It is experimentally and theoretically confirmed that the ordering of the interstitial nitrogen involves nucleation, growth and, after impingement, coarsening of the ordered domains [5].The domain size decreases with the Cr content and increases with the N content in the alloys studied.In previous studies, most of the antiphase domains observed in low-temperature nitrided austenitic stainless steel with about 18 wt% Cr are present as an isotropic structure [6][7][8].However, in the TEM observation by Jiang and Meletis [9], the superlattice diffraction spots show very weak elongation, which usually indicates an abnormal structure of the ordered antiphase domain in the nitrided layer.Since the low-temperature nitrided layer has a very complicated state, such as high stress, high concentration gradient, and high defect density and so on [10][11][12][13], it is important to discover the core reason for the elongation of superlattice diffraction spots or the abnormal in antiphase domain structure.These high defects, stress, strain and gradient are not the nature of the nitrogen-expanded austenite, therefore, it is difficult to exclude the constrained expansion during low-temperature nitriding [14][15][16].However, a perfect nitrogen-expanded austenite can be obtained, based on the low-temperature nitriding of the TEM sample, to study the phase nature purely [17].
In present work, an abnormal anisotropic antiphase domain has been characterized in perfect nitrogenexpanded austenite without stress and plastic strain prepared by unconstrained nitriding.The microstructure and nanostructure of the antiphase domain are analyzed by electron diffraction and corresponding dark-field image under TEM observation, and thus its origin and formation mechanism are also discussed.

Experimental
Two austenitic Fe-Cr-Ni alloys were prepared by melting high-purity iron, chromium, and nickel.The desired Fe-Cr-Ni alloy composition from the Schaeffler phase diagram is obtained as alloy-lx (Fe-66 wt% and Ni-34 wt%) and alloy-m (Fe-55.34wt%, Cr-17.66 wt% and Ni-27 wt%).The composition was chosen to keep the alloys in an austenitic state and to have different Cr contents.The TEM foils were prepared by the standard method: (1) the alloys were cut into 2 mm thick slices, and both sides of the samples were ground by watergrinding abrasive papers from coarse to fine in sequence until the thickness reached about 30 μm; (2) using a Gatan dimpling grinding system, the thickness was further reduced locally until the thickness reached about 10 μm, and the thinned area was cut into a 3 mm diameter chip; (3) using a Gatan precision ion polishing system to further thin the film to the nanoscale by a single-sided double-gun mode, ensuring that one side of the thinned area was not damaged by the bombardment of the frontal ion beam.The nitriding process was performed by plasma-based low-energy ion implantation at 380 °C for 10 min.The TEM specimens were fixed in a specially designed holder to keep the specimen upright, the details of which are described in [17].

Results
Figure 1 shows the TEM bright field images and electron diffraction patterns of the perfect nitrogen-expanded austenite prepared for alloys-lx and -m.The bright field images show the observable thin region at the free edges that can expand freely without constraint from the austenite matrix during nitriding.The contrast in the bright field image is equal thickness and/or equal inclination fringes, not twins and faults from constrained strain.The Fe 4 N-like superstructure spots are observed in the electron diffraction patterns of both nitrided alloys.The electron diffraction of alloy-lx shows a standard 〈110〉 axial diffraction pattern of Fe 4 N ordering.In comparison, in the diffraction patterns of alloy-m, a clear phenomenon is observed that the superstructure spots are scattered with a clear elongation along the (001) fundamental spots.The diffraction spot scattering is very sensitive to local inhomogeneity and can be used to directly probe the local structures.Since the preparation and observation processes of both nitrided alloys are under the same conditions, the superstructure spot distortions of the γ′ N phase in alloy-m are real diffraction phenomena that could rule out the possibility of experimental error.Otherwise, since the two samples are all perfect nitrogen-expanded austenite without stress and plastic deformation, the possibility of twins, stacking faults, ε-martensite and stress can also be ruled out.
To further understand the distortion shape of the superstructure spots in the γ′ N phase for alloy-m, both 〈100〉 and 〈110〉 axial diffraction patterns of alloy-m were examined to reconstruct the diffraction intensity distribution in reciprocal space.Figure 2 shows the diffraction patterns of γ′ N phase for alloy-m with the pattern reconstruction process.Figures 2(a When at least one dimension of the observed object is smaller than about 10 nm, its diffraction spots are significantly broadened.The extension and shape of the diffraction spots in the reciprocal lattice are directly related to the external shape of the observed object.As the superstructure spots are egg-shaped elongated while the fundamental spots are unchanged, this indicates that the nitrogen-ordered arrangement presents has separated antiphase domains with small pancake shape [18].To further confirm the origin of the egg-shaped elongated superstructure spots, figure 3 shows the TEM dark-field images of the perfect nitrogen-expanded austenite of the elongated (010) and (001) superstructure spots.The bright part corresponds to the Fe 4 N -like ordered antiphase domains with the (010) superstructure diffraction [figure 3(a)] and the (001) superstructure diffraction [figure 3(b)], respectively.The bright antiphase domains appear as dense parallel lines with a clear orientation, which is the cross section of a lamellar structure.The distribution of antiphase domains is not uniform, but the bright zones distributed in the two dark-field images are partially complementary and could fill most of the sample.Figure 4 shows the schematic diagram of the diffraction patterns with elongated superstructure spots depending on the shape and distribution for the antiphase domains.The lamellar antiphase domains are perpendicularly arranged and distributed in different zones along three (100) planes, resulting in the elongated superstructure spots in the diffraction patterns of γ′ N phase.

Discussion
Compare with the nitrided layer reported by Jiang and Meletis [9], the elongation spots in perfect expanded austenite are much clearer.The different distortion behavior of the superstructure spots in the different   reports may be caused by three reasons: (1) the TEM observation of conventional nitrided layer samples contained high-density stacking faults and dislocations, which may interfere with the diffraction behavior of antiphase domains, resulting in less or no distortion of the superstructure spots [3,6,7,9]; (2) the plastic deformation and stress affect the nucleation and coarsening of the ordered phase, which eliminates the anisotropic growth; (3) the different nitriding temperature and time lead to variations in the elastic and stress state, and further affect the shape of the antiphase domains in the nucleation and growth process.
Based on the high-resolution TEM observation of antiphase domains, most of the reported antiphase domains are isotropic, corresponding to undistorted superstructure spots.Therefore, the reason (1) could be basically be excluded.Based on the previous reports, the influencing factors on the shape of the ordered second phase precipitated from the matrix are summarized as diffusivity, interfacial energy, elastic energy, misfit strain, plastic deformation, applied stress, and crystallographic orientation [19][20][21][22].Since the perfect nitrogenexpanded austenite is free from plastic deformation and constrained stress during and after the nitriding process, the diffusivity, elastic energy and misfit strain are the main factors influencing the shape of γ′ N antiphase domains.Although the large macroscopic compressive stresses are eliminated in the perfect nitrogen-expanded austenite, the nanoscale stresses and strains in the γ N and γ′ N phases from inhomogeneous nitrogen concentration still exist.The elastic modulus in the 〈100〉 direction is largest for the Fe 4 N-like ordered γ′ N phase, but smallest for the nitrogen disordered γ N phase [3,23,24].Therefore, the γ′ N phase preferentially grows along the 〈100〉 direction, which corresponds to the soft direction in the γ N phase and maintains a lowest elastic strain energy [20].This is expected to be the main driving force for the formation of the lamellar antiphase domain structure.The elastic interaction between the parallel adjacent antiphase domains also affects the growing and coarsening process of the antiphase domains to form the lamellar structure [20,25].However, the perfect expanded austenite was nitrided for only 10 min, which mainly remains in the nucleation stage.In the conventional samples nitrided for hours, the ordered phase has coalesced and undergoes a long coarsening process.The entire SRO γ N phase is basically transformed in ordered γ′ N phase, and the different ordered γ′ N domains are separated by antiphase domain boundaries.During the coarsening process, the elastic modulus between different ordered γ′ N phase domains are same, and the driving force for anisotropic growth disappears.Therefore, their coarsening process will not go on anisotropic growth and prefers to form isotropic antiphase domain structure, which should be one of the main reason of disappear of anisotropic antiphase domain.

Conclusions
In summary, the perfect nitrogen-expanded austenite of the designed Fe-Cr-Ni austenitic alloys without stress and plastic strain is prepared by low-temperature nitrided TEM foils.The structure and origin of Fe 4 N-like ordered γ′ N antiphase domains are explored by TEM observation of the perfect nitrogen-expanded austenite.Based on the elongated electron diffraction and corresponding dark field image, the lamellar γ′ N antiphase domains along (100) planes in the γ N matrix are confirmed.The pancake-like feature of the antiphase domains is explained by anisotropic elasticity modulus of the γ′ N and γ N phases.It is unambiguously demonstrated that the formation of antiphase domains in the unconstrainedly nitrided layer is determined by Cr-N SRO, not by stress and plastic strain.

Figure 1 .
Figure 1.TEM bright field images and diffraction patterns of the perfect nitrogen-expanded austenite on two nitrided alloys: alloy-lx, alloy-m.
) and (b) show the 〈100〉 and 〈110〉 axial diffraction patterns, and figures 2(c) and (d) show the reproduced diffraction patterns in the 〈100〉 and 〈110〉 axial directions, respectively.Based on the 〈100〉 and 〈110〉 diffraction patterns, the reciprocal lattice with superstructure spots in the reciprocal space is reconstructed as shown in figure 2(e).The intensity of the superstructure peaks has an approximately eggshaped distribution that extends along the two closest fundamental spots.In the 〈100〉 diffraction pattern, the (100) superstructure spot is a cross section of the egg-shaped distribution along the elongated direction, while the (110) superstructure spot is a cross section of the egg-shaped distribution along the elongated direction, as shown by the green line in figures 2(c) and (e).In the 〈110〉 diffraction pattern, all superstructure spots are cross sections of the egg distribution along the elongated direction, as shown by the red line in figures 2(d) and (e).

Figure 3 .
Figure 3. TEM Dark field image of (010) and (001) in the 〈100〉 axis of the perfect nitrogen-expanded austenite for the nitrided alloy-m.

Figure 4 .
Figure 4.The schematic diagram of the diffraction patterns with elongated superstructure spots depending on the shape and distribution for the antiphase domains.