Synthesis and characterization of metal-ceramic composite 316l/sycro

The composite formed by 316L steel with 10% of Sr 1.8 Y 0.2 CrReO 6 (SYCRO) double perovskite was successfully sintered using Top Down nanotechnology associated with powder metallurgy. SEM images showed that the SYCRO nanoparticles were deposited on the steel surface homogeneously. The scattering spectra of the images confirmed the presence of the Sr and Re elements scattered homogeneously on the surface of the composite, which corroborates the images observed by SEM. X-ray microtomography showed a solid build, with few pores and cracks, and a 3D distribution of the main components. X-ray diffraction measurements showed that SYCRO particles were embedded in the 316L steel matrix. Magnetic susceptibility a.c. showed that the composite has ferromagnetism and Tc close to that found in the literature for the ceramic composite Sr 1.8 Y 0.2 CrReO 6 , which certifies that the sintering of the composite was obtained successfully.


Introduction
Austenitic steel 316L (S31603) contains a low carbon content (0.03%) and has high resistance to general corrosion, PIT corrosion, intragranular corrosion and corrosion embrittlement under high mechanical stress (1,2).Because it is a widely used steel in everyday applications of corrosion resistance and easy to acquire, it becomes a good candidate for scientific applications.Furthermore, the steel in question has no magnetic response in the annealed condition according to data supplied by the manufacturer.Double Perovskites, in turn, have been of great interest recently because of their wide range of properties (3).Among them, the Sr2CrReO6 compound is one of the ones that has been outstanding due to its properties: conductive and ferromagnetic at room temperature (4).Recent studies (4) have shown that a small amount (10%) of the Y element substituting Sr at its site leads to an increase in magnetization, in addition to making the material Sr1,8Y0,2CrReO6 (SYCRO) more susceptible to field change magnetic.However, the double perovskites in general present some type of degradation.SYCRO, in particular, presents problems with humidity and presence of carbon at high temperature.For the production of a composite that is resistant to corrosion and has a magnetic response, it may be of interest for the production of structural devices, actuators and magnetic sensors that need to be immersed in extreme environments, such as: petrochemical, food, pharmaceutical industry and nuclear power plant.That is, it is intended to combine mechanical and chemical properties of the 316L steel with the magnetic properties of the perovskite SYCRO to obtain controllably magnetic properties in the composition and working temperature range for applications in chemically and thermally aggressive environments.The composite obtained can present chemical and mechanical resistance characteristics controlled by the percentage composition of each one.
2 Experimental Setup AISI 316L / SYCRO composite sintering was performed by solid state diffusion.The ceramic compound SYCRO was obtained by a method described by Orlando et al. (5), its powder was then macerated and sieved in a ball mill until grains of less than or equal to 1μm were obtained.Powder of the AISI 316L brand Master-Melt PLUS was then mixed via high energy ball mill (Top Down nanotechnology) with the SYCRO powder, so that the ceramic represents 10% of the total mass.The obtained nanostructured homogeneous mixture was then compacted cylindrically into a pelletizer under a pressure of 2GPa.The obtained 5mm diameter and 12mm high cylinder was wrapped in a tantalum sheet and sealed under vacuum in a quartz tube prepressurized with Argon gas, as shown in Figure 1, in order to preserve the initial stoichiometry of the ceramics.The heat treatment was carried out in a programmable tubular furnace (6), and the sample was heated for 24 hours at a temperature of 1020 ° C, with a heating rate of 20 ° C / min.The cooling rate was 30 ° C / min Figure 1.Homogeneous mixture of the compacted composite wrapped in a tantalum sheet and sealed under vacuum in a quartz tube.Scanning Electron Microscopy (SEM) and Dispersive Energy Spectroscopy (EDS) were made with the purpose of observing the microstructure and chemical composition of the composite.SEM measurements were performed on a Zeiss machine, model EVO MA10, equipped with an X-MaxN detector from Oxford Instruments.In order to observe the crystallinity of the ceramic in the middle of the composite, X-ray diffraction (XRD) measurements were performed on a conventional Bruker D8 Advance X-ray machine.The length used was λ = 1.5418Å with a pitch of 0.02 ° in a range of 15 ° -100 °.Rietveld refinement was performed using the software FullProf Suite (7).Magnetic susceptibility a.c. at room temperature were performed on the AISI316L / SYCRO composite, in order to determine if there was a change in the magnetic response of the SYCRO ceramics in the composite.These measures were performed in a programmable susceptometer (6).X-ray computed tomography (μCT) was performed to analyze the 3D distribution of particles and defects in the sample.The measurement was done in a Bruker micro-model, model 1173, of high energy.The measurement parameters used were: energy of 130 kV and current of 61 μA; detector array of 2240 x 2240 pixels; magnified pixel size of 6 μm; 0.5 ° angular pitch; number of 5 frames for each projection and 0.50 mm Copper filter

Results
The composite AISI316L / SYCRO was well formed, with metallic and dense appearance.SEM images (Figure 2) showed a good formation of the composite with a considerable presence of pores.The white regions observed in the backscattered electron image (Figure 2b) indicate the presence of SYCRO particles (light regions) deposited in the steel (dark regions).ESD measurements (Figure 2c and Figure 2d) demonstrate a concentration of the Re element in the lighter parts.The high concentration of Cr element in relation to Re in the dispersion spectrum, observed in Figures 2c and 2d, is also present in the steel composition (between 16% -18%), which makes it impossible to distinguish between its concentration in the steel and the ceramic compound.In order to investigate better than the clear regions, a larger approximation was performed in one of these regions (Figure 3a and Figure 3b).A measure of dispersive electron spectroscopy in a continuous-line form was performed (Figure 3c) from a point A, outside the light region, to a point B. The result of the measurement is shown in Figure 3d.We can observe that the concentration of Sr, Y and Re increases as the line approaches the light region, which proves that the region is composed of perovskite SYCRO.An EDS mapping of the same region (Figure 4a) showed that the dark region observed in Figure 3b is mostly composed of steel and that the lighter region is composed of ceramics.Images of μCT (Figures 5 and 6) showed the presence of few defects (pores or cracks) in the 3D structure of the sample (Figures 5b, 6b and 6c).In these images, by default, black pixels represent empty structures or very low density, being treated as background.Pixels with varying shades of gray represent structures of significant density, where the increase of tonality to the white represents a greater density of the structure.It is possible to distinguish, in Figure 5a, two distinct phases of the materials composing the sample.In Figure 5c and 6d (in blue) it is possible to visualize the 2D and 3D distribution, respectively, of the whitest pixels identified in the sample.
The processing of the images obtained by μCT basically followed the following steps: Smoothing through a Non-Local Means filter (window: 21 pixels; neighborhood: 5 pixels; similarity: 0.6), to better homogenize the image tones, so as to allow a better separation of the phases; interactive thresholding, separating the structures according to the gray-scale histogram intensities of the image; and morphological operations to eliminate false structures due to noise

Conclusions
The composite formed of 316L steel with 10% of Sr1,8Y0,2CrReO6 double perovskite (SYCRO) was successfully sintered using the Top Down technique associated with powder metallurgy.SEM images showed good formation of the composite with a considerable presence of pores and that the SYCRO nanoparticles were deposited in the composite.The dispersion spectra of the images confirmed the presence of perovskite elements in the steel medium in the composite.X-ray diffraction measurements confirmed the presence of SYCRO in the composite, but with a low percentage of phase due to its small grain size.The calculated network parameters for perovskite were lower than those observed in the literature, however, more precise measurements are necessary to prove this deformation.Magnetic susceptibility a.c.showed that the composite has ferromagnetism and Tc close to that found in the literature for the ceramic composite Sr 1.8 Y 0.2 CrReO 6 , which certifies that the sintering of the composite was successfully obtained.This work indicates opportunities for the production of new composites that can be elaborated from this idea for several other applications: biomaterials, microheaters, sensors and magnetic actuators to be used even in chemically aggressive environments

Figure 2 .
Figure 2. 316L / SYCRO Composite Scanning Electron Microscopy Images.(a) Image of secondary electrons in 500 X.(b) Image of backscattered electrons in 500 X.(c) Map of dispersive energy spectroscopy in 500 X and dispersion spectrum (d).

Figure 3 .
Figure 3. Backscattered electron image in 500 X (a) with approximation in 3000 X of the region indicated with arrow (red) (b).EDS in line from the approximate region (c) from point A to point B. Point C stands out for the boundary between the 316L steel grain and the SYCRO perovskite grain.(d) Scatter plot as a function of the distance traveled by the line.Points A, B and C are the highlights in Figure c.

Figure 4 .
Figure 4. Map of SDS of the region in the approximation of 3000 X (a) highlighting the presence of a perovskite grain in the center (light region), where the Re element is highlighted, and steel in the region external to the perovskite grain (dark region).(b) Scatter plot of the ESD map.

Figure 5 .Figure 6 :
Figure 5. 2D images of original μCT (a); (b) pore and crack cracking; (c) segmentation of the structures of higher density

Figure 7 .
Figure 7. Rietveld refinement of the composite.The dots in red represent the spectra observed experimentally, the black curve indicates the spectrum calculated by the Rietveld refinement, the blue curve indicates the observed and calculated spectrum difference, the traces below the spectrum (red) indicate possible reflection planes of perovskite SYCRO (in blue) and steel (in green).The reliability parameters generated by the refinement were Rp = 7.96, Rwp = 11.4 and χ2 = 4.4.

Figure 8
Figure8shows that the ferromagnetic arrangement ceases around 650 K, and the 1st derivative indicates a transition temperature Tc≈625 K.The result found corroborates with the results found in the literature for the compound SYCRO (4,5), which certifies that the composite has the presence of the double perovskite.

Figure 8 .
Figure 8. A.c. susceptibility depending on the temperature of the composite.