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This paper discusses the experimental methods of the study began, the process of preparing materials, Synthesis evaluation of nanostructured  Fe₈₀Cr₂₀ alloys and morphology characterization of specimens. Implementation of surface treatment via ion implantation process and the morphology characterization of  Fe₈₀Cr₂₀ alloys during exposure at 1173 K-1373 K  oxidation test will be explained in detail.

Preparation and Evaluation of Starting Materials

Fe₈₀Cr₂₀ alloys were developed by using mechanical alloying process, high purity (99.9%) powders (average particle size of 2 μm) of Fe and Cr were loaded into the milling jar under the protection of Argon. 5% of Acid Stearic from overall mixture weight was used as process control agent (PCA) to prevent excessive cold welding, the proposed amount is to 5%.Then the milling jar was sealed under argon atmosphere and was pumped with additional argon gas. High energy ball milling was conducted in different milling time for 40, 60 and 80 hours in RETCH PM400 planetary ball mil (Figure. 3.1) using hardened steels balls with vary diameters of 10, 20, 30, 40 and 50mm, and vials rotated at a speed 300 rpm. The ball to powder weight ratio was fixed at 20:1.

Figure :  Planetary Ball Mill PM400

Synthesis Evaluation of nanocrystalline Fe₈₀Cr₂₀ alloys as Milled Powders

The milled powders of  Fe₈₀Cr₂₀ alloys with milling time 7 (Fe₈₀Cr₂₀ 7 h), 15 (Fe₈₀Cr₂₀ 15 h), 20 (Fe₈₀Cr₂₀ 20 h),  40 (Fe₈₀Cr₂₀ 40 h), 60 (Fe₈₀Cr₂₀ 60 h)and 80 (Fe₈₀Cr₂₀ 80 h) hours were evaluate by using x-ray diffractometer (XRD) to identify and characterize nanostructured and intermetallic phase of milled powders.

Williamso-Hall method  was applied to examining the crystallite size and microstrains, the average crystallite size of as milled powders of FeCr 40 h, FeCr 60 h, FeCr 80 h were calculate  by measuring the value of FWHM, 2θ (angle of diffraction), and λ (wavelength) which obtained from XRD data. X-Ray powder diffraction (XRD) measurements were recorded at room temperature in Bruker D8 ADVANCE diffractometer equipped with Cu-Kα radiation (λ= 1.54056 Å). The intensities of diffraction lines were collected with a constant step of 0.02o of 2θ and with a constant counting time of 20 seconds at each step. The peak breadth due to sample (strain and size), B was calculated according to Gaussian profile (Schafler, E. and Zehetbauer, M, 2005, Cullity, B.D. and Stock, S.R, 2001, Dinnebier, R.E. and Bilinge, S.J.L, 2008, and Han, Bing Q, Lavernia, E. J. and Mohamed,F.A, 2005).

X-rays diffraction line broadening profile analysis was used to analyze the crystallite size and microstrains of milled powders. The peak breadth due to sample (strain and size), B was calculated according to Gaussian profile (Schafler, E. and Zehetbauer, M, 2005, Cullity, B.D. and Stock, S.R, 2001).

Where B is the FWHM at half maximum of the powder, B_inst is the FWHM of the standard reference materials (LaB₆: NIST SRM 660a) used for calibration and B_exp is the FWHM evaluated (Cullity, B.D. and Stock, S.R, 2001).

Figure: FWHM of the standard reference material for LaB₆ (Cline, J.P . et al., 2000).

 The average crystallite size and internal micro strain are calculated by the Williamson-Hall method as expressed in Eqution :

Where K is the Scherrer constant, D is crystallite size, λ is the wavelength, and ε is microstrain. The effective crystallite size taking strain into account is estimated by plotting (B cos θ) vs. (sin θ). From Eq. (2), a plot of (B cos θ) versus (sin θ) is a straight line with a slope of 4e and an intercept of Kλ/D. In virtue of (110), (200) and (211) diffraction peaks from XRD profiles, the crystallite size and microstrains of milled powders phase could be estimated.

Chemical composition analysis of Ferritic steel

As comparison one of available commercial Ferritic steel was selected to compare with Fe₈₀Cr₂₀ alloys. Chemical composition of available Ferritic steel was examined by using Glow discharged spectrometry (GDS). A glow discharge optical emission spectrometer (GDOES) is available for measuring bulk chemical composition and quantitative depth profiles on a range of steels commonly used in industrial manufacture. The spectrometer uniformly sputters material from the sample surface by applying a controlled voltage, current and argon pressure, and photomultiplier tube detectors are used to identify the specific concentrations of Fe, C, Si, Mn, P, S, Cr, Mo, Ni, N, O and W, based on the wavelength and intensity of the light emitted by the excited electrons in each element when they return to the ground state.

Figure : Glow Discharge Optical Emission Spectroscopy

Hot compacting process of Ball Milled Powders

Fe₈₀Cr₂₀ 40 h, Fe₈₀Cr₂₀ 60 h and Fe₈₀Cr₂₀ 80 h as milled  powders were selected  to compacted at 1273 K into pellets (diameter = 33 mm and thickness = 3 mm) under a uniaxial pressure of 25 MPa pressure in vacuum atmosphere within 45 minutes, detail explanation of consolidation process as shown in Figure below.

Figure: Cycle of hot compaction process

Figure: Hot Compacting Mach

Synthesis evaluation of Compacted Powders and Commercial Ferritic steel

Synthesis evaluation of compacted powders and commercial Ferritic steel were evaluated based on nanocrystalline structure and micro strain, the average crystallite size of as compacted powders of FeCr 40 h, FeCr 60 h, FeCr 80 h and commercial Ferritic steel were calculate  by measuring the value of FWHM, 2θ (angle of diffraction), and λ (wavelength) which obtained from XRD data. X-Ray powder diffraction (XRD) measurements were recorded at room temperature in Bruker D8 ADVANCE diffractometer equipped with Cu-Kα radiation (λ= 1.54056 Å).

Same like milled powders, X-rays diffraction line broadening profile analysis was used to analyze the crystallite size and microstrains of compacted powders. The peak breadth due to sample (strain and size), B was calculated according to Gaussian profile (Schafler, E. and Zehetbauer, M, 2005, Cullity, B.D. and Stock, S.R, 2001), and the average crystallite size and internal micro strain are calculated by the Williamson-Hall method as expressed in Eqution below.

Surface Treatment via Ion Implantation Process

The effect variation milling time hours on mechanical alloying process followed by hot compaction process were influence the characterization of solid materials. One of them is the value of density will be different in the specimens that are processed with different milling times. In this study, the densities of developed Fe₈₀Cr₂₀ alloys (Fe₈₀Cr₂₀ 40 h, Fe₈₀Cr₂₀ 60 h and Fe₈₀Cr₂₀ 80 h) were examined by using Archimedes test. The density values will affect the level depth profile of dopant ions on the surface of Fe-Cr system; therefore the density needs to be known to determine the parameters of the ion implantation process.

Figure: Archimedes Test

The selected of developed Fe₈₀Cr₂₀ alloys and the available commercial Ferritic steel were cut from the ingots to the size of surface area of about 8 cm². Before implantation, all specimens were polished on silicon carbide paper up to the 1500 grit and final polishing using 0.05 μm diamond pastes. The polished specimens were then ultrasonically cleaned with ethanol over 30 minutes, rinsed with deionised water, and dried.

Figure: Polishing Machine

After cleaning, the implantation of Lanthanum and Titanium dopants with nominal doses 1x10¹⁷ ions/cm² was undertaken using 100 keV ion beam energy and 200 kV acceleration potential. The beam current density was maintained at 10 μA/cm². Implantation of specimens was performed using ion implantation accelerator of Cockcroft-Walton Type which located in BATAN Yogyakarta Indonesia which could generate gas ions with maximum of 200 keV/200 μA (H. Saryanto, D. Sebayang, and P. Untoro, 2009).

The distribution of implanted ions was simulated by TRIM-SRIM Simulation software. The number of ions used in the simulation was 99,999. In addition entered ion types, target elements, target density, and energy range, and the projected distance could be calculated. The required time process of ion implantation, then, can be estimated by the following equation:

Where D is the implanting dose, I represents the beam current (µA), t equals to beam time (s), q is the charge state of the ion, and A is defined as the striking area (cm²) (Marest, G. 1998, Sujitno, T. 2002, and Wena, F. L, Lo, Y.L. and Yu Y.C. 2007). Theoretical calculations have been made using the simulation program TRIM-SRIM Simulation software to provide an approach of the depth profile, the prediction of dopants concentration into surface of the alloys was provided from that software, and can calculate the profile depth of Ions through the use of equation as follows:

x = R_p +σR_p        

Where, x is the depth of profile of ions, R_p is ion range and σR_p is the longitudinal straggling

High temperature oxidation test
For oxidation resistance studies, the as compacted blank specimens (before implantation) with different milling time were submitted to oxidation tests in laboratory air at atmospheric pressure in a PROTHERM box furnace. The isothermal oxidation tests were carried out at high temperature from 1173-1373 K within 100 h. The heating and cooling rates were controlled at 5^oC/min. For weight measurements the exposures were interrupted every 20 h. The mass gain of the specimens was determined after each cycle on an electronic microbalance with an accuracy of 0.01 mg. For more detailed analysis of the oxidation kinetics, the mass gain-oxidation time curves were plotted. In the same way, the selected of developed Fe₈₀Cr₂₀ and the available commercial Ferritic steel as the implanted specimens also performed to oxidation tests for more detailed analysis of comparison of oxidation behaviour.

Figure: Protherm Box Furnace

Oxidation kinetics
The oxidation kinetics were plotting as Mass gain curve, is usually the measurement of the oxidation resistance of heat resistant alloys. After being oxidized for certain time (t) at certain temperature (1173 K, 1273 K, and 1373 K in this experiment), the samples are cooled to room temperature along with the furnace cooling. In this study, the weight gain resulted from mass gain per unit surface area of specimen that can be calculated by the following function:

y(t) = (W_t −W₀)/S

In this function, W₀, W_t, and S, respectively, represent the initial mass, oxidized mass after oxidation for (t) hours and the initial surface areas of the samples.

Parabolic Rate
The parabolic rate law was considered primarily as the basis for data processing and interpretation of results in this study. The mechanism of oxidation test by which thickening proceeds on the surface of specimens of the oxidation test has been agree with Wagner theory of oxidation (Young, D .J. In: Burstein, Tim Editor, 2008). At high temperature, oxide film thicken could be approximated to a near parabolic rate law and the mechanism has been explained by Wagner. The parabolic growth equation of film thickness with time was examined by equation:

(ΔW/A)² = k_pt   

Where k_p is the parabolic rate constant. The k_p was obtained from the slope of a linear regression fitted line of (ΔW/A)² vs. t plot.

Oxide Morphology Characterization
X-ray Diffraction (XRD) and scanning electronic microscopy (SEM) coupled to an Energy dispersive spectrometer (EDX) were used to examine the surface morphology and phase composition of oxide films.  The surface of oxide scales were subsequently examined by scanning electron microscopy (SEM) equipped with an EDX analyzer to elucidate the microstructure and the chemical composition. The chemical phases of the scales were determined by X-ray diffraction (XRD) which performed using a BRUKER D8 Advance diffractometer and as referred to the ICDD PDF-2 database (http://www.icdd.com/products/pdf2.htm, 2009) which attached in the system.

Identification of Oxide Phase

X-ray Diffractometer (XRD) and Table phase diagram were used to identify the sequence of thermodynamically stable oxides from the alloy or scale interface to the reaction gas. In order to predict and explain the oxidation phase accurately for a qualitative interpretation of the oxidation mechanism, Ellingham diagram was used. This is to identity the oxides phase formed during oxidation at different temperatures.

In order to predict and justify the oxidation phase accurately for a qualitative interpretation of the oxidation mechanism, it was applied the Ellingham diagram due to identify the oxides phase formed during oxidation at different temperatures. The equilibrium oxygen partial pressure (pO₂) was used to determine the oxygen activity due to the temperature.  

During the high temperature oxidation of an alloy, a complex interplay between kinetic and thermodynamic considerations arises. After the formation of dense oxide scales, an oxygen potential gradient will evolve across the oxide scale. At high temperatures, one can usually assume a condition of local equilibrium at the Alloy/scale and scale/gas interface. Hence, the oxygen gradient across the oxide scale is given by the decomposition pressure of the oxide in equilibrium with the alloy and the oxygen activity in the reaction atmosphere. The sequence of oxides, which can exist in equilibrium as oxidation products, can be determined from a ternary Fe-Cr-O phase diagram. Thus, phase diagrams are important for a qualitative interpretation of the oxidation mechanism. The present phase diagrams are presented as isothermal alloy p(O₂) phase diagrams. The phase diagrams can be used to identify the sequence of thermodynamically stable oxides from the alloy/scale interface to the reaction gas.

The phase diagrams can be used to identify the sequence of thermodynamically stable oxides from the alloy/scale interface to the reaction gas. The phase diagrams can be used to identify the sequence of thermodynamically stable oxides from the alloy/scale interface to the reaction gas. 

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