Structural, thermal and optical characteristics of Fe 1−X Zr X alloy by using mechanical milling approach: influence of Zr4+ ion

By changing Zr concentrations, Fe1-x-Zrx(x = 0.25 & 1 at%) alloys were successfully produced in an argon atmosphere using the mechanical alloying method. The produced Fe1-x-Zrxalloys were characterized by x-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersion x-ray analysis (EDAX), fourier transform infrared spectroscopy (FTIR), thermogravimetry analyzer (TG-DTA), ultraviolet-visible spectroscopy (UV–vis) absorption, and photoluminescence (PL) spectroscopy. According to XRD pattern analysis, the prepared alloys had a cubic crystalline structure and complete solid solution formation. The prepared alloy samples the average crystallite size was calculated using the Scherrer formula. The average crystallite size for the 0.25 at% Zr concentration is found to be 7.79 nm and 11.8 nm for the 1 at% Zr concentration. Lattice parameter changes are a very important tool for confirming the complete dissolution of the Zr atom in the Fe matrix.The TEM-dark field image confirms that the grain size is in the nanometric range (<100 nm). TEM-SAED spotty continuous ring pattern confirmed the complete solution formation is well correlated with the XRD results. The elemental composition of materials can be determined quantitatively through energy dispersive x-ray analysis in the Fe1-x-Zrxalloy. The elemental distribution of the mechanically alloyed samples shows that Zr elements are homogeneously distributed in the Fe matrix. Bands at 3428 cm−1 in the FTIR spectrum have been linked to O–H stretching vibrations. CH2 and CH stretching vibrations were associated with peaks of about 2920 cm−1 and 2850 cm−1. The weight loss and gain changes were observed and represented in the TG-DST graph; we found that overall weight changes are + 10.7% (gain) at 1023 °C for Fe 1-x-Zr x (x = 0.25 at%) alloy. However, when compared to 0.25 at% of Zr alloy, Fe-Zr (x = 1 at%) exhibits a low weight gain (+ 6.54%). The UV–visible absorbance edge revealed a blue shift when Zr was added, indicating alloy production. The energy band gap of materials was calculated using UV–vis, and it has been observed that the band gap reduces as Zr concentration increases. Zr was added to Fe1-x-Zrxalloy nanoparticles, resulting in 514 nm and 775 nm emission wavelengths. The greatest emission wavelengths and strong flawless sharp emission peaks were discovered to be between 450 nm and 550 nm. The higher PL emission peak was 514 nm at 0.25 at% of Zr.


Introduction
In recent decades, Iron (Fe)-based alloys have become increasingly important in several industries due to their unique properties.Furthermore, scientists are interested in Fe alloys because of their extensive uses in a variety of fields, including nanoscience, biomedical, mechanical and aeronautical engineering.Because of their superior mechanical qualities, excellent wear and corrosion resistance at a reasonable cost, iron-based alloys are frequently employed in high-temperature applications.Particularly, nanosized Fe materials,which include optoelectronic, mechanical and magnetic properties are tunable with favorable doping elements (transition metals or rare earth metals) [1][2][3].Among the transition metal (TM) elements, Zirconium (Zr) is a significant TM from Group IV-B of the periodic table.Due to its high hardness, corrosion resistance, excellent biocompatibility and ductility properties, it has been used in multiple applications.In addition, its physical and chemical properties are similar to those of titanium elements and it is also a transparent optical material.Due to these properties, they are used in the manufacture of transparent optical devices, fuel cells and electrodes [4][5][6].Nowadays, developing novel materials and determining their performance is one of the most promising areas of research for contemporary needs.There are several research papers demonstrating that the binary alloy system Fe-X (where X = Cu, Co, Nb, Ni, Zr, Zn, Mn) is used in a variety of engineering applications.YidiLi et al [7] reported that Fe-Cu alloys have excellent mechanical properties such as strength and ductility.Fe-Co alloys in the form of thin films are also widely used in lightweight microwave absorbers (LMA) for electronic communications and military defence [8].The high strength and electrical properties of the Fe-Cu-Nb alloy lead to an increase in the electrical industry, as reported by Ping Zhang [9].According to a literature survey by Mengmeng Wang et al Ni-Fe alloys exhibit high corrosion resistance [10,11].Fe-Zr alloy shows that the greatest wear resistance has been widely discussed by Cong et al [12] And also, Yanan Xu et al [13] have shown high corrosion rates and biodegradable properties in Fe-Zn alloy.Gustavo Figueira et al [14] Fe-Mn alloys have proven to be bio-absorbable materials in biomedical applications.On the other hand, non-equilibrium processing methods like Mechanical Alloying (MA) [15], inert gas condensation (IGC) [16] and rapid solidification processing (RSP) [17] can be used to bring thermodynamically insoluble alloying elements into the lattice of solvent up to some extent to produce a disordered solid solution.The MA is preferred among the nonequilibrium processes, because it is very effective in producing highly supersaturated solid solutions effortlessly.This solid-state powder processing technology includes cold welding, fracturing and re-welding of powder particles to bring them into nanostructures.Highly metastable materials such as nanostructured materials can be prepared by the process.Scaling up to industrial quantities seems straight forward.In addition to attrition and agglomeration, high energy milling can induce chemical reactions, which can be used to influence the milling process and the properties of the product.The MA is also reported as a feasible solid state processing route for the synthesis of large quantities of nanostructured materials [15][16][17][18].The main purpose of this work is to investigate the structural, thermal and optical properties of a Fe-Zr alloy with two different Zr concentrations.So far, various routes for producing Fe-based alloys have been presented, including vacuum hot pressing, mechanical alloying, hydrothermal, microwave irradiation, sol-gel, thermal decomposition, precipitation and powder consolidation by spark plasma sintering techniques.The mechanical alloying technique is preferred as a result of its convenient and effective methodology to perpare the alloys.

Experimental details 2.1. Synthesis of Fe 1-x Zr x alloy
The Fe-Zr x alloys with two different amounts of Zr (0.25 & 1 at%) elemental powders have been synthesized by the mechanical alloying method.Commercially available elemental powder (Alfa Aesar-325 mesh) of zirconium andiron of high purity (99.7%) have been used for the preparation of Fe-Zr alloys.Raw materials (Fe & Zr) were weighed and sealed in a hardened stainless steel container with a high-purity argon environment (99.9 wt%).Hardened stainless steel grinding media and balls were used for mechanical milling.The grinding media carries 8 mm and 6 mm diameter balls and a ball-to-powder weight ratio of 10:1 was maintained throughout the milling time.The vial was sealed in high-purity argon atmosphere (purity <10 ppm O 2 ) prior to milling and the milling was carried out for 25 h at room temperature.These elemental powders were blended by using SPEX 8000 M high-energy ball mill.Finally, the milled powder samples (Fe 1-x -Zr x ) were received and the graphical representation is shown in figure 1.

Characterization
The Fe 1-x Zr x alloy (x = 0.25 and 1 at%) alloy was developed and its structural characteristics determined using XRD and FTIR.Transmission electron microscopy was used to examine the surface morphology of the samples (TEM).The chemical composition of nanoparticles was determined by energy dispersion x-ray analysis (EDAX).Using a thermos gravimetric analyzer (TGA), the thermal performance of the Fe-Zr alloy was examined.A commercial UV spectrophotometer is used to record the optical absorption spectra.Using a commercial PL spectrometer and a 325 nm He-Cd laser excitation, the PL spectra of all the materials were captured at room temperature.2 shows the XRD pattern analysis with Cu-Kα 2 radiation of wavelength λ = 1.5406Å from 2θ°= 20°to 80°the scan rate is 0.2°per min and the data were analyzed by using of X'pert high score plus software.From the XRD pattern analysis, it can be observed that the Zr peak is not detectedin the two alloy compositions are shown in figures 2 (a) and(b).And also, at a higher angle 2θ, it clearly noticed that peak intensity is gradually decreased and full width half maximum (FWHM) or width of the peak is increasing due to the addition of 1 at% Zr in the Fe matrix are shown in figures 2 (a) and (b).Moreover, the XRD JCPDS no:06-0696 for the Fe-Zr alloy.It is mainly attributed due to two reasons: crystallite size reduction and increase in the residual strain because of their atomic radius difference of iron (Fe = 0.126 Å) is smaller than that of zirconium (Zr = 0.160 Å) and its dissolution of larger-sized solute (Zr) atoms in the Fe matrix [18][19][20][21][22].The complete dissolution of solute (Zr) atoms in the Fe matrix after 25 h of mechanical alloying and also it increases the defects in the Fe-Zr alloy particularly dislocation density, point defects, etc.The crystallite size is reduced to a minimal range, which is reflected in the peak broadening.The higher magnified view (110) peak was shown in figure 2(c), the peak shifts towards the left (lower 2θ angle) and it is also mainly attributed due to Zr concentration increases [23].It is clearly indicated that the increase in the lattice parameter is due to the peak shift towards the lower 2θ angle and also it confirms the solid solution formation.The complete dissolution of the alloying element in the matrix can generally be observed through changes in the lattice parameter.Furthermore, lattice parameter change is a very important tool to confirm the complete dissolution of alloying elements in the matrix or solid solution [15,16].XRD phase analysis concluded that there is a complete solid solution or single phase formation and also there is no amorphous or impurity or Fe 2 O 3 phases present in the Fe-Zr alloy with the variation of Zr concentration.

Result and discussions
Moreover, the average crystallite size (D), lattice parameter (a) and lattice micro-strain (ε) of Fe 1-x -Zr x alloy samples were calculated by using the Scherrer formula.The XRD pattern was analyzed by using X'pert high score plus software, Cu-Kα 2 peak stripping, and background subtraction was accomplished.XRD line profile analysis of three major peaks was used to determine the crystallite size after instrumental peak broadening (Bi) and strain-related peak broadening were removed (Bs).The standard polycrystalline silicon sample is run to obtain the instrumental broadening (Bi).The geometric mean of the Gaussian and Lorentzian profiles is then used to subtract Bi from the measured peak width (Bo).This results in Br, which is broadening due to strain (Bs) + broadening due to crystallite size (Bc).[18][19][20][21][22]. Using Scherrer equation (1), based on the FWHM values, the average crystallite size (D 110 ) of Fe 1-x-Zr x alloy was determined.
Where λ is the x-ray diffraction wavelength, K is a constant of 0.94, θ is the Bragg angle and β is the diffraction peak full width at half maximum (FWHM).It is significant that table 1 reports on the average crystallite size of 0.25 at% Zr alloy to be 7.79 nm, while for 1 at% Zr were found 11.80 nm as shown in the figure 2(d).The examined Fe 1-x -Zr x alloys lattice parameter was estimated using the equation (2) below.
The Williamson-Hall method is used to eliminate the strain-related widening in the plot between BrCosθ versus Sinθ.The lattice spacing is d, the lattice parameters are a, b, and c, and the Miller indices are h, k, and l.Calculated with formula (2), the lattice parameters of 2.811 Å for 0.25 at% Zr alloy and 2.877 Å for 1 at% Zr.It is clearly indicated that the lattice parameter of Fe1-x -Zr x alloy decreases with the increase of Zr content, which may be attributed to the complete solution formation after 25 h of mechanical alloying.It can be observed that the unit cell volume (V = a3) of the alloy increased for Zr concentration high, in agreement with atomic radii of Zr   (0.160 nm) larger than atomic radii of Fe (0.126 nm).The lattice micro-strain of Fe 1-x -Zr x alloy was obtained using the below given formula (3).
It can be observed from the table 1 that the micro-strain decreases with increasing Zr concentrations.The obtained micro-strain values for the 0.25 at%Zr alloy were about 3.05 × 10 −3 , which value increased to 4.45 × 10 −3 with increasing Zr concentration.The increasing of lattice micro strain with increasing of Zrcontent due to the reduction of crystallite size (D) with increase in the dislocation density (δ) [24].

Microstructure analysis of Fe 1-x -Zr x alloy
Transmission electron microscopy (TEM) analysis was carried out for the Fe 1-x -Zr x alloy (1 at%Zr) as shown in figure 3. Selected area electron diffraction (SAED), dark-field and bright-field images were given the details of grain size, phase and structure of the prepared Fe-Zr (1 at% Zr) alloy were shown in figures 3(a)-(c).The TEM-SAED pattern of the 25 h mechanically alloyed (Fe-1 Zrat%) sample is shown in figure 3(a).It can be clearly noticed that all the major peaks of Fe are visible from the TEM-SAED pattern and indicates that there is no formation ofamorphous phase and impurities present after 25 h mechanical alloyed samples.In figure 2, XRD phase analysis confirmed that there is no amorphous phase formation and it is well collaborates with the TEM-SAED ring pattern.The spotty continuous rings (i.e.diffracted spots closed enough to form an almost continuous ring) revealed no presence of free Zr or amorphous phase formation.And also, the spotty continuous ring patterns indicate the nanocrystalline structure and grain size in the nanometric range.
TEM bright field imaging could not reveal the grain size due to the lower contrast of the powder particles.The dark field imaging method is recognised to be more prominent than the bright field imaging technique due to the higher contrast obtained [25].As a result, dark field TEM imaging clearly revealed the shape and size of the grain of the 25 h milled samples.It clearly shows that nano-size grains formed after 25 h of mechanical alloying and a complete solid solution, as confirmed by the SAED pattern.Soorja et al [18] observed similar kinds of TEM results.The TEM grain size result is fully consistent with the average crystallite size, which is calculated using XRD results.
The elemental composition of materials can be determined quantitatively through energy dispersive x-ray analysis (figure 4) in the Fe 1-x-Zr x alloy.The elemental distribution of the mechanically alloyed samples shows that Zr elements are homogeneously distributed in the Fe matrix.On the (110) plane, it shows transparent and robust segregation of Fe.It appears to show that the synthesized samples are made up of Fe and Zr, no other peaks associated with any odd components could be seen, confirming the purity.The quantitative EDAX analysis confirmed that the elemental distribution in the samples is highly homogeneous and the estimated composition from the EDAX analysis is found to be almost the same as the blend composition of the corresponding alloy.

FTIR functional group analysis of Fe 1-x -Zr x alloy
A quick, inexpensive, simple and non-destructive approach is fourier transform infrared (FTIR) spectroscopy.It is a facilitative approach for verifying the identity of pure chemicals and information on molecules.As well as, FTIR method is based on locating functional groups in molecules that vibrate in response to exposure to particular light wavelengths.The result of FTIR characterization of Fe 1-x Zr x alloy is shown in figure 5. Bands at 3428 cm −1 have been identified as being associated with O-H stretching vibrations.Peaks around 2920 cm −1 and 2850 cm −1 were linked to CH 2 and CH stretching vibrations.The peaks observed near 1630 cm

Thermal properties of Fe 1-x -Zr x alloy
To assess the thermal activities and to establish the structure of the Fe 1-x -Zr x alloy, a thermogravimetric analysis (TGA) was used.Furthermore, TGA allows for distinguishing physical and chemical properties that occur when a material is heated.It is possible to investigate heat stability and degradation mechanisms in detail that can be seen in the figure 6 for Fe-Zr alloy samples, the top one is the DTG, the middle one is the DTA and the bottom one is the TG curve.At 26.52 °C, the TG curve shows that the prepared Fe-Zr (1 at%) alloy has 100 percent mass.When heating from 26.52 °C to 100.3 °C, it loses 0.08 percent of its weight.Desorption of moisture and loss of water physically adsorbed on the metal's surface are the main causes of deterioration.However, the mass gain is exponentially enhanced from 100.3 °C to 400.2 °C (gain in mass = 1.12 percent).Beyond 400.2 °C mass gain grew linearly up to 1200 °C with a 4.1 percent increment of mass received in this region (500.4°C-1203 °C).Differential thermal analysis (DTA) curves reveal a modest exothermic peak around 350 °C with temperatures ranging between 300 °C and 400 °C [29][30][31].The DTG curve exhibits one peak as temperature rises and the contamination point of the peak is around 355 °C.
Figure 7 shows the Fe-Zr (0.25%) samples simultaneous TG-DTA-DTG results heating temperature range from 0 to 1205 °C.The TG curve showed the initial weight loss (0.08%) at a temperature of 200 °C and additionally two-step weight-gain observed between 300 °C-1203 °C.In DTA curves a clear exothermic peak was detected compare to 1 at% of Zr alloy at 340 °C, with temperatures ranging between 300 °C and 400 °C.Maximum DTA intensity observed ∼ 60 uV and beyond 400 °C the DTA intensity is gradually decreased.Thermal gravimetric data information of Fe 1-x-Zr x alloy are displayed in table 2. The change in weightloss and weightgain have been noticed and described as total weight changes are + 10.7% (gain) at 1023 °C for Fe-Zr (0.25%) alloy.However, Fe-Zr (1%) exhibits low weight gain (+ 6.54%) compared to 0.25% Zr alloy.3.3.Optical properties analysis of Fe 1-x -Zr x alloys 3.3.1.UV-Visible absorption spectra analysis At room temperature, the absorption spectra of Fe 1-x -Zr x alloy generated by mechanically alloyed samples were recorded spanning the wavelength range of 300-800 nm. Figure 8 shows the UV-vis absorption spectra of Fe-Zr alloy, which has a characteristic peak of about 400 nm.Fe-Ag bimetallic nanoparticles made by a simple solution-based technique produced similar findings.Furthermore, a deep absorption edge was formed at approximately 370 nm, which corresponds to 0.25 percent Zr doped with Fe, as seen in figure 8.When compared to 0.25 at% of Zr doped with Fe, the absorption edge of the 1 at% of Zr doped materials is moved towards shorter wavelengths.The shift toward shorter wavelengths shows the rise in the optical band gap.[24][25][26][27][28][29][30][31][32].

UV-Visible transmittance spectra analysis
The influence of Zr doping on the transmittance characteristics of Fe 1-x-Zr x alloy is investigated using UV-Transmittance spectra.Figure 9 shows the transmission spectra of Fe-Zr alloy nanoparticles.Transmission spectra have been observed in the 300-800 nm range.The transmittance behavior of samples has been sharply increased in the lower wavelength side(300-350 nm) and maintained at a higher wavelength.For 0.25 at%Zr alloy, a very high transmittance (T > 80%) was obtained in all regions of the EM spectrum.With an increase in Zr concentration (Zr = 1 at%), transmittance dropped due to increased, light dispersion on the rough surfaces of the produced alloy, the transmittance was reduced.

Optical energy gap (E g )
The optical band gap energy of the Fe 1-x-Zr x alloy sample is calculated by Tauc plot (figure 10).It gives the relation between absorption coefficients as a function of incident photon energy.
Where h is a Planck constant (6.626 × 10 −34 m 2 kg/s), hυ is the input photon energy, Eg is the band gap energy and n is a constant that depends on the transition type.The optical absorption index (n) value is 0.5, 1.5, 2, or 3 for direct allowed, direct prohibited, indirect allowed and indirect forbidden transitions.The Fe 1-x-Zr x alloy semiconductor display allowed direct transitions, hence the n value was selected to be 0.5.The straight portion of the (hυ) 2 versus hυ curve was extrapolated on the hυ axis at α = 0 to get the direct band gap.For x = 1 at.% Zr concentration doping on Fe 1-x-Zr x alloy generated by mechanical milling was determined to be 3.81 eV.Because of the larger crystallite size, which is larger than x = 0.25% doped alloy (Eg = 3.72 eV) [33,34].

Photoluminescence properties of Fe 1-x Zr x alloy
Figure 11 shows the photoluminescence (PL) spectra of Fe 1-x-Zr x alloy nanoparticles at ambient temperature, ranging from 300 to 900 nm.Fe was incorporated Fe 1-x-Zr x alloy nanoparticles, resulting in emissions at 514 nm and 775 nm.The strong perfect crisp emission peak and maximum emission wavelengths were discovered to be between 450 nm and 550 nm.At 0.25 at% Zr, the PL emission peak was 514 nm which was higher.The transition from the Fe ions conduction band to the exciting states of the Zr ion band gap was blamed for the increased emission.Furthermore, high Zr concentration (Zr = 1 at%) suppresses this green emission peak (= 514 nm) of electromagnetic radiation.Because of the Zr doping concentration, a non-radiative recombination event is occurred [35].

Conclusion
The Fe 1-x -Zr x alloy with the variation of Zr (0.25 & 1 at%) concentrations of the alloys were prepared in the controlled argon atmosphere by using a mechanical alloying process.According to XRD pattern analysis confirmed that the Fe-Zralloys in a cubic structure with typical average crystallite sizes were found to be in the range of 7.79 nm to 11.8 nm.Lattice parameter changes are a very important tool for confirming the complete dissolution Zr atom in the Fe matrix.The TEM-dark field image confirms that the grain size is in the nanometric range (<100 nm).TEM-SAED spotty continuous ring pattern confirmed the complete solution formation is well correlated with the XRD results.UV-vis spectrum shows that with increasing Zr concentration the absorption intensity is increased.With respect to x, the optical band gap (Eg) values might be modified from 3.72 eV to 3.81 4 eV.The PL emission intensity of Fe-Zr alloy was found to be greatest at 0.25 at% Zr, by adding 1 at% Zr, the PL emission is greatly suppressed and the peak is shifted in the range of 640 to 225 a.u.

3. 1 .
Structural and functional group analysis of Fe 1-x -Zr x alloys 3.1.1.Fe 1-x -Zr x alloys Structural characterization by XRD XRD analysis was carried out of Fe 1-x -Zr x alloy(x = 0.25 & 1 at%) with varying the Zr concentrations after 25 h of mechanically alloyed samples.In figure

− 1
were caused by water molecule deformation vibrations [26].The C-C vibrational mode was accountable for the bands detected at 1384 cm −1 .The stretching vibration of the C-O bond which is weaker in the spectrum of composite nanoparticles has a band approximately 1025 cm −1 .The stretching vibration of Metal-O bonds like Fe-O and Zr-O was assigned to the adsorption band between 500 and 650 cm −1 [27, 28].