Increasing Cr Content Research Articles

Effects of Al and Co contents on the microstructure and properties of AlxCoCryFeNi high-entropy alloys

AlxCoCryFeNi high-entropy alloys (HEAs) were prepared by vacuum arc melting. The effects of Al and Cr contents on the microstructure, corrosion resistance and mechanical properties of the alloys were evaluated. Results showed that with an increase in Cr content, the phase of the alloys changed from FCC + B2 + BCC to FCC + B2 + BCC + FeCr when the Al content was 0.75 (X = 0.75). Coarse FCC dendrites decreased. The alloys exhibited two mixed BCC/B2 structures with different morphologies: (i) the granular B2 phase distributed in the BCC matrix and (ii) the maze-like spinodal decomposition structure. At X = 1.5, the FCC and FeCr disappeared, and the microstructure of the alloy transformed into a mixed structure with a nanoparticle-like BCC phase distributed in the B2 matrix. During compression testing, the fracture type was brittle fracture for all alloys, except for X0.75Y1, which exhibited excellent mechanical properties with a fracture strain of 40% due to partial plastic tearing. Both B2 and FeCr phases, which were rich in AlNi and Cr, respectively, showed reduced corrosion resistance. For the former, the loose structure of Al2O3 reduced the compactness of the passivated film; for the latter, the precipitation of the Cr-rich phase led to a Cr-poor region in the periphery. The Al0.75CoCrFeNi alloy exhibited the highest corrosion resistance, which was attributed to its reduced FeCr and B2 phases.

Cr3+ doping-controlled magnetic and dielectric properties of polycrystalline Y-type BaSrCo2Fe11-xCrxAlO22 hexaferrite

The trend towards multifunctionality and integrating electronic devices has led to the exploration of Y-type hexaferrite with excellent magnetic and dielectric properties. The Y-type hexaferrite BaSrCo2Fe11-xCrxAlO22 (0.0 ≤ x ≤ 0.5, with step of 0.1) ceramics have been prepared by conventional solid state reaction route. The phase structure, microscopic morphology, magnetic and dielectric properties of Y-type hexaferrite with chromium substitution are studied in detail. The analysis of XRD, SEM and EDS reveals that the lattice parameters and grain size decrease with the increase of Cr content. XPS results show the coexistence of Fe2+ and Fe3+. All samples exhibit well-defined ferromagnetic hysteresis loops at room temperature and the Cr3+ doping enhances the saturation magnetization and coercivity. The dielectric constants for all samples are found in the range of 10–15 and the dielectric loss is less than 0.02 at high frequency. The resistivity demonstrates that the samples possess high insulating nature with all above 2 × 109 Ω·cm, with a maximum value of 9.5 × 109 Ω·cm for x = 0.2. These results reveal that Cr3+ doped Y type hexaferrite could have potential applications in multifunctional electronic devices.

Effect of Chromium on Microstructure and Mechanical Properties of Hot-Dip Galvanized Dual-Phase (DP980) Steel

Generally, the addition of Chromium (Cr) into the dual-phase (DP) steels can suppress bainitic transformation. In this work, the continuous hot-dip galvanization of DP980 steel is thermally simulated with various Cr contents of 0/0.3/0.6 wt.%, and the effect of Cr on bainitic transformation and properties of steels is studied. The results indicate that the bainitic transformation is obviously inhibited. The fraction of bainite decreases with the increasing Cr content. The incubation time is prolonged in the 0.6Cr steel with a slower bainitic transformation rate. Compared to that of 0 and 0.3Cr steels, the 0.6Cr steel exhibits a high tensile strength of 1033 MPa and uniform elongation of 9.1% due to the rapid strain-hardening rate. As a result, the mechanical properties of 0.6Cr steel satisfy the requirements of hot-dip galvanized DP980 steel.

Spatial dynamics of pH in the rhizosphere of Leersia hexandra Swartz at different chromium exposure

The roots of hyperaccumulators can significantly alter soil pH and thus change the chromium (Cr) availability in the rhizosphere. The pH dynamics in the rhizosphere of Cr hyperaccumulator Leersia hexandra Swartz remains unknown. In this study, the spatial dynamics of pH in the rhizosphere of L. hexandra at different Cr exposure were examined using planar optode (PO). The effects of different Cr concentrations on the biomass, physiological parameters, and soil enzyme activity were investigated. The results showed that pH in the rhizosphere of L. hexandra was highly heterogeneous and followed the root shape. There were obvious soil acidification in all groups and the average pH values in the control, Cr50, and Cr100 groups decreased by 0.26, 0.27, and 0.35 pH unit, respectively. At a certain concentration (50 mg kg–1), Cr significantly increased the plant height and biomass of L. hexandra compared to the control (p < 0.05). The concentrations of chlorophyll a, chlorophyll b, and total chlorophyll in the leaves increased with increasing Cr concentrations. The acid phosphatase, urease, and catalase activities in the rhizosphere were higher than those in the bulk soil. These results provide new insights into elucidating the hyperaccumulating mechanism of Cr and improving the phytoremediation efficiency.

Effect of Au-ions irradiation on microstructure and mechanical properties of FeCrAl coating

Six kinds of FeCrAl coatings with different Cr and Al contents were irradiated by 6 MeV Au-ions with a damage level of 20 dpa. The results showed that the surface roughness and grain size of the coatings increased significantly after irradiation. Nanoscale precipitation and irradiation-induced twins appeared in the coating with relatively lower Cr content. The increase of Cr content promotes the transformation from irradiation-induced softening to hardening. Changed Al content can cause the change in hardness, but cannot cause the transition of softening and hardening. The underlying evolution mechanism of microstructure and mechanical properties induced by Au-ions irradiation is discussed.

Combination of In Situ Confocal Microscopy and Calorimetry to Investigate Solidification of Super‐ and Hyper‐Duplex Stainless Steels

The solidification processes of super‐ and hyper‐duplex stainless steels (i.e., UNS S32750 and S 33 207) are investigated in situ by a high‐temperature confocal laser scanning microscope (HT‐CLSM) and differential scanning calorimetry (DSC). The variations of δ‐ferrite phase fraction during solidification are measured quantitatively. The results show that liquid L→δ‐ferrite transformation first occurs at a certain degree of supercooling during the solidification process of steel. UNS S3DSS 3207 with a higher Cr content can result in a higher nucleation temperature and faster growth of δ‐ferrite compared to those of UNS S332750 steel. Moreover, both the liquidus (TL) and solidus temperatures (TS) are increased with the increasing Cr content, while TL increases greater than TS. Electron microscopies are used to quantify the fraction and composition of each phase. Scheil equation is employed to predict the distribution behavior of the main alloying elements in the solidification process, and the predicted results are consistent with the experimental findings. This study aims to provide real‐time experimental insights into the solidification kinetics of state‐of‐the‐art high‐alloy‐grade duplex steels and benefits for controlling the casting process in the real production of stainless steels.

Elucidating Primary Degradation Mechanisms in High-Cycling-Capacity, Compositionally Tunable High-Entropy Hydrides.

The hydrogen sorption properties of single-phase bcc (TiVNb)100-xCrx alloys (x = 0-35) are reported. All alloys absorb hydrogen quickly at 25 °C, forming fcc hydrides with storage capacity depending on the Cr content. A thermodynamic destabilization of the fcc hydride is observed with increasing Cr concentration, which agrees well with previous compositional machine learning models for metal hydride thermodynamics. The steric effect or repulsive interactions between Cr-H might be responsible for this behavior. The cycling performances of the TiVNbCr alloy show an initial decrease in capacity, which cannot be explained by a structural change. Pair distribution function analysis of the total X-ray scattering on the first and last cycled hydrides demonstrated an average random fcc structure without lattice distortion at short-range order. If the as-cast alloy contains a very low density of defects, the first hydrogen absorption introduces dislocations and vacancies that cumulate into small vacancy clusters, as revealed by positron annihilation spectroscopy. Finally, the main reason for the capacity drop seems to be due to dislocations formed during cycling, while the presence of vacancy clusters might be related to the lattice relaxation. Having identified the major contribution to the capacity loss, compositional modifications to the TiVNbCr system can now be explored that minimize defect formation and maximize material cycling performance.

Microstructure and properties of AlCrxFeNi1.2Cu0.8 high-entropy alloys at high temperature

With the development of space technology, high-temperature resistant materials have become one of the hot spots in space research. The research of new high-temperature materials has become the key to breakthrough in aviation field. Research on microstructure and properties of AlCrxFeNi1.2Cu0.8 high-entropy alloys at high temperature (900, 1000 and 1100°C) were studied in this paper. After high-temperature annealing, the structural stability of alloys is maintained and the microstructure of alloys is dendrite. The grains are refined with the increase of Cr content. The alloy exhibits excellent comprehensive mechanical properties after heat treatment. The compressive strength of the alloy is above 1000 MPa and the plasticity of the alloy is above 10%. The alloy changes from brittle to tough with the increase in temperature. The hardness of Cr1.25 specimen is doubled after annealing, and the corrosion resistance of Cr 0.5 specimen is the best.

Facile Synthesis of Chromium-Doped Fe1.1Mn1.9O4 Nanoparticles and the Effect of Cr Content on Their Magnetic and Structural Properties.

In the present study, Fe1.1(CrxMn1-x)1.9O4 nanoparticles (0 ≤ x ≤ 0.5) were successfully synthesized by a combustion method, and the influence of Cr substitution on the structural and magnetic properties of the obtained nanoparticles was studied by various methods. The structural analysis revealed that the sample with x = 0 has a tetragonal structure, while all Cr-doped samples crystallize into a cubic structure. Additionally, the results of TEM show that doping with chromium leads to an increase in particle size. The magnetic hysteresis loops demonstrate the behavior typical for soft magnetic materials with low coercivity and remanence magnetization. The magnetic measurements revealed that the saturation magnetization of the obtained nanoparticles demonstrates a decreasing trend with increasing Cr content. The influence of chromium doping on the observation change in saturation magnetization is discussed. Based on the results of temperature-dependent magnetization measurements, it was found that the temperature of a magnetic transition in synthesized nanoparticles depends on Cr content.

Processing and Investigation of Cd0.5Zn0.5Fe2−xCrxO4 (0 ≤ x ≤ 2) Spinel Nanoparticles

This study presents the synthesis of Cd0.5Zn0.5Fe2−xCrxO4 nanoparticles via the sol–gel method, along with a comprehensive characterization of their morphological, structural, infrared, and magnetic properties. The X-ray diffraction pattern confirms the formation of the spinel structure, and the cation distribution is estimated using X-ray analysis and confirmed by magnetization measurements. The crystalline size, ranging from 152 to 189 nm, and lattice parameter, varying from 8.51134 Å to 8.42067 Å, decrease with increasing Cr content. The saturation magnetization decreases from 55 emu/g to 10.8 emu/g, while the remanent magnetization increases (3.5 emu/g ≤ Mr ≤ 6.27 emu/g), and the coercivity increases (82 Oe ≤ HC ≤ 422.15 Oe) with the addition of Cr ions. Fourier transform infrared (FTIR) spectroscopy reveals two absorption bands at ν1 and ν2, located near 600 and 400 cm−1, respectively, which correspond to the vibrations of the metal–oxygen bonds in the spinel structure.

First‐Principle Study of Bandgap Engineering and Optical Properties of Monolayer WSe2 in Second Near‐Infrared Windows

AbstractFluorescence imaging in the second near‐infrared II (NIR‐II) window is opening up new possibilities in bioimaging due to its low scattering rate within the tissue. The integration of 2D materials with NIR‐II fluorescence will enable the development of multifunctional imaging probes. However, there are very few 2D materials that can fluoresce in the NIR‐II range. Monolayer WSe2 is a potential 2D material, but its photoluminescence (PL) around 790 nm is still far from the NIR‐II range due to its bandgap of 1.54 eV. In this study, it is investigated the electronic structures, dielectric functions, and PL spectra for Te, I, and Cr‐doped monolayer WSe2, as well as W and S vacant monolayer WSe2. Most of the defected monolayer WSe2 remain semiconductors, except for a few configurations exhibiting metallic properties after making vacancies. Among the monolayer WSe2 under investigation, the Cr‐doped WSe2 performs the best, exhibiting a strong PL peak in NIR‐II with a decreased bandgap around 1.0 eV. As increasing Cr concentration, the peak shifts further toward the red end of the spectrum due to an enhancement of p–d transition. The results provide a useful guideline for material synthesis applied in NIR‐II bioimaging and other biophysics.

Comparative Study of Chromium Phytoremediation by Two Aquatic Macrophytes.

Chromium (Cr) occurs in several oxidation states from trivalent to hexavalent. However, hexavalent forms are more toxic and mainly produced by anthropogenic activities. A hydroponic experiment was conducted to analyse the comparative remediation of Cr by Marsilea minuta and Pistia stratiotes. Plants were exposed to four concentrations of Cr (0.5, 1.0, 1.5, and 2.0 mM) for 3 days. The highest accumulation of Cr was seen at the 1.5 mM concentration after 3 days in Marsilea (11.96mg/g) and Pistia (18.78mg/g). Dry weights decreased and malondialdehyde (MDA) levels increased in response to increasing Cr concentrations. Results indicate that both macrophytes are suitable candidates for Cr phytoremediation. Antioxidant-enzyme activity as a function of metal tolerance is imperative for a coherent understanding of plant physiology under metal stress.

Characterization and corrosion behavior of Ni-Cr coatings by using pulse current electrodeposition

PurposeThe purpose of this paper is to study the effect of chromium content on the microstructure and corrosion resistance of Ni-Cr coating.Design/methodology/approachNi-Cr coating was prepared by pulse current electrodeposition with trivalent chromium. On the basis of studying effect of electroplating parameters on composition and morphology, Ni-Cr alloy coatings with various chromium contents were obtained. The microstructure was characterized by scanning electron microscopy, X-ray diffractometer and transmission electron microscopy. Corrosion behavior was studied by potentiodynamic polarization and electrochemical impedance spectroscopy techniques.FindingsElectrodeposited chromium was solidly dissolved in nickel and refined the grain of the coating. With the increase of Cr content, the corrosion resistance of Ni-Cr coating was enhanced, which is due to the formation of continuous nickel hydroxide and compact chromium oxide passive films.Originality/valueNi-Cr alloy coating without penetration crack was prepared in trivalent chromium electrolyte, and the mechanism of its excellent corrosion resistance was proposed.

Effects of Domestic Sewage on the Photosynthesis and Chromium Migration of Coix lacryma-jobi L. in Chromium-Contaminated Constructed Wetlands

To investigate the effects of domestic sewage on the photosynthesis and chromium migration of plants in chromium-contaminated constructed wetlands, small vertical flow constructed wetlands of Coix lacryma-jobi L. were set up. These wetlands were used to treat wastewater containing 0, 20, and 40 mg/L of hexavalent chromium (Cr (VI)), prepared with domestic sewage (DS), 1/2 Hoagland nutrient solution (NS), and 1/2 Hoagland nutrient solution prepared with domestic wastewater (DN), respectively. The aim was to investigate the effects of domestic sewage on indicators, such as plant growth and chromium accumulation. The results were as follows: (1) Plant heights were significantly inhibited under 20 mg/L and 40 mg/L Cr (VI) treatments, and stem diameters were not significantly affected. The use of domestic sewage in treatment alleviated the inhibition of Cr (VI) on the growth of Coix lacryma-jobi L. (2) Indicators such as root activity, photosynthetic gas exchange, and chlorophyll fluorescence properties significantly decreased with the increase in Cr (VI) concentration. The values of these photosynthetic gas exchange parameters under the DN treatment were the greatest, followed by NS and DS. On the 70th day of Cr (VI) treatment, the net photosynthetic rate (Pn) under the DN treatment was significantly higher than that under NS and DS treatments. (3) Glutathione (GSH) content in roots, stems, and leaves of Coix lacryma-jobi L. significantly increased with the increase in Cr concentration, and it increased more significantly under the DN and DS treatments than under the NS treatment. (4) With the same Cr treatment, the Cr content in roots, stems, and leaves of Coix lacryma-jobi L. under the NS treatment was the highest, followed by DS and DN. The total Cr content in the substrate under the DN treatment was the highest, followed by DS and NS. (5) The addition of domestic sewage reduced the Cr (VI) content in the water sample and increased the organic matter content. The Cr (VI) content in the water sample under the NS treatment was the highest, followed by DS and DN. The addition of domestic sewage increased the accumulation of chromium in the substrate, decreased the absorption of chromium by plants, increased GSH content in roots, stems, and leaves, alleviated the damage of Cr (VI) to plants, and thus benefited the growth of Coix lacryma-jobi L. in the constructed wetlands and ensured the sustainable and stable operation of the wetlands.

Proline-mediated activation of glyoxalase II improve methylglyoxal detoxification in Oryza sativa L. under chromium injury: Clarification via vector analysis of enzymatic activities and gene expression

Environmental factors affect plants in several ways including the excessive accumulation of methylglyoxal (MG), resulting in dysfunctions of many biological processes. Exogenous proline (Pro) application is one of the successful strategies to increase plant tolerance against various environmental stresses, including chromium (Cr). This study highlights the alleviation role of exogenous Pro on MG detoxification in rice plants induced by Cr(Vl) through modifying the expression of glyoxalase I (Gly I)- and glyoxalase II (Gly II)-related genes. The MG content in rice roots was significantly reduced by Pro application under Cr(VI) stress, however, there was little effect on the MG content in shoots. In this connection, the vector analysis was used to compare the involvement of Gly l and Gly II on MG detoxification in ‘Cr(VI)’ and ‘Pro+Cr(VI)’ treatments. Results exhibited that vector strength in rice roots increased with an increase in Cr concentrations, while there was a negligible difference in the shoots. The comparative analysis demonstrated that the vector strengths in roots under ‘Pro+Cr(VI)’ treatments were higher than ‘Cr(VI)’ treatments, suggesting that Pro improved Gly II activity more efficiently to reduce MG content in roots. Calculation of the gene expression variation factors (GEFs) indicated a positive effect of Pro application on the expression of Gly I and Gly ll-related genes, wherein a stronger impact was in roots than the shoots. Together, the vector analysis and gene expression data reveal that exogenous Pro chiefly improved Gly ll activity in rice roots which subsequently enhanced MG detoxification under Cr(VI) stress.

Improved optical, dielectric, and nonlinear properties of CaCu3Ti4O12 thin films by chromium doping

In the present work, the optical and dielectric properties, microstructures, and nonlinear electrical performances were systematically investigated for Ca1-xCrxCu3Ti4O12 (x = 0.00 - 0.40) thin films prepared on Pt/Ti/SiO2/Si substrates using the sol-gel method. X-ray photoelectron spectroscopy (XPS) verified that Cr3+ and Cr6+ were present in the thin films. Additionally, the content of secondary phase of titanium oxide (TiO2) increased, the grain size decreased, and the surface roughness improved with the increase in the doped content of Cr. Due to the reduced grain size and oxygen vacancy concentration, the optical energy band gap increased from 2.92 to 3.54 eV. The change in the dielectric constants (Ɛr) of the thin films followed the internal barrier layer capacitor model. Doping was found to decrease the dielectric loss (tan δ) and enhance the nonlinear properties, which was due to the presence of TiO2 secondary phase, high density, reduced grain size, incremental potential barrier height and grain boundary resistance. The Ca0.8Cr0.2Cu3Ti4O12 thin film had a high Ɛr of around 9340, low tan δ of around 0.012 at 1 kHz, nonlinear coefficient of about 4.64, and breakdown field of ca. 89 kV/mm. These results indicated that the doping of an appropriate amount of Cr could effectively modify the microstructure of CaCu3Ti4O12 thin films and improve their optical and electrical properties.

Investigation of the effect of Cr co-doping on the refractive index, spectroscopic parameters and lasing of Nd:GdVO4 crystals

Cr co-doped (0.5, 1.0, 1.2 and 1.5 at.%) Nd (0.5 at.%):GdVO4 single crystals, used as laser gain medium, were grown by the optical floating zone technique to investigate the effect of Cr on the refractive index and spectroscopic properties. The ordinary (no) and extra-ordinary (ne) refractive index of 0.5 at.% Cr co-doped Nd:GdVO4 crystals was found to be lower than the 1.0 at.% Cr co-doped crystals, and thereafter it becomes almost constant up to 1.5 at.% of Cr co-doping. Further, the thermo-optic coefficient (dn/dT) of Cr co-doped Nd:GdVO4 crystals is positive with values ∼10−5/°C and the value is higher for ne compared to no for all the Cr doping concentrations. Further, the measured fluorescence lifetime of Nd ion decreases from 98 to 85 μs with the increase in Cr concentration. The Judd-Ofelt (JO) analysis revealed that the JO parameters for all the samples are Ω2 ∼9.6–10.6 × 10−20 cm2, Ω4 ∼6.5–7.0 × 10−20 cm2 and Ω6 ∼6.5–7.0 × 10−20 cm2. The radiative lifetime (∼100 μs) and emission cross section (∼3.2–4.1 × 10−19 cm2) are not affected significantly by the Cr co-doping. The investigation suggests that the inclusion of Cr ion in the crystal lattice does not deteriorate the overall spectroscopic properties of the Nd ions, thus Cr may act as an excellent activator for Q-switching in the Nd:GdVO4 laser gain medium. Further, the self Q-switching in the Cr co-doped Nd:GdVO4 laser element was demonstrated. A stable pulsed laser output at 1066 nm with power ∼105 mW, pulse width ∼477 ns and repetition rate ∼550 kHz was obtained in the 1.0 at.% Cr co-doped crystal.

Cumulative effects of primary radiation damage in alloy 800H: An atomistic simulation study

Fe-Ni-Cr-based alloys, including alloy 800H, are used as structural materials in nuclear reactors such as the PWRs and BWRs, but also are considered as candidate materials for advanced reactors. In this work, we report a molecular dynamics (MD) study of the effect of irradiation and alloy composition on the microstructure and mechanical behavior of alloy 800H. Our results of primary irradiation damage suggest a larger spontaneous recombination distance for Cr compared to other alloy components, thereby suggesting that Cr may increase the tolerance of the alloy 800H to primary defect production. The progressive accumulation of irradiation events through cascade overlap results in the growth of defects and their agglomeration into clusters, but also induces the formation of dislocations. Irradiation-induced mechanical degradation was also investigated for the single crystal, and based on our observations two degradation mechanisms were proposed: the nucleation and glide of dislocations, promoted by the dynamics of interstitial atoms and vacancies defects under applied strain field, and the rapid activation of slip systems by the combined effect of the foregoing and generated dislocations, vacancies and interstitial atoms. We also found that increasing Ni or Cr substantially limits the irradiation-induced mechanical degradation. Finally, our MD findings highlight the strengthening of alloy 800H by increasing Ni content, and the degradation of the Young modulus induced by the increase of Cr content.

Microstructure, mechanical properties and tribocorrosion characteristics of (Mo1-xCrx)5Si3 alloys

(Mo1-xCrx)5Si3 alloys were prepared by vacuum arc melting technique. The microstructure and mechanical properties of (Mo1-xCrx)5Si3 alloys were characterized using a scanning electron microscope (SEM), X-ray diffraction (XRD) and nano-indentation tests. The tribocorrosion characteristics of the alloys were investigated using a tribocorrosion tester in 3.5 wt% NaCl solution. The results revealed that the thermal expansion anisotropy ((CTE(c)/CTE(a))) of the (Mo1-xCrx)5Si3 alloys decreased from 2.25 to 1.45 with increasing Cr content. The Cr-alloyed (Mo1-xCrx)5Si3 alloys exhibited a high hardness, and their fracture toughness, ratios of H3/E2 and H/E were improved. (Mo1-xCrx)5Si3 alloys showed a strong wear-corrosion combined effect, and an opposite trend between friction coefficient and OCP values was observed during the tribocorrosion tests. The tribocorrosion resistances of (Mo1-xCrx)5Si3 alloys were better than that of 0Cr18Ni9 stainless steel due to the formation of Cr2O3-MoO3-SiOx protective films. The (Mo0.8Cr0.2)5Si3 alloy exhibited the best tribocorrosion characteristics with a low friction coefficient and wear rate. For the 0Cr18Ni9 and Mo5Si3 alloy, pure wear (Vw) was dominant, nevertheless, corrosion-induced wear (Vcw) was the main tribocorrosion mechanism of (Mo0.8Cr0.2)5Si3 alloy.

Impact of Cr doping on the structure, optical and magnetic properties of nanocrystalline ZnO particles

The doping of Cr ions on the structure, optical and magnetic properties of ZnO have been investigated. Zn1−xCrxO (0 ≤ x ≤ 0.05) samples were prepared by the solution combustion method. Rietveld refined x-ray diffraction results reveal that all the samples are in a hexagonal structure. Transmission electron microscopy (TEM) revealed that the particle size was 48.2 ±2.1 nm, 29.4 ± 2.2 nm and 25.3 ± 2.1 nm for the Zn1−xCrxO with x = 0, 0.03 and 0.05, respectively. The band-gap decreases slightly from 3.308 ± 0.003 eV to 3.291 ± 0.003 eV with increase of Cr content from x = 0 to 0.05, respectively. The field-dependent magnetization M(μοH) measurements of ZnO and Cr-doped ZnO exhibit room temperature ferromagnetism (RTFM). Photoluminescence and electron paramagnetic resonance exhibits negatively charged zinc vacancies (VZn−) and singly ionized oxygen vacancies (Vo+) in ZnO and these defects are responsible for RTFM. And the VZn− accountable for RTFM in Cr-doped ZnO. EPR results confirmed that Cr3+ ions replaces the Zn2+ sites in the hexagonal ZnO. The observed long-range ferromagnetism ordering is explained based on the bound magnetic polaron (BMP) mechanism. This work elucidates the origin of RTFM in synthesized ZnO and Cr-doped ZnO.