Kourosh Shirvani - Academia.edu (original) (raw)

Papers by Kourosh Shirvani

Ni-based superalloy Rene80 is used for manufacturing gas turbine blades in jet engines. The lifet... more Ni-based superalloy Rene80 is used for manufacturing gas turbine blades in jet engines. The lifetime of some jet engine turbine blades is limited by low cycle fatigue and this property has been strongly affected by coatings. Ductile to Brittle Transition Temperature (DBTT) is the most important factor which affects the mechanical properties of coated alloys. In this study, high temperature-low cycle fatigue behavior of uncoated and coated Rene80 by platinum-aluminide (Pt-Al) was evaluated at temperatures 871 °C (near the DBTT) and 982 °C (above the DBTT). Results of low cycle fatigue tests under strain-controlled condition at 871 °C for R =0 and strain rate of 2×10 s, at a total strain range of 0.8% showed a decrease in fatigue strength of coated specimens about 14%, compared to the uncoated ones. However, increasing the testing temperature from 871 °C to 982 °C, led to an increase in the low cycle fatigue behavior of coated Rene80 about 10% as compared to the

Corrosion Science, 2010

Simple and Si-modified aluminide coatings having medium-thickness (40-60 lm) have been applied on... more Simple and Si-modified aluminide coatings having medium-thickness (40-60 lm) have been applied on the superalloy GTD-111 by a slurry technique. Hot corrosion and cyclic oxidation performance of the uncoated and the coated superalloy were investigated by exposing samples to a molten film of Na 2 SO 4-40 %wt NaVO 3-10%wt NaCl at 780°C and 1 h cyclic oxidation at 1100°C in air, respectively. The presence of silicon in the aluminide structure increased the oxidation resistance by a factor of 1.7 times. In addition, a SiO 2-containing scale, which formed on the Si-containing coating surface, was stable during of the hot corrosion testing.

MATERIALS TRANSACTIONS, 2002

The microstructures of slurry Si-modified aluminide coatings on Ni-base superalloy In-738LC have ... more The microstructures of slurry Si-modified aluminide coatings on Ni-base superalloy In-738LC have been investigated using SEM, TEM, EDS and XRD. The corrosion performance of the coatings has been also investigated by hot corrosion and cyclic oxidation tests. In order to evaluate the hot corrosion performance, the anodic polarization curves were measured in Na 2 SO 4-25 mol%NaVO 3-5 mol%NaCl at 1053 K. The cyclic oxidation test was carried out by exposure to alternate conditions of air atmosphere at 1373 K and room temperature. The analyses of SEM, EDS and XRD were also used to characterize attack morphologies of the coatings exposed to the test conditions. The results showed that the heat-treated slurry Si-modified aluminide coating was composed of β-NiAl phase including fine precipitates, which were found to be complex silicides. The results of hot corrosion tests indicated that at least 10-13 mass%Si in the coating layer is needed to improve the corrosion resistance of the aluminide coating. The coatings that met this criterion also showed a good cyclic oxidation resistance.

Coatings

Refractory high-entropy alloys (RHEAs) contain alloying elements with a high melting point, promi... more Refractory high-entropy alloys (RHEAs) contain alloying elements with a high melting point, promising high-temperature applications due to their unique properties. In this work, laser cladding is used to prepare RHEAS based on NbMoTaTiNi. At the same time as laser cladding, the ultrasonic field is used, and then the microstructural characteristics, grain size, residual stress, wear, and hardness of the coating are evaluated. The results show that the coating is biphasic and includes the γ (Ni) and NbMoTaTiNi phase. The NbMoTaTiNi phase had a uniform distribution throughout the coating when the ultrasonic field was applied, so that when the ultrasonic field was not used, the NbMoTaTiNi powder, in addition to spreading uniformly, had the un-melting of large particles. This caused an increase in the residual tension of the coating. The conversion of columnar grains to the equiaxed, and the reduction in structural defects, were other characteristics of using the ultrasonic field. The fo...

International Journal of Minerals, Metallurgy, and Materials, 2019

The weldability of the ZhS6U nickel-based superalloy, which is prone to solidification cracking d... more The weldability of the ZhS6U nickel-based superalloy, which is prone to solidification cracking during electron-beam welding (EBW) repair processes, was investigated. The effects of two different pre-weld heat-treatment cycles on the final microstructure before and after welding were examined. Welds were made on flat coupons using an EBW machine, and the two heat-treatment cycles were designed to reduce γ′ liquation before welding. Microstructural features were also examined by optical and scanning electron microscopy. The results showed that the change in the morphology and size of the γ′ precipitates in the pre-weld heat-treatment cycles changed the ability of the superalloy to release the tensile stresses caused by the matrix phase cooling after EBW. The high hardness in the welded coupons subjected to the first heat-treatment cycle resulted in greater resistance to stress release by the base alloy, and the concentration of stress in the base metal caused liquation cracks in the heat-affected zone and solidification cracks in the weld area.

Corrosion Science, 2012

Aluminizing of bare and 3μm-Pt-electroplated specimens has been utilised to prepare NiAl and low ... more Aluminizing of bare and 3μm-Pt-electroplated specimens has been utilised to prepare NiAl and low platinum (Ni,Pt)Al coatings. Cyclic oxidation of the coatings was investigated by exposing samples to 1h cyclic oxidation at 1100°C. The modified coating exhibited an external layer of NiAl-25 vol.% PtAl2 above a three-zone structure. This structure endured over the whole testing time, while the NiAl coating

Journal of Central South University

The prediction of fatigue life of metallic alloys is justly accepted as one of the most important... more The prediction of fatigue life of metallic alloys is justly accepted as one of the most important phenomena in the field of metallurgical and mechanical engineering. At elevated temperatures, oxidation of the surfaces has an effective role in the fatigue strength and ductility of the alloys. In the present work, the effect of prior cyclic oxidation on the high temperature low cycle fatigue (HTLCF) properties of nickel-based superalloy Rene®80 has been assessed in the uncoated state and in the Pt-aluminide (Pt-Al) coated condition at 930 °C. To apply cyclic oxidation, simulation of engine thermal exposure was carried out by exposing coated and uncoated fatigue specimens in the burner rig (120 cycles at 1100°C). The cyclic oxidation procedure led to a changing in the coating microstructure from the dual-phase ( ξ -PtAl 2 + β -(Ni, Pt) Al) to single phase ( β -(Ni, Pt)Al). Results of HTLCF tests showed an improvement in the HTLCF life around 11.5% in the unexposed coated specimen (pre-cyclic oxidation) as compared to unexposed bare specimen, while this rise for exposed coated specimen (post-cyclic oxidation) was only 5%. Although a mixed mode fracture morphology (ductile and brittle) was observed on the fracture surfaces of failed specimens, the wider regions of brittle fracture belonged to exposed coated/uncoated ones. 预测金属合金的疲劳寿命是冶金和机械工程领域中最重要的研究之一。在高温下, 表面氧化对 合金的疲劳强度和延展性有重要影响。本文研究了预循环氧化对无涂层和有Pt-Al 涂层的镍基高温合金 Rene®80 在930 °C 时的高温低循环疲劳(HTLCF)性能的影响。将有涂层和无涂层的疲劳试样在燃烧器 钻机中进行发动机热暴露模拟(1100 °C, 120 次循环)。循环氧化过程中涂层的微观结构由双相( ξ -PtAl 2 + β -(Ni、Pt)Al)转变为单相( β -(Ni、Pt)Al)。结果显示, 与未暴露的无涂层试样相比, 未暴露的涂层试样 (预循环氧化)的HTLCF 寿命延长了约11.5%, 暴露涂层试样(后循环氧化)的仅延长了5%。虽然断裂试 样的断口表现为混合模式断裂形态(延性和脆性), 但暴露涂层/无涂层的试样断口大部分属于脆性断裂。

Journal of Materials Engineering and Performance, 2022

In the present study, cast-aluminizing (CA) and subsequent diffusion annealing processes were exp... more In the present study, cast-aluminizing (CA) and subsequent diffusion annealing processes were exploited to develop simple and Si-modified aluminide coatings on HH309 stainless steel. The CA method involved casting of HH309 melt into a mold, and Al-alloy sheets (pure Al and Al-12wt.%Si) were already inserted at the bottom of the mold. Afterward, the CA samples were diffusion-annealed at 1050 °C for 1 h and 1000 °C for 4h during two stages of heat treatment. The morphology, composition, and phase analysis of the coating were characterized by FE-SEM, EDS, and XRD. The results indicated that a multilayer aluminide coating containing (FeCrNi)2Al5 + (FeCrNi)Al2 and (FeCrNi)Al sub-layers was formed on the samples in the first stage heat treatment, while an integrated (FeCrNi)Al layer replaced the primary multilayer coating after the second stage. Then, the wear resistance of the coatings was evaluated. It was found that the applied aluminizing technique improved the wear resistance of HH309 alloy, significantly. The wear rate values for uncoated steel, aluminide, and Si-modified aluminide coatings were about 4.48 × 10−7, 1.44 × 10−7, and 0.74 × 10−7 mm3/N mm, respectively. Wear mechanisms analysis demonstrated that the type of wear for HH309 alloy was abrasive and adhesive and for aluminized steel was dominated by fatigue wear. Chemical composition analysis of the worn aluminide coatings showed that the aluminized samples also suffer oxidation wear.

Surface and Coatings Technology, 2020

This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Intermetallics, 2020

In the present work, a novel technique has been introduced to obtain an aluminide coating by cast... more In the present work, a novel technique has been introduced to obtain an aluminide coating by casting process and subsequent heat treatment. To do so, the aluminum sheet was placed at the bottom of a copper mold, then HH309 SS melt was poured into the mold. This technique was named Cast-Aluminizing (CA). The CA samples were heat-treated at the temperature range of 900-1050 � C for 0.5-5 h. The FE-SEM, XRD, and EDS were utilized to characterize the microstructure, phase analysis and chemical composition of cast-aluminized samples, respectively. Results showed that (Fe,Cr,Ni)Al 3 and (Fe,Cr,Ni) 2 Al 5 layers were formed at the Al/HH309 interface. FE-SEM analysis demonstrated a multi-layer aluminide coating on the heat-treated specimens. This coating consisted of (Fe,Cr,Ni) 2 Al 5 þ(Fe,Cr,Ni)Al 2 , (Fe,Cr,Ni)Al and α-Fe,Cr,Ni(Al) sub-layers. The growth kinetics investigation showed that the thickness of layers increased with the increase of the annealing temperature and time. The growth rate of layers obeyed a parabolic law. The activation energies for the growth of (Fe,Cr, Ni) 2 Al 5 þ(Fe,Cr,Ni)Al 2 , (Fe,Cr,Ni)Al and α-Fe,Cr,Ni(Al) layers were about 203, 250 and 247 kJ/mol, respectively. Microhardness measurements revealed that (Fe,Cr,Ni) 2 Al 5 þ(Fe,Cr,Ni)Al 2 , (Fe,Cr,Ni)Al and α-Fe,Cr,Ni(Al) layers had a hardness of about 820-1040, 580-710 and 380-470 HV, respectively. The resistance to oxidation of castaluminized and heat-treated (CA þ HT) samples in the air at 1000 � C was studied. The CA þ HT samples exhibited higher oxidation resistance than uncoated samples due to the formation of a protective Al 2 O 3 layer on the surface.

Journal of Advanced Materials In Engineering, 2019

In this study, the effect of platinum-aluminide coating parameters on surface roughness of nickel... more In this study, the effect of platinum-aluminide coating parameters on surface roughness of nickel-based superalloy Rene ® 80 was evaluated. For this purpose, different thicknesses of Pt-layer (2, 4, 6 and 8µm) were plated on the

Rare Metals, 2019

The effect of platinum (a rare metal)-aluminide coating parameters on the tensile properties of n... more The effect of platinum (a rare metal)-aluminide coating parameters on the tensile properties of nickel-based superalloy Rene Ò 80 was evaluated at 871°C. For this purpose, initial layers of platinum with different thicknesses (2, 4, 6 and 8 lm) were coated on tensile samples. Then, low-temperature high-activity (LTHA) and hightemperature low-activity (HTLA) processes were used for aluminizing. Results of microstructural evaluations using scanning electron microscope (SEM) and phase analysis by X-ray diffraction (XRD) showed a three-layer structure coating for different platinum layer thicknesses and both aluminizing processes. Increasing the thickness of the platinum layer from 2 to 8 lm led to the improvement in the final coating thickness from 91.6 to 102.1 lm in HTLA. This increase was from 128.1 to 148.6 lm in LTHA. The results of hot tensile tests at 871°C showed a decrease in strength properties of the coated samples compared to the uncoated ones. However, HTLA and high thicknesses of the initial platinum layer showed an intense reduction. The results of fractographic evaluations about uncoated samples showed a ductile fracture. On the other hand, coated samples showed a simultaneous ductile and brittle fracture failure mechanism. But the main fracture morphology was brittle cleavage fracture which was for the HTLA.

Materials Research Express, 2019

Journal of Mining and Metallurgy, Section B: Metallurgy, 2019

Low cycle fatigue is the most important failure mode in Aviation/Industrial engine rotary turbine... more Low cycle fatigue is the most important failure mode in Aviation/Industrial engine rotary turbine parts. In this paper, the influence of Pt-aluminide coating parameters on high temperature low cycle fatigue behavior of superalloy Rene?80 which is used to manufacture turbine blades, has been investigated. For this purpose, initial platinum layers of different thicknesses (2?m and 8?m) were coated on fatigue specimens. Then the aluminizing process was performed with two conditions of low temperature-high activity and high temperature-low activity. The results of microstructure investigations performed by scanning electron microscope and X-ray diffraction phase analysis indicated a three-layer structure for the coating (bi-phase (Ni,Pt)Al+PtAl2, single-phase (Ni,Pt)Al and interdiffusion zone) with different chemical compositions at both thicknesses of the platinum layer and using both aluminizing methods. Also, the results of low cycle fatigue tests at 871?C, R=0 and strain rate of 2?1...

Materials Chemistry and Physics, 2019

Abstract In this study, the effect of heat treatment on characteristics of the γ'-Ni3(Al, Ti)... more Abstract In this study, the effect of heat treatment on characteristics of the γ'-Ni3(Al, Ti) phase, as the main strengthening precipitates of the nickel-based superalloy Rene®80 was evaluated in terms of morphology and distribution of the γ' phase, γ (matrix)-γ′(strengthening precipitates) lattice mismatch, and hardness of the alloy. For this purpose, at first, the solution heat treatment was performed at 1204 °C on the alloy. Then, heat treatment cycles were carried out at three temperatures, namely 871 °C, 1095 °C, and 1130 °C, for different periods of time. The microstructures were analyzed by optical microscope and scanning electron microscope. Also, X-ray diffraction analysis was conducted to determine variations of the γ-γ′ lattice mismatch, followed by hardness measurement test, which was performed after each stage of heat treatments. The results showed an increase in the amount and size of the γ' phase with holding time at 871 °C, while the values were seen to remain constant and even decrease at 1095 °C and 1130 °C, respectively, due to simultaneous dissolution of the γ' phase during extended periods of time. At all of the three temperatures, the γ-γ′ lattice mismatch increased with time and showed larger negative values, while an increase in temperature from 871 °C to 1130 °C led to lower γ-γ′ lattice mismatch. The largest lattice mismatch (−0.653%) was observed at 871 °C after a heat treatment time of 1000 s. Hardness of the Rene®80 alloy at 871 °C increased with time, while different behaviors were observed at 1095 °C and 1130 °C, where the hardness decreased for extended heat treatment times. The most significant cause of such changes in hardness might be the coherency elastic strain between the γ and γ' phases, which is affected by the characteristics of the γ' phase and its lattice mismatch with the γ phase.

Oxidation of Metals, 2012

ABSTRACT Conventional and gradient CoNiCrAlYSi coatings were produced by using low vacuum plasma ... more ABSTRACT Conventional and gradient CoNiCrAlYSi coatings were produced by using low vacuum plasma spray and an additional step of diffusional over aluminizing (pack cementation) techniques on an Inconel-738 substrate. Hot corrosion of these coatings was investigated using Na2SO4–20wt%NaVO3 molten salt at 880 °C for 800 h. Hot corrosion rate was determined by measuring the weight gain of the specimens at regular intervals for a duration of 20 h. X-ray diffraction, field emission scanning electron microscopy and electron probe micro analysis techniques were used to determine the nature of phases, investigation of the thermally grown oxide, examination of the surface attack and determination of the elemental distribution. The gradient coating showed better performance by re-healing alumina scale due to its possession of more β phase as Al reservoir. Results indicated that pack cementation process caused an increase in amount of aluminum-rich β phase and better hot corrosion properties of gradient coatings owing to the Al enrichment in the outer layer and rapid formation of protective oxide on the surface.

Revista de Metalurgia, 2021

La superaleación Rene®80 a base de Ni se utiliza para fabricar álabes de turbinas de gas en motor... more La superaleación Rene®80 a base de Ni se utiliza para fabricar álabes de turbinas de gas en motores a reacción. La vida útil de algunas palas de turbina de motores a reacción está limitada por la fatiga de baja frecuencia y esta propiedad se ha visto fuertemente afectada por los revestimientos. La temperatura de transición de dúctil a frágil (DBTT) es el factor más importante que afecta las propiedades mecánicas de las aleaciones recubiertas. En este estudio, se evaluó el comportamiento de fatiga a alta temperatura y baja frecuencia del compuesto Rene®80 sin recubrimiento y recubierto con aluminuro de platino (Pt-Al) a temperaturas de 871 °C (cerca del DBTT) y 982 °C (por encima del DBTT). Los resultados de las pruebas de fatiga, en condiciones de deformación controlada a 871 °C para R = 0 y una tasa de deformación de 2×10-3 s-1, en un intervalo de deformación total de 0,8%, mostraron una disminución de la resistencia a la fatiga de las muestras recubiertas de aproximadamente un 14%...

Journal of The Electrochemical Society, 2011

Chromate conversion coatings (CCCs) were synthesized on AA7075 alloy. The effects of sodium sacch... more Chromate conversion coatings (CCCs) were synthesized on AA7075 alloy. The effects of sodium saccharin as a grain refining agent (GRA) and coating time on the coating morphology, topography, and alloy corrosion resistance were studied in this work. Morphologies and topographies of CCCs were examined using optical, scanning electron and atomic force microscopes. The corrosion behaviour was evaluated using potentiodynamic polarization and electrochemical impedance spectroscopy in 0.5 M NaCl solution. Major decreases in both coating grain size and corrosion rate occur at a GRA concentration of 1 g/l. Coating roughness increased to a limited nano-scale extent with increasing immersion time.

Ni-based superalloy Rene80 is used for manufacturing gas turbine blades in jet engines. The lifet... more Ni-based superalloy Rene80 is used for manufacturing gas turbine blades in jet engines. The lifetime of some jet engine turbine blades is limited by low cycle fatigue and this property has been strongly affected by coatings. Ductile to Brittle Transition Temperature (DBTT) is the most important factor which affects the mechanical properties of coated alloys. In this study, high temperature-low cycle fatigue behavior of uncoated and coated Rene80 by platinum-aluminide (Pt-Al) was evaluated at temperatures 871 °C (near the DBTT) and 982 °C (above the DBTT). Results of low cycle fatigue tests under strain-controlled condition at 871 °C for R =0 and strain rate of 2×10 s, at a total strain range of 0.8% showed a decrease in fatigue strength of coated specimens about 14%, compared to the uncoated ones. However, increasing the testing temperature from 871 °C to 982 °C, led to an increase in the low cycle fatigue behavior of coated Rene80 about 10% as compared to the

Corrosion Science, 2010

Simple and Si-modified aluminide coatings having medium-thickness (40-60 lm) have been applied on... more Simple and Si-modified aluminide coatings having medium-thickness (40-60 lm) have been applied on the superalloy GTD-111 by a slurry technique. Hot corrosion and cyclic oxidation performance of the uncoated and the coated superalloy were investigated by exposing samples to a molten film of Na 2 SO 4-40 %wt NaVO 3-10%wt NaCl at 780°C and 1 h cyclic oxidation at 1100°C in air, respectively. The presence of silicon in the aluminide structure increased the oxidation resistance by a factor of 1.7 times. In addition, a SiO 2-containing scale, which formed on the Si-containing coating surface, was stable during of the hot corrosion testing.

MATERIALS TRANSACTIONS, 2002

The microstructures of slurry Si-modified aluminide coatings on Ni-base superalloy In-738LC have ... more The microstructures of slurry Si-modified aluminide coatings on Ni-base superalloy In-738LC have been investigated using SEM, TEM, EDS and XRD. The corrosion performance of the coatings has been also investigated by hot corrosion and cyclic oxidation tests. In order to evaluate the hot corrosion performance, the anodic polarization curves were measured in Na 2 SO 4-25 mol%NaVO 3-5 mol%NaCl at 1053 K. The cyclic oxidation test was carried out by exposure to alternate conditions of air atmosphere at 1373 K and room temperature. The analyses of SEM, EDS and XRD were also used to characterize attack morphologies of the coatings exposed to the test conditions. The results showed that the heat-treated slurry Si-modified aluminide coating was composed of β-NiAl phase including fine precipitates, which were found to be complex silicides. The results of hot corrosion tests indicated that at least 10-13 mass%Si in the coating layer is needed to improve the corrosion resistance of the aluminide coating. The coatings that met this criterion also showed a good cyclic oxidation resistance.

Coatings

Refractory high-entropy alloys (RHEAs) contain alloying elements with a high melting point, promi... more Refractory high-entropy alloys (RHEAs) contain alloying elements with a high melting point, promising high-temperature applications due to their unique properties. In this work, laser cladding is used to prepare RHEAS based on NbMoTaTiNi. At the same time as laser cladding, the ultrasonic field is used, and then the microstructural characteristics, grain size, residual stress, wear, and hardness of the coating are evaluated. The results show that the coating is biphasic and includes the γ (Ni) and NbMoTaTiNi phase. The NbMoTaTiNi phase had a uniform distribution throughout the coating when the ultrasonic field was applied, so that when the ultrasonic field was not used, the NbMoTaTiNi powder, in addition to spreading uniformly, had the un-melting of large particles. This caused an increase in the residual tension of the coating. The conversion of columnar grains to the equiaxed, and the reduction in structural defects, were other characteristics of using the ultrasonic field. The fo...

International Journal of Minerals, Metallurgy, and Materials, 2019

The weldability of the ZhS6U nickel-based superalloy, which is prone to solidification cracking d... more The weldability of the ZhS6U nickel-based superalloy, which is prone to solidification cracking during electron-beam welding (EBW) repair processes, was investigated. The effects of two different pre-weld heat-treatment cycles on the final microstructure before and after welding were examined. Welds were made on flat coupons using an EBW machine, and the two heat-treatment cycles were designed to reduce γ′ liquation before welding. Microstructural features were also examined by optical and scanning electron microscopy. The results showed that the change in the morphology and size of the γ′ precipitates in the pre-weld heat-treatment cycles changed the ability of the superalloy to release the tensile stresses caused by the matrix phase cooling after EBW. The high hardness in the welded coupons subjected to the first heat-treatment cycle resulted in greater resistance to stress release by the base alloy, and the concentration of stress in the base metal caused liquation cracks in the heat-affected zone and solidification cracks in the weld area.

Corrosion Science, 2012

Aluminizing of bare and 3μm-Pt-electroplated specimens has been utilised to prepare NiAl and low ... more Aluminizing of bare and 3μm-Pt-electroplated specimens has been utilised to prepare NiAl and low platinum (Ni,Pt)Al coatings. Cyclic oxidation of the coatings was investigated by exposing samples to 1h cyclic oxidation at 1100°C. The modified coating exhibited an external layer of NiAl-25 vol.% PtAl2 above a three-zone structure. This structure endured over the whole testing time, while the NiAl coating

Journal of Central South University

The prediction of fatigue life of metallic alloys is justly accepted as one of the most important... more The prediction of fatigue life of metallic alloys is justly accepted as one of the most important phenomena in the field of metallurgical and mechanical engineering. At elevated temperatures, oxidation of the surfaces has an effective role in the fatigue strength and ductility of the alloys. In the present work, the effect of prior cyclic oxidation on the high temperature low cycle fatigue (HTLCF) properties of nickel-based superalloy Rene®80 has been assessed in the uncoated state and in the Pt-aluminide (Pt-Al) coated condition at 930 °C. To apply cyclic oxidation, simulation of engine thermal exposure was carried out by exposing coated and uncoated fatigue specimens in the burner rig (120 cycles at 1100°C). The cyclic oxidation procedure led to a changing in the coating microstructure from the dual-phase ( ξ -PtAl 2 + β -(Ni, Pt) Al) to single phase ( β -(Ni, Pt)Al). Results of HTLCF tests showed an improvement in the HTLCF life around 11.5% in the unexposed coated specimen (pre-cyclic oxidation) as compared to unexposed bare specimen, while this rise for exposed coated specimen (post-cyclic oxidation) was only 5%. Although a mixed mode fracture morphology (ductile and brittle) was observed on the fracture surfaces of failed specimens, the wider regions of brittle fracture belonged to exposed coated/uncoated ones. 预测金属合金的疲劳寿命是冶金和机械工程领域中最重要的研究之一。在高温下, 表面氧化对 合金的疲劳强度和延展性有重要影响。本文研究了预循环氧化对无涂层和有Pt-Al 涂层的镍基高温合金 Rene®80 在930 °C 时的高温低循环疲劳(HTLCF)性能的影响。将有涂层和无涂层的疲劳试样在燃烧器 钻机中进行发动机热暴露模拟(1100 °C, 120 次循环)。循环氧化过程中涂层的微观结构由双相( ξ -PtAl 2 + β -(Ni、Pt)Al)转变为单相( β -(Ni、Pt)Al)。结果显示, 与未暴露的无涂层试样相比, 未暴露的涂层试样 (预循环氧化)的HTLCF 寿命延长了约11.5%, 暴露涂层试样(后循环氧化)的仅延长了5%。虽然断裂试 样的断口表现为混合模式断裂形态(延性和脆性), 但暴露涂层/无涂层的试样断口大部分属于脆性断裂。

Journal of Materials Engineering and Performance, 2022

In the present study, cast-aluminizing (CA) and subsequent diffusion annealing processes were exp... more In the present study, cast-aluminizing (CA) and subsequent diffusion annealing processes were exploited to develop simple and Si-modified aluminide coatings on HH309 stainless steel. The CA method involved casting of HH309 melt into a mold, and Al-alloy sheets (pure Al and Al-12wt.%Si) were already inserted at the bottom of the mold. Afterward, the CA samples were diffusion-annealed at 1050 °C for 1 h and 1000 °C for 4h during two stages of heat treatment. The morphology, composition, and phase analysis of the coating were characterized by FE-SEM, EDS, and XRD. The results indicated that a multilayer aluminide coating containing (FeCrNi)2Al5 + (FeCrNi)Al2 and (FeCrNi)Al sub-layers was formed on the samples in the first stage heat treatment, while an integrated (FeCrNi)Al layer replaced the primary multilayer coating after the second stage. Then, the wear resistance of the coatings was evaluated. It was found that the applied aluminizing technique improved the wear resistance of HH309 alloy, significantly. The wear rate values for uncoated steel, aluminide, and Si-modified aluminide coatings were about 4.48 × 10−7, 1.44 × 10−7, and 0.74 × 10−7 mm3/N mm, respectively. Wear mechanisms analysis demonstrated that the type of wear for HH309 alloy was abrasive and adhesive and for aluminized steel was dominated by fatigue wear. Chemical composition analysis of the worn aluminide coatings showed that the aluminized samples also suffer oxidation wear.

Surface and Coatings Technology, 2020

This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Intermetallics, 2020

In the present work, a novel technique has been introduced to obtain an aluminide coating by cast... more In the present work, a novel technique has been introduced to obtain an aluminide coating by casting process and subsequent heat treatment. To do so, the aluminum sheet was placed at the bottom of a copper mold, then HH309 SS melt was poured into the mold. This technique was named Cast-Aluminizing (CA). The CA samples were heat-treated at the temperature range of 900-1050 � C for 0.5-5 h. The FE-SEM, XRD, and EDS were utilized to characterize the microstructure, phase analysis and chemical composition of cast-aluminized samples, respectively. Results showed that (Fe,Cr,Ni)Al 3 and (Fe,Cr,Ni) 2 Al 5 layers were formed at the Al/HH309 interface. FE-SEM analysis demonstrated a multi-layer aluminide coating on the heat-treated specimens. This coating consisted of (Fe,Cr,Ni) 2 Al 5 þ(Fe,Cr,Ni)Al 2 , (Fe,Cr,Ni)Al and α-Fe,Cr,Ni(Al) sub-layers. The growth kinetics investigation showed that the thickness of layers increased with the increase of the annealing temperature and time. The growth rate of layers obeyed a parabolic law. The activation energies for the growth of (Fe,Cr, Ni) 2 Al 5 þ(Fe,Cr,Ni)Al 2 , (Fe,Cr,Ni)Al and α-Fe,Cr,Ni(Al) layers were about 203, 250 and 247 kJ/mol, respectively. Microhardness measurements revealed that (Fe,Cr,Ni) 2 Al 5 þ(Fe,Cr,Ni)Al 2 , (Fe,Cr,Ni)Al and α-Fe,Cr,Ni(Al) layers had a hardness of about 820-1040, 580-710 and 380-470 HV, respectively. The resistance to oxidation of castaluminized and heat-treated (CA þ HT) samples in the air at 1000 � C was studied. The CA þ HT samples exhibited higher oxidation resistance than uncoated samples due to the formation of a protective Al 2 O 3 layer on the surface.

Journal of Advanced Materials In Engineering, 2019

In this study, the effect of platinum-aluminide coating parameters on surface roughness of nickel... more In this study, the effect of platinum-aluminide coating parameters on surface roughness of nickel-based superalloy Rene ® 80 was evaluated. For this purpose, different thicknesses of Pt-layer (2, 4, 6 and 8µm) were plated on the

Rare Metals, 2019

The effect of platinum (a rare metal)-aluminide coating parameters on the tensile properties of n... more The effect of platinum (a rare metal)-aluminide coating parameters on the tensile properties of nickel-based superalloy Rene Ò 80 was evaluated at 871°C. For this purpose, initial layers of platinum with different thicknesses (2, 4, 6 and 8 lm) were coated on tensile samples. Then, low-temperature high-activity (LTHA) and hightemperature low-activity (HTLA) processes were used for aluminizing. Results of microstructural evaluations using scanning electron microscope (SEM) and phase analysis by X-ray diffraction (XRD) showed a three-layer structure coating for different platinum layer thicknesses and both aluminizing processes. Increasing the thickness of the platinum layer from 2 to 8 lm led to the improvement in the final coating thickness from 91.6 to 102.1 lm in HTLA. This increase was from 128.1 to 148.6 lm in LTHA. The results of hot tensile tests at 871°C showed a decrease in strength properties of the coated samples compared to the uncoated ones. However, HTLA and high thicknesses of the initial platinum layer showed an intense reduction. The results of fractographic evaluations about uncoated samples showed a ductile fracture. On the other hand, coated samples showed a simultaneous ductile and brittle fracture failure mechanism. But the main fracture morphology was brittle cleavage fracture which was for the HTLA.

Materials Research Express, 2019

Journal of Mining and Metallurgy, Section B: Metallurgy, 2019

Low cycle fatigue is the most important failure mode in Aviation/Industrial engine rotary turbine... more Low cycle fatigue is the most important failure mode in Aviation/Industrial engine rotary turbine parts. In this paper, the influence of Pt-aluminide coating parameters on high temperature low cycle fatigue behavior of superalloy Rene?80 which is used to manufacture turbine blades, has been investigated. For this purpose, initial platinum layers of different thicknesses (2?m and 8?m) were coated on fatigue specimens. Then the aluminizing process was performed with two conditions of low temperature-high activity and high temperature-low activity. The results of microstructure investigations performed by scanning electron microscope and X-ray diffraction phase analysis indicated a three-layer structure for the coating (bi-phase (Ni,Pt)Al+PtAl2, single-phase (Ni,Pt)Al and interdiffusion zone) with different chemical compositions at both thicknesses of the platinum layer and using both aluminizing methods. Also, the results of low cycle fatigue tests at 871?C, R=0 and strain rate of 2?1...

Materials Chemistry and Physics, 2019

Abstract In this study, the effect of heat treatment on characteristics of the γ'-Ni3(Al, Ti)... more Abstract In this study, the effect of heat treatment on characteristics of the γ'-Ni3(Al, Ti) phase, as the main strengthening precipitates of the nickel-based superalloy Rene®80 was evaluated in terms of morphology and distribution of the γ' phase, γ (matrix)-γ′(strengthening precipitates) lattice mismatch, and hardness of the alloy. For this purpose, at first, the solution heat treatment was performed at 1204 °C on the alloy. Then, heat treatment cycles were carried out at three temperatures, namely 871 °C, 1095 °C, and 1130 °C, for different periods of time. The microstructures were analyzed by optical microscope and scanning electron microscope. Also, X-ray diffraction analysis was conducted to determine variations of the γ-γ′ lattice mismatch, followed by hardness measurement test, which was performed after each stage of heat treatments. The results showed an increase in the amount and size of the γ' phase with holding time at 871 °C, while the values were seen to remain constant and even decrease at 1095 °C and 1130 °C, respectively, due to simultaneous dissolution of the γ' phase during extended periods of time. At all of the three temperatures, the γ-γ′ lattice mismatch increased with time and showed larger negative values, while an increase in temperature from 871 °C to 1130 °C led to lower γ-γ′ lattice mismatch. The largest lattice mismatch (−0.653%) was observed at 871 °C after a heat treatment time of 1000 s. Hardness of the Rene®80 alloy at 871 °C increased with time, while different behaviors were observed at 1095 °C and 1130 °C, where the hardness decreased for extended heat treatment times. The most significant cause of such changes in hardness might be the coherency elastic strain between the γ and γ' phases, which is affected by the characteristics of the γ' phase and its lattice mismatch with the γ phase.

Oxidation of Metals, 2012

ABSTRACT Conventional and gradient CoNiCrAlYSi coatings were produced by using low vacuum plasma ... more ABSTRACT Conventional and gradient CoNiCrAlYSi coatings were produced by using low vacuum plasma spray and an additional step of diffusional over aluminizing (pack cementation) techniques on an Inconel-738 substrate. Hot corrosion of these coatings was investigated using Na2SO4–20wt%NaVO3 molten salt at 880 °C for 800 h. Hot corrosion rate was determined by measuring the weight gain of the specimens at regular intervals for a duration of 20 h. X-ray diffraction, field emission scanning electron microscopy and electron probe micro analysis techniques were used to determine the nature of phases, investigation of the thermally grown oxide, examination of the surface attack and determination of the elemental distribution. The gradient coating showed better performance by re-healing alumina scale due to its possession of more β phase as Al reservoir. Results indicated that pack cementation process caused an increase in amount of aluminum-rich β phase and better hot corrosion properties of gradient coatings owing to the Al enrichment in the outer layer and rapid formation of protective oxide on the surface.

Revista de Metalurgia, 2021

La superaleación Rene®80 a base de Ni se utiliza para fabricar álabes de turbinas de gas en motor... more La superaleación Rene®80 a base de Ni se utiliza para fabricar álabes de turbinas de gas en motores a reacción. La vida útil de algunas palas de turbina de motores a reacción está limitada por la fatiga de baja frecuencia y esta propiedad se ha visto fuertemente afectada por los revestimientos. La temperatura de transición de dúctil a frágil (DBTT) es el factor más importante que afecta las propiedades mecánicas de las aleaciones recubiertas. En este estudio, se evaluó el comportamiento de fatiga a alta temperatura y baja frecuencia del compuesto Rene®80 sin recubrimiento y recubierto con aluminuro de platino (Pt-Al) a temperaturas de 871 °C (cerca del DBTT) y 982 °C (por encima del DBTT). Los resultados de las pruebas de fatiga, en condiciones de deformación controlada a 871 °C para R = 0 y una tasa de deformación de 2×10-3 s-1, en un intervalo de deformación total de 0,8%, mostraron una disminución de la resistencia a la fatiga de las muestras recubiertas de aproximadamente un 14%...

Journal of The Electrochemical Society, 2011

Chromate conversion coatings (CCCs) were synthesized on AA7075 alloy. The effects of sodium sacch... more Chromate conversion coatings (CCCs) were synthesized on AA7075 alloy. The effects of sodium saccharin as a grain refining agent (GRA) and coating time on the coating morphology, topography, and alloy corrosion resistance were studied in this work. Morphologies and topographies of CCCs were examined using optical, scanning electron and atomic force microscopes. The corrosion behaviour was evaluated using potentiodynamic polarization and electrochemical impedance spectroscopy in 0.5 M NaCl solution. Major decreases in both coating grain size and corrosion rate occur at a GRA concentration of 1 g/l. Coating roughness increased to a limited nano-scale extent with increasing immersion time.