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Öğe CONSOLIDATION OF THERMALLY STABILIZED FE BASED FERRITIC STEELS VIA HOT PRESSING AND HOT EXTRUSION (ECAE)(Gazi Univ, Fac Engineering Architecture, 2016) Kotan, HasanThe key characteristic of ball-milled powders consolidation process is to achieve densification and particle bonding without degradation in mechanical properties. Recent demonstrations of novel processing methods involving temperature, high shear and high pressure have shown promise for bonding high strength particulate materials. In this study, we report the ability of multi-pass high temperature equal channel angular extrusion (ECAE) and hot pressing to produce fully dense and well-bonded bulk Ferritic alloys from nanocrystalline Fe-Ni-Zr powders. Subsequent microstructural characterizations indicate full consolidation via hot extrusion with grain growth to micron sizes. The consolidated alloys demonstrate an extremely high strength (4-6 GPa) at room temperature.Öğe Effect of Composition, Mechanical Alloying Temperature and Cooling Rate on Martensitic Transformation and Its Reversion in Mechanically Alloyed Stainless Steels(Korean Inst Metals Materials, 2021) Polat, Gokhan; Kotan, HasanStainless steels with Fe/Cr/Ni ratios of 74/18/8, 71/17/12, and 55/20/25 were produced from elemental powders by high energy mechanical alloying at both room and cryogenic temperatures. The effect of mechanical alloying temperature on martensitic transformation, the reversion of deformation-induced martensite-to-austenite upon annealing, and the influence of cooling rate on the thermal stability of reversed austenite upon cooling to room temperature were investigated in detail by in-situ and ex-situ X-ray diffraction (XRD) experiments, transmission electron microscopy (TEM) and Thermo-Calc simulations. A relative comparison of stainless steels after room temperature mechanical alloying indicated that the low nickel-containing steel underwent an almost complete martensitic transformation. However, martensitic transformation by deformation through mechanical alloying at room temperature would not be possible with increasing nickel contents but was created partially at cryogenic temperature, the degree of which depended on the steel composition. The in-situ XRD studies exhibited that the deformation-induced martensite completely transformed to austenite at elevated temperatures. The complete reverse transformation temperature simulated by Thermo-Calc software was found to be lower than that of the experimentally determined ones. Additionally, the different cooling rates from the reversed austenite demonstrated that the slower cooling increased the thermal stability of reversed austenite at room temperature. GraphicÖğe Effect of Hf additions on phase transformation, microstructural stability, and hardness of nanocrystalline 304L stainless steels synthesized by mechanical alloying(Elsevier, 2021) Kotan, Hasan; Polat, Gokhan; Yildiz, A. Busra304L stainless steels with Hf additions were nanostructured by mechanical alloying (MA) and annealed at temperatures up to 1100 degrees C. The results showed that face-centered cubic (fcc) phase in 304L transformed to body-centered cubic (bcc) phase during MA. The in-situ studies revealed that bcc-to-fcc phase trans -formation completed after 105 min annealing at 900 degrees C for 304L, whereas Hf addition increased the required time and temperature for the complete transformation. The grain size of 304L stainless steel was-10 nm after MA and remained-167 and-293 nm after annealing at 900 and 1100 degrees C, respectively, with Hf addition in comparison to 960 nm average grain size of base 304L stainless steel after annealing at 900 degrees C. The hardness of 304L increased from-200 HV to 408 HV after MA and remained 329 HV after annealing at 1100 degrees C with Hf addition as opposed to 195 HV hardness of 304L. (c) 2021 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.Öğe Effect of milling time, MWCNT content, and annealing temperature on microstructure and hardness of Fe/MWCNT nanocomposites synthesized by high-energy ball milling(Elsevier, 2021) Polat, Gokhan; Canbolat, I. Emre; Uzunoglu, Aytekin; Kotan, HasanNanocrystalline pure Fe and Fe/MWCNT nanocomposites powders with 0.25, 0.5, 1, and 10 wt% MWCNT contents were synthesized by high-energy ball milling (HEBM). The as-milled powders were coldcompacted and annealed at 400 degrees C and 600 degrees C for 1 h in Ar atmosphere. The effect of ball milling on pristine MWCNT and Fe/MWCNT composite powders was also investigated as a function of milling time up to 20 h. The physical properties of MWCNT were imaged by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) before and after HEBM. The structural damage of MWCNT as a function of milling time and MWCNT content was studied using Raman spectroscopy. The structural characterization of MWCNT and Fe/MWCNT composites was conducted by X-ray diffraction (XRD) as a function of milling time, MWCNT content, and annealing temperature. The chemical properties of the synthesized composite powders were investigated using X-ray photoelectron spectroscopy (XPS). The microhardness test was performed to assess the effect of milling time, annealing temperature, and MWCNT content on the mechanical properties. The results indicated that after the ball milling process, the structure of MWCNT was destroyed, and the formation of the amorphous carbon phase was observed, which was confirmed by XRD and TEM analyses. In addition, decreased defect and carbon intensity ratios (ID/IG) were calculated from the Raman results with longer ball milling processes, which is attributed to the destruction of carbon bonds. The XPS results confirmed the presence of FeAC bonds as a result of the formation of carbide phases. A fine dispersion of precipitated carbides determined by TEM is found to promote the grain size stability below 100 nm in the nanocrystalline Fe matrix. The results from the micro-hardness tests showed that Orowan particle strengthening resulting from the carbide formation, as well as grain size hardening, is an important contributor to strengthening in Fe/MWCNT composites. (C) 2021 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.Öğe Grain size stabilization of oxide dispersion strengthened CoCrFeNi-Y2O3 high entropy alloys synthesized by mechanical alloying(Elsevier Science Sa, 2021) Tekin, Mustafa; Polat, Gokhan; Kalay, Y. Eren; Kotan, HasanNanocrystalline CoCrFeNi high entropy alloys (HEAs) with 1 and 4 wt% nanosized Y2O3 were synthesized by high energy mechanical alloying and subjected to annealing treatments at different temperatures up to 1100 degrees C. X-ray diffraction (XRD), focused ion beam microscopy (FIB), and transmission electron microscopy (TEM) were used to investigate the microstructures of as-milled and annealed HEAs as a function of annealing temperature and Y2O3 content. The results have shown that the as-milled HEAs were solid solutions with face-centered cubic (fcc) crystal structure, which remained stable even after annealing at 1100 degrees C. The as-milled nanocrystalline CoCrFeNi HEA revealed grain growth upon annealing, reaching 293 nm and 1.45 mu m after annealing at 900 and 1100 degrees C, respectively. This suggests that the nanocrystalline microstructure of CoCrFeNi is not thermally stable at high temperatures. The grain size stability was found to reach around 72 nm with nanosized Y2O3 particles after annealing at 1100 degrees C. Accordingly, 477 +/- 20 HV asmilled hardness of CoCrFeNi was dramatically reduced to 220 +/- 14 HV after annealing at 1100 degrees C due to severe grain coarsening but retained around 450 +/- 23 HV with 4 wt% Y2O3 addition. The correlation between microstructure and hardness was utilized to evaluate the mechanical properties. (c) 2021 Elsevier B.V. All rights reserved.Öğe High Temperature Mechanical Properties and Microstructures of Thermally Stabilized Fe-Based Alloys Synthesized by Mechanical Alloying Followed by Hot Extrusion(Korean Inst Metals Materials, 2021) Kotan, Hasan; Darling, Kris A.; Luckenbaugh, TomThe key requirement to consolidate high-energy mechanically alloyed nanocrystalline powders is to achieve densification and particle bonding without impairment in the mechanical properties. Recent demonstrations of consolidation methods involving high shear, pressure and temperature have shown promising results for bonding high strength particulate materials produced by mechanical alloying. In this study, we report the ability of multi-pass high temperature equal channel angular extrusion to produce bulk ferritic alloys from nanocrystalline Fe-Ni-Zr powders. Subsequent microstructural characterizations indicate limited grain growth as the average grain sizes remain smaller than 100 nm after processing temperatures of 600 degrees C and 700 degrees C, above which grains reach micron sizes. The compression test results reveal that the alloys exhibit high mechanical strength at room and moderately high temperatures compared to the pure Fe and Fe-Ni alloys without Zr addition. GraphicÖğe Influence of Zr and nano-Y2O3 additions on thermal stability and improved hardness in mechanically alloyed Fe base ferritic alloys(Elsevier Science Sa, 2014) Kotan, Hasan; Darling, Kris A.; Scattergood, Ronald O.; Koch, Carl C.The motivation of this work was driven to improve the thermal stability in systems where polymorphic transformations can result in an additional driving force, upsetting the expected thermodynamic stability. In this study, Fe92Ni8 alloys with Zr and nano-Y2O3 additions were produced by ball milling and then annealed at high temperatures. Emphasis was placed on understanding the effects of dispersed nano-Y2O3 particle additions and their effect on microstructural stability at and above the bcc-to-fcc transformation occurring at 700 degrees C in Fe-Ni systems. Results reveal that microstructural stability and hardness can be promoted by a combination of Zr and Y2O3 additions, that being mostly effective for stability before and after phase transition, respectively. The mechanical strength of these alloys is achieved by a unique microstructure comprised a ultra-fine grain Fe base matrix, which contains dispersions of both nano-scale in-situ formed Zr base intermetallics and ex-situ added Y2O3 secondary oxide phases. Both of these were found to be essential for a combination of high thermal stability and high mechanical strength properties. (C) 2014 Elsevier B.V. All rights reserved.Öğe Inkjet printing of highly dispersed, shortened, and defect-rich MWCNTs to construct flexible electrochemical sensors for the detection of bisphenol A in milk samples(Pergamon-Elsevier Science Ltd, 2023) Sarikaya, Sumeyye; Ipekci, Hasan Huseyin; Kotan, Hasan; Uzunoglu, AytekinAn inkjet-printed electrochemical bisphenol A (BPA) sensor was developed by exploiting the high dispersibility of shortened and defect-rich nitrogen-doped multi-walled carbon nanotubes (MWCNTs). To enhance the dispersibility of MWCNTs in water-based dispersant, the defect concentration was increased by both creating shortened CNT and heteroatom doping, and modification of their surfaces with sulfonate (SO3H) groups. CNTbased inks were prepared by dispersing modified CNT structures in deionized water and the stability and physical properties of the inks were determined. The results suggested that the shortening of MWCNT and SO3H functionalization strategies improved the stability of CNT-based aqueous inks significantly, which enabled the preparation of highly concentrated inks up to 20 mg/ml with high stability. The CNT-based inks were printed on flexible polyethylene tetraflate (PET) and used as an electrochemical BPA sensor. The electrochemical analytical performance of the printed sensors was evaluated using chronoamperometry and differential pulse voltammetry methods. The sensors showed a wide linear range of 5-100 (CA) and 60-700 (DPV) with a LOD of 0.7 mu M. The real sample analysis was conducted in milk and high recoveries were obtained, suggesting the applicability of the sensors in real media.Öğe An investigation of abnormal grain growth in Zr doped CoCrFeNi HEAs through in-situ formed oxide phases(Elsevier Sci Ltd, 2022) Tekin, Mustafa; Polat, Gokhan; Kotan, HasanAbnormal grain growth (AGG) in nanocrystalline (CoCrFeNi)(100-x)Zr-x (x = 1 and 4 at. %) HEAs, prepared through high energy mechanical alloying, was comprehensively investigated upon annealing. Transmission electron microscopy (TEM), including high angle annular dark field imaging (HAADF) and energy dispersive spectroscopy (EDS) mapping, focused ion beam microscopy (FIB), and X-ray diffraction experiments (XRD) were utilized to investigate the microstructures as a function of added Zr content and temperature exposures. The results showed that nanocrystalline grains of the as-milled HEAs did not increase significantly upon annealing up to 700 degrees C as the nanocrystalline grain sizes were retained. However, grain growth was observed in (CoCrFeNi)(99)Zr-1 after annealing at 900 degrees C, which turned into AGG after annealing at a higher temperature of 1100 degrees C, disrupting the equiaxed grain structures observed at 900 degrees C. Although the increased amount of Zr doping reduced the average grain size in (CoCrFeNi)(96)Zr-4, bimodal grain structure existed in the microstructure composed of a matrix with 255 nm grain size and abnormally grown grains up to 3 mu m. The observed AGG was attributed to the pinning effect of in-situ formed secondary oxide phases. The microstructural evolution as a function of Zr doping and annealing temperatures was also correlated with the microhardness test results. The AGG and bimodal grain structure reported for the Zr doped CoCrFeNi HEA may open a new avenue to produce HEAs with the enhanced strength-ductility combination due to the incorporation of larger grains and in-situ formed oxide phases in a fine-grained matrix.Öğe Isothermal Annealing of a Thermally Stabilized Fe-Based Ferritic Alloy(Springer, 2015) Kotan, Hasan; Darling, Kris A.In this study, the stability and microstructural evolution, including grain size and hardness of nanocrystalline Fe91Ni8Zr1 alloyed powders, produced by ball milling, were investigated after annealing at 900 and 1000 A degrees C for up to 24 h. Results indicate that rapid grain growth to the micron scale occurs within the first few minutes of exposure to the elevated annealing temperatures. However, despite the loss of nanocrystallinity, an extremely stable and efficient hardening effect persists, which has been found to be equal to that predicted by Hall-Petch strengthening even at the smallest grain sizes. The mechanical properties of the samples consolidated to bulk via equal channel angular extrusion at 900 A degrees C were evaluated by uniaxial compression at room and elevated temperatures. Results reveal high compressive yield stress as well as the appearance and disappearance of a yield drop indicating the presence of coherent (GP zone like) precipitates within the microstructure. Such a hardening mechanism has implications for developing new Fe-Ni-based alloys exhibiting a combination of high strength and ductility for high temperature applications.Öğe Microstructural Characterization and Hardness Study of Nanostructured CoCrFeNi High Entropy Alloys with Dual Effect of Y and Nano-Sized Y2O3 Additions(Springer India, 2022) Tekin, Mustafa; Kotan, HasanThe motivation of this work is to increase the grain size stability of nanocrystalline CoCrFeNi high entropy alloys (HEA) with Y and Y2O3 additions to strike a balance between thermodynamic and kinetic stabilizations. The nanocrystalline HEAs prepared by mechanical alloying were annealed at different temperatures and characterized by X-ray diffraction, focused ion beam microscopy and micro-hardness test. The results revealed that as-milled nanostructured grain size yielded grain growth upon annealing reaching to 350 nm and 1.3 mu m after annealing at 900 degrees C and 1100 degrees C, respectively, while the addition of Y and Y2O3 appeared to stabilize the grain size in the nano-range after annealing at the same temperatures. Consequently, while the as-milled hardness of CoCrFeNi HEA dropped from 475 HV to around 200 HV after annealing at 1100 degrees C, the elevated hardness of 430 HV was retained with Y and Y2O3 additions after annealing at the same temperature.Öğe Microstructural Characterization of Improved Formability of Ti-6Al-4V Sheet by Pulsating Hydraulic Bulging at Room Temperature(Springer, 2023) Yapan, Yusuf Furkan; Ozturk, Osman; Turkoz, Mevlut; Dilmec, Murat; Livatyali, Haydar; Halkaci, Huseyin Selcuk; Kotan, HasanThis study aims to increase the limited formability of the Ti-6Al-4V sheet at room temperature using the pulsating hydraulic bulging and to investigate the microstructural reasons for the increased formability. Accordingly, monotonic and pulsating hydraulic bulge tests (HBTs) were applied to Ti-6Al-4V sheets with 0.55 mm thickness at room temperature, and the underlying microstructural reasons for the improved formability were investigated by conducting detailed microstructural characterizations. The experimental results showed that the pulsating HBT samples reached 15.4% higher bulge dome height than the monotonic HBT samples, and a more homogeneous thickness distribution was achieved by pulsating bulging. The thickness and microhardness distributions, dislocation density estimations, and fracture surface analysis were utilized to correlate the increased formability with the microstructure. It was determined that stress relaxation occurred during the pulsating HBT, resulting in the achieved improved formability. The decrease in the dislocation density in the early stage of forming prevented the locking of the dislocations and delayed the occurrence of the damage, i.e., leading to the increased bulge dome height. The higher plastic deformation resulting from the increased formability also increased the hardness along the cross-section. The increased formability of the Ti-6Al-4V sheet during the pulsating HBT and the underlying microstructural phenomena elaborated in this study are expected to make significant contributions to the current literature on the sheet metals with limited formability at room temperature.Öğe Microstructural evolution of 316L stainless steels with yttrium addition after mechanical milling and heat treatment(Elsevier Science Sa, 2015) Kotan, HasanNanocrystalline 316L stainless steels with yttrium addition were prepared by mechanical milling at cryogenic temperature and subjected to annealing treatments at various temperatures up to 1200 degrees C. The dependence of hardness on the microstructure was utilized to study the mechanical changes in the steels occurring during annealing. The microstructural evolution of the as-milled and annealed steels was characterized by means of X-ray diffraction (XRD), focused ion beam microscopy (FIB) and transmission electron microscopy (TEM) techniques. The results have revealed that austenite in as-received powder partially transformed to martensite phase during mechanical milling whereas the annealing induced reverse transformation of martensite-to-austenite. Furthermore, while the austenite-to-martensite phase ratio increased with increasing annealing temperature, the equilibrium structure was not achieved after three hours heat treatments up to 1200 degrees C resulting in a dual-phased steels with around 10% martensite. The grain size of 316L steel was 19 nm after mechanical milling and remained around 116 nm at 1100 degrees C with yttrium addition as opposed to micron size grains of plain 316L steel at the same annealing temperature. Such microstructural features facilitate the use of these materials at elevated temperatures, as well as the development of scalable processing routes into a dense nanocrystalline compact. (C) 2015 Elsevier B.V. All rights reserved.Öğe Phase transformation and grain growth behavior of a nanocrystalline 18/8 stainless steel(Elsevier Science Sa, 2017) Kotan, Hasan; Darling, Kris A.Fe-18Cr-8Ni and Fe-18Cr-8Ni-1Y (at%) stainless steel powders were nanostructured by mechanical alloying from elemental powders and subjected to 90 min annealing treatments at various temperatures. The microstructural evolutions as a function of alloy compositions and temperatures were investigated by in-situ and ex-situ x-ray diffraction experiments, transmission electron microscopy and focused ion beam microscopy. The dependence of hardness on the microstructure was utilized to study the mechanical changes. It was found that the resulting microstructures by mechanical alloying were bcc solid solution, the so-called alpha'-martensite structure. The high temperature in-situ x-ray diffraction experiments showed that the martensite-to-austenite reverse phase transformation was completed above 800 and 900 degrees C for Fe-18Cr-8Ni and Fe-18Cr-8Ni-1Y steels, respectively. A partial or complete retransformation to martensite was observed upon cooling to room temperature. Annealing of nanocrystalline Fe-18Cr-8Ni steel yielded grain growth reaching to micron sizes at 1100 degrees C while addition of 1 at% yttrium stabilized the microstructure around 160 nm grain size and 6 GPa hardness after 90 min annealing at 1200 degrees C.Öğe Preparation of defect-rich, N-doped activated carbons via high-energy ball milling and investigation of their electrochemical performances towards hydrogen peroxide sensing(Springer Heidelberg, 2022) Uzunoglu, Aytekin; Kotan, Hasan; Karaagac, Rumeysa; Ipekci, Hasan H.N-doped, defect-rich activated carbons (ACs) were prepared using high-energy ball milling (BM) followed by a heat treatment to prepare metal-free, highly active carbon-based electrocatalysts. To investigate the effect of milling process on the physicochemical and electrochemical properties, a series of milling durations ranging from 2.5 to 15 min were applied. It was seen that while 2.5-min milling process was insufficient to destroy the AC structure and achieve high defect concentration, 5-min milling process yielded a significant change in the X-ray diffraction reflection. X-ray photoelectron spectroscopy results suggested that contents of N-binding sites were altered depending on the milling time. Electrochemical performance results indicated that both N doping and ball milling processes had significant effects on the H2O2 reduction capabilities. N-AC-5BM-based sensors showed a high sensitivity of 301.24 mu A mM(-1) cm(-2), which is 2.5 and 1.7 times greater than pristine AC (119.3 mu A mM(-1) cm(-2)) and N-doped AC (180.6 mu A mM(-1) cm(-2)), respectively. In addition, 5-min ball-milling process widened the linear range profoundly, and a wide linear range of 0.1-52 mM was obtained. The performance of the sensors was compared with both metal-free and noble-metal containing sensors and the findings displayed that N-AC-5BM-based sensors had higher performances than even noble metal-containing sensors.Öğe Role of yttrium addition and annealing temperature on thermal stability and hardness of nanocrystalline CoCrFeNi high entropy alloy(Elsevier Sci Ltd, 2022) Polat, Gokhan; Tekin, Mustafa; Kotan, HasanCoCrFeNi high entropy alloys (HEAs) with yttrium (Y) additions (1 and 4 at. %) were nanostructured by mechanical alloying process and annealed at various temperatures between 500 degrees C and 1100 degrees C. The structure, grain growth, and hardness were studied as a function of solute addition and annealing temperature using X-ray diffraction (XRD), focused ion beam (FIB), and scanning transmission electron microscope (S/TEM) techniques, and hardness test. The thermo-physical calculations were utilized to discuss the phase evolution after mechanical alloying and annealing with respect to added solutes. The results showed that Y additions did not affect the main crystal structure of the base CoCrFeNi HEA as the solid solution with a single face-centered cubic (fcc) crystal structure was maintained even after 1 h annealing at 1100 degrees C. The as-milled nanocrystalline grain size of CoCrFeNi HEA yielded extensive grain growth with the temperature exposures reaching 291 nm and 1.4 mu m after annealing at 900 degrees C and 1100 degrees C, respectively. However, Y additions retarded the grain growth and decreased the average grain size upon annealing as compared to the base HEA. That is, 1 and 4 at. % Y additions stabilized the grain size around 88 nm and 95 nm (both determined by TEM) after annealing at 900 degrees C and 1100 degrees C, respectively. Accordingly, the as-milled hardness of CoCrFeNi HEA dropped from 475 HV to 220 HV after annealing at 1100 degrees C, while the reduction in hardness was relatively gradual with Y additions and retained around 435 HV with 4 at. % Y addition even after annealing at 1100 degrees C. Such thermal stability may facilitate the use of HEAs at high temperatures and enable the consolidation routes of powders into dense nanocrystalline compact HEAs.Öğe A study of microstructural evolution of Fe-18Cr-8Ni, Fe-17Cr-12Ni, and Fe-20Cr-25Ni stainless steels after mechanical alloying and annealing(Elsevier Science Inc, 2018) Kotan, Hasan; Darling, Kris A.In this study, high-energy mechanical alloying technique was used to produce nanocrystalline stainless steels of three different compositions from elemental powders. The microstructural evolution (grain growth and phase transformation) as a function of alloy compositions and annealing temperatures were investigated by room and high temperature x-ray diffraction experiments, transmission electron microscopy and focused ion beam microscopy. The results revealed that stainless steels with low nickel content (i.e., Fe-18Cr-8Ni) underwent a deformation-induced martensitic transformation during room temperature mechanical alloying. Deformation induced martensitic transformation with increasing nickel content (i.e., Fe-17Cr-12Ni and Fe-20Cr-25Ni) would not be possible by room temperature milling but was created by high strain rate cryogenic processing, the degree to which was compositional dependent. Post process annealing induced the reverse transformation from martensite-to-austenite the ratio of which was found to be a factor of alloy composition and annealing temperature. The real time in-situ x-ray studies showed that the martensite-to-austenite reverse transformation was completed at around 600 degrees C and 800 degrees C for Fe-18Cr-8Ni and Fe-20Cr-25Ni steels, respectively. Microscopy studies revealed a significant enhancement in the resistance to grain growth for Fe-17Cr-12Ni steel over other compositions at elevated temperatures as high as 1200 degrees C. As such, cryogenic processing following by reverse martensitic transformation of high Ni containing alloys provides a pathway for developing higher heat resistant stainless steel alloys.Öğe Termal kararlılığı arttırılmış demir esaslı ferritik çelik tozlarının sıcak pres ve sıcak ekstrüzyon (ECAE) yöntemleri ile konsolidasyonu(2016) Kotan, HasanBilyeli değirmenler ile üretilen yüksek mukavemetli tozların konsolidasyonunda temel hedef mekanik özelliklerinde önemli bir kayıp olmadan bu tozların birleştirilerek porozite içermeyen ürüne dönüştürülebilmesidir. Son yıllarda yapılan araştırmalar sıcak ekstrüzyon ile yüksek kayma ve basınca maruz bırakılan tozların porozite içermeden, teorik yoğunluklarında birleştirilebileceğini göstermiştir. Bu çalışmada sıcak presleme ve yüksek sıcaklıklarda çok geçişli eş kanallı açısal ekstrüzyon (ECAE - equal channel angular extrusion) teknikleri kullanılarak nano-kristalin Fe-Ni-Zr tozlarından konsolide edilmiş ferritik alaşımların üretimi araştırılmıştır. Elde edilen sonuçlar sıcak presleme metodu ile teorik yoğunluğa ulaşılamadığını, ancak sıcak ekstrüzyon yöntemi ile yapısında porozite içermeyen ve teorik yoğunlukta malzeme üretilebileceğini göstermiştir. Konsolidasyon sonucunda mikroyapıda mikrometre seviyelerine ulaşan tane büyümesi gözlenmesine rağmen üretilen bu çeliklerin sertliği 4-6 GPa civarındadır.Öğe Thermal stability, phase transformation and hardness of mechanically alloyed nanocrystalline Fe-18Cr-8Ni stainless steel with Zr and Y2O3 additions(Elsevier Science Sa, 2018) Kotan, HasanNanocrystalline Fe-18Cr-8Ni (at%) stainless steels were produced by mechanical alloying from elemental powders with Zr and Y2O3 additions and subjected to annealing treatments at various temperatures. Xray diffraction experiments, transmission electron microscopy and focused ion beam microscopy were used to investigate the microstructural evolution as a function of alloy composition and annealing temperature. The dependence of hardness on the microstructure was utilized to study the mechanical changes with temperature. It was found that the resulting microstructures by mechanical alloying were bcc solid solution, the so-called alpha'-martensite structure whereas, depending on the composition and temperature, partial reverse transformation from martensite to austenite was induced by annealing. Asmilled nanocrystalline Fe-18Cr-8Ni stainless steel yielded grain growth upon annealing reaching to micron sizes at 1100 degrees C while addition of Zr and Y2O3 stabilized the microstructure below approximately 250 nm grain size and close to 5 GPa hardness after annealing at the same temperature. Such microstructural features may facilitate the consolidation process of nanocrystalline stainless steel powders as well as enabling the use of these materials at elevated temperatures. (C) 2018 Elsevier B.V. All rights reserved.Öğe Understanding microstructural evolution and hardness of nanostructured Fe89.5Ni8Zr2.5 alloy produced by mechanical alloying and pressureless sintering(Elsevier - Division Reed Elsevier India Pvt Ltd, 2020) Polat, Gokhan; Batibay, Ahmet B.; Kotan, HasanFe89.5Ni8Zr2.5 alloy was synthesized by mechanical alloying followed by pressureless sintering at various temperatures up to 900 degrees C. Microstructural evolution as a function of processing temperature was characterized using focused ion beam microscopy, transmission electron microscopy and X-ray diffraction techniques. The dependence of hardness on the microstructure was utilized to study the mechanical changes. The experimental results showed that microstructural stability can be enhanced by segregation of solutes to grain boundaries at low temperatures and by precipitation of second phases at elevated temperatures. Eventually, at higher processing temperatures the stability was lost due to the coarsening of the precipitated second phases leaving behind ultra-fined grained microstructure. Despite the coarsening of the grain size with increasing processing temperatures, the in-situ formed second phases were found to induce an Orowan strengthening effect leading to approximately 5.5 GPa hardness after 1 h sintering at 900 degrees C. (c) 2020 Karabuk University. Publishing services by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
