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Nanocrystalline 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 cold-compacted and annealed at 400 °C and 600 °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 Fe C 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.

304L stainless steels with Hf additions were nanostructured by mechanical alloying (MA) and annealed at temperatures up to 1100 °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 transformation completed after 105 min annealing at 900 °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 °C, respectively, with Hf addition in comparison to 960 nm average grain size of base 304L stainless steel after annealing at 900 °C. The hardness of 304L increased from ~200 HV to 408 HV after MA and remained 329 HV after annealing at 1100 °C with Hf addition as opposed to 195 HV hardness of 304L.

In this study, a Ti-22Al-25Nb alloy with nanocrystalline structure was produced by high energy mechanical alloying (HEMA) and 1 at.-% yttrium was added as a thermal stabilizer. The as-milled samples were annealed at various temperatures up to 900 °C in a protective gas atmosphere, and the samples were allowed to cool to room temperature in the furnace. The phase transformations and microstructural changes as a function of the annealing temperatures and alloy compositions were studied using room- and high-temperature X-ray diffraction (XRD), focused ion beam microscopy (FIB), and scanning electron microscopy (SEM). The mechanical properties of the samples were interpreted based on the hardness results and their correlation with the microstructures. The results showed that the as-milled nanocrystalline structure of Ti-22Al-25Nb alloy increased from 3.4 nm to 350 nm after annealing at 800 °C due to the high driving force induced by the large grain boundary area. Consequently, the as-milled hardness of the Ti-22Al-25Nb alloy dropped from 7.63 ± 0.18 GPa to 5.37 ± 0.28 GPa. The grain size stability of the Ti-22Al-25Nb alloy after annealing at elevated temperature was ensured through the addition of yttrium. Thus, the grain size remained at the level of 125 nm, and the hardness value was maintained at around 6.98 ± 0.43 GPa after annealing at 800 °C.

Fe89.5Ni8Zr2.5 alloy was synthesized by mechanical alloying followed by pressureless sintering at various temperatures up to 900 °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 °C.

Stainless 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. Graphic

The 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 °C and 700 °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.

Nanocrystalline 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.

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.

A series of bulk nanostructured Fe–Ni–Zr oxide-dispersion-strengthened (ODS) alloys were synthesized using high energy mechanical alloying and consolidated using high temperature equal channel angular extrusion. The resultant microstructures are composed of nano/ultrafine or micrometer-sized grains with larger intermetallic precipitates and small Zr oxide clusters (

Nanocrystalline 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.

Bu calismada, Fe 74 Cr 18 Ni 8 paslanmaz celik alasimi Y elementi ve nano boyutta Y 2 O 3 ilaveleri ile mekanik alasimlama yontemi kullanilarak nanokristal yapida uretilmis ve 1100 oC’ye kadar farkli sicakliklarda tavlama islemine tabii tutulmustur. X-isinlari kirinimi, gecirimli elektron mikroskobu ve odaklanmis iyon demeti mikroskobu kullanilarak sicakliga bagli olarak mikroyapida meydana gelen degisimler incelenmistir. Mikroyapinin malzeme sertligine etkisinden faydalanilarak tavlama sicakligina bagli olarak mekanik ozelliklerdeki degisimler tespit edilmistir. Elde edilen sonuclar denge durumunda ymk kafes yapisina sahip olan bu paslanmaz celik kristal yapilarinin mekanik alasimlama sirasindaki yogun plastik deformasyondan dolayi hmk kafes yapisindaki a’ – martenzite donustugunu gostermektedir. Mekanik alasimlamadan sonra uygulanan tavlama islemi ile sicakliga bagli olarak farkli oranlarda martenzit – ostenit tersine faz donusumunun gerceklestigi tespit edilmistir. Nanokristal yapili Fe 74 Cr 18 Ni 8 paslanmaz celik alasiminda yuksek tane siniri alanindan dolayi yuksek sicakliklarda meydana gelen ve mikrometre seviyelerine ulasan tane boyutunun, Y ve Y 2 O 3 ilaveleri ile 1100 °C’de yapilan tavlamadan sonra bile yaklasik olarak 150 nm seviyesinde tutuldugu ve sertliginin ise 6,5 GPa seviyelerinde korundugu tespit edilmistir.

Nanocrystalline 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.

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 °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 °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.

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 °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.

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.

It is well established that the severe plastic deformation of metastable austenitic stainless steels causes martensitic transformation and the annealing of deformation induced martensitic phase induces reverse transformation from martensite to austenite the ratio of which has been found to be a factor of alloy compositions and annealing / sintering temperatures. In this study, nanocrystalline austenitic stainless steels of different compositions will be synthesized from elemental powders by using high energy mechanical milling. As-milled powders will be cold-consolidated and sintered at elevated temperatures up to 1100 ºC in order to facilitate a better densification for the subsequent experiments and alter the microstructure i.e., grain size and austenite to martensite phase ratio. Microstructural evolutions as a function of alloy compositions and sintering temperatures will be investigated using focused ion beam, transmission electron microscopy and in-situ / ex-situ x-ray diffraction techniques to determine the grain sizes and phases upon milling and sintering. The corrosion behavior of austenitic stainless steels will be determined using immersion and electrochemical tests at various temperatures. The electrochemical corrosion tests will be conducted using a flat corrosion test and a potentiostat. The relative importance of compositions, grain sizes and austenite to martensite phase ratios will be discussed with respect to corrosion behavior of the studied stainless steels.

We report recent research in our laboratory on the thermal stabilization of nanocrystalline binary alloys with ternary additions. Fe-Cr and Fe-Ni alloys with the ternary addition of Zr are studied. The thermal stability of these nanocrystalline alloys, prepared by mechanical alloying of powders, is studied by XRD, TEM and hardness as a function of annealing temperature. The relative importance of thermodynamic or kinetic stabilization in various temperature ranges is discussed for the different alloys. In agreement with our recent model for thermodynamic stabilization, it is found that Zr solute additions are more effective in stabilizing the nanocrystalline grain size in the Fe-Cr than in the Fe-Ni system.

Grain growth of ball-milled pure Fe, Fe 92 Ni 8 , and Fe 91 Ni 8 Zr 1 alloys has been studied using X-ray diffractometry (XRD), focused ion beam (FIB) microscopy and transmission electron microscopy (TEM). Mechanical properties with respect to compositional changes and annealing temperatures have been investigated using microhardness and shear punch tests. We found the rate of grain growth of the Fe 91 Ni 8 Zr 1 alloy to be much less than that of pure Fe and the Fe 92 Ni 8 alloy at elevated temperatures. The microstructure of the ternary Fe 91 Ni 8 Zr 1 alloy remains nanoscale up to 700 °C where only a few grains grow abnormally whereas annealing of pure iron and the Fe 92 Ni 8 alloy leads to extensive grain growth. The grain growth of the ternary alloy at high annealing temperatures is coupled with precipitation of Fe 2 Zr. A fine dispersion of precipitated second phase is found to promote the microstructural stability at high annealing temperatures and to increase the hardness and ultimate shear strength of ternary Fe 91 Ni 8 Zr 1 alloy drastically when the grain size is above nanoscale.

Data from the literature and our laboratory have been reviewed regarding the maximum homologous temperatures that can be attained by the addition of solute elements that may induce thermodynamic or kinetic (Zener pinning) stabilization of a nanocrystalline grain size (

Grain growth and microstructural evolution of thermally stabilized Fe91Ni8Zr1 were investigated by in situ and ex situ studies. Our investigations suggest that the microstructural evolution is fairly slow and the microstructure shows stabilization up to about 700 °C. Above this temperature, a certain fraction of grains grow abnormally into the nanocrystalline matrix, resulting in a bimodal microstructure and causing the complete loss of thermal stability. The reason for abnormal grain growth and the loss of thermal stability is identified as the appearance of the fcc γ-phase and consequent reduction in the total area of grain boundaries and the overall stored energy.

The primary objective of this work was to determine the influence of 1–4 at% Zr additions on the thermal stability of mechanically alloyed nanocrystalline Fe–Cr alloys containing 10 and 18 at% Cr. Grain sizes based on XRD, along with microhardness changes, are reported for isochronal annealing treatments up to 1000 °C. Microstructure investigations were done using optical microscopy, channeling contrast FIB imaging, and TEM. Grain size stabilization in the nanaoscale range was maintained up to 900 °C by adding 2 at% Zr. Kinetic pinning by nanoscale intermetallic particles was identified as one source of high temperature grain size stabilization. Intermetallic particles also contribute to strengthening in addition to the Hall–Petch effect. The analysis of microhardness, XRD data, and measured values from the TEM image for Fe-10 at% Cr with 2 at% Zr suggested that both thermodynamic and kinetic mechanisms would contribute to grain size stabilization. There was no significant difference in the results for the 10 and 18 at% Cr alloys, which indicates that the α→γ transformation does not influence the grain size stabilization.

Fe–xNi alloys from x = 0 to x = 15 with an as-milled grain size and hardness in the range of 8–11 nm and 8.5–9.5 GPa, respectively, were synthesized by ball milling. Microstructural changes, hardness, and grain growth due to annealing were characterized using X-ray diffractometry, microhardness, focused ion beam channeling contrast imaging, and optical microscopy. It was found that the composition range of single bcc phase was extended by ball milling. Subsequent annealing of MA samples resulted in reduction of hardness and extensive grain growth. It indicates that nickel has no significant effect on thermal stabilization of iron. Retained austenite was observed for Fe–8Ni and Fe–10Ni alloys annealed in the two-phase region and effect of as-milled structure on retained austenite formation was discussed.

Bilyeli 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., The 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 FeNi-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.

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.

This work presents a model for evaluating thermodynamic stabilization of ternary nanocrystalline alloys. It is applicable to alloy systems containing strongly segregating size-misfit solutes with a significant enthalpy of elastic strain and/or immiscible solutes with a positive mixing enthalpy. On the basis of a regular solution model, the chemical and elastic strain energy contributions are incorporated into the mixing enthalpy ΔHmix, and the mixing entropy ΔSmix is obtained using the ideal solution approximation. The Gibbs mixing free energy ΔGmix is minimized with respect to simultaneous variations in grain size and solute segregation parameters. The Lagrange multiplier method is used to obtain numerical solutions for the minimum ΔGmix corresponding to an equilibrium grain size for given alloy compositions. The numerical solutions will serve as a guideline for choosing solutes and assessing the possibility of thermodynamic stabilization. The temperature dependence of the nanocrystalline grain size and ...

The work presented here was motivated by the need to develop a predictive model for thermodynamic stabilization of binary alloys that is applicable to strongly segregating size-misfit solutes, and that can use available input data for a wide range of solvent-solute combinations. This will serve as a benchmark for selecting solutes and assessing the possible contribution of thermodynamic stabilization for development of high-temperature nanocrystalline alloys. Following a regular solution model that distinguishes the grain boundary and grain interior volume fractions by a transitional interface in a closed system, we include both the chemical and elastic strain energy contributions to the mixing enthalpy ΔHmix using an appropriately scaled linear superposition. The total Gibbs mixing free energy ΔGmix is minimized with respect to simultaneous variations in the grain-boundary volume fraction and the solute contents in the grain boundary and grain interior. The Lagrange multiplier method was used to obtain numerical solutions with the constraint of fixed total solute content. The model predictions are presented using a parametric variation of the required input parameters. Applications are then given for the dependence of the nanocrystalline grain size on temperature and total solute content for selected binary systems where experimental results suggest that thermodynamic stabilization could be effective.

Makale, WOS:000959908100001, In this study, in-situ oxide dispersion strengthened (ODS) Fe91Ni8Zr1 and Fe88Ni8Zr4 alloys were produced by combination of high energy mechanical alloying (HEMA) and high temperature equal channel angular extrusion (HT-ECAE). The wear behaviors of the consolidated samples were investigated under different loads from 1 N to 4 N by reciprocating wear tests at room temperature. The Scanning electron microscopy (SEM) was used to examine the wear tracks to analyze the wear characteristics as a function of applied loads. The relative comparison of the wear results showed that under the lower loads of 1 N and 2 N, Fe88Ni8Zr4 alloy has lower wear rate than Fe91Ni8Zr1 alloy whereas under the higher loads of 3 N and 4 N, it is vice versa. Additionally, the friction coefficient of Fe91Ni8Zr1 alloy was found to be lower than that of Fe88Ni8Zr4 alloy under all the applied loads. The results were comparatively discussed with respect to microstructural features of 1 at% Zr and 4 at% Zr containing ODS alloys produced by HEMA followed by ECAE. The obtained results of ODS alloys with different grain size, precipitate size, and number density of the precipitates, may disclose a new sight for using such alloys in wear applications just as cutting tools, turbine blades, and discs.