Evaluation of Microstructure and Mechanical Properties of Nano-Y2O3-Dispersed Ferritic Alloy Synthesized by Mechanical Alloying and Consolidated by High-Pressure Sintering

In this study, an attempt has been made to synthesize 1.0 wt pct nano-Y₂O₃-dispersed ferritic alloys with nominal compositions: 83.0 Fe-13.5 Cr-2.0 Al-0.5 Ti (alloy A), 79.0 Fe-17.5 Cr-2.0 Al-0.5 Ti (alloy B), 75.0 Fe-21.5 Cr-2.0 Al-0.5 Ti (alloy C), and 71.0 Fe-25.5 Cr-2.0 Al-0.5 Ti (alloy D) steels (all in wt pct) by solid-state mechanical alloying route and consolidation the milled powder by high-pressure sintering at 873 K, 1073 K, and 1273 K (600°C, 800°C, and 1000°C) using 8 GPa uniaxial pressure for 3 minutes. Subsequently, an extensive effort has been undertaken to characterize the microstructural and phase evolution by X-ray diffraction, scanning and transmission electron microscopy, and energy dispersive spectroscopy. Mechanical properties including hardness, compressive strength, Young’s modulus, and fracture toughness were determined using micro/nano-indentation unit and universal testing machine. The present ferritic alloys record extraordinary levels of compressive strength (from 1150 to 2550 MPa), Young’s modulus (from 200 to 240 GPa), indentation fracture toughness (from 3.6 to 15.4 MPa√m), and hardness (from13.5 to 18.5 GPa) and measure up to 1.5 through 2 times greater strength but with a lower density (~7.4 Mg/m³) than other oxide dispersion-strengthened ferritic steels (<1200 MPa) or tungsten-based alloys (<2200 MPa). Besides superior mechanical strength, the novelty of these alloys lies in the unique microstructure comprising uniform distribution of either nanometric (~10 nm) oxide (Y₂Ti₂O₇/Y₂TiO₅ or un-reacted Y₂O₃) or intermetallic (Fe₁₁TiY and Al_9.22Cr_2.78Y) particles' ferritic matrix useful for grain boundary pinning and creep resistance.     

Phase transformations during tempering of the Fe-15Cr-1Mo martensites containing nitrogen or carbon

Hardness measurements, dilatometry, internal friction measurements, Mössbauer spectroscopy and transmission electron microscopy are utilized in order to study the effect of tempering on the microstructure of a stainless martensitic steel containing 15% Cr, 1% Mo and 0.6% N. A similar carbon steel containing 15% Cr, 1% Mo and 0.6% C is used for comparison. Tempering of alloy Fe-15Cr-1Mo-0.6N in the low temperature range of 353-473 K leads to formation of hexagonal ?-nitride (Fe,Cr)₂N, which is followed by precipitation of the orthorombic ?-nitride (Fe,Cr)₂N at temperatures of 573-773 K. The hexagonal nitride Cr₂N is precipitated at 923 K and preferably formed at grain boundaries. The alloy Fe-15Cr-1Mo-0.6C shows the expected tempering behaviour. ?-carbide (Fe,Cr)₂C and cementite (Fe,Cr)₃C are precipitated during low temperature ageing, followed by the formation of Cr₇C₃ carbides after the temperature has risen to 873 K. With a similar interstitial content the amount of retained austenite in the nitrogen martensite is nearly twice as high as in the carbon one. Furthermore, the thermal stability of the retained austenite of the nitrogen alloy is substantially higher than that of the carbon steel.     

Oxidation Behavior of NiAl-30.75Cr-3Mo-0.25Ho Alloy at High Temperatures

The oxidation behavior of NiAl-30.75Cr-3Mo-0.25Ho alloy from 1300 to 1500 K in air atmosphere was investigated. The results reveal that oxidation resistance of the alloy is improved by the addition of Ho. At 1500 K, the oxidation kinetic curve obeys the parabolic law (n≈0.5), whereas the oxidation kinetic curve of the tested alloy follows the cubic relations (n≈0.3～0.4) from 1300 to 1450 K. An activation energy of about 261 kJ·mol-1 was determined for the tested alloy. It is found that a continuous and compact Al2O3 layer has formed on the surface of NiAl-30.75Cr-3Mo-0.25Ho alloy after oxidation 100 h at various tested temperatures. A rich-Ho solution formed on the boundaries of Cr(Mo) phase. Doped little amount of Ho in NiAl-31Cr-3Mo alloy promotes the transformation from θ-Al2O3 phase to α-Al2O3 phase, and decreases the size of Al2O3 and the crack forming in the oxidation scale prolongs the spalling time of the film at high temperature. The volatilizing oxides of Cr, Mo and the reactive element effects (REEs) make the mass gain lower than that of pure NiAl.     

Oxidation of Fe-7Cr-12Ni-(0-6)Al-(0-7)Si alloys

A series of Fe-7Cr-12Ni-(0-6)Al-(0-7)Si alloys was oxidized at test temperatures between 800° and 1000 °C. The master alloy, Fe-7Cr-12Ni, had no oxidation resistance at temperatures above 800 °C. Aluminum additions to the master alloy produced a protective A1₂O₃ layer over the substrate metal which prevented rapid oxidation and allowed time for a Cr₂O₃ layer to develop underneath. Silicon addition to the master alloy produced an SiO₂ oxide sublayer at the metal-oxide interface which, when formed, provided an effective barrier to oxidation. A combination of 1.5-pct-Al and 2.0-pct-Si additions provided the best oxidation protection.     

Heterogeneous Creep Deformations and Correlation to Microstructures in Fe-30Cr-3Al Alloys Strengthened by an Fe<Subscript>2</Subscript>Nb Laves Phase

A new Fe-Cr-Al (FCA) alloy system has been developed with good oxidation resistance and creep strength at high temperature. The alloy system is a candidate for use in future fossil-fueled power plants. The creep strength of these alloys at 973 K (700 °C) was found to be comparable with traditional 9 pct Cr ferritic–martensitic steels. A few FCA alloys with general composition of Fe-30Cr-3Al-.2Si-xNb (x = 0, 1, or 2) with a ferrite matrix and Fe₂Nb-type Laves precipitates were prepared. The detailed microstructural characterization of samples, before and after creep rupture testing, indicated precipitation of the Laves phase within the matrix, Laves phase at the grain boundaries, and a 0.5 to 1.5 μm wide precipitate-free zone (PFZ) parallel to all the grain boundaries. In these alloys, the areal fraction of grain boundary Laves phase and the width of the PFZ controlled the cavitation nucleation and eventual grain boundary ductile failure. A phenomenological model was used to compare the creep strain rates controlled by the effects of the particles on the dislocations within the grain and at grain boundaries. (The research sponsored by US-DOE, Office of Fossil Energy, the Crosscutting Research Program).     

Grain-size control in Ti-48Al-2Cr-2Nb with yttrium additions

A gas-atomized (GA) prealloyed powder of the Ti-48Al-2Cr-2Nb intermetallic and 1.6 wt pct Y were mechanically alloyed (MA) and hot isostatically pressed (hipped) to produce a fully dense nanocrystalline material. Mechanical alloying of the as-blended powder for 16 hours resulted in the formation of a disordered fcc phase. Hipping of the alloy powder produced a single-phase nanocrystalline TiAl intermetallic, containing a distribution of 20 to 35-nm-sized Al₂Y₄O₉ particles. The formation of oxide particles occurred by the chemical combination of Al and Y with oxygen, which entered as a contaminant during milling. Oxide particles increased the hardness of the intermetallic compound and minimized grain growth even at 0.8 T _m , where T _m is the melting point of the compound.     

Effect of small concentrations of zirconium on high temperature oxidation behaviour of Fe-15 Cr-4 Al

Effect of 1% Zr on oxidation behaviour of Fe-15 Cr-4 Al alloy under isothermal conditions in air, O₂ and O₂-10% H₂O environments in the temperature range 1000–1150°C was investigated. The effect of zirconium concentration was studied at 1 200°C in air. Oxidation rate increases with increase in zirconium concentration. Parabolic rate of oxidation was observed. Limited study on cyclic oxidation was carried out at 1150°C in air. The cycle consisted of one hour holding at isothermal temperature followed by half an hour air cooling. The oxidised samples were examined by X-Ray diffractometry, SEM, EDAX. Extensive spalling was observed in the base alloy, Fe-15 Cr-4 Al in all environments. Zirconium additions eliminated the spalling of the scale. The EDAX analysis of a spalled region shows the presence of iron and chromium while the unspalled region is aluminium rich. A common structural feature, localised formation of granules/nodules was observed in the scale of zirconium containing alloys in all the environments. The number of granules increased with increase in zirconium concentration and was observed to be a maximum in 1% Zr and also increases with increasing temperature. The observations reveal that 1% Zr alloy shows lower oxidation rate in O₂-H₂O environment under isothermal conditions. X-Ray diffraction analysis shows the additional presence of Fe₂O₃ and Cr₂O₃ in α-Al₂O₃ scale which have not been detected in the α-Al₂O₃ scale formed in other environments, air and O₂. 0.2% Zr is most effective in increasing oxidation resistance of Fe-15 Cr-4 Al alloy both under isothermal and cyclic oxidation conditions.     

A Novel Ni-Containing Powder Metallurgy Steel with Ultrahigh Impact,Fatigue, and Tensile Properties

The impact toughness of powder metallurgy (PM) steel is typically inferior, and it is further impaired when the microstructure is strengthened. To formulate a versatile PM steel with superior impact, fatigue, and tensile properties, the influences of various microstructures, including ferrite, pearlite, bainite, and Ni-rich areas, were identified. The correlations between impact toughness with other mechanical properties were also studied. The results demonstrated that ferrite provides more resistance to impact loading than Ni-rich martensite, followed by bainite and pearlite. However, Ni-rich martensite presents the highest transverse rupture strength (TRS), fatigue strength, tensile strength, and hardness, followed by bainite, pearlite, and ferrite. With 74 pct Ni-rich martensite and 14 pct bainite, Fe-3Cr-0.5Mo-4Ni-0.5C steel achieves the optimal combination of impact energy (39 J), TRS (2170 MPa), bending fatigue strength at 2 × 10⁶ cycles (770 MPa), tensile strength (1323 MPa), and apparent hardness (38 HRC). The impact energy of Fe-3Cr-0.5Mo-4Ni-0.5C steel is twice as high as those of the ordinary high-strength PM steels. These findings demonstrate that a high-strength PM steel with high-toughness can be produced by optimized alloy design and microstructure.     

Preparation and properties of some ODS Fe-Cr-Al alloys

Several alloys based on Fe-25Cr-6Al and Fe-25Cr-11Al (wt pct) with additions of yttrium, Al₂O₃, and Y₂O₃ have been prepared by mechanical alloying of elemental, master alloy and oxide powders. The powders were consolidated by extrusion at 1000°C with a reduction ratio of 36:1. The resulting oxide contents were all approximately either 3 vol pct or 8 vol pct of mixed Al₂O₃-Y₂O₃ oxides or of Al₂O₃. The alloys exhibited substantial ductility at 600°C: an alloy containing 3 vol pct oxide could be readily warm worked to sheet without intermediate annealing; an 8 vol pct alloy required intermediate annealing at 1100°C. The 3 vol pct alloys could be recrystallized to produce large elongated grains by isothermal annealing of as-extruded material at 1450°C, but the high temperature strength properties were not improved. However, these alloys, together with some of the 8 vol pct materials, could be more readily recrystallized after rod (or sheet) rolling; sub-stantially improved tensile and stress rupture properties were obtained following 9 pct rod rolling at 620°C and isothermal annealing for 2 h at 1350°C. In this condition, the rup-ture strengths of selected alloys at 1000 and 1100°C were superior to those of competitive nickel-and cobalt-base superalloys. The oxidation resistance of all the alloys was ex-cellent. F. G. WILSON and C. D. DESFORGES, formerly with Fulmer Re-search Institute     

Development of Wear-Resistant Cu-10Cr-3Ag Electrical Contacts with Nano-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Dispersion by Mechanical Alloying and High Pressure Sintering

Cu-10Cr-3Ag (wt pct) alloy with nanocrystalline Al₂O₃ dispersion was prepared by mechanical alloying and consolidated by high pressure sintering at different temperatures. Characterization by X-ray diffraction and scanning electron microscopy or transmission electron microscopy shows the formation of nanocrystalline matrix grains of about 40 nm after 25 hours of milling with nanometric (<20 nm) Al₂O₃ particles dispersed in it. After consolidation by high pressure sintering (8 GPa at 400 °C to 800 °C), the dispersoids retain their ultrafine size and uniform distribution, while the alloyed matrix undergoes significant grain growth. The hardness and wear resistance of the pellets increase significantly with the addition of nano-Al₂O₃ particles. The electrical conductivity of the pellets without and with nano-Al₂O₃ dispersion is about 30 pct IACS (international annealing copper standard) and 25 pct IACS, respectively. Thus, mechanical alloying followed by high pressure sintering seems a potential route for developing nano-Al₂O₃ dispersed Cu-Cr-Ag alloy for heavy duty electrical contact.     

Oxidation Behavior of Nb-20Mo-15Si-25Cr and Nb-20Mo-15Si-25Cr-5B Alloys

Nb-20Mo-15Si-25Cr (25Cr alloy) and Nb-20Mo-15Si-25Cr-5B (25Cr/5B alloy) alloys have been subjected to oxidation in air for 24 hours from 973 K to 1673 K (700 °C to 1400 °C). Even though B additions do not improve oxidation resistance at temperatures higher than 1473 K (1200 °C), the lower temperature oxidation resistance is superior with B by influencing the microstructure. Porous oxide scale development at lower temperatures has been attributed to the dominant growth of Nb₂O₅ and the vaporization of MoO₃. An intermediate oxidation layer is developed between the scale and the metal for the 25Cr/5B alloy at temperatures above 1173 K (900 °C). Scale densification at elevated temperatures results in higher stress development as a result of the mismatch of coefficients of thermal expansion, ultimately resulting in oxide spallation.     

Tensile and creep properties of the experimental oxide dispersion strengthened iron-base sheet alloy MA-956E at 1365 K

A study of the 1365 K tensile properties, creep characteristics and residual room temperature properties after creep testing of the experimental oxide dispersion strengthened iron-base alloy MA-956E (Fe-20Cr-4.5Al-0.5Ti-0.5Y₂O₃) was conducted. The 1365 K tensile properties, particularly ductility, are strongly dependent on strain rate. It appears that MA-956E does not easily undergo slow plastic deformation. Rather than deform under creep loading conditions, the alloy apparently fails by a crack nucleation and growth mechanism. Fortunately, there appears to be a threshold stress below which crack nucleation andJor growth does not occur.     

HRTEM Study of Irradiation-Induced Cavities in Oxide-Dispersed Ferritic Steel

Structures of oxide nanoparticles and the effects of matrix/nanoparticle interfaces on irradiation-induced cavity nucleation and distribution in Fe-16Cr-4.5Al-0.3Ti-2W-0.37Y₂O₃ oxide-dispersed ferritic steel have been studied using high-resolution transmission electron microscopy techniques. The frequent observations of partially crystallized complex-oxide nanoparticles in as-fabricated steel provide an implication into the formation mechanism of nanoparticles. The mechanism involves the solid-state mixing of pre-alloyed metallic powder and Y₂O₃ powder to form an amorphous solid solution and from which the nucleation of high density complex-oxide nanoparticles (on the order of ~1 × 10²² m^?3). Simultaneous dual ion beams consisting of iron and helium were employed to irradiate the oxide-dispersed steel at 698 K (425 °C). The result shows that the defective oxide nanoparticles have a positive effect on the mitigation of dimensional swelling as a result of the preferred nucleation of helium-filled cavities at the matrix/nanoparticle interfaces.     

Effect of Y2O3 on Spark Plasma Sintering Kinetics of Nanocrystalline 9Cr-1Mo Ferritic Oxide Dispersion-Strengthened Steels

Nanocrystalline mechanically alloyed powders of 9Cr-1Mo ferritic steels with and without yttria dispersoids were densified using spark plasma sintering (SPS) to near-theoretical density at a temperature of 1073 K (800 °C). Studies on densification behaviour revealed that steels with dispersoids densified faster when compared to Fe-9Cr-1Mo steel. The evaluation of densification mechanisms during SPS reveals that grain boundary and lattice diffusion to be predominant at relative densities ranging from >0.7 to 0.9 in both the alloys.     

Structural Studies of Dispersoids in Fe–15 wt% Y<Subscript>2</Subscript>O<Subscript>3</Subscript>–5 wt% Ti Model ODS Alloys During Milling and Subsequent Annealing

Oxide dispersion strengthened (ODS) steels have very high thermal stability and creep resistance due to reinforcement of hard and stable nano-sized ceramic dispersoids in metallic matrix which act as barriers to dislocation motion. This study established the role of Ti in the structural evolution of yttria during mechanical milling and subsequent annealing in a Fe–15 wt% Y₂O₃–5 wt% Ti model ODS alloy, using electron microscopy and XRD techniques. The alloy was synthesized in a high energy planetary ball mill in Ar atmosphere by varying the milling durations in the range of 0 (un-milled) to 60 h. The XRD result revealed amorphisation of Y₂O₃/Ti during milling and evolution of YTiO₃ complex oxide upon annealing at 1273 K for 1 h. The electron microscopy studies revealed the refinement of alloy powders from ~50  μm to few nanometers during milling. Electron diffraction analysis and high resolution transmission electron microscopy of 60 h milled as well as and annealed powder showed formation of different types of Y–Ti–O complex oxides such as Y₂Ti₂O₇, Y₂TiO₅ and YTiO₃.     

Fatigue Crack Growth Behaviour of Conventional and Modified IN 718 Superalloys at 650 °C

In the present investigation, the fatigue crack growth behaviour of modified IN 718 superalloy (Ni-0.02 %, C-19.04 %, Cr-19.31 %, Fe-3.04 %, Mo-4.73 %, Nb-1.01 %, Al-1.16 %, Ti-0.0033 %, B, all in wt%) has been compared with conventional IN 718 superalloy (Ni-0.02 %, C-19.0 %, Cr-19.35 %, Fe-3.0 %, Mo-5.10 %, Nb-0.50 %, Al-1.00 %, Ti-0.0033 %, B, all in wt%) at 650 °C. Modified IN 718 superalloy exhibits marginally lower crack growth rate as compared to conventional alloy and was attributed to roughness induced crack closure.     

Phase Evolution and Mechanical Properties of Nano-TiO2 Dispersed Zr-Based Alloys by Mechanical Alloying and Conventional Sintering

The present study deals with the synthesis of 1.0 to 2.0 wt pct nano-TiO₂ dispersed Zr-based alloy with nominal compositions 45.0Zr-30.0Fe-20.0Ni-5.0Mo (alloy A), 44.0Zr-30.0 Fe-20.0Ni-5.0Mo-1.0TiO₂ (alloy B), 44.0Zr-30.0Fe-20.0Ni-4.5Mo-1.5TiO₂ (alloy C), and 44.0Zr-30.0Fe-20.0Ni-4.0Mo-2.0TiO₂ (alloy D) by mechanical alloying and consolidation of the milled powders using 1 GPa uniaxial pressure for 5 minutes and conventional sintering at 1673 K (1400 °C). The microstructural and phase evolution during each stage of milling and the consolidated products were studied by X-ray diffraction (XRD), scanning electron microscopy and transmission electron microscopy (TEM), and energy-dispersive spectroscopy. The particle size of the milled powder was also analyzed at systemic intervals during milling, and it showed a rapid decrease in particle size in the initial hours of milling. XRD analysis showed a fine crystallite size of 10 to 20 nm after 20 hours of milling and was confirmed by TEM. The recrystallization behavior of the milled powder was studied by differential scanning calorimetry. The hardness of the sintered Zr-based alloys was recorded in the range of 5.1 to 7.0 GPa, which is much higher than that of similar alloys, developed via the melting casting route.     

Improving High-Temperature Strength of Ti-6Al-2.5Mo-1.5Cr-0.5Fe-0.3Si Titanium Alloy by Cryogenic Pre-treatment and Laser Peening

Metallurgical and Materials Transactions A - The plastic deformation behavior and microscopic strengthening mechanism of Ti-6Al-2.5Mo-1.5Cr-0.5Fe-0.3Si (TC6) titanium alloy treated by the...     

Electrochemical polarization and passive film of a cryogenic and non-magnetic steel in aqueous solution

The corrosion behaviour of the austenitic steel Fe-23Mn-4Al-5Cr-0.3C in different aqueous solutions of pH-0.8 to 15.3 and the corrosion protection mechanism induced by adding Al or Al and Cr have been investigated by electrochemical measurements and AES/XPS analysis. The corrosion behaviour of Fe-Mn base steel have been compared with those of mild steel, cryogenic 9 % Ni steel, stainless steels 1Cr13 and 1Cr18Ni9Ti. The addition of manganese to mild steel is very detrimental to the corrosion resistance. Fe-25Mn steel passivates with difficulty even in such neutral aqueous electrolytes as 1 M Na₂SO₄ solution. The addition of 5 % aluminum to Fe-25Mn steel confers passivity to the steel in neutral or oxidizing, chloride-free solution. The addition of 5 % Cr to Fe-Mn-Al steel further improves resistance to corrosion. The passivity of Fe-23.5Mn-4Al-5Cr-0.3C steel in aqueous electrolytes tested is superior to that of 9 % Ni steel and approximate to that of 1Cr13 stainless steel. The corrosion resistance is probably imparted by a thin barrier film of oxides. The outer part of the passive film formed on the surface of Fe-23.5Mn-4Al-5Cr-0.3C steel in 1 M Na₂SO₄ solution is enriched in Al³⁺, Cr³⁺ and Fe³⁺, and this means that the film is probably made up of a mixture of Al₂O₃, Cr₂C₃ and Fe₂O₃.     

Dry Sliding Wear Behavior of Ti-6.5Al-3.5Mo-1.5Zr-0.3Si Alloy

Dry sliding wear tests were performed for Ti-6.5Al-3.5Mo-1.5Zr-0.3Si alloy against AISI 52100 steel under the loads of 50 to 250 N at 298 K to 873 K (25 °C to 600 °C). The wear behavior of the alloy varied with the change of test conditions. More or less tribo-oxides TiO₂ and Fe₂O₃ formed on worn surfaces under various conditions. At lower temperature [298 K to 473 K (25 °C to 200 °C)], less and scattered tribo-oxide layers did not show wear-reduced effect. As more number of and continuous tribo-oxide layers appeared at higher temperatures [773 K to 873 K (500 °C to 600 °C)], the wear rate would be substantially reduced. It can be suggested that Ti-6.5Al-3.5Mo-1.5Zr-0.3Si alloy possessed excellent wear resistance at 773 K to 873 K (500 °C to 600 °C). The wear-reduced effect of tribo-oxides seemed to depend on the appearance of Fe₂O₃ and the amount of tribo-oxides.