Effect of Temperature and Composition on NiAl Precipitation and Morphology in Fe-Ni-Al Alloys

NiAl-strengthened ferritic alloys have been of particular interest because of their possibilities as a high-temperature material for power-generation purposes. In the present work, the effect of temperature and composition on the precipitation of the NiAl (β′) phase was studied using a diffusion couple made of alloys Fe_0.50Ni_0.25Al_0.25 and Fe_0.80Ni_0.08Al_0.12. The composition gradient was obtained with a diffusion annealing treatment at 1373 K (1100°C), and the precipitation was promoted by aging at 1123 K, 1173 K and 1223 K (850 °C, 900 °C and 950 °C) for 5 hours. The formation of a supersaturated solid solution and the precipitation of the β′ phase were obtained after the diffusion annealing and aging treatments, respectively. A gradual increase in the size and volumetric fraction of the precipitates was evident in compositions with higher NiAl. This promoted changes in the precipitate morphology that followed the sequence: rounded cuboids → plates → irregular (maze-like). Compositions with a low β′ volumetric fraction followed the predicted size distributions of the Lifshitz-Slyozov-Wagner (LSW) theory. Finally, it was observed that the hardness tends to increase inversely to the Fe content and decreases only because of precipitate coarsening. The achieved results provide new information regarding the dependency of the morphology of β′ precipitates with composition and temperature.     

The effect of heat treatment on Mg<Subscript>2</Subscript>Si coarsening in aluminum 6105 alloy

The coarsening behavior of Mg₂Si intermetallic particles was studied during homogenization of aluminum 6105 alloy. An industrially homogenized 6105 aluminum alloy contained large globular undissolved Mg₂Si. With subsequent heat treatment at 590 °C over 1 week, it was found that the large Mg₂Si precipitates coarsened with time. The coarsening of the large Mg₂Si precipitates was quantified by statistical analysis of micrographs. A linear relationship between and time fits the experimental measurements quite well, indicating that the coarsening is diffusion controlled, with grain-boundary diffusion dominating volume diffusion.     

Microanalysis of age-hardening precipitates in aluminium alloys

The microdiffraction and X-ray emission spectroscopy facilities of a field emission gun STEM have been used to study precipitates formed on aging Al-1.16 wt% (Mg₂Si) and Al-1.16 wt% (Mg₂Ge) at 250°C after solution treatment. In the former, a new intermediate precipitate structure, β″ (a = 0.30 nm, b = 0.33 nm, c = 0.40 nm; α = β = 90°, γ = 71°: [001]_β′'//[001]_matrix [100]_β′' ^ [100]_matrix = 5°), formed after 3 h, has been observed and a previously suggested structure of hexagonal symmetry, formed after 5 h, has been confirmed. Magnesium concentrations in precipitates formed after 5 h in both alloys have been found to be substantially lower, relative to the other alloying element, than in the equilibrium precipitate (the atomic ratios observed being Mg_0,44Si and Mg_0.29Ge). It is suggested that, in the very early stages, precipitation is controlled by only one, the least soluble, of the two alloying elements.     

The thermodynamic activity of ZnO in silicate melts

The activity of ZnO in ZnO-SiO₂ and CaO-ZnO-SiO₂ melts has been measured at 1560 °C using a transpiration technique with CO-CO₂ mixtures as the carrier gas. The activities of ZnO in dilute solution in 42 wt pct SiO₂?38 wt pct CaO-20 wt pct A1₂O₃ in the range 1400° to 1550 °C and in 62 wt pct SiO₂?23.3 wt pct CaO?14.7 wt pct A1₂O₃ at 1550 °C have also been measured. The measured free energies of formation of ZnO-SiO₂ melts are significantly more negative than published estimated values and this, together with the behavior observed in the system CaO-Al₂O₃-SiO₂, indicate that ZnO is a relatively basic oxide. The results are discussed in terms of the polymerization model of binary silicate melts and ideal silicate mixing in ternary silicate melts. The behavior of ZnO in dilute solution in CaO-Al₂O₃-SiO₂ melts is discussed in terms of the possibility of the fluxing of ZnO by iron blast furnace slags.     

Melt-Processed Ni3Al matrix composites reinforced with TiC particles

TiC-reinforced Ni₃Al metal matrix composites have been investigated. The composites were prepared by in situ precipitation of either 0.01, 0.05, or 0.1 vol fraction TiC in molten Ni3Al. These precipitates are thermodynamically stable in the Ni₃Al at 1000 °C up to 50 hours. Although almost ideal precipitate distribution was achieved at low volume fraction of TiC, at higher volume fraction, there was a tendency toward agglomeration. Room and hightemperature mechanical testing showed significant improvement in modulus, yield strength, and ultimate tensile strength, while still maintaining a 2.5 pct elongation at 0.1 vol. fraction TiC and 500 °C. The density of precipitates (number/m³) is strongly dependent on the fraction of TiC, on temperature, and on the holding time at that temperature. The critical nucleation temperature for these precipitates was determined to be at 1475 °C. Experimentally evaluated growth rate constants indicate a diffusion-controlled coarsening mechanism along with coalescence and multiparticle interaction at higher volume fractions of TiC. This is in accordance with either the Lifshitz Slyozov Encounter Modified (LSEM) model or with the Voorhees and Glicksman (VG) model.     

Microstructural evolution during thermomechanical processing of a Ti-Nb interstitial-free steel just below the Ar 3 temperature

Laboratory thermomechanical processing (TMP) experiments have been carried out to study the austenite transformation characteristics, precipitation behavior, and recrystallization of deformed ferrite for an interstitial-free (IF) steel in the temperature range just below Ar ₃. For cooling rates in the range 0.1 °C s⁻¹ to 130 °C s⁻¹, austenite transforms to either polygonal ferrite (PF) or massive ferrite (MF). The transformation temperatures vary systematically with cooling rate and austenite condition. There is indirect evidence that the transformation rates for both PF and MF are decreased by the presence of substitutional solute atoms and precipitate particles. When unstable austenite is deformed at 850 °C, it transforms to an extremely fine strain-induced MF. Under conditions of high supersaturation of Ti, Nb, and S, (Ti,Nb) _x S _y precipitates form at 850 °C as coprecipitates on pre-existing (Ti,Nb)N particles and as discrete precipitates within PF grains. Pre-existing intragranular (Ti,Nb) _x S _y precipitates retard recrystallization and grain coarsening of PF deformed at 850 °C and result in a stable, recovered subgrain structure. The results are relevant to the design of TMP schedules for warm rolling of IF steels.     

Influence of various heat treatments on the microstructure of polycrystalline IN738LC

IN738LC is a modern, nickel-based superalloy utilized at high temperatures in aggressive environments. Durability of this superalloy is dependent on the strengthening of γ′ precipitates. This study focuses on the microstructural development of IN738LC during various heat treatments. The 1120 °C/2 h/accelerated air-cooled (AAC) solution treatment, given in the literature, already produces a bimodal precipitate microstructure, which is, thus, not an adequate solutionizing procedure to yield a single-phase solid solution in the alloy at the outset. However, the 1235 °C/4 h/water quenched (WQ) solution treatment does produce the single-phase condition. A microstructure with fine precipitates develops if solutionizing is carried out under 1200 °C/4 h/AAC conditions. Agings at lower temperatures after 1200 °C/4 h/AAC or 1250 °C/4 h/AAC or WQ conditions yield analogous microstructures. Agings below ∼950 °C for 24 hours yield nearly spheroidal precipitates, and single aging for 24 hours at 1050 °C or 1120 °C produces cuboidal precipitates. Two different γ′ precipitate growth processes are observed: merging of smaller precipitates to produce larger ones (in duplex precipitate-size microstructures) and growth through solute absorption from the matrix. Average activation energies for the precipitate growth processes are 191 and 350 kJ/mol in the ranges of 850 °C to 1050 °C and 1050 °C to 1120 °C, respectively, calculated using the precipitate sizes from microstructures in the WQ condition, and 150 and 298 kJ/mol in the analogous temperature ranges, calculated from precipitate sizes in the microstructures in the slow furnace-cooled condition.     

Influence of long- term aging and superimposed creep

In an effort to understand the effects of long-term aging at high temperatures and those of the superimposed creep stresses on the microstructural variations in a 0.50Cr-0.50Mo-0.25V steel, the shoulder and gage portions of the specimens subjected to stress rupture tests at 500 °C and 540 °C have been studied by transmission electron microscopy. The optical microstructure, in the normalized and tempered condition, consists of about 95 pct ferrite with the remainder bainite. The ferrite has an extremely fine precipitation of vanadium carbide (VC) as interphase precipitate as well as in random distribution. In the bainitic area, M₃C is present in the globular as well as plate-like morphology. Specimens ruptured at 500 °C and 540 °C were selected for transmission electron microscopic study. Both the interphase precipitates and randomly distrib- uted precipitates of VC coarsen considerably due to prolonged exposure at elevated tempera- tures, and ultimately fine M₂C platelets nucleate and grow on either side of these precipitates, resulting in the formation of the so-called H-carbides. The superimposed creep stress has a marginal effect on the coarsening kinetics of VC but has enhanced its precipitation along dis- locations and has promoted the formation of H-carbides.     

A Morphological Study of the Carburization/Reduction of Tungsten Oxides with Carbon Monoxide

Thermogravimetry, X-ray diffraction analysis and scanning electron microscopy have been used to study the gas phase carburization/reduction by CO of WO₃, W₁₈O₄₉ and WO₂ over the temperature range 600 to 1100°C. The lower oxides W₁₈O₄₉ and WO₂ were produced by controlled reduction of WO₃ in H₂-H₂O mixtures. A wide range of particle morphologies and sizes have been obtained for the WC product and these have been related to the roles played by the lower oxides. The total carbon content of the carburized products is shown to be dependent upon both the temperature and time of processing.     

The surface energy of metastable Al3Li precipitates from coarsening kinetics

Small angle X-ray scattering has been used to measure the coarsening kinetics of metastable Al₃Li precipitates in a dilute AlLi binary alloy. The results are compared to previous transmission electron microscopy studies of similar alloys. Lifschitz-Slyozov-Wagner coarsening theory was invoked as a means of obtaining the α/Al₃Li interfacial energy. However, corrections accounting for effects due to finite volume fractions and limited solubility of Li in the precipitate phase are included. The surface energy was found to be ∼0.005J/m², significantly less than previously reported. A possible explanation for the discrepancy is discussed.     

Effect of heat-treatment conditions on the structure and physicomechanical and chemical properties of a Ni-Cr-Cu-Ti maraging steel

The effects of quenching (from 950°C or from 950 and 850°C) and the aging conditions on the structure, properties, and delayed fracture (DF) of 03Kh11N10M2DT maraging steel has been studied by dilatometry, X-ray diffraction, and fracture tests. The DF-crack growth rate is maximal after aging at 400°C irrespective of the quenching conditions, and the corrosion rate is maximal after aging at 350–400°C in the case of single quenching and at 350°C after double quenching. The kinetics and mechanism of the early stages of the decomposition of a supersaturated α solid solution are investigated by electrical-resistance measurements and transmission electron microscopy. In the state after single quenching, aging occurs in two stages at all isothermal heat treatments; in the state after double quenching, aging occurs in one stage at a time exponent n = 0.2 in the Johnson-Mehl equation. Upon aging at 400°C, the intermediate ordered Fe₃(Ni,Ti) phase with a complex cubic lattice precipitates, and the intermetallic compound Ni₃Ti precipitates upon subsequent aging. Moreover, copper-rich ε-phase precipitates form only in the case of single quenching. The substantial increase in the crack growth rate during DF with n < 0.2 is likely to be caused by the formation of Guinier-Preston zones enriched in nickel and titanium.     

Structure and properties of rapidly solidified dispersion-strengthened titanium alloys: Part I. Characterization of dispersoid distribution,structure, and chemistry

The feasibility of developing dispersion-strengthened powder metallurgy Ti alloys was determined in Ti-RE (RE = Ce, Dy, Er, Gd, La, Nd, or Y) alloys prepared by rapid solidification processing. The alloys were produced by electron-beam melting and splat quenching. Dispersoid precipitation and growth were studied as functions of annealing temperature, 700 to 1000 °C, for annealing times between 5 and 50,000 minutes. Dispersoid diameters, spacings, compositions, and crystal structures were characterized by transmission and scanning electron microscopy, X-ray and electron diffraction, energy-dispersive X-ray analysis, and scanning Auger microscopy. Two classes of dispersoid coarsening behavior at temperatures below theβ-transus were identified. In Ti-Ce, Ti-Gd, and Ti-Nd alloys, equilibrium rare earth sesquioxide (RE₂O₃) dispersoids form early in the annealing process and coarsen rapidly to > 1 μm diameter. The Ti-Nd alloys additionally contain large volume fractions of small (< 100 nm diameter) dispersoids. In the other Ti-RE alloys, dispersoids identified as Ti-RE-O-C compounds coarsen relatively slowly. Ti-Er is the most promising of the investigated systems for application in a multicomponent dispersion-strengthened alloy because long-time annealing at 700 to 800 °C produces stable dispersoids of 50 to 150 nm average diameter and 300 to 600 nm inter-particle spacing.     

M2C precipitates in isothermal tempering of high Co-Ni secondary hardening steel

The effects of isothermal tempering on the coarsening behavior of hexagonal M₂C precipitates and the secondary hardening reaction in ultrahigh-strength AerMet 100 steel were investigated. The tempering temperatures were 468 °C, 482 °C, and 510 °C, and the tempering time spanned the range from 1 to 400 hours. Experimental studies of the coarsening behavior of the carbides were made by utilizing transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray diffractometry (XRD). The hardness at the secondary hardening peak was about HRc 55. The average length and diameter of M₂C carbides were 4 to 8 nm and 1.5 to 2.5 nm, respectively, at all three tempering temperatures; hence, the aspect ratio was almost 3, an equilibrium value in this case. The size of the M₂C carbides increased monotonically with time, but the growth kinetics did not exactly follow the classical coarsening behavior. The amount of precipitated austenite increased with tempering time and temperature. M₂C precipitates were still relatively fine even after 200 hours of tempering. This feature seemed to be closely related to the high hardness maintained after prolonged tempering.     

Phase Relations in CaO-SiO2-Al2O3-15 mass pct CaF2 System at 1523 K (1250 °C)

CaO-SiO₂-Al₂O₃-CaF₂ is a base system of mold flux for high Al steels. Phase equilibrium in CaO-SiO₂-Al₂O₃-15 mass pct CaF₂ system at 1523 K (1250 °C) was investigated using quenching method followed by X-ray diffraction and Scanning electron microscopy equipped with energy dispersive X-ray spectroscopy. Isothermal section in this system at 1523 K (1250 °C) with Al₂O₃ being less than 25 mass pct and CaO/SiO₂ (mass pct) being between 0.43 and 1.25 was experimentally constructed. The liquidus composition and seven solid-liquid coexistence regions at 1523 K (1250 °C) were determined.     

The effect of thermal exposure on the microstructure and mechanical properties of nickel-base superalloys

The three nickel-base superalloys B-1900, TRW-NASA VIA, and René 80 were studied utilizing metallographic and residue analysis techniques in conjuction with mechanical property tests to determine the effect of thermal exposure on the microstructure and mechanical properties. Exposure times of 10, 100, 1000, and 5000 h at temperatures from 1400 to 2000°F (760 to 1093°C) were evaluated. Four minor phases-MC, M₆C, M₂₃C₆, and M₃B₂-plus gamma-prime were observed in the gamma matrix of these alloys. Significant variations in the mechanical properties were observed to occur with thermal exposure. Microstructural evaluation indicated that these variations in properties were due primarily to gamma-prime agglomeration or ripening. Perturbations noted in a number of the mechanical property vs exposure temperature curves in the 1500 to 1900°F (816 to 1038°C) temperature range appeared to be due to the precipitation and growth of M₆C and/or M₂₃C₆ carbides.     

Phase Equilibria and Thermodynamic Studies in the System CaO−FeO−Fe2O3−SiO2

Phase equilibria and thermodynamic properties of the system CaO?FeO?Fe₂O₃?SiO₂ were studied at 1450° and 1550°C, over a range ofpO₂ from 1 to about 10^?11 atm. Isothermal phase diagrams and activity-composition diagrams were constructed for 0, 5, 10, 20, and 30 wt pct SiO₂ sections. The data are applicable to further understanding the behavior of simple BOF steelmaking slags.     

Rate of change of oxygen chemical potential in liquid PbO−SiO2 binary solution

The rate of the chemical potential change of oxygen in a liquid PbO?SiO₂ binary solution, with SiO₂ contents of 10, 20, and 30 mol pct, and in pure PbO, has been measured at temperatures of 900°, 950°, 1000°, 1050°, and 1100°C. The rate increased with temperature according to the Arrhenius type relation and decreased with the increase of the silica content. It is suggested that the rate-controlling step is the counter diffusion rate of Pb²⁺ and Pb⁴⁺ ions, which are considered to be the most easily movable ions in the PbO?SiO₂ solution. The relation between the rate of oxygen chemical potential change and the electrical conductivity is also discussed for the liquid PbO?SiO₂ system.     

Thermodynamic activity of Na2O in Na2O-CaO-SiO2, Na2O-MgO-SiO2, and Na2O-CaO-SiO2-Al2O3 melts at 1400°C

A gas phase equilibration technique was used to generate Na₂O isoactivity data at high-silica compositions in the systems Na₂O-CaO-SiO₂, Na₂O-MgO-SiO₂, and Na₂O-CaO-SiO₂-(10 and 20 wt pct) A1₂O₃ at 1400 °. Loga _{Na 2 O} values referenced to pure liquid Na₂O at 1400 ° as standard state ranged from about ?8.0 to ?7.0. Silica activities were calculated in the Na₂O-CaO-SiO₂ system using the Gibbs-Duhem equation. Richardson’s model for ideal mixing of basic oxides in silica was applied to the Na₂O-CaO-SiO₂ system. The model shows best fit when cationic mixing is assumed to occur on divalent sites. The alkali retention in slags has been described using a defined “sodium capacity”. The temperature variation of alkali retention was estimated, and the resulting sodium capacity was used to evaluate alkali stability in blast furnace slags.