Microstructural Evolutions During Thermal Aging of Alloy 625: Impact of Temperature and Forming Process

The microstructural evolutions occurring upon thermal aging of alloy 625 sheets were studied in the 823 K to 1173 K (550 °C to 900 °C) temperature range and for durations up to 2000 hours. TTT diagrams of the δ and γ″ phases were established based on high-resolution scanning electron microscopy and associated quantitative image analysis approaches. The evolutions of secondary carbide volume fraction were also characterized. It was observed that the precipitation domains of the γ″ and δ phases are, respectively, 823 K to 1023 K (550 °C to 750 °C) and 923 K to 1173 K (650 °C to 900 °C) and that the γ″ coarsening follows the LSW theory once these particles have an ellipsoidal morphology. The onset of grain growth, accompanied with an increase of the texture index, was observed at a temperature as low as 1173 K (900 °C). It results from the progressive dissolution of grain boundaries’ secondary carbides (especially M₆C carbides) at this temperature, a process that favors a greater mobility of grain boundaries. It is also shown that the forming process (shear spinning), even after a relaxation heat treatment, enhances and stabilizes the precipitation of the δ phase compared to as-rolled + solution heat-treated sheets. It hence slows down the precipitation of the γ″ phase, a result that is in good agreement with a thermal aging that was performed under load (i.e., during a creep test).     

Characterization of Continuous and Discontinuous Precipitation Phases in Pd-Rich Precious Metal Alloys

Aberration-corrected scanning transmission electron microscopy (AC-STEM), X-ray diffraction (XRD), electron backscatter diffraction, and electron probe microanalysis were applied to characterize continuous and discontinuous phase formation in precious metal alloys used in electrical contacts. The Pd-rich Paliney^® (^®Paliney is tradename of Deringer-Ney Inc., Bloomfield, CT) alloys contain Pd, Ag, Cu, Au, Pt (and Zn or Ni). With aging at 755 K (482 °C), nanometer-scale chemistry modulation was observed indicating spinodal decomposition. An ordered body-centered tetragonal (bct) structure was also observed with AC-STEM after the 755 K (482 °C) aging treatment and another phase, tentatively identified as β-Cu₃Pd₄Zn, was found by microscopy and XRD after prolonged holds at higher temperatures. During slow cooling or isothermal holds at high temperature [755 K to 973 K (482 °C to 700 °C)], a two-phase lamellar structure develops along grain boundaries by discontinuous precipitation. XRD and AC-STEM showed that the lamellar structure was comprised of Ag-rich and Cu-rich fcc phases (α ₁ and α ₂). The phases are discussed in relation to a pseudo-ternary diagram based on Ag-Cu-Pd, which provides a simplified representation of the discontinuous phase compositions in the multi-component alloy system.     

Development of a High-Strength Ultrafine-Grained Ferritic Steel Nanocomposite

This article describes the microstructural and mechanical properties of 12YWT oxide-dispersion-strengthened (ODS)-ferritic steel nanocomposite. According to the annealing results obtained from X-ray diffraction line profile analysis on mechanically alloyed powders milled for 80 hours, the hot extrusion at 1123 K (850 °C) resulted in a nearly equiaxed ultrafine structure with an ultimate tensile strength of 1470 MPa, yield strength of 1390 MPa, and total elongation of 13 pct at room temperature comparable with high-strength 14YWT ODS steel. Maximum total elongation was found at 973 K (600 °C) where fractography of the tensile specimen showed a fully ductile dimple feature compared with the splitting cracks and very fine dimpled structure observed at room temperature. The presence of very small particles on the wall of dimples at 1073 K (800 °C) with nearly chemical composition of the matrix alloy was attributed to the activation of the boundaries decohesion mechanism as a result of diffusion of solute atoms. The results of Charpy impact test also indicated significant improvement of transition temperature with respect to predecessor 12YWT because of the decreased grain size and more homogeneity of grain size distribution. Hence, this alloy represented a good compromise between the strength and Charpy impact properties.     

Effect of Nd Additions on Extrusion Texture Development and on Slip Activity in a Mg-Mn Alloy

Two Mg-1 wt pct Mn alloys containing 0.5 wt pct and 1 wt pct Nd have been processed by indirect extrusion at temperatures ranging from 548 K (275 °C) to 633 K (360 °C) and speeds between 2.8 and 11 mm/s. The microstructure and the texture of the extruded bars were analyzed in order to understand the effect of the processing parameters and of the rare-earth (RE) alloying additions on the texture development. Increasing the Nd content results in weak textures in which the predominant orientations are a function of the extrusion conditions. This may be explained by the occurrence of particle pinning of grain boundaries and by the nucleation of grains with a wider range of orientations. Mechanical tests were carried out in tension and in compression in all the processed samples at 10^?3 s^?1 and room temperature. It was found that larger RE amounts give rise to the disappearance of the yield asymmetry and to an anomalously high activity of tensile twinning, especially at the lowest extrusion temperatures. This has been attributed to an increase of the critical resolved shear stress of basal slip due to the presence of Mg₃Nd coherent and semi-coherent intermetallic prismatic plates.     

Effect of Thermal Aging on Ductile-Brittle Transition Temperature of Modified 9Cr-1Mo Steel Evaluated with Reference Temperature Approach Under Dynamic Loading Condition

The effect of thermal aging on the ductile-brittle transition behavior has been assessed for a modified 9Cr-1Mo steel (P91) using the reference temperature approach under dynamic loading condition (T ₀ ^dy ). The steel in normalized and tempered (NT) condition and in different levels of subsequent cold work (CW) was subjected to thermal aging at temperatures of 873 K and 923 K (600 °C and 650 °C) for 5000 and 10,000 hours. For the NT and all the cold work conditions of the starting material, a drastic increase in T ₀ ^dy has been noticed after aging at 923 K (650 °C) for 10,000 h. A moderate increase was observed for the NT steel aged at 873 K (600 °C) for 5000 hours and for the 10 pct CW steel aged at 873 K (600 °C) for 10,000 h. A detailed transmission electron microscope (TEM) study of the embrittled materials aged at 923 K (650 °C)/10,000 hours and 873 K (600 °C)/10,000 hours has indicated presence of hexagonal Laves phase of Fe₂(Mo,Nb) type with different size and spatial distributions. The increase in the T ₀ ^dy is attributed to the embrittling effect of a network of Laves phase precipitates along the grain boundaries.     

Diffusion Bonding of 17-4 Precipitation Hardening Stainless Steel to Ti Alloy With and Without Ni Alloy Interlayer: Interface Microstructure and Mechanical Properties

In the present study, the diffusion bonding of 17-4 precipitation hardening stainless steel to Ti alloy with and without nickel alloy as intermediate material was carried out in the temperature range of 1073 K to 1223 K (800 °C to 950 °C) in steps of 298 K (25 °C) for 60 minutes in vacuum. The effects of bonding temperature on interfaces microstructures of bonded joint were analyzed by light optical and scanning electron microscopy. In the case of directly bonded stainless steel and titanium alloy, the layerwise α-Fe + χ, χ, FeTi + λ, FeTi + β-Ti phase, and phase mixture were observed at the bond interface. However, when nickel alloy was used as an interlayer, the interfaces indicate that Ni₃Ti, NiTi, and NiTi₂ are formed at the nickel alloy-titanium alloy interface and the PHSS-nickel alloy interface is free from intermetallics up to 1148 K (875 °C) and above this temperature, intermetallics were formed. The irregular-shaped particles of Fe₅Cr₃₅Ni₄₀Ti₁₅ have been observed within the Ni₃Ti intermetallic layer. The joint tensile and shear strength were measured; a maximum tensile strength of ~477 MPa and shear strength of ~356.9 MPa along with ~4.2 pct elongation were obtained for the direct bonded joint when processed at 1173 K (900 °C). However, when nickel base alloy was used as an interlayer in the same materials at the bonding temperature of 1148 K (875 °C), the bond tensile and shear strengths increase to ~523.6 and ~389.6 MPa, respectively, along with 6.2 pct elongation.     

Structure and Properties of Nano-Scale Oxide-Dispersed Iron

Bulk samples of pure iron and yttria dispersed iron with and without titanium (i.e., Fe, Fe-Y₂O₃, and Fe-Y₂O₃-Ti) were prepared by hot extrusion of high-energy ball-milled powders. An examination of the microstructure using TEM revealed that the addition of titanium resulted in the reduction of the dispersoid size with a concomitant increase in the volume fraction of the dispersoids. As a result, Fe-Y₂O₃-Ti exhibited a substantial increase in hardness and tensile properties as compared to Fe and Fe-Y₂O₃. The higher hardness and strength of Fe-Y₂O₃-Ti is shown to be due to the presence of finer and higher number density of Y-Ti-O complex oxides. Dynamic strain aging in the temperature range of 423 K to 573 K (150 °C to 300 °C) was observed in all the compositions studied.     

Reduction Behavior of Panzhihua Titanomagnetite Concentrates with Coal

The reduction behavior of the Panzhihua titanomagnetite concentrates (PTC) briquette with coal was investigated by temperature-programmed heating under argon atmosphere in a vertical tube electric furnace. The mass loss behavior of the PTC-coal mixture was checked by thermogravimetric analysis method in argon with a heating rate of 5 K (5 °C)/ min. It was found that there are five stages during the carbothermic reduction process of the PTC. The devolatilization of coal occurred in the first stage, and reductions of iron oxides mainly occurred in the second and third stages. The reduction rate of iron oxide in the third stage was much higher than that in the second stage because of the significant rate of carbon gasification reaction. The iron in the ilmenite was reduced in the fourth stage. In the final stage, the rutile was partially reduced to lower valence oxides. The phase transformation of the briquette reduced at different temperatures was investigated by X-ray diffraction (XRD). The main phases of sample reduced at 1173 K (900 °C) are metallic iron, ilmenite (FeTiO₃), and titanomagnetite (Fe_3–x Ti _x O₄). The traces of rutile (TiO₂) were observed at 1273 K (1000 °C). The iron carbide (Fe₃C) and ferrous-pseudobrookite (FeTi₂O₅) appeared at 1473 K (1200 °C). The titanium carbide was found in the sample reduced at 1623 K (1350 °C). The shrinkages of reduced briquettes, which increased with increase in the temperature, were found to depend greatly on the temperature. With increasing the reduction temperature to 1573 K (1300 °C), the iron nuggets were observed outside of the samples reduced. The nugget formation can indicate a new process of ironmaking with titanomagnetite similar to ITmk3 (Ironmaking Technology Mark 3).     

Acceleration or Suppression of α-Phase Precipitation Using Isothermal ω Phase in Ti-20 at.pct Nb Alloy

Homogeneous precipitation of a fine α phase in the β matrix of Ti alloys is a promising method for obtaining a highly strengthened Ti-based alloy. Isothermal ω particles are known to be the nucleation sites for fine α-phase precipitation, but an understanding of the kinetics of α-phase formation on isothermal ω particles is still lacking. This study aimed to reveal the effect of isothermal ω particles on α-phase precipitation onset time. Two-step isothermal aging of a Ti-20 at.pct Nb alloy after solid solution treatment at 1273 K (1000 °C) was carried out. The first step of the aging at 633 K (360 °C) involved the formation of isothermal ω particles in the β matrix. This was followed by a second aging step at 673 K, 723 K, and 773 K (400 °C, 450 °C, and 500 °C) for α-phase precipitation. Suppression of α-phase nucleation on the isothermal ω particles occurred at 673 K (400 °C), whereas acceleration of α-phase nucleation on the isothermal ω particles was observed at 723 K and 773 K (450 °C and 500 °C). Thermodynamic stability of the isothermal ω particles and solute partitioning were controlling factors for the α-phase precipitation kinetics.     

Effects of Thermomechanical Processing and Heat Treatment on the Tensile and Creep Properties of Boron-Modified Near Alpha Titanium Alloy Ti-1100

Tensile properties of boron-modified near alpha titanium alloy were evaluated in ??-??- and ??-processed conditions after heat treatments at different solution treatment temperatures at both room temperature and 873 K (600 °C). Creep behavior was also investigated in these processing and heat-treatment conditions at 873 K (600 °C). While the yield strength (YS) and the ultimate tensile strength (UTS) did not show significant dependence on the processing history, a marginal improvement in elongation-to-failure values were observed in ??-??-processed condition as compared with ??-processed condition at both temperatures. Creep resistance of the alloy at 873 K (600 °C) was found to be significantly superior in ??-processed condition as compared with ??-??-processed condition.     

Kinetics of Delta-Ferrite to Austenite Phase Transformation in a Two-Phase Fe-Al-C Alloy

The kinetics of delta-ferrite to austenite phase transformation was investigated using a quenching dilatometer in a Fe-Al-C alloy. The results showed that the austenite phase nucleated along the delta grain boundaries. The transformed austenite morphology changed from cellular to Widmanstätten pattern when the holding temperature decreased from 1398 K to 1123 K (1125 °C to 850 °C). Full partitioning of the substitutional alloying elements was observed and the spacing of the austenite plates was controlled by the diffusing distance of the substitutional elements. Interestingly, growth of the austenite front was controlled by the long-range diffusion of carbon from the center of the delta grains to the growing front. Deformation of the delta phase enhanced the nucleation of austenite at existing grain boundaries and newly formed subgrain boundaries.     

The Influence of Precipitation of Alpha2 on Properties and Microstructure in TIMETAL 6-4

Samples of Hot Isostatically Pressed (HIPped) powder of TIMETAL 6-4 (Ti-6Al-4V, compositions in wt pct unless indicated), which was HIPped at 1203 K (930 °C), and of forged bar stock, which was slowly cooled from above the beta transus, were both subsequently held at 773 K (500 °C) for times up to 5 weeks and analyzed using scanning and transmission electron microscopy and atom probe analysis. It has been shown that in the samples aged for 5 weeks at 773 K (500 °C), there is a high density of alpha2 (α₂, an ordered phase based on the composition Ti₃Al) precipitates, which are typically 5 nm in size, and a significantly smaller density was present in the slowly cooled samples. The fatigue and tensile properties of samples aged for 5 weeks at 773 K (500 °C) have been compared with those of the HIPped powder and of the forged samples which were slowly cooled from just above the transus, and although no significant difference was found between the fatigue properties, the tensile strength of the aged samples was 5 pct higher than that of the as-HIPped and slowly cooled forged samples. The ductility of the forged samples did not decrease after aging at 773 K (500 °C) despite the strength increase. Transmission electron microscopy has been used to assess the nature of dislocations generated during tensile and fatigue deformation and it has been found that not just is planar slip observed, but dislocation pairs are not uncommon in samples aged at 773 K (500 °C) and some are seen in slowly cooled Ti6Al4V.     

Partitioning of Calcium Between Liquid Silver and Liquid Iron

The partitioning of calcium between liquid silver and liquid iron at 1823 K and 1873 K (1550 °C and 1600 °C) was studied experimentally using a closed molybdenum container. The calcium potential in the container was controlled by the composition of the alloys in equilibrium. The results agreed well with previous experimental measurements and indicated that the effect of temperature was not very pronounced in the temperature range studied.     

Effect of Extrusion Temperature on the Plastic Deformation of an Mg-Y-Zn Alloy Containing LPSO Phase Using <Emphasis Type="Italic">In Situ</Emphasis> Neutron Diffraction

The evolution of the internal strains during in situ tension and compression tests has been measured in an MgY₂Zn₁ alloy containing long-period stacking ordered (LPSO) phase using neutron diffraction. The alloy was extruded at two different temperatures to study the influence of the microstructure and texture of the magnesium and the LPSO phases on the deformation mechanisms. The alloy extruded at 623 K (350 °C) exhibits a strong fiber texture with the basal plane parallel to the extrusion direction due to the presence of areas of coarse non-recrystallised grains. However, at 723 K (450 °C), the magnesium phase is fully recrystallised with grains randomly oriented. On the other hand, at the two extrusion temperatures, the LPSO phase orients their basal plane parallel to the extrusion direction. Yield stress is always slightly higher in compression than in tension. Independently on the stress sign and the extrusion temperature, the beginning of plasticity is controlled by the activation of the basal slip system in the dynamic recrystallized grains. Therefore, the elongated fiber-shaped LPSO phase which behaves as the reinforcement in a metal matrix composite is responsible for this tension–compression asymmetry.     

Experimental Investigation and Analytical Prediction of σ-Phase Precipitation in AISI 316L Austenitic Stainless Steel

The phase precipitation in industrial AISI 316L stainless steel during aging for up to 80,000 hours between 823 K and 1073 K (550 °C and 800 °C) has been studied using transmission electron microscopy, scanning transmission electron microscopy, and carbon replica energy-dispersive X-ray microanalysis. Three phases were identified: Chromium carbides (M₂₃C₆), Laves phase (η), and σ-phase (Fe-Cr). M₂₃C₆ carbide precipitation occurred firstly and was followed by the η and σ-phases at grain boundaries when the aging temperature is higher than 873 K (600 °C). Precipitation and growth of M₂₃C₆ create chromium depletion zones at the grain boundaries and also retard the σ-phase formation. Thus, the σ-phase is controlled by the kinetic of chromium bulk diffusion and can appear only when the chromium reaches, at grain boundaries and at the M₂₃C₆/γ and M₂₃C₆/η/γ interfaces, content higher than a critical value obtained by self-healing. An analytical model, based on equivalent chromium content, has been established in this study and successfully validated to predict the time–temperature–precipitation diagram of the σ-phase. The obtained diagram is in good agreement with the experimental results.     

Thermal Behavior and Phase Transformation of TiO2 Nanocrystallites Prepared by a Coprecipitation Route

TiO₂ freeze-dried precursor powders were synthesized using a coprecipitation route that includes titanium tetrachloride (TiCl₄) as initial material prepared at 348 K (75 °C) and pH 7. Differential scanning calorimetry/thermogravimetry (DSC/TG), X-ray diffraction (XRD), transmission electron microscopy (TEM), and selected area electron diffraction (SAED) and high resolution TEM were utilized to characterize the thermal behavior and phase transformation of the TiO₂ freeze-dried precursor powders after calcination. The main compound of the TiO₂ freeze-dried precursor powders was TiO₂·H₂O based on a TG analysis conducted at a heating rate of 20 K (20 °C)/min. The anatase TiO₂ (a-TiO₂) first appeared at 473 K (200 °C), then from a-TiO₂ transformed to rutile TiO₂ (r-TiO₂) at 773 K (500 °C). The activation energy of a-TiO₂ formation from TiO₂ freeze-dried precursor powders was 242.4 ± 33.9 kJ/mol, whereas, the activation energy of phase transformation from a-TiO₂ to r-TiO₂ was 267.5 ± 19.1 kJ/mol. The crystallite size of a-TiO₂ grew from 3.5 to 23.2 nm when raising the calcination temperature from 473 K to 873 K (200 °C to 600 °C). In addition, the crystallite size of r-TiO₂ increased from 17.4 to 48.1 nm when calcination temperature increased from 773 K to 1073 K (500 °C to 800 °C).     

Transition in Failure Mechanism Under Cyclic Creep in 316LN Austenitic Stainless Steel

Cyclic creep behavior of a type 316LN austenitic stainless steel was investigated in the temperature range from 823 K to 923 K (550 °C to 650 °C). A transition from fatigue-dominated to creep-dominated failure mode was observed with an increase in the mean stress. The threshold value of mean stress for the transition was seen to be a strong function of the test temperature. Occurrence of dynamic strain aging proved beneficial owing to a substantial reduction in the strain accumulation during cyclic loading.     

Cryogenic Mechanical Properties of Warm Multi-Pass Caliber-Rolled Fine-Grained Titanium Alloys: Ti-6Al-4V (Normal and ELI Grades) and VT14

The effect of microstructural refinement and the β phase fraction, V _β, on the mechanical properties at cryogenic temperatures (up to 20 K) of two commercially important aerospace titanium alloys: Ti-6Al-4V (normal as well as extra low interstitial grades) and VT14 was examined. Multi-pass caliber rolling in the temperature range of 973 K to 1223 K (700 °C to 950 °C) was employed to refine the microstructure, as V _β was found to increase nonlinearly with the rolling temperature. Detailed microstructural characterization of the alloys after caliber rolling was carried out using optical microscopy (OM), scanning electron microscopy (SEM), electron back-scatter diffraction (EBSD), and transmission electron microscopy (TEM). Complete spheroidization of the primary α laths along with formation of bimodal microstructure occurred when the alloys are rolled at temperatures above 1123 K (850 °C). For rolling temperatures less than 1123 K (850 °C), complete fragmentation of the β phase with limited spheroidization of α laths was observed. The microstructural refinement due to caliber rolling was found to significantly enhance the strength with no penalty on ductility both at room and cryogenic temperatures. This was attributed to a complex interplay between microstructural refinement and reduced transformed β phase fraction. TEM suggests that the serrated stress–strain responses observed in the post-yield deformation regime of specimens tested at 20 K were due to the activation of \( \left\{ {10\bar{1}2} \right\} \) tensile twins.