Microstructural evolution in damaged IN738LC alloy during various steps of rejuvenation heat treatments

IN738LC is one of the superior nickel base superalloys utilized at high temperatures in aggressive environments. However, experiencing high temperatures and stresses during service causes microstructure deterioration and degradation of mechanical properties in this alloy. To restore the microstructure and mechanical properties of the degraded alloy, rejuvenation heat treatments can be considered. In this study, the evolution of microstructural features in a creep damaged IN738LC superalloy during different stages of rejuvenation heat treatment cycles was investigated. During solution treatment stage, dissolution of coarsened γ′ precipitates, grain boundary films and transition zone around primary MC carbides took place. Solution treatment at high temperature led to lower volume fraction and smaller size of remnant γ′ precipitates and further dissolution of grain boundary films and caused MC carbides to be remained without the surrounding transition zone. In addition, fine γ′ precipitates formed during the subsequent cooling, namely cooling precipitates, were detected in higher contents after solutionizing at higher temperatures. It was found that slower cooling rates after solution treatment gave rise to coarser γ′ precipitates with higher volume fraction. Among all heat treatment cycles investigated, double solution treatment at 1190 °C/4 h/FC (furnace cooling) + 1120 °C/2 h/AC (air cooling) followed by aging at 845 °C/24 h/AC was successful to revert the microstructure back to its virgin state.     

Microstructural evolution of a Ni-based superalloy (617B) at 700 °C studied by electron microscopy and atom probe tomography

We report on the microstructural evolution of a polycrystalline Ni-based superalloy (Alloy 617B) for power plant applications at a service temperature of 700 °C. The formation of secondary M₂₃C₆-carbides close to grain boundaries (GBs) and around primary Ti(C,N) particles is observed upon annealing at 700 °C, where γ′ is found to nucleate heterogeneously at M₂₃C₆ carbides. Using atom probe tomography, elemental partitioning to the phases and composition profiles across phase and grain boundaries are determined. Enrichments of B at γ/M₂₃C₆ and γ′/M₂₃C₆ interfaces as well as at grain boundaries are detected, while no B enrichment is found at γ/γ′ interfaces. It is suggested that segregation of B in conjunction with γ′ formation stabilizes a network of secondary M₂₃C₆ precipitates near GBs and thus increases the creep rupture life of Alloy 617B. Calculations of the equilibrium phase compositions by Thermo-Calc confirm the chemical compositions measured by atom probe tomography.     

Hot corrosion and oxidation behavior of a novel Pt + Hf-modified γ′-Ni3Al + γ-Ni-based coating

A new type of Pt + Hf-modified γ′-Ni₃Al + γ-Ni-based coating has been developed in which deposition involves Pt electroplating followed by combined aluminizing and hafnizing using a pack cementation process. Cyclic oxidation testing of both Pt + Hf-modified γ′ + γ and Pt-modified β-NiAl coatings at 1150 °C (2102 °F), in air, resulted in the formation of a continuous and adherent α-Al₂O₃ scale; however, the latter developed unwanted surface undulations after thermal cycling. Type I (i.e. 900 °C/1652 °F) and Type II (i.e. 705 °C/1300 °F) hot corrosion behavior of the Pt + Hf-modified γ′ + γ coating were studied and compared to Pt-modified β and γ + β-CoCrAlY coatings. Both types of hot corrosion conditions were simulated by depositing Na₂SO₄ salt on the coated samples and then exposing the samples to a laboratory-based furnace rig. It was found that the Pt + Hf-modified γ′ + γ and Pt-modified β coatings exhibited superior Type II hot corrosion resistance compared to the γ + β-CoCrAlY coating; while the Pt + Hf-modified γ′ + γ and γ + β-CoCrAlY coatings showed improved Type I hot corrosion performance than the Pt-modified β.     

Effect of cooling rate on microstructure and tensile properties of powder metallurgy Ni-based superalloy

The effects of size distribution, morphology and volume fraction of γ′ phase and grain size on tensile properties of powder processed Ni-based superalloy were investigated by using two different quenching methods. Oil quenching and air cooling were adopted with cooling rate of 183 °C/s and 4–15 °C/s, respectively. The experimental results show that the average size of the secondary γ′ after oil quenching is 24.5 nm compared with 49.8 nm under air cooling, and corresponding volume fractions of γ′ are 29% and 34%, respectively. Meanwhile, the average grain size remains nearly equivalent from both oil-quenching and air-cooling specimens. The tensile strength at room temperature is higher for the oil-quenched specimen than the equivalent from the air-cooled specimen, but the difference approaches each other as the temperature increases to 650 °C. The fractography clearly demonstrates that transgranular fracture governs the failure process at ambient temperature, in contrast to the intergranular fracture at 650 °C or even higher temperature. These two mechanical responses indicate the strengthening effects of γ′ precipitates and grain boundary for polycrystalline Ni-based superalloys at different temperatures.     

Influence of titanium addition on the microstructure of the novel ferrous-based stainless steel

The microstructural characteristics of the Fe-9Al-30Mn-1C-5Ti (wt.%) alloy were determined by scanning electron microscopy, transmission electron microscopy, and energy-dispersive X-ray spectrometry. The microstructure of the alloy was essentially a mixture of (γ + TiC_x + (α + B2 + DO₃)) phases during solution treatment between 950 °C and 1150 °C. The TiC_x carbide had a face-center-cubic structure with a lattice parameter a of 0.432 nm. When the as-quenched alloy was subjected to aging treatment at temperatures of 450-850 °C, the following microstructural transformation occurred: (γ + TiC_x + κ + (α + DO₃)) → (γ + TiC_x + κ + (α + B2 + DO₃ + TiC_x)) → (γ + TiC_x + κ + κ′ + (α + B2 + DO₃)) → (γ + TiC_x + (α + B2 + DO₃)). Addition of Ti promotes the formation of the α phase at high temperatures.     

Increase of misfit during creep of superalloys and its correlation with deformation

In superalloys the loss of coherency during creep results in the increase of misfit of the γ/γ′-interface. The kinetics of this process were measured locally by TEM (Moiré fringes) and X-ray diffraction. Two materials were creep tested (SRR99 and CMSX-4) in two temperature ranges (stable γ′-morphology and rafting), and the morphology changes were quantified. A microstructural model allows calculation of the equilibrium misfit and the increase of plastic strain on the basis of these data. At high temperatures and low stresses the model describes quantitatively creep kinetics up to 30 h. Here the processes controlling primary creep are propagation of dislocation loops along matrix channels and thickening of the matrix channels orientated perpendicular to the load direction.     

In situ analysis of the tensile and tensile-creep deformation mechanisms in rolled AZ31

A rolled AZ31 alloy was tensile tested in a scanning electron microscope at 323 K (50 °C), 423 K (150 °C), and 523 K (250 °C) in order to analyze the deformation mechanisms in situ. Electron backscatter diffraction was performed both before and after straining. There was a significant difference in the activity of the various deformation modes at the three test temperatures and the mechanical anisotropy was considerably reduced with temperature. At 323 K (50 °C) extension twinning, basal, prismatic 〈a〉, and pyramidal 〈c+a〉 slip were active. Twinning disappeared above 323 K (50 °C), suggesting that the critical resolved shear stress (CRSS) of non-basal systems becomes less than that of twinning at T < 423 K (150 °C). Plasticity was controlled at high temperature by a combination of basal and prismatic 〈a〉 slip. From 423 K (150 °C) to 523 K (250 °C), a transition occurs in the dominant deformation mechanism from basal + prismatic 〈a〉 to mainly prismatic 〈a〉 slip. This is consistent with a decrease of the CRSS of non-basal slip systems with increasing temperature. These results suggest that the observed drop in normal anisotropy with increasing temperature is likely to be the consequence of an increase in non-basal slip activity. In situ tensile-creep experiments, performed at approximately the yield stress at 423 K (150 °C), indicated that less slip and more grain boundary cracking occurs during creep deformation compared with the higher-stress tensile experiments.     

An in-situ study of plasma nitriding

Direct in-situ observation of phase generation and growth during heat treatment cycles gives information independent e.g. of effects resulting from cooling and atmospheric changes of properties. In this investigation time resolved in-situ X-ray diffraction (XRD) analysis of growing nitride layers during plasma nitriding was conducted to gain experimental data of growing compound layers for different plasma nitriding parameters. With two gas mixtures of 5% N₂-95% H₂ and 25% N₂-75% H₂. plasma nitriding of an AISI 1045 steel was performed in the temperature range of 450 °C < T < 560 °C. The in-situ XRD-observation consisted of series of 50 to 60 single runs of phase analysis during a 3-h plasma nitriding treatment. Nitriding with the formation of nitride phases starts at different times, depending on the nitriding temperature and the gas composition in the plasma for the given plasma parameters pressure, voltage and current density. The higher the nitriding temperature and the higher the nitrogen content in the process gas the shorter is the time for the first detection of the γ′-Fe₄N-phase. Single phase γ′nitride layers were detected for the 5% N₂-95% H₂ gas mixture in a temperature range 450 °C < T < 560 °C. For the highest temperatures 540 °C and 560 °C and the gas mixture 25% N₂-75% H₂ the ε-Fe_2-3N phase occurred later in the plasma nitriding process. Assuming that nitride layers in plasma nitriding also grow by nucleation of small γ′ particles up to a complete layer, the experimental data fitted in a reasonable way in plots calculated for the incubation time of the γ′-phase during gas nitriding.     

Influence of pressure and surface roughness on the heat transfer efficiency during water spray quenching of 6082 aluminum alloy

The heat flux (q) and heat transfer coefficient (h) at the interface between hot aluminum surface and spray water were determined by using an inverse heat conduction method. Good agreements between numerically calculated temperatures with the inverse identified h and experimentally measurements demonstrate that the method is valid for solving the q and h of spray quenching process. The estimated heat flux consists of three main stages of transition boiling, nucleate boiling and single-phase cooling. The results show that both the heat flux and heat transfer coefficient increase with the increasing of spray pressure. When the surface temperature is lower than 170 °C, the q, h and the maximum heat transfer coefficient (h_max) decrease and then increase as surface roughness increases. However, when the surface temperature is higher than 170 °C, the influence of surface is insignificant. This phenomenon may be attributed to the variation of nucleation site density with surface roughness.     

Structure and electrical conduction behavior of LiZnVO4 ceramic prepared by solution-based chemical route

In the present work, an evaluation of the structural and electrical properties of a compound (LiZnVO₄) has been undertaken. This compound was prepared by solution-based chemical route. The electrical properties were measured using a.c. impedance spectroscopy method in the frequency range of 10³-10⁶ Hz at various temperatures from 28 to 300 °C. X-ray diffraction study indicates a rhombohedral unit cell structure with lattice parameters a = 14.1934 Å, b = 14.1934 Å, c = 9.4926 Å, V = 1656.12 (Å)³, α = 90°, β = 90° and γ = 120°. A field emission scanning electron micrograph reveals a polycrystalline texture of the compound with grains of unequal sizes ∼0.2-2.0 μm. The electrical conduction in the material is a thermally activated process due to the bulk effect. Frequency dependence of a.c. conductivity obeys Jonscher's universal law (σ_ac = σ_dc + Aωⁿ).     

Pb-free glass frits prepared by spray pyrolysis as inorganic binders of Al electrodes in Si solar cells

Pb-free glass frits prepared by spray pyrolysis for Al electrodes were of fine size, spherical morphology and dense structure. Their mean size and geometric standard deviation when prepared at 1,200 °C were 1.0 μm and 1.4, respectively. Their glass transition temperature (T_g) was 374 °C. An Al electrode formed from Al paste with glass frits had a dense structure and good adhesion to the Si substrate. It had a well-developed back-surface field layer of 17.5 μm thickness. Al electrodes formed from Al paste without glass frits had sheet resistances between 21 and 32 mΩ sq⁻¹ as the firing temperature changed from 600 to 900 °C. This compared with values from electrodes formed with frits that decreased from 20 to 7 mΩ sq⁻¹ over the same range of firing temperatures.     

Effects of boron on phase transformation of high Nb containing TiAl-based alloy

The effect of 0.7 at% boron on the phase transformation of Ti–46.5Al–8Nb during a range of different continuous cooling rates from the alpha single phase region has been investigated. In addition the microstructure of Ti–46.5Al–8Nb and Ti–46.5Al–8Nb–0.7B during water quenching at a range of different temperatures near the alpha transus temperature (T_α) has been examined. It has been found that the trend of the variation of microstructures with different cooling rates is the same in the two alloys varying from massive gamma (γ_m) to lamellar, but boron addition increases the cooling rate to obtain these microstructures. During water quenching at different temperatures (T₁ ≅ T_α, T₂ < T_α, and T₃ ≪ T_α) boron addition has a very strong effect on the trend of the variation of microstructures.     

Development of high speed steel sintered using concentrated solar energy

The steel powders were sintered under N₂–H₂ atmosphere in a solar furnace and in a Fresnel lens, after compaction of the green parts, using much higher heating and/or cooling rates as compared to conventional tubular furnace. The effects of processing parameters and the use of concentrated solar energy on densification and mechanical properties were analyzed. Experimental results demonstrated the activating effect of concentrated solar energy on the sintering process showing that an optimum densification is achieved at 1150 °C on both solar installations in just 90 min for the solar furnace and in 30 min in the case of Fresnel lens installation compared with an optimum temperature of 1290 °C in ∼10 h of total cycle in the conventional tubular furnace. Better mechanical properties were obtained using concentrated solar energy with microhardness measurements ranging between 800 and 900 HV. Microstructural analyses by scanning and transmission electron microscopy reveal the presence of submicron sized vanadium nitrides and other nanometer sized particles in the samples that could be responsible for the high hardness values obtained.     

Effect of β grain growth on variant selection and texture memory effect during α → β → α phase transformation in Ti-6 Al-4 V

In the present study, the texture evolution and the role of β grain growth on variant selection during β → α phase transformation have been investigated in Ti-6 Al-4 V with and without 0.4 wt.% yttrium addition. The aim of adding yttrium was to control β grain growth above the β transus by pinning grain boundaries with yttria. Both materials were first thermomechanically processed to generate similar starting microstructures and crystallographic textures. Subsequently, both materials were solution-heat-treated above the β transus followed by slow cooling to promote growth of the α lath structure from grain boundary α. Additional interrupted slow cooling experiments were carried out to identify the α lamellae that nucleate first from β grain boundaries. Detailed electron backscatter diffraction analysis was carried out and it was found that the β heat treatment did not generate new texture components although the intensities of the individual components changed dramatically depending on the alloy/β grain size. Variant selection was assessed by comparing measured α texture components with predicted α texture components based on the high-temperature β texture assuming equal variant selection. It was found that with increasing β grain size variant selection intensified favouring the {φ1, Φ, φ2} {90°, 30°, 0°} texture component. Interrupted cooling experiments revealed that α nucleates first on β grain boundaries that are formed by two β grains having a common (1 1 0) normal and that these α lamellae display almost exclusively a {φ1, Φ, φ2} {90°, 30°, 0°} orientation. Consequently, the dominance of this variant with increasing β grain size can be related to the relative free growth of this particular α texture component into an “empty” β grain.     

Identification of microstructural mechanisms during densification of a TiAl alloy by spark plasma sintering

This work aims at identifying, by coupled scanning and transmission electron microscopy (SEM and TEM) observations, the densification mechanisms occurring when an atomized Ti-47Al-1W-1Re-0.2Si powder is densified by spark plasma sintering (SPS). For this purpose, interruptions of the SPS cycle have been performed to follow the evolution of the microstructure step by step. The powder particles exhibit a classical dendritic microstructure containing a large amount of out-of-equilibrium α phase. During heating-up, the microstructure undergoes successive transformations. At T = 525-875 °C the α phase transforms into γ. The γ phase formed is supersaturated in W and Re. It de-saturates for T above 875 °C by discontinuous precipitation of W and Re-rich B2 phase. Densification takes place for T between 900 °C and 1150 °C by plastic deformation of the powder particles. TEM observations show that the repartition of the plastic deformation is correlated to the dendritic microstructure, and that dynamic recrystallization mechanisms occur. Microstructural phenomena directly resulting from the high currents involved in the SPS process have not been observed.     

Die quenching limit of AA2024 aluminum alloy billet on servo press

Die quenching of AA2024 aluminum alloy billets was carried out on a servo press with ram-motion control of WC-20 mass%Co dies directly after solution heat treatment (SHT). To clarify the dependence on billet size for die quenching, two billets with a height of h₀ = 8 mm or 16 mm and with the same diameter of 16 mm were prepared. The cylindrical billets were heated in an electric furnace at 823 K and transferred to the press. Then the billets were uniaxially compressed with a reduction in height (Δh/h₀) of 2% or 5%, and further held between the dies. The sandwiching duration by dies (t_d.q.) was varied from 0 to 8 s. Based a measured temperature change, hardness and TG–DTA analysis, it is found that die quenching is successfully carried out without precipitation hardening only in the case of the billet with a height of 8 mm and t_d.q. > 6 s. The reduction in height is limited less than 5% by intergranular fracture on side surface of billet during the die quenching process.     

Characterization of the hot deformation behavior of a Ti-22Al-25Nb alloy using processing maps based on the Murty criterion

Isothermal compression testing of Ti-22Al-25Nb alloy was carried out at deformation temperatures between 940 and 1060 °C with strain rate between 0.001 and 10 s⁻¹, and a height reduction of 50%. The hot deformation behavior of Ti-22Al-25Nb alloy was characterized based on an analysis of the stress-strain behavior, kinetics and the processing map, for obtaining optimum processing windows and achieving desired microstructures during hot working. The constitutive equation was established, which described the flow stress as a function of the strain rate and deformation temperature. The apparent activation energies were calculated to be 788.77 kJ/mol in the α₂ + β/B2 + O phase region and 436.23 kJ/mol in the α₂ + B2 phase region, respectively. Based on Dynamic Material Model and the Murty instability criterion, the processing map for the Ti-22Al-25Nb alloy was constructed for strain of 0.6. The map exhibits a stable domain for the temperature range of 940-1060 °C and strain rate range of 0.001-0.1 s⁻¹ with two peaks in power dissipation of 51 and 56%, occurring at 940 °C/0.001 s⁻¹ and 1060 °C/0.001 s⁻¹, respectively. One is associated with lamellar globularization, and the other displays a phenomenon of recrystallization. Therefore, the desired processing condition of the Ti-22Al-25Nb alloy is 940 °C/0.001 s⁻¹ in the α₂ + β/B2 + O phase field. Moreover, the material also undergoes flow instabilities at strain rates higher than 1 s⁻¹. This instability domain exhibits flow localization and adiabatic shear bands which should be avoided during hot processing in order to obtain satisfactory properties.     

Synthesis of highly tetragonal BaTiO3 nanopowders by a two-step alkoxide-hydroxide route

BaTiO₃ (BT) nanopowders were synthesized using the alkoxide-hydroxide route. Formation of BT nanopowders commenced at 60 °C and their amount increased with increasing temperature. However, a TiO₂ second phase was always developed at temperatures higher than 100 °C due to the insufficient amount of H₂O caused by its evaporation. Therefore, a two-step process is presented herein for the synthesis of homogeneous, highly tetragonal BT nanopowders. The cubic BT nanopowders were synthesized at 100 °C and subsequently heated to high temperatures (>200 °C) to increase their tetragonality. Homogeneous BT nanopowders with an average size of 94.8 nm and a high c/a ratio of 1.0081 were obtained for the specimens synthesized at 260 °C for 60 h after the formation of the cubic BT nanopowders at 100 °C for 20 h.     

Hot deformation behavior of Co-base ODS alloys

Co-base ODS alloys, strengthened by nanosized oxide dispersion and γ′ precipitates, were prepared by mechanical alloying. Hot compression tests were performed in the temperature range of 25-970 °C with the strain rate varying from 0.0001 to 0.01 s⁻¹. The influence of deformation parameters and Y₂O₃ content on the flow behavior and the microstructure of compressive specimens were investigated. It was observed that Co-base ODS alloys had exhibited the homogeneous distribution of ultrafine γ′ precipitates (0.26 μm), nanosized oxide particle (12.5 nm), and small grains (1.1 μm). The enhanced flow stress at low temperatures and high strain rates was attributed to the rapid multiplication of dislocations due to the pining effect of oxide dispersion, γ′ precipitates, and fine grains. The peak stress had decreased with increasing the deformation temperature as well as by decreasing the strain rate. The obvious improvement in flow stress was acquired by increasing the Y₂O₃ contents. Grain boundary sliding and porosity growth had been considered to be the main softening mechanisms during the hot deformation at elevated temperature. Moreover, It was found that the flow softening could be accelerated by decreasing the strain rate. The activation energy had increased with increasing temperature, but decreased with decreasing strain rate.