Dissolution Of Laves Phase Research Articles

Unexpected formation of delta (δ) phase in as-cast niobium-bearing superalloy at solution annealing temperatures

The annealing of as-cast Inconel 718 superalloy at high solution annealing temperatures for short holding durations was studied. It was found that the needle-shaped δ phase formed from the eutectic constituent, which was followed by the dissolution of this phase by continued annealing. In fact, the formation of δ phase was characterized as an intermediate stage for the dissolution of Laves phase and back-diffusion of the segregated elements from the interdendritic regions. Thermodynamics analysis revealed that the upper temperature limit for the formation of δ phase is dependent on the Nb content. During annealing, the Laves phase started to dissolve, which resulted in the rejection of Nb to the interlamellar austenite, which was characterized as the primary Nb source for the observed local increase in the formation temperature of δ phase

Investigation on the effect of post weld heat treatment on microwave joining of the Alloy-718 weldment

In the present work, the butt joining of Alloy-718 (980 °C solution treated condition) was achieved in an industrial microwave oven using the principles of microwave hybrid heating (MHH) technique. The weldment reveals perfect diffusion bonding between base materials through fully melting of powder particles. The fabricated weldment was heat treated with standard heat treatments such as direct aged, 980 °C solution treated and aged, and solution treated at 1080 °C and ageing. The microstructural behaviors of all weldment were investigated along with their mechanical properties such as microhardness, tensile, and flexural strength at room temperature. The improvement in mechanical properties of post heat treated weldment was observed due to the dissolution of the Laves phase and precipitation of coherent precipitates in the fusion zone microstructure. The microhardness in the fusion zone of 1080 °C solution treated and aged weldment was higher among all welded condition due to complete Laves phase dissolution after this treatment. The flexural and tensile strength was higher at 980 °C solution treated and aged weldment among all welded condition.

Homogenization kinetics of a typical nickel-based superalloy

The kinetics analysis of homogenization in Inconel 718 superalloy needs much more attention. For instance, the activation energy of Laves phase dissolution has not been determined so far, which is required for characterization of the underlying mechanisms. In the present work, based on Johnson–Mehl–Avrami–Kolmogorov (JMAK) analysis, diffusion calculations, and experimental investigation of homogenization at different temperatures and holding durations, the kinetics of Laves phase dissolution was studied. The activation energy of 274.5 kJ/mol was determined in this respect, which is close to the activation energy for diffusion of Mo in Ni (288.15 kJ/mol). Moreover based on the diffusion coefficients, it was found that the diffusion of Mo in Ni is slower than those of Nb and Ti in Ni. These results were also supported by the calculations based on the diffusion distance and it was concluded that the back-diffusion of molybdenum in austenite is the controlling micro-mechanism for dissolution of the undesirable Laves phase in this common superalloy.

Solidification behavior and Laves phase dissolution during homogenization heat treatment of Inconel 718 superalloy

The solidification behavior and microstructural evolutions during homogenization heat treatment of Inconel 718 superalloy were studied. The microstructure of the as-cast alloy was characterized by optical micrographs, scanning electron microscopy (SEM) images, and elemental analysis based on energy dispersive spectroscopy (EDS). The optimum etching techniques, especially electroetching, were characterized to reveal the dendritic nickel matrix, the microsegregation between dendrite arms, and the austenite/Laves eutectic structure. The elemental analysis of different microstructural features, especially the variation of the chemical composition of austenite, was successfully related to the pseudo-binary solidification constitution diagram. Moreover, the extension of the nonequilibrium eutectic isotherm was evaluated to unravel the solidification path of this important engineering alloy. The dissolution of the Laves phase and its kinetics during exposure at elevated temperatures with the purpose of homogenization were also discussed.

An electrochemical method for detection and quantification of Laves phase in 12Cr martensitic stainless steel

An electrochemical method for the detection and quantification of Laves phase in 12Cr martensitic stainless steel is proposed in the present work. Laves phase dissolution peak can be observed at the lower potential range which can be obtained in anodic polarization curves in a specific solution with specific pH values. The amount of Laves phase was calculated by the integration of anodic current with time according to the polarization curve and also independently measured by using scanning electron spectroscopy. A good correlation of these two methods was obtained.

Microstructure, Creep and Thermomechanical Fatigue of Novel Solid Solution and Laves Phase Strengthened Cr2O3 and Al2O3 Forming Ferrites for Car Engine Exhaust and Heat Exchanger Systems

Increasing efficiency of internal combustion engines by downsizing leads to a rise in exhaust gas temperature up to 1000 °C accompanied with higher mechanical loading of exhaust systems, approaching the limits of currently used Cr2O3 forming ferrites. Similarly, the conditions in solid oxide fuel cell (SOFC) heat exchanger systems exceed the limits of commercial Al2O3 forming heat resistant ferrites with respect to temperature and thermomechanical fatigue (TMF) loading. Aim of the presented work was the development of Cr2O3 and Al2O3 forming ferrites with improved mechanical strength and TMF properties up to 900–1000 °C. Baseline material was the Crofer® 22H alloy, which should be further hardened by solid solution and Laves phase precipitates. The paper gives an overview of the key results achieved at Research Center Julich on correlation between microstructure evolution, deformation and lifetime in hot tensile, creep and TMF tests. Increasing Laves phase dissolution temperature elevates short time stability of precipitates and long term stability of the grain structure up to 950 °C. Furthermore, a lower Laves phase content of the Al2O3 forming material could be compensated by dynamic strain ageing via Al diffusion resulting in superior high temperature strength, creep behavior and TMF life.

INVITED] Laser treatment of Inconel 718 alloy and surface characteristics

Laser surface texturing of Inconel 718 alloy is carried out under the high pressure nitrogen assisting gas. The combination of evaporation and melting at the irradiated surface is achieved by controlling the laser scanning speed and the laser output power. Morphological and metallurgical changes in the treated surface are analyzed using the analytical tools including optical, electron scanning, and atomic force microscopes, energy dispersive spectroscopy, and X-ray diffraction. Microhardnes and friction coefficient of the laser treated surface are measured. Residual stress formed in the surface region is determined from the X-ray diffraction data. Surface hydrophobicity of the laser treated layer is assessed incorporating the sessile drop method. It is found that laser treated surface is free from large size asperities including cracks and the voids. Surface microhardness increases significantly after the laser treatment process, which is attributed to the dense layer formation at the surface under the high cooling rates, dissolution of Laves phase in the surface region, and formation of nitride species at the surface. Residual stress formed is compressive in the laser treated surface and friction coefficient reduces at the surface after the laser treatment process. The combination of evaporation and melting at the irradiated surface results in surface texture composes of micro/nano-poles and pillars, which enhance the surface hydrophobicity.

Effect of phosphorus segregation on as-cast microstructure and homogenization treatment of IN706 alloy

The distribution of phosphorus in IN706 alloy was studied, with particular emphasis on the as-cast microstructure and homogenization treatment. It is found that phosphorus is segregated in Laves phase and markedly influences the as-cast microstructure of IN706 alloy. Phosphorus retards the formation of Laves phase with the eutectic form, but promotes the Laves formation with the blocky form. Phosphorus refines the dendritic structure and noticeably enlarges the interdendritic area. The reason for phosphorus influencing the as-cast microstructure is attributed to its extremely low solubility in γ matrix and great retarding effect on γ matrix solidification. In addition, phosphorus slightly decreases the melting temperature of Laves phase, and retards the dissolution of Laves phase and the element diffusion between dendrite core and interdendritic area during the homogenization treatment. And it is thought that phosphorus delays the homogenization process by inhibiting the element diffusion.

Structure–Property Correlations in Microwave Joining of Inconel 718

The butt joining of Inconel 718 plates at 981°C solution treated and aged (981STA) condition was carried out using the microwave hybrid heating technique with Inconel 718 powder as a filler material. The developed joints were free from any microfissures (cracks) and were metallurgically bonded through complete melting of the powder particles. The as-welded joints were subjected to postweld heat treatments, including direct-aged, 981STA and 1080STA. The microstructural features of the welded joints were investigated using a field emission-scanning electron microscope equipped with x-ray elemental analysis. Microhardness and room-temperature tensile properties of the welded joints were evaluated. The postweld heat-treated specimens exhibited higher microhardness and tensile strength than the as-welded specimens due to the formation of strengthening precipitates in the microstructure after postweld heat treatments. The microhardness of the fusion zone of the joint in 1080STA condition was higher than all welded conditions due to the complete dissolution of Laves phase after 1080STA treatment. However, the tensile strength of the welded specimen in 981STA condition was higher than all welded conditions. The tensile strength in 1080STA condition was lower than that in 981STA condition because of the grain coarsening that took place after 1080STA condition. The fractography of the fractured surfaces was carried out to determine the structure–property–fracture correlation.

Long-term behaviour of solid oxide fuel cell interconnect materials in contact with Ni-mesh during exposure in simulated anode gas at 700 and 800 °C

In the present study the long-term behaviour of two ferritic steels, Crofer 22 APU and Crofer 22H, in contact with a Ni-mesh during exposure in simulated anode gas, Ar–4%H2–2%H2O, at 700 and 800 °C for exposure times up to 3000 h was investigated. Ni diffusion from the Ni-mesh into the steel resulted in the formation of an austenitic zone whereas diffusion of iron and chromium from the steel into the Ni-mesh resulted in the formation of chromia base oxides in the Ni-mesh. Depending on the chemical composition of the steel, the temperature and the exposure time, interdiffusion processes between ferritic steel and Ni-mesh also resulted in σ-phase formation at the austenite–ferrite interface and in Laves-phase dissolution in the austenitic zone. The extent and morphology of the σ-phase formation are discussed on the basis of thermodynamic considerations, including reaction paths in the ternary alloy system Fe–Ni–Cr.

Laser controlled melting of pre-prepared inconel 718 alloy surface

Laser treatment of Inconel 718 alloy surface is carried out. The alloy surface is coated with a carbon layer containing 7% TiC particles prior to the laser treatment. The carbon coating provides increased absorption of the incident laser beam and holds TiC particles. The microstrutural and morphological changes in the laser treated region are examined using optical and scanning electron microscopes, energy dispersive spectroscopy, and X-ray diffraction. The microhardness of the surface is measured and the residual stress formed at the surface vicinity is determined from the XRD technique. It is found that partial dissolution of carbide particles takes place at the surface. The composition of fine grains at the surface vicinity, nitride compounds formed, and dissolution of Laves phase at the surface region enhances the hardness at the treated surface. In addition, laser treated surface is free from the micro-crack network and cavities.

Effects of Phosphorus on Laves Phase Dissolution and Elemental Diffusion in GH706 Alloy

The effects of P on element segregation, Laves phase dissolution and elemental diffusion in GH706 alloy have been investigated. The results show that P promotes the segregation of Nb and Ti. Phosphorus was enriched in Laves phase, which enhanced the precipitation of blocky Laves phase, while constrained the formation of eutectic Laves phase. Moreover, more needle-like phase was formed around blocky Laves phase in high P-doped alloy. It also demonstrated that P slightly lowered the melting-point of Laves phase. During the homogenization process, P hindered the diffusion of Nb which blocks the diffusion of Nb and dissolution of Laves phase, and complicated the homogenization treatment.

Microstructure and mechanical properties of Inconel 718 electron beam welds

Bead on plate, full penetration electron beam welds were produced in 2 mm thickness sheets of Inconel 718 in the solution treated condition. Welds were subjected to an aging treatment with and without post-weld solution treatment. Weld microstructures, high temperature tensile properties and stress rupture properties were evaluated. The as welded fusion zone showed a considerable amount of interdendritic niobium segregation and brittle intermetallic Laves phase. The tensile and stress rupture properties of the welds after post-weld aging treatment were found to be inferior in relation to the base metal. Post-weld solution treatment at 980°C was found to result in partial dissolution of Laves phase, some reduction in niobium segregation and the formation of δ phase needles around the Laves particles. The use of 980°C solution treatment was found to improve the weld properties to some extent, although not to the level of the base metal. The reasons for this behaviour are discussed, correlating microstructures, fracture features and mechanical properties.

Microstructure and tensile properties of Inconel 718 pulsed Nd-YAG laser welds

Pulsed Nd-YAG laser welding characteristics of Inconel 718 sheet material (2 mm thick) were investigated. Welds were subjected to three different post-weld heat treatments: direct aging, 980 °C solution treatment + aging (980STA), and 1080 °C solution treatment + aging (1080STA). Weld microstructures and room temperature tensile properties were evaluated. Weld tensile properties in direct aged condition were found to be inferior in relation to the base metal due to the presence of Nb-rich brittle intermetallic Laves phase in fusion zone microstructure. Solution treatment at 980 °C was found to result in considerable dissolution of Laves phase, leading to some improvement in weld properties, though not to the level of base metal. The treatment was also found to result in precipitation of needle-like δ-phase around partially dissolved Laves particles. Solution treatment at 1080 °C was found to result in complete dissolution of Laves phase, but also in significant grain coarsening in the base metal, which led to considerable deterioration in base metal properties.

Special features of dissolution of the laves phase under the conditions of rapid electron heating for quenching

1. The dissolution of the excess Laves phase in alloy 36NKhTYuM8 under the conditions of rapid quenching is maximum in the stage of heating to 1200°C, and the efficiency of the treatment increases in proportion to the heating rate. We have suggested an explanation of the determining role of the heating rate in the dissolution of the excess phase, which is based on the concept of high thermal stresses that appear on the interphase boundaries because of the difference in the coefficients of thermal expansion of the matrix and the phase. 2. High plastic deformation of alloy 36NKhTYuM8 (e=70%) is accompanied by deformation and cracking of the particles of the excess Laves phase, but the nonequilibrium nature of the structure of these particles does not influence noticeably their solubility under the conditions of rapid heating. 3. The process of primary recrystallization that occurs in deformed alloy in the stage of rapid heating does not stimulate the dissolution of the excess phase, which means that the efficiency of rapid heating in quenching of alloys can be used independently of their initial state.

Ｗ強化型９％Ｃｒフェライト系耐熱鋼の熱時効ぜい化と電気化学的手法によるその評価

Thermal aging embrittlement of newly developed W alloyed 9%Cr ferritic steel KA-STBA29/KA-STPA29 (ASME T92/P92) has been investigated. In order to clarify the controlling factor of the embrittlement, Charpy impact tests and determination of precipitates have been carried out using materials aged under various conditions. The decrease in the upper and lower shelf energies and the increase in the fracture appearance transition temperature (FATT) due to thermal aging were caused by the precipitation and coarsening of Laves phase. The decrease in the impact toughness was found to be uniquely correlated with the increase in the area fraction of Laves phase. In order to develop a non-destructive technique for detection of the thermal aging embrittlement, changes in electrochemical properties of KA-STBA29/KA STPA29 due to thermal aging have been investigated. The electrochemical polarization technique has been applied to thermally aged materials. Experimental results on electrochemical polarization measurements revealed that the peak current density “Ip” which appeared at a specific potential during potentiodynamic polarization measurements in 1N KOH solution linearly increased with the degree of embrittlement evaluated by the impact energy at 0°C. The increase in Ip corresponded to the increase in selective dissolution of Laves phase. As a consequence, the degree of thermal aging embrittlement can be estimated by the electrochemical technique.