Evolution Of Laves Phase Research Articles

Impact of Nb and Zr on the precipitation evolution of Laves phase and the properties of laser cladding coating of AlCoCrFeNi high entropy alloy

The impact of Nb, Zr, and their synergies on the phase composition, wear and high oxidation resistance of a high entropy laser cladding coating consisting of AlCoCrFeNi on Q235 steel was thoroughly investigated in this work. From our analysis, it was demonstrated that the addition of Nb and Zr promoted the formation of the Laves phase except the BCC phase, and a small amount of MC phase was formed in the coatings containing Zr. Although both of them can refine the grain to different degrees, the grain refinement degree was the highest when they were added together. The solid solution strengthening, fine crystal strengthening, and Laves phase precipitation strengthening induced by the addition of Nb and Zr can significantly improve the microhardness of the coating, which was 1.1–1.5 times that of S1-S3 coatings. The formation of ternary Laves phase, grain refinement, and increase of the microhardness enhanced the plastic deformation and micro-cutting resistance of S4 coating, and the wear resistance was 1.3–2.5 times that of S1-S3 coatings. The unstable oxide Nb2O5 produced by the addition of Nb led to a reduction in the stability of the oxide layer. However, when the two were added cooperatively, the formation of unstable oxide Nb2O5 was reduced by the presence of Zr. Moreover, grain refinement was induced by the collaboration of the two making the protective oxide layer form faster, and stronger bonding with the coating, the high temperature oxidation resistance was significantly improved, which was 1.3–4.3 times that of S1-S3 coatings.

Heterostructure microstructure and laves phase evolution mechanisms during inter-layer hammering hybrid directed energy deposition (DED) process

Directed energy deposition (DED) is suitable for fabrication of large Inconel 718 components, however, large columnar dendrites and several laves phase impair the mechanical properties. In this work, the formation mechanism of heterostructure microstructure with coarse grain (CG) and fine grain (FG) alternating distribution was explored, and the evolution process of Laves phase during inter-layer hammering hybrid directed energy deposition (HDED) was revealed. The results show that the heterostructure microstructure composed of CG region and FG region with a ratio of 1:1 was obtained due to the work hardening and recrystallization mechanisms, and the average grain size of CG region and FG region are 68.9 μm and 12.0 μm, respectively. The Laves phase was first fractured into small pieces by hammering and then resolved into the matrix during the subsequent heat input process, which promotes the precipitation of γ″ phase from the matrix. The strength and plasticity of the specimens fabricated by HDED are synchronously improved. The dislocation strengthening and solid solution strengthening are the mainly contributions to the strength improvement, and the grain refinement and heterostructure microstructure increased the plasticity of the hybrid manufactured specimens.

Minimizing intermetallic Laves phase evolution and enhancing precipitation strengthening of Superalloy-718 joints using InterPulse magnetic arc constriction and high frequency pulsation

The techniques of InterPulse magnetic constriction and high frequency pulsation of arc in gas tungsten arc welding (GTAW) were deployed to minimize the detrimental intermetallic laves phase evolution in fusion zone (FZ) of Superalloy-718 welds and enhance the precipitation strengthening of joints. The Superalloy-718 joints were developed by using an advanced variant of GTAW popularly known as InterPulse gas tungsten constricted arc welding (IP-GTCAW). The joints were subjected to the post weld heat treatment (PWHT) cycles of direct aging (DA), solution annealing at 980 °C and 1065 °C followed by aging (980STA and 1065STA) respectively. The tensile properties and hardness of joints were evaluated. The microstructures of joints were analyzed using optical (OM) and scanning electron microscopy (SEM). The elemental analysis of Laves phase and dendrite core of FZ of joints was performed by using energy dispersive spectroscopy (EDS). The tensile fractured surfaces were analyzed using SEM. Results showed that the Superalloy-718 joints developed using IP-GTCAW showed better response to PWHT than GTAW joints because of the greater refinement of FZ microstructure and evolution of finer and discrete Laves phases in FZ. The 980STA joints exhibited superior tensile properties than DA and 1065STA joints. It is correlated to the greater dissolution of Laves phases in nickel austenitic (γ) matrix of FZ resulting in more niobium (Nb) accessible for the precipitation strengthening of FZ. The 1065STA joints showed slightly higher hardness of FZ than 980STA joints because of the almost complete dissolution of Laves phases in FZ. However, the tensile properties of 1065STA joints are lower than 980STA joints because of severe grain growth in FZ and BM. The evolution of microvoids at the interface of Laves phase/weld matrix leading to the development and coalescence of microcracks in FZ on tensile loading is the main mechanism responsible for the premature fracture of joints.

Laves phase evolution in 10Cr-3W-3Co steel during thermal exposure and creep

The formation of Laves phase was studied in experimental 10Cr-3 W-3Co creep-resistant steel. The steel was exposed to the temperature of 650 °C, and just thermal and creep exposure was compared. Laves phase content, particle size, and morphology were quantitatively analysed by image analysis of micrographs. Significant differences were observed between the creep and thermal exposure. Thermal exposure resulted in fine Laves phase particle nucleation and their gradual coarsening. On the other hand, creep exposure induced the formation of coarse particles from the early stages of exposure. The differences seem to originate in a different state of the matrix, where martensite crystal boundaries facilitate the precipitation of fine Laves phase particles.

Boron influence on the development of Laves phase in 10Cr creep resistant steel

• Enhanced boron content refines precipitating Laves phase. • Laves phase content remains the same. • Enhanced boron content causes formation of thin Laves phase films. • The films does not cause loss of plasticity at 650 °C. The evolution and coarsening of Laves phase in 10Cr-3W-3Co-1Mo nitrogen free creep resistant steels at temperature 650 °C was quantified by use of scanning electron microscopy and image analysis. Influence of B was followed by comparison of two experimental steels with 90 ppm and 120 ppm of B. Results showed the refining influence of enhanced B content on the Laves phase evolution and the formation of thin Laves phase films due to higher B content. Quasistatic tensile tests at temperature 650 °C did not show detrimental effect of these films on the material plasticity.

Effects of Long-Term Aging on Properties and Laves Phase Evolution of A New Type of Fe-Based Superalloy

Effects of Long-Term Aging on Properties and Laves Phase Evolution of A New Type of Fe-Based Superalloy

Synergistic effect of Laves phase evolution and porosity defects in nuclear-grade FeCrAl alloy laser welded joints: Experiments and crystal plasticity modeling

The Fukushima Daiichi nuclear accident in Japan in 2011 created a strong push for the exploration of advanced accident-tolerant fuel materials. For this purpose, FeCrAl-based alloys are a promising class of materials. However, no study has been conducted on the Laves phase evolution in the weld, which is proved to be directly related to weld softening based on the observations in this study. Moreover, the failure mechanism after laser welding remains unknown. In this study, laser welded nuclear-grade FeCrAl alloys were investigated through experiments and crystal plasticity modeling. The maximum ultimate tensile strength and elongation of the welds were 652 MPa (strength coefficient of ∼70.7%) and 9.7%, respectively. Obvious softening was observed in the laser-welded FeCrAl alloy joints. The re-precipitated nanoscale Fe2(Nb,Mo) Laves-phase particles in the fusion zone (FZ) were smaller than 200 nm, whereas the particles in the heat-affected zone (HAZ) were coarsened over 1.6 μm owing to Ostwald ripening. The bipolar size distribution of the Laves phase was the intrinsic reason for the weld softening. In addition, porosity defects were detected in the FZ and fusion line. Most of them were less than 4 × 10−5 mm3 in volume and were distributed left-of-center or right-of-center instead of in the exact center of the FZ. The in situ mechanical response confirmed that pore-induced stress concentration triggered off crack initiation and propagation, leading to the weld failure. Therefore, enhancing the precipitation of Laves phase with concurrent elimination of the porosity defects can serve as a promising strategy to achieve high-strength FeCrAl alloy joints. The fundamental discoveries in this study can facilitate the application of laser welding in joining the nuclear-grade FeCrAl-based alloys.

Dynamic recrystallization, Laves phase evolution and mechanical performance of nuclear-grade Nb containing FeCrAl alloy joints fabricated by friction stir welding

Nb containing FeCrAl alloys are a new class of promising candidates for accident-tolerant-fuel (ATF) materials used in nuclear power plants. The macrostructure, dynamic recrystallization, Laves phase evolution, tensile properties, and their correlations of the Nb containing FeCrAl alloy after friction stir welding (FSW) were systematically investigated at different welding parameters. FSW led to grain refinement and the stir zones (SZs) were mainly composed of fine equiaxed grains. The second-phase particles in the base materials (BM) were C14-type Fe2(Nb,Mo) with a diameter range of 500–900 nm. Under low heat input conditions, the majority of the Fe2(Nb,Mo) Laves phase particles in the SZs were dissolved into the matrix. With heat input increasing, very fine Fe2(Nb,Mo) Laves phase dispersions and Y2O3 in the nanoscale were re-precipitated. Microhardness distributions of Nb containing FeCrAl alloy joints obtained at different welding parameters exhibited identical “U-shaped” profiles. The average hardness of the base material (BM) was ∼352HV, while that in the stir zone (SZ) was reduced to 224-229HV. The joints obtained at 400rpm-80 mm/min held optimal yield strength (389 ± 11 MPa) and ultimate tensile strength (492±6 MPa). Micron-scale Fe2(Nb,Mo) Laves phase particles in the BM helped accumulate dislocation pile-ups. In contrast, the re-precipitated nanoscale Fe2(Nb,Mo) and Y2O3 precipitates could not pin dislocations as effectively as those larger particles in the BM. As a result, the mechanical properties of FSW joints were lower than those of the BM.

The effect of Laves phase on heavy-ion radiation response of Nb-containing FeCrAl alloy for accident-tolerant fuel cladding

As a promising candidate material for the accident tolerant fuel cladding in light water reactors, the Nb-containing FeCrAl alloy has shown outstanding out-of-pile service performance due to the Laves phase precipitation. In this work, the radiation response in FeCrAl alloys with gradient Nb content under heavy ion radiation has been investigated. The focus is on the effect of the Laves phase on irradiation-induced defects and hardening. We found that the phase boundary between the matrix and Laves phase can play a critical role in capturing radiation defects, as verified by in-situ heavy-ion radiation experiments and molecular dynamic simulations. Additionally, the evolution of Laves phase under radiation is analyzed. Radiation-induced amorphization and segregations observed at high radiation doses will deepen the fundamental understanding of the stability of Laves phases in the radiation environment.

Effect of Alloying on the Nucleation and Growth of Laves Phase in the 9–10%Cr-3%Co Martensitic Steels during Creep

Five Co-modified P92-type steels with different contents of Cr, W, Mo, B, N, and Re have been examined to evaluate the effect of the chemical composition on the evolution of Laves phase during creep at 650 °C. The creep tests have been carried out at 650 °C under various applied initial stresses ranging from 80 to 200 MPa until rupture. An increase in the B and Cr contents leads to a decrease in the size and volume fraction of M23C6 carbides precipitated during tempering and an increase in their number particle density along the boundaries. In turns, this affects the amount of the nucleation sites for Laves phase during creep. The (W+Mo) content determines the diffusion growth and coarsening of Laves phase during creep. Susceptibility of Laves phase to coarsening with a high rate is caused by the large difference in Gibbs energy between fine and large particles located at the low-angle and high-angle boundaries, respectively, and can cause the creep strength breakdown. The addition of Re to the 10%Cr steel with low N and high B contents provides the slowest coarsening of Laves phase among the steels studied.

Microstructural characteristics and tensile properties of gas tungsten constricted arc (GTCA) welded Inconel 718 superalloy sheets for aeroengine components

Abstract Inconel 718 is a nickel-based superalloy, typically used in high temperature aerospace applications due to its exceptional mechanical properties and weldability. However, it is susceptible to solute segregation and coarser interconnected laves phase evolution in weld metal due to the high heat input in the gas tungsten arc welding (GTAW) process which drastically reduces the tensile properties of welded joints. It is also susceptible to HAZ microfissuring and liquation cracking issues owing to the evolution of low melting temperature eutectic film at the grain boundaries. To overcome this problem, a newly emerged novel gas tungsten constricted arc welding (GTCAW) process, principally differentiated by magnetic arc constriction and high frequency accentuation of the arc, is used to join Inconel 718 superalloy. The primary objective of this investigation is to evaluate the microstructural characteristics and tensile properties of GTCA welded Inconel 718 superalloy sheets (2 mm thick), fabricated using optimized process parameters, and compare its performance with the base metal. Results showed that GTCA welded joints exhibited 99.20 % and 73.5 % of base metal strength and ductility claiming significant advantages over the GTAW process. It is correlated to the grain refinement in the fusion zone and the evolution of finer discrete laves phase in interdendritic areas of the weld. The basic mechanism responsible for fusion zone grain refinement and corresponding influence on tensile properties of joints is also discussed in brief with regard to the mechanics of arc constriction and pulsing.

Microstructural characteristics and tensile properties of gas tungsten constricted arc (GTCA) welded Inconel 718 superalloy sheets for aeroengine components

Abstract Inconel 718 is a nickel-based superalloy, typically used in high temperature aerospace applications due to its exceptional mechanical properties and weldability. However, it is susceptible to solute segregation and coarser interconnected laves phase evolution in weld metal due to the high heat input in the gas tungsten arc welding (GTAW) process which drastically reduces the tensile properties of welded joints. It is also susceptible to HAZ microfissuring and liquation cracking issues owing to the evolution of low melting temperature eutectic film at the grain boundaries. To overcome this problem, a newly emerged novel gas tungsten constricted arc welding (GTCAW) process, principally differentiated by magnetic arc constriction and high frequency accentuation of the arc, is used to join Inconel 718 superalloy. The primary objective of this investigation is to evaluate the microstructural characteristics and tensile properties of GTCA welded Inconel 718 superalloy sheets (2 mm thick), fabricated using optimized process parameters, and compare its performance with the base metal. Results showed that GTCA welded joints exhibited 99.20 % and 73.5 % of base metal strength and ductility claiming significant advantages over the GTAW process. It is correlated to the grain refinement in the fusion zone and the evolution of finer discrete laves phase in interdendritic areas of the weld. The basic mechanism responsible for fusion zone grain refinement and corresponding influence on tensile properties of joints is also discussed in brief with regard to the mechanics of arc constriction and pulsing.

Influence of arc constriction current (ACC) on microstructural evolution and tensile properties of tungsten inert gas welded thin sheets of aerospace alloy

ABSTRACT The electromagnetic arc constriction technique was implemented to mitigate the heat input related weldability issues in Tungsten Inert Gas (TIG) welding of Alloy 718 particularly solute segregation and substantial laves phase evolution in fusion zone which significantly deteriorates the strength and elongation of welded joints. This paper reports the direct effect of arc constriction current (ACC) on microstructural evolution and tensile properties of Constricted Arc TIG (CA-TIG) welded Alloy 718 joints. ACC is the important parameter which induces the electromagnetic filed and controls the arc constriction characteristics. The basic mechanism responsible for microstructural modification and corresponding influence on the tensile properties of joints is studied both in quantitative and qualitative manner related to the mechanics of electromagnetic arc constriction and correlated to the cooling rate. The joints made by applying ACC of 50 A yielded superior tensile properties. It is correlated to the grain refinement of fusion zone resulting in the evolution of laves phase with finer discrete morphology in interdendritic spacings. Increase in ACC above 50 A causes severe growth of grains in fusion zone and substantial decrement in tensile properties of joints. Beneficial effects of ACC were not perceived at maximum level owing to the high heat input.

Influence of magnetically constricted arc traverse speed (MCATS) on tensile properties and microstructural characteristics of welded Inconel 718 alloy sheets

The magnetically constricted arc technique was implemented to mitigate the heat input related metallurgical problems in Gas Tungsten Arc Welding (GTAW) of Inconel 718 alloy particularly Nb segregation and subsequent laves phase evolution in fusion zone. This paper reports the direct effect of magnetically constricted arc traverse speed (MCATS) on bead profile, tensile properties and microstructural evolution of Inconel 718 alloy sheets joined by Gas Tungsten Constricted Arc Welding (GTCAW) process. The mechanism amenable for the microstructural modification and corresponding influence on the tensile properties of joints is investigated both in qualitative and quantitative manner related to the mechanics of arc constriction and pulsing. It is correlated to the solidification conditions during welding. The relationship between MCATS and Arc Constriction Current (ACC) was derived. Its interaction effect on the magnetic arc constriction and joint performance was analysed. Results showed that the joints fabricated using CATS of 70 mm/min exhibited superior tensile properties (98.39% of base metal strength with 31.50% elongation). It is attributed to the grain refinement in fusion zone microstructure leading to the evolution of finer, discrete laves phase in interdendritic areas.

Effect of Heat Input on Evolution of Microstructure and Tensile Properties of Gas Tungsten Constricted Arc (GTCA) Welded Inconel 718 Alloy Sheets

The significant effect of heat input on the microstructural characteristics and tensile properties of 2-mm-thick Inconel 718 alloy sheets joined by gas tungsten constricted arc welding process was investigated systematically. It involves the application of magnetic arc constriction technique to solve the heat input-related metallurgical problems in gas tungsten arc welding of Inconel 718 alloy such as segregation of Nb and laves phase evolution in weld metal region which drastically reduces the weld mechanical properties. Heat input range was used from 495 to 585 J/mm by varying Main Current from 60 to 80 A at distinct levels of 5 A. The average secondary dendrite arm spacing was increased from 6.31 to 8.92 μm, revealing the corresponding cooling rate between 2087 and 720 °C/s. Superior tensile properties were obtained at an optimum heat input of 518 J/mm. It exhibited 98.16 and 78% of base metal strength and ductility. It is attributed to the grain refinement in fusion zone microstructure and the evolution of lower amount of laves phase with finer and discrete morphology. The average size and volume fraction of laves phase for the current sample were 3.12 μm and 7.68%. The benefits of magnetic arc constriction and Delta Current pulsing were not achieved at higher heat input.

Laves Phase Evolution in China Low-Activation Martensitic (CLAM) Steel during Long-Term Aging at 550 °C.

To clarify the precipitation and evolution law of the Laves phase in China low-activation martensitic (CLAM) steel during long-term aging at high temperature, this paper carried out an aging treatment of CLAM steel at 550 °C for up to 30,000 h. The segregation behavior of alloy elements and the precipitation amount and average size of the Laves phase were quantitatively characterized by transmission electron microscopy (TEM) and scanning electron microscopy (SEM), and the precipitation and coarsening behavior of the Laves phase were obtained. The results show that the Laves phase begins to precipitate within 5000 h after aging and mainly depends on M23C6 carbides to nucleate and grow at the grain boundary and subgrain boundary. During the aging process, the average size of the Laves phase grows continuously. After more than 25,000 h, the growth rate of the Laves phase decreases. After 30,000 h of aging, the average size reaches 439.9 nm, and the maximum size exceeds 800 nm. The area fraction of the Laves phase increases continuously during the 20,000 h aging process and tends to be stable after aging for 20,000 h. The area fraction is approximately 1.85%.

The Characterization of Soft Fine‐Grained Heat‐Affected Zones in P91 Weldments under Creep Exposure Conditions

The martensitic 10CrMoVNb 91 (P91) steel welded joint is subjected to reaustenitizing‐based post‐weld heating, referred to as post‐weld normalizing and tempering (PWNT). The various zones of P91 weldments in PWNT treatment and after creep exposure at 620 °C for 150 MPa are studied. The evolution of intermetallic Laves phase in the various zones of P91 weldments is observed after the creep rupture tests. The evolution of Laves phase is characterized using field‐emission scanning electron microscopy (FESEM) with energy‐dispersive X‐ray spectroscopy.

The evolution behavior of second phases during long-term creep rupture process for modified 9Cr-1.5Mo-1Co steel welded joint

In this paper, evolution of Laves phase in weld metal (WM) during long-term creep exposure was investigated for modified 9Cr-1.5Mo-1Co steel welded joint (WJ) by microstructure characterization and theoretical calculation based on a series of creep tests at 620 °C. It was found that some rod-like Mo-rich Laves phase precipitated adjacent to M23C6 carbides on grain boundary and other isolated polygonal Laves phase particles formed in grains after long-term creep exposure. Number and size of Laves phase increased simultaneously at early stage of precipitation. The volume fraction nearly approached to equilibrium fraction while coarsening was still ongoing after 2500 h. Johnson-Mehl-Avrami and Ostwald ripening model indicated that precipitation of Laves phase was controlled by grain boundary diffusion with a decreasing rate. EDS results showed Laves phase in WM had the highest Mo content in the whole WJ. The pinning effects on grain boundaries by Laves phase became weaker during creep exposure due to its coarsening. More high angle boundaries and sub-structure in WM from EBSD results provided more potential sites for Laves phase nucleating. With Laves phase particles growth on grain boundaries in WM, solid solution elements like Mo in matrix were severely depleted, resulting in loss of solute strengthening effect and precipitation strengthening effect, thus leading to the rupture in WM.

M <sub>23</sub>C <sub>6</sub>-Assisted Laves Phase Growth Mechanism in a Novel 9Cr-3W-3Co-1CuVNbB Steel Under Creep Deformation

Laves phase particles are an important precipitate in tempered martensite ferritic steels that can strongly affect the microstructure and mechanical properties. We systematically investigated the evolution of Laves phase in G115 steel. Experimental results showed that the amount of Laves phases increases, whereas that of M23C6 carbides decreases as creep deformation proceeds. Further characterizations were conducted to thoroughly explore the evolutionary mechanism of Laves phase. Interestingly, a hybrid Laves-M23C6 particle was observed, and Cr segregation occurred near the remaining M23C6 carbides. Further, we found that C and Cr were also segregated near a Laves phase that was formed. The Cr concentration of the Laves phase was much lower than that of M23C6 carbide, but slightly higher than that of the ferrite matrix. The result confirms that Laves phase is transformed from M23C6 carbide, and it can also explain reasonably well the decrease in M23C6 carbide during creep deformation. We also proposed a M23C623C6 carbides, which is considered to be an interface-controlled M23C623C6-assisted Laves phase growth model was established assuming that the Gibbs energy is balanced at the M23C6/Laves phase interface. Furthermore, a guideline for development of novel tempered martensite ferritic steels was proposed. Decreasing the Si content of the material can effectively postpone Laves phase nucleation. In addition, the fraction of the Laves phase decreases, which helps improve the creep strength.

Study on the effect of ageing on laves phase evolution and their effect on mechanical properties of P92 steel

P92 steel is candidate material for application in reactor pressure vessels in nuclear power plants. In present investigation, Laves phase evolution (at 650°C) with varying ageing time (upto 3000h) in P92 steel and their effect on mechanical properties have been investigated. During thermal ageing, the microstructure analysis showed the evolution of Laves phase that degrades the strength of P92 steel. The formation of Laves phase was observed after the thermal ageing of 720h and it showed higher coarsening rate in ageing time range of 720h −1440h. The Laves phase formation was also confirmed by the XRD analysis, and line mapping. The strength and ductility decreased as a result of deprivation of solid solution strengthening and formation of Laves phase. The hardness of P92 steel was also affected by ageing time but less pronounced as compared to strength. Charpy toughness was also reduced continuously with increase in ageing time as a result of thermal straining of particles and Laves phase formation.