Growth Of Laves Phase Research Articles

A novel precipitation mechanism of Laves phase in Fe-30Cr-2Mo super ferritic stainless steel: In-situ phase transformation

Laves phase is generally prejudicial to the performance of ferritic stainless steels, so understanding the underlying formation mechanism should be technologically useful. Here we demonstrate a new precipitation behavior of Laves phase in Fe-30Cr-2Mo super ferritic stainless steel by using transmission electron microscopy. It is revealed that the MC carbides firstly precipitate adhere to the M2SC carbosulphides with an orientation relationship (OR) of (0001)M2SC//(111¯)MC and [112¯0]M2SC//[011]MC. Subsequently, the interfaces between MC carbides and ferrite matrix provide preferential nucleation sites for Laves phases, which hold OR with MC as follows: (111¯)MC//(0003)Laves and [011]MC//[112¯0]Laves. Notably, the transformation from MC to Laves phases is an in-situ process based on the dissolution of MC carbide, which supplies the niobium element for nucleation and growth of Laves phase. This work enriches the recognition of the precipitation mechanism of Laves phase, and should therefore facilitate the design of high-performance ferritic stainless steels.

Precipitation and growth of Laves phase and NbC during aging and its effect on tensile properties of a novel 15Cr–22Ni–1Nb austenitic heat-resistant steel

The microstructure evolution and the mechanical properties of 15Cr–22Ni–1Nb austenitic heat-resistant steel were investigated during the aging up to 24 h at 780 °C, 820 °C and 860 °C. A large amount of Laves phase and nano-sized NbC were precipitated during the aging. The NbC particles exhibit excellent coarsening resistance, and the coarsening rate constant of nano-sized NbC particles is four orders of magnitude smaller than that of Laves phase during the aging at different temperatures. The room temperature tensile strength of the heat-resistant steel after aging at 820 °C for 4 h is highest among different aging treatments. The volume fraction of Laves phase increase with the increase in aging temperature and holding time, resulting in a decrease in the ductility of the heat-resistant steel. Ostwald ripening was observed during the aging for 8 h at different aging temperatures. Consequently, the high temperature ultimate tensile strength of heat-resistant steel after the aging for 8 h is lower than that for 4 h and 24 h. The precipitation strengthening contributed by Laves phase and nano-sized NbC was estimated. The yield strength contributed by submicron Laves phase particles is smaller than that provided by nano-sized NbC particles.

Alleviating segregation and enhancing tensile properties of Inconel 718 superalloy by twin-roll casting and two-stage cold rolling

A novel processing route involving twin-roll casting and two-stage cold rolling was proposed to manufacture Inconel 718 alloy. Due to the sub-rapid solidification rate (412.5 °C/s) of as-cast strip, the element segregation can be alleviated and the growth of Laves phase was effectively suppressed. It was also observed that the dissolution of Laves phases during subsequent homogenization was significantly accelerated because of their refined size. The corresponding homogenization time can be shortened from dozens of hours to 1 h. Additionally, the non-uniform microstructure of cold-rolled and annealed samples caused by coarse grains from homogenization treatment can be eliminated by two-stage cold rolling method. Furthermore, a large amount of granular δ phases precipitated during intermediate annealing. After double aging treatment, the final products exhibited an excellent combination of strength and elongation. The ultimate tensile strength when deformed at room temperature and at 650 °C were about 1475 and 1120 MPa, and the corresponding total elongation were ∼ 15% and ∼ 22%. These results indicated that the twin-roll casting was a promising route to produce Inconel 718 alloy.

Maximizing strength and corrosion resistance of InterPulsed TIG welded Superalloy 718 joints by RSM for aerospace applications

Inconel 718 (IN-718) is a nickel-based alloy, mostly used in the aeroengine applications due to its good mechanical strength and resistance to corrosion. However, it is amenable to alloying segregation and Laves phase growth in fusion zone (FZ) which severely deteriorates the strength and corrosion resistance of IN-718 joints. To overcome this problem, InterPulsed TIG (IP-TIG) welding process, characterized by high frequency current pulsation and arc constriction, was attempted to join thin sheets of IN-718 alloy. The 3D graphs of response surfaces were built to establish the optimized welding parameters for maximizing the joint strength. The parametric mathematical equation was constructed to predict the strength of IN-718 joints. The mathematical equation was also built to predict the tensile strength from Laves phase. The potentiodynamic corrosion test was done in a 3.56 wt.% NaCl solution for the selected joints to determine the corrosion behavior of IN-718 joints. The IP-TIG welded IN-718 joints exhibited 99.20% of base metal strength and superior corrosion resistance at optimized condition of welding parameters. This refers to the refining of FZ dendritic structure which aids in minimizing the Laves phase. The IN-718 joints developed using optimized IP-TIG parameters exhibited 22–32% higher strength and superior corrosion resistance than TIG welded IN-718 joints and comparable to EBW and LBW IN-718 joints.

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.

Study on W-rich M3B2 borides in a 9Cr3W3CoB heat-resistant steel

In a 9Cr3W3CoB heat-resistant steel with 150 ppm B, the evolution of W-rich M3B2 type borides during aging at temperatures of 650, 700, 750 °C for different times was investigated. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron probe microanalysis (EPMA) and secondary ion mass spectroscopy (SIMS) were employed to characterize the morphology evolution of the W-rich M3B2 type borides. Most of B was discovered to be present in M3B2-type (M = Fe, W, Cr, V and Nb) borides which were unevenly distributed in the matrix and could not be eliminated by normal heat-treatment. However, these borides could dissolve during prolonged isothermal exposure, where the dissolution process was accelerated by increasing aging temperature. It is indicated that the W-rich M3B2 type borides are thermodynamically metastable at high temperature. Their temporary existence postpones the nucleation and growth of Laves phase and prevents B from segregating in M23C6 carbides. W-rich M3B2 type borides are not responsible for cleavage fracture initiation when they are dissolved after aging at high temperatures for 2000 h.

Spontaneous Crystallization in Systems of Binary Hard Sphere Colloids.

Computer simulations of the fluid-to-solid phase transition in the hard sphere system were instrumental for our understanding of crystallization processes. But while colloid experiments and theory have been predicting the stability of several binary hard sphere crystals for many years, simulations were not successful to confirm this phenomenon. Here, we report the growth of binary hard sphere crystals isostructural to Laves phases, AlB_{2}, and NaZn_{13} in simulation directly from the fluid. We analyze particle kinetics during Laves phase growth using event-driven molecular dynamics simulations with and without swap moves that speed up diffusion. The crystallization process transitions from nucleation and growth to spinodal decomposition already deep within the fluid-solid coexistence regime. Finally, we present packing fraction-size ratio state diagrams in the vicinity of the stability regions of three binary crystals.

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.

Microstructural Evolution of P92 Steel During Long-Term Aging

The mechanical properties and microstructure evolution of P92 steel were investigated after aging up to 11,000 h at 923 K. Charpy impact and tensile tests were carried out to study the strength and ductility of aged P92 steel. In addition, microstructure evolution of the samples during long-term aging was investigated with X-ray diffraction, optical microscopy and scanning electron microscope (SEM). Then, statistical quantitative image analysis based on SEM images was used to evaluate the precipitation during long-term aging. The mechanical properties were found to be associated with the evolution of precipitation, especially the coarsening of Laves-phase. Results show that the ductility and strength of P92 steel decrease with the growth of Laves-phase during short-term aging (shorter than 1000 h). However, as the aging time further increasing, the ductility and strength of P92 steel decrease slowly with the coarsening of Laves-phase.

Precipitate formations with self-adaptive elemental diffusion and segregation in T92 steel

A detailed characterization of the composition and evolution of precipitates during thermal exposure (0–10000 h at 650 °C) of T92 steel was carried out using a combination of transmission electron microscopy (TEM), high angle annular dark field (HAADF) imaging, energy-dispersive X-ray spectrometry (EDS) analysis, scanning electron microscopy (SEM) and first-principles calculations. It is shown that MX in T92 steel can be expressed as V/Nb(C,N) rather than complete solid-solution (V,Nb)(C,N) in terms of composition. The segregation of P and S elements in the MX precipitates was found after normalization. Meanwhile the slow segregation of Si from the matrix into the MX took place during a long-term thermal exposure. The nucleation and growth of M23C6 carbides adjacent to the MX micrograin boundary carbonitrides always accompany by the diffusion of C and S from the neighboring MX, and the segregation of W and Mo from the matrix, respectively. Similarly this elemental diffusion behavior of W and Mo, as well as the segregation of Si, P and S were also found for the nucleation and growth of Laves phase in contact with M23C6. Our results indicate that these precipitates formation in T92 steel is a special process with self-adaptive elemental diffusion and segregation.

Study on the nucleation and growth of Laves phase in a 10% Cr martensite ferritic steel after long-term aging

The nucleation and growth of Laves phase in a 10% Cr martensite ferritic steel after long-term aging have been investigated in this paper. Laves phase, (Fe, Cr)2 (Mo, W), was observed after long-term (>750h) aging at 650°C. It is found that Laves phases prefer to locate at prior austenite grain boundaries and martensite lath boundaries, especially, most of them precipitate at grain boundaries with a misorientation angle of 40–60° and only a small amount of them at 3–10°. Moreover, the size of Laves phase at 40–60° grain boundaries is larger than that at 3–10° grain boundaries. In addition, some Laves phases are formed in the regions adjacent to M23C6 particles, with increasing aging time they will gradually swallow the Cr-rich M23C6 carbides in close vicinity, resulting in carbon atoms segregation on the vicinity of phase interfaces between Laves phase and α-Fe.

Influence of precipitation on dislocation substructure and creep properties of P91 steel weld joints

Two trial weld joints were prepared using the GTAW and SMAW methods and they underwent creep testing at temperatures between 525 and 625°C. The longest time to rupture was 45,811 h. Two main processes occurred during creep exposures: recovery and precipitation of secondary phases. Slight coarsenings of the M23C6 carbide, precipitation of Laves phase and Z-phase were observed after long tests at high temperatures. Some differences in microstructure and creep failure were found in the individual zones of weldments. After long exposure at temperatures up to 600°C, fractures occurred in the fine-grain heat-affected zone as a result of a low density of fine vanadium nitride and a high density of coarse particles at grain and subgrain boundaries. At 625°C, growth of Laves phase caused a softening of the ferritic matrix and crack propagation in the weld metal.

Effect of Chromium, Aluminum and Nickel on Microstructure and Reverse-S type Creep Rupture Strength of High Cr Ferritic Heat Resisting Steels

Microstructure and creep strength at 650°C of two high-Cr ferritic heat resisting steels were investigated. 11Cr steel containing relatively high carbon, aluminum and nickel showed a significant drop in creep strength after 1000 h while 9Cr steel containing relatively low carbon, aluminum and nickel showed a recovery of creep strength after a slight drop. The 11Cr steel contained M23(CB)6, Z phase and relatively coarsened Laves phase as precipitates after 7000 h at 650°C, while the 9Cr steel contained fine MX and ultrafine M6C in addition to M23(CB)6 and Laves phase. Z phase was not found and the growth of Laves phase was retarded in the 9Cr steel. It was concluded that the drop of creep strength in the 11Cr steel was attributed to the coarsened Laves and Z phases and the recovery of the strength in the 9Cr steel to fine MX and M6C. Experimental results on the effects of steel compositions on the stability of precipitates in the steels were thermodynamically consistent with Thermo-Calc calculation results using an existing database. It was suggested that chromium and aluminum increased the kinetics of the formation and coarsening of the precipitates.

Investigation of Dissolution and Precipitation Behaviours of Precipitates in Welded Joints of High Cr Ferritic Heat Resistant Steels

This paper is aimed at investigating dissolution and precipitation behaviour in fine grained heat-affected zones (FGHAZ), coarse grained heat-affected zones (CGHAZ), and base metals for the welded joints of high Cr ferrite heat resistant steels. The simulated HAZ (SHAZ) specimens were used, whose thermal cycles were controlled to be the same as those in the actual welded joint with peak temperatures of 1 173 and 1 523 K to represent FGHAZ and CGHAZ, respectively. The behaviours of dissolution and precipitation during welding, PWHT and ageing processes were observed. SEM observation showed that after long time ageing the precipitates in FGHAZ specimens (1 173 K) were fewer and larger than those in CGHAZ (1 523 K) specimens and base metal specimens. This phenomenon implied that growth and coarsening of precipitates in FGHAZ may play a role in the deterioration of creep property and Type IV cracking, which was observed in previous creep tests. X-ray diffraction analysis for the electrolytic extraction showed that the types of precipitates were M23C6, M7C3, MX, Laves phase, and μ phase. By using software of Thermo-Calc and Dictra, a 3-D model was performed to simulate the dissolution process of M23C6 during welding. It was found that the dissolution of M23C6 can lead to high Cr and high W concentration in a local region. On the other hand, the precipitation behaviour of M23C6 was simulated, and it was found that the growth of M23C6 was fast and in the period of PWHT and creep process it can reach to the equilibrium concentration in a short time of about 2 000 seconds. At last, the growth of Laves phase was simulated and it was found that it grew very slowly and it reached the equilibrium concentration in about 3 000 hours. As an important point, the simulation result showed that Laves phase was easy to grow in the region where M23C6 had dissolved due to the high residual concentration of Cr and W.

Use of atom probe techniques to support thermodynamic and atomistic modelling of phase transformations

Atom probe field ion microscopy (APFIM) gives accurate data on phase composition for all elements and has a good sensitivity and excellent spatial resolution. APFIM data have therefore been used as support for thermodynamic modelling of phase equilibria and phase transformations. This paper describes a number of cases where atom probe data from Chalmers University have been used to judge the accuracy of modelling: the solubility of W and C in Co, the equilibrium volume fraction of VN in a complex chromium steel, the content of B in M 23C 6 precipitates, the growth of Laves phase during ageing of a chromium steel, and the growth of secondary NbC precipitates in a stabilised austenitic stainless steel. Atomistic modelling is now emerging as a tool for materials science, in particular for modelling of interfacial structure and energies. The status of the activities in this field at Chalmers University is described, and the role of APFIM in atomistic modelling is discussed.