Showing papers on "Laves phase published in 2023"

Dissolution kinetics of Laves phase during homogenization heat treatment of additively manufactured Inconel 718 superalloy

Abstract: The Laves phase in the as-built Inconel 718 (IN718) superalloy is known as undesirable phase, which limits the industrial applications of the manufactured parts. Post-processing in the form of the homogenization heat treatment is known as an effective method to eliminate the deleterious Laves phase, where understanding its dissolution kinetics is crucial for optimization of the heat treatment schedule. Accordingly, the IN718 nickel-based superalloy manufactured by the laser powder bed fusion (LPBF), also known as selective laser melting (SLM), was homogenized at temperatures from 950 to 1150 °C for 30 to 120 s, and the Laves phase dissolution was studied by microstructural analysis, its kinetics was modeled by Johnson-Mehl-Avrami-Kolmogorov (JMAK) analysis, and the model was validated by the second Fick's law. The activation energy for Laves phase dissolution was determined as ∼160 kJ/mol with an Avrami exponent of ∼1, which was attributed to the high dislocation density and fine microstructure of the LPBF material, and accordingly, the lattice and grain boundary diffusion of Nb in Ni were characterized as the underlying atomistic mechanisms during the dissolution of the Laves phase. These findings are based on the underlying atomistic mechanisms, which can shed light on the principles of dissolution kinetics of Laves phase in IN718 for additive manufactured parts. Moreover, the conducted comparison among LPBF, directed energy deposition (DED), and conventional casting indicated that the fabricating method and the initial segregation play key roles. In other words, due to its lower segregation degree, LPBF has faster Laves phase dissolution kinetics in comparison with other fabrication methods.

Electrochemical corrosion behavior in sulfuric acid solution and dry sliding friction and wear properties of laser-cladded CoCrFeNiNb high entropy alloy coatings

Abstract: 316 stainless steel (SS) was coated with CoCrFeNiNbx (x: atomic ratio, x = 0, 0.1, 0.2, and 0.3) high entropy alloy coatings (HEACs) by laser cladding (LC). The impacts of Nb content on the phase composition, microstructure, microhardness, wear resistance and corrosion properties of CoCrFeNi based HEACs were examined. Different from the canonical dendritic (DR) - interdendritic (ID) microstructure presented by the other three HEACs, the ID of Nb0.3 HEACs also existed nanoscale Nb-rich Laves phase composed of NbNi and NbCo. The inclusion of Nb significantly reduced the average grain size of CoCrFeNi based HEACs from 59.43 μm to 11.38 μm. It also greatly diminished the formation of high angle grain boundaries (HAGBs), increasing the percentage of low angle grain boundaries (LAGBs) from 31 % to 61 %. The microhardness of Nb-containing HEACs was significantly promoted due to the solid solution strengthening and fine grain strengthening caused by large size Nb atoms. Affected by Laves phase with high coordination number and space filling ratio and higher proportion of LAGBs, the microhardness of Nb0.3 HEAC was even as high as 530 HV. The increase in Nb concentration also strengthened the HEACs' resistance to wear. Nb0, Nb0.1, Nb0.2, and Nb0.3 HEACs had corresponding specific wear rates of 0.36, 0.32, 0.23, and 0.20 mm3/Nm. Unlike 316 SS, Nb0 and Nb0.1 HEACs, which had both adhesive wear and abrasive wear, Nb0.2 and Nb0.3 HEACs were mainly wear by abrasive wear. The corrosion behavior of CoCrFeNiNbx HEACs in 0.5 M H2SO4 solution was similar to 316 SS, but the corrosion resistance was stronger than 316 SS. Nb0.3 HEAC with the Ecorr of −164 mV and Icorr of 6.40 × 10−7 A/cm2 showed the best acid corrosion resistance, which could be attributed to the stable and dense passivation film on its surface. Distinct from the intergranular corrosion of other coatings, the corrosion of Nb0.3 HEAC mainly occurred in DR, because the FCC phase acted as the anode site in it when FCC and Laves phase co-existed.

Tailoring the Plasticity of Topologically Close‐Packed Phases via the Crystals’ Fundamental Building Blocks

Abstract: Brittle topologically close‐packed precipitates form in many advanced alloys. Due to their complex structures, little is known about their plasticity. Here, a strategy is presented to understand and tailor the deformability of these complex phases by considering the Nb–Co µ‐phase as an archetypal material. The plasticity of the Nb–Co µ‐phase is controlled by the Laves phase building block that forms parts of its unit cell. It is found that between the bulk C15–NbCo2 Laves and Nb–Co µ‐phases, the interplanar spacing and local stiffness of the Laves phase building block change, leading to a strong reduction in hardness and stiffness, as well as a transition from synchroshear to crystallographic slip. Furthermore, as the composition changes from Nb6Co7 to Nb7Co6, the Co atoms in the triple layer are substituted such that the triple layer of the Laves phase building block becomes a slab of pure Nb, resulting in inhomogeneous changes in elasticity and a transition from crystallographic slip to a glide‐and‐shuffle mechanism. These findings open opportunities to purposefully tailor the plasticity of these topologically close‐packed phases in the bulk by manipulating the interplanar spacing and local shear modulus of the fundamental crystal building blocks at the atomic scale.

Microstructural evolution and strengthening mechanisms of Inconel 718 alloy with different W addition fabricated by laser cladding

Abstract: In this study, Inconel 718 alloys with various W additions (0, 6, 12, and 18 wt%) were fabricated by laser cladding. The influences of W addition on the microstructure and mechanical properties of the as-deposited Inconel 718 alloy were studied. The results show that with the rise of W addition, the fraction of Laves phase, the low-angle grain boundaries (LAGBs), and the dislocation density monotonically increase, whereas the grain size and interdendritic region of Inconel 718 alloy decrease, and the alloy with no interdendritic region can be obtained at a W addition of 18 wt%. Additionally, Fe2W phase is observed above the striped Laves phase in Inconel 718-18 W alloy. As the introduction of W increases from 0 to 18 wt%, the microhardness of alloys enhances from 273.1 ± 19.4 HV to 380.2 ± 12.6 HV. Moreover, the ultimate tensile strength (UTS) and yield strength tensile (YS) of the Inconel 718 are constantly enhanced by increasing W addition, while the elongation (El) decreases. The as-deposited alloy sample with 12 wt% W addition exhibits excellent comprehensive properties compared to the other samples. The YS, UTS, and El of Inconel 718-12 W alloy are 749.96 ± 16.25 MPa, 1106.71 ± 30.39 MPa, and 18.61 ± 1.47%, respectively. The excellent comprehensive mechanical properties of the Inconel 718-12 W alloy is mainly attributed to dislocation strengthening and solid solution strengthening. The present study provides a new way to achieve as-deposited Inconel 718 alloy with excellent synthetical mechanical properties.

Laves Phase Precipitation Behavior in HiperFer (High Performance Ferritic) Steel with and without Boron Alloying

Abstract: High-chromium ferritic stainless HiperFer steels were developed for high-temperature applications in power conversion equipment. The presented research describes the precipitation behavior of the Laves phase after the thermomechanical treatment of Fe-17Cr-0.6Nb-2.4W HiperFer alloys with and without the addition of 55 ppm boron. The boron-alloyed variant was produced with the aim of enhancing grain boundary strengthening and consequently increasing creep resistance. The focus is set on the effect of boron on the thermomechanically induced precipitation of (Fe,Cr,Si)2(Nb,W) Laves phase at grain boundaries. The addition of boron modifies the diffusion conditions in the area of grain boundaries. Consequently, the formation of Laves phase is promoted and the particle growth and coarsening process are suppressed. The impact of boron addition was validated by performing creep and thermomechanical fatigue testing in the standard processing state of HiperFer steel. In the B-alloyed variant, increased creep ductility through the modification of the particle-free zone widths at high-angle grain boundaries was encountered. Nevertheless, an optimized thermomechanical treatment is necessary to fully utilize the increased ductility effect for the creep strength optimization of the B-alloyed grade.

Comparison of thermal expansion path of the Fe2Mo Laves phase calculated using the DFT-based phonon and Debye-Grüneisen approaches

Abstract: Precipitation of the intermetallic Fe2Mo Laves phase at the interface between the nuclear fuel and the fuel element cladding can significantly weaken the strength characteristics of the cladding and fuel. Despite the importance of designing materials for the cladding of fuel rods, the thermodynamic properties and the trajectory of the thermal expansion of the Fe2Mo remain poorly understood. The thermodynamic properties of the Fe2Mo have been studied using the finite-temperature quantum mechanical calculations within the frame of the density functional theory under the quasiharmonic approximation. The vibrational contribution to the free energy was obtained using phonon calculations. The thermal expansion of Fe2Mo was predicted by comparing between free energies calculated in different directions. A path with the least energy was chosen as the trajectory of thermal expansion. The obtained result was compared with the direction calculated in previous theoretical work used the Debye–Grüneisen approach and accounted magnetic subsystem to calculate the vibrational and magnetic contributions to the free energy. This comparison reveals that these two approaches are in good agreement with each other. The work shows that the Fe2Mo possesses a non-isotropic thermal expansion. The heat capacity and volumetric expansion at constant pressure are modeled. The calculated results analyzed and are in satisfactory agreement with the experimental data. Obtained results can be useful for further design of fuel element cladding materials intended for generation IV reactors, the operating temperature of which should be above 873 K.

Phase Equilibria of the Ti-Nb-Mn Ternary System at 1173K, 1273K and 1373K

Abstract: Phase equilibria in the Ti-Nb-Mn ternary system at 1173K, 1273K and 1373K were studied through the equilibrated alloy method by using scanning electron microscopy (SEM), electron probe microanalysis (EPMA) and X-ray diffraction (XRD) techniques. A new stable ternary phase K was confirmed and the composition was around Ti50Nb7Mn43. A wide-range continuous solid solution phase (Ti,Nb)Mn2 with the C14 Laves structure had been found at these temperatures due to the same phase structures of TiMn2 and NbMn2 phases. The solubility of Nb in TiMn4, αTiMn and βTiMn intermetallic compounds was determined. Based on the experimental results and reasonable extrapolations, the isothermal sections of Ti-Nb-Mn ternary system at 1173K, 1273K and 1373K were constructed.

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

Abstract: The techniques of InterPulse magnetic compression and high frequency pulsation of arc in gas tungsten arc welding (GTAW) were deployed to minimize intermetallic laves phase evolvement in fusion zone (FZ) of Superalloy-718 welds and enhance the precipitation strengthening of joints. The Superalloy-718 joints were developed by deploying an advanced variant of GTAW popularly known as InterPulse gas tungsten compressed 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 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 refining of FZ and evolvement of finer and discrete Laves phase in FZ. The 980STA joints exhibited superior tensile properties than DA and 1065STA joints. It is correlated to the greater dissolution of Laves phase in nickel austenitic (γ) matrix 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 phase in FZ. However, the tensile properties of 1065STA joints are lower than 980STA joints because of severe grain growth in FZ and BM. The evolvement of microvoids at the interface of Laves phase/weld matrix leading to development and coalescence of microcracks in FZ on tensile loading is main mechanism responsible for the premature fracture of joints.

Rate dependence of damage formation in metallic-intermetallic Mg-Al-Ca composites

Abstract: We study a cast Mg-4.65Al-2.82Ca alloy with a microstructure containing $\alpha$-Mg matrix reinforced with a C36 Laves phase skeleton. Such ternary alloys are targeted for elevated temperature applications in automotive engines since they possess excellent creep properties. However, in application, the alloy may be subjected to a wide range of strain rates and in material development, accelerated testing is often of essence. It is therefore crucial to understand the effect of such rate variations. Here, we focus on their impact on damage formation. Due to the locally highly variable skeleton forming the reinforcement in this alloy, we employ an analysis based on high resolution panoramic imaging by scanning electron microscopy coupled with automated damage analysis by deep learning-based object detection and classification convolutional neural network algorithm (YOLOV5). We find that with decreasing strain rate the dominant damage mechanism for a given strain level changes: at a strain rate of $5\cdot10^{-4}/s$ the evolution of microcracks in the C36 Laves phase governs damage formation. However , when the strain rate is decreased to $5\cdot10^{-6}/s$, interface decohesion at the $\alpha$-Mg/Laves phase interfaces becomes equally important. We also observe a change in crack orientation indicating an increasing influence of plastic co-deformation of the {\alpha}-Mg matrix and Laves phase. We attribute this transition in leading damage mechanism to thermally activated processes at the interface.