Showing papers on "Laves phase published in 2019"

Heat treatment of Inconel 718 produced by selective laser melting: Microstructure and mechanical properties

Abstract: Inconel 718 (IN718) samples have been produced by selective laser melting (SLM). The effects of solution temperature, time and cooling rate as well as aging hardening on the microstructure and mechanical properties of SLMed IN718 have been studied. It is found that the as-fabricated IN718 is characterized with fine cellular dendrites with Laves phase precipitating in the subgrain boundaries, which is profoundly different from cast and wrought materials and needs different heat treatment schedules. The relationship between the minimum solution time and solution temperature is established and it provides a basis for the selection of solution treatment parameters. In addition, decreasing the cooling rate of solution treatment will contribute to the precipitation of strengthening phases. The precipitation temperatures of γ′ and γ″ are about the same for SLMed and wrought IN718, but the former has a faster aging response. The tensile properties of SLMed IN718 can be tuned in a large range by properly varying the microstructure. The highest elongation of 39.1% can be obtained after solution treatment (water quenching) without aging treatment and the highest yield and tensile strength (1374/1545 MPa) can be obtained after the direct aging treatment. The match of strength and ductility is able to be tailored by controlling the amount of strengthening phases, which can be realized by adjusting the cooling rate of solution treatment and aging time.

Study on the element segregation and Laves phase formation in the laser metal deposited IN718 superalloy by flat top laser and gaussian distribution laser

Abstract: The element segregation, Laves phase formation, and mechanical properties of the laser metal deposited IN718 superalloy by the flat top laser beam (FTLB) and gaussian distribution laser beam (GDLB) are studied. It is found that the Laves phase formation in the gaussian distribution laser deposited IN718 (GDLD-IN718) is substantially higher than that in the flat top laser deposited IN718 (FTLD-IN718). The higher production of the Laves phase in the GDLD-IN718 contributes to the higher microhardness and lower tensile strength (about 20% reduction) of the as-deposited IN718 than that of the FTLD-IN718. The element redistribution behavior in the laser rapid solidification under both of the lasers are also studied through the finite element simulation. The results show that the severe laser energy concentration in the center of the GDLB produces higher molten pool temperature, lower horizontal thermal gradient to vertical thermal gradient ratio (GX/GZ) of the solid-liquid interface. These typical thermal characteristic of the GDLB generated molten pool eventually results in a lower redistribution coefficient of the alloying elements and as a result, the interdendritic element segregation and Laves phase formation are dramatically improved in the GDLD-IN718. The present comparative study proves that the FTLB is more superior for the laser additive manufacturing than that of the GDLB.

Investigation on Microsegregation of IN718 Alloy During Additive Manufacturing via Integrated Phase-Field and Finite-Element Modeling

Abstract: In this work, we apply a multi-scale model combining finite-element method (FEM) and phase-field model (PFM) to simulate the evolution of solidification microstructures at different locations within a molten pool of an additively manufactured IN718 alloy. Specifically, the FEM is used to calculate the shape of molten pool and the relative thermal gradient G at the macroscale. Then, the calculated thermal information is input into PFM for microstructure simulation. Finally, the morphology of solidification structures and formation of Laves phase at different sites are studied and compared. We found that the solidification site with a large angle between the temperature gradient and the preferred crystalline orientation could build up a high niobium (Nb) concentration in the liquid during solidification but has less possibility of forming continuous long chain morphology of Laves phase particles. This finding provides an understanding of the microstructure evolution inside the molten pool of IN718 alloy during solidification. Further, the finding indicates that the site with a large misorientation angle will have a good hot cracking resistance after solidification.

On the role of Laves phases on the mechanical properties of Mg-Al-Ca alloys

Abstract: As-cast Mg-Al-Ca alloys are among the most promising alloys for elevated temperature applications (≤200 °C) due to their superior creep properties when compared to conventional AZ or AM series Mg alloys. The microstructures of Mg-Al-Ca alloys consist of a soft α-Mg phase reinforced with hard interconnected Laves phases. These interconnected Laves phases are the main reason for the good creep resistance of these alloys as they impede creep deformation. The volume fraction, type and morphology of Laves phases can be controlled through the Ca/Al ratio. Consequently, the Ca/Al ratio can be used to manipulate the mechanical properties of this alloy system in order to achieve optimum creep resistance. We show here that a higher Ca/Al ratio results in i) higher volume fraction of intermetallic Laves phases in the microstructure, ii) improvement in the yield strength (YS), and iii) enhancement in creep resistance at a stress level of 50–70 MPa and a temperature of 170 °C of the as-cast alloys. Moreover, the local strain distribution and partitioning at the microstructural level occurring during high temperature tensile deformation (at ∼170 °C) was measured using quasi in-situ DIC in SEM revealing stress localisation at the α-Mg Laves phase interfaces.

Knowledge of Process-Structure-Property Relationships to Engineer Better Heat Treatments for Laser Powder Bed Fusion Additive Manufactured Inconel 718

Abstract: Dislocation structures, chemical segregation, {\gamma ^{\prime}, {\gamma ^{\prime \prime}}, {\delta} precipitates and Laves phase were quantified within the microstructures of Inconel 718 (IN718) produced by laser powder bed fusion additive manufacturing (AM) and subjected to standard, direct aging, and modified multi-step heat treatments. Additionally, heat-treated samples still attached to the build plates vs. those removed were also documented for a standard heat treatment. The effects of the different resulting microstructures on room temperature strengths and elongations to failure is revealed. Knowledge derived from these process structure property relationships was used to engineer a super solvus solution anneal at 1020 degC for 15 minutes, followed by aging at 720 degC for 24 hours heat treatment for AM-IN718 that eliminates Laves and {\delta} phases, preserves AM specific dislocation cells that are shown to be stabilized by MC carbide particles, and precipitates dense {\gamma ^{\prime} and {\gamma ^{\prime \prime}} nanoparticle populations. This 'optimized for AM-IN718 heat treatment' results in superior properties relative to wrought/additively manufactured, then industry standard heat treated IN718: relative increases of 7/10 percent in yield strength, 2/7 percent in ultimate strength, and 23/57 percent in elongation to failure are realized, respectively, regardless of as-built vs. machined surface finishes.

Improved trade-off between strength and plasticity in titanium based metastable beta type Ti-Zr-Fe-Sn alloys

Abstract: An impressive strengthening ability of Laves phases is favorable to develop titanium alloys with an improved trade-off between strength and plasticity. Therefore, the Ti-xZr-7Fe-ySn (x = 25, 30, 35 wt% and y = 1, 2 wt%) alloys were first designed in such a manner that a Laves phase would precipitate in these alloys and then the investigated alloys were produced by cold crucible levitation melting. A hexagonal close-packed C14 type Laves phase along with a dominant fraction of body-centered cubic β phase are formed in all the as-cast Ti-xZr-7Fe-ySn alloys except in Ti-25Zr-7Fe-2Sn. The volume fraction of the Laves-C14 phase is found to be sensitive to the quantities of Zr and Sn. Amongst all the investigated alloys, Ti-35Zr-7Fe-2Sn shows a better dislocation-pinning ability in terms of dislocation density (3.96 × 1015 m−2), yield strength (1359 MPa) and hardness (437 HV), whereas Ti-25Zr-7Fe-1Sn shows a better deformation ability in terms of compressive strain at failure (36.2%) and plastic strain (31.9%). Crack propagation, regions of dimples and deformation bands are examined in the fracture analyses. Moreover, in this work, Ti-25Zr-7Fe-1Sn exhibits the best strength and plasticity trade-off in terms of a product of ultimate strength and compressive strain at failure (77.4 GPa %).

Thermal Stability of the HfNbTiVZr High-Entropy Alloy.

Abstract: The multicomponent alloy HfNbTiVZr has been described as a single-phase high-entropy alloy (HEA) in the literature, although some authors have reported that additional phases can form during annealing. The thermal stability of this alloy has therefore been investigated with a combination of experimental annealing studies and thermodynamic calculations using the CALPHAD approach. The thermodynamic calculations show that a single-phase HEA is stable above about 830 °C. At lower temperatures, the most stable state is a phase mixture of bcc, hcp, and a cubic C15 Laves phase. Annealing experiments followed by quenching confirm the results from thermodynamic calculations with the exception of the Laves phase structure, which was identified as a hexagonal C14 type instead of the cubic C15 type. Limitations of the applied CALPHAD thermodynamic description of the system could be an explanation for this discrepancy. As-synthesized HfNbTiVZr alloys prepared by arc-melting form a single-phase bcc HEA at room temperature. In situ annealing studies of this alloy show that additional phases start to form above 600 °C. This indicates that the observed HEA is metastable at room temperature and stabilized by a slow kinetics during cooling. X-ray diffraction analyses using different cooling rates and annealing times show that the phase transformations in this HEA are slow and that completely different phase compositions can be obtained depending on the annealing procedure. In addition, it has been shown that the sample preparation method (mortar grinding, heat treatment, etc.) has a significant influence on the collected diffraction patterns and therefore on the phase identification and analysis.

Plastic deformation of single crystalline C14 Mg2Ca Laves phase at room temperature

Abstract: Intermetallic phases, such as Mg2Ca, have been shown to significantly improve the creep strength of magnesium alloys. However, the relevant deformation mechanisms of the intermetallics for further alloy development are largely unknown as the application temperature of the intermetallic-metallic composites lies in their brittle low temperature regime. In this study, we investigated the deformation mechanisms of the hexagonal Mg2Ca Laves phase at the same size scale (μm) as in intermetallic-metallic alloys and at room temperature using nanomechanical test methods. We identified active slip planes by a statistical evaluation of slip traces formed around nanoindentations and measured the corresponding critical resolved shear stresses for each slip system by compression of single crystalline micropillars in selected orientations. Deformation occurs on basal, 1st and 2nd order pyramidal as well as 1st and 2nd order prismatic planes and the critical stresses are of the order of 0.44 GPa–0.59 GPa with the lowest value obtained on the 1st order prismatic planes. Finally, we discuss the possible slip systems on a theoretical basis in terms of their local atomic structure.

Precipitates in Additively Manufactured Inconel 625 Superalloy

Abstract: Laser-based additive manufacturing processes are increasingly used for fabricating components made of nickel-based superalloys. The microstructure development, and in particular the precipitation of secondary phases, is of great importance for the properties of additively manufactured nickel-based superalloys. This paper summarizes the literature data on the microstructure of Inconel 625 superalloy manufactured using laser-based powder-bed fusion and directed energy deposition processes, with particular emphasis on the phase identification of precipitates. The microstructure of Inconel 625 manufactured by laser-based directed energy deposition in as-built condition is investigated by means of light microscopy and transmission electron microscopy. Phase analysis of precipitates is performed by the combination of selected area electron diffraction and microanalysis of chemical composition. Precipitates present in the interdendritic areas of as-built Inconel 625 are identified as MC and M23C₆ carbides as well as the Laves phase.

Microstructures and Wear Resistance of AlCrFeNi 2 W 0.2 Nb x High-Entropy Alloy Coatings Prepared by Laser Cladding

Abstract: The AlCrFeNi2W0.2Nbx high-entropy alloy (HEA) coatings were synthesized on the 304 stainless steel by laser cladding. The microstructure, microhardness and wear resistance of HEA coatings were investigated. The HEA coatings show a good metallurgical bonding to the substrate, and they consist of the cladding zone, bonding zone and heat-affected zone. The phase structures of the HEA coatings are the BCC solid solution phase and the Fe2Nb-type Laves phase. The Nb0.5 composition shows a hypo-eutectic microstructure; the primary phase is the BCC solid solution phase. While Nb1.0, Nb1.5 and Nb2.0 coatings show hyper-eutectic microstructures, the primary dendrites show the Laves phase. The microhardness of AlCrFeNi2W0.2Nbx coatings increases with increasing Nb content and that of the Nb2.0 coating is up to 890.7 HV, about 4.5 times as the 304 stainless steel. Dry sliding model wear testing has been performed. The AlCrFeNi2W0.2Nbx (x = 1.5, 2.0) high-entropy alloy coatings exhibit an order of magnitude lower wear than 304 stainless steel under the same loading conditions. It is attributed to the larger volume fraction of hard Laves phase and the anti-attrition of newly formed oxidation films during friction process.

CoCrFeMnNi high-entropy alloys reinforced with Laves phase by adding Nb and Ti elements

Abstract: Laves phase plays a positive role in improving the strength of high-entropy alloys (HEAs); Nb and Ti elements have potential to promote Laves phase formation in some HEAs. For improving the strength of the face-centered cubic (FCC) CoCrFeMnNi HEA, a series of (CoCrFeMnNi)100−xNbx (atomic ratio: x = 0, 4, 8, 12, 16) and (CoCrFeMnNi)100−xTix (atomic ratio: x = 0, 2, 4, 6, 8, 12) HEAs were prepared by melting. The effects of Nb and Ti on the microstructure evolution and compressive properties of the CoCrFeMnNi HEAs were investigated. For (CoCrFeMnNi)100−xNbx HEAs, the second-phase (Laves and σ phase) volume fraction increased from 0 to 42%. The yield strength also increased gradually from 202 to 1010 MPa. However, the fracture strain decreased from 60% (no fracture) to 12% with increasing Nb content. For (CoCrFeMnNi)100−xTix HEAs, the yield strength increased from 202 to 1322 MPa. The Laves phase volume fraction also increased from 0 to 27%. However, the fracture strain decreased from 60% (no fracture) to 7.5% with increasing Ti content. Addition of Nb and Ti has a good effect on improving the strength of FCC CoCrFeMnNi HEA.

Design of a Seven-Component Eutectic High-Entropy Alloy

Abstract: A eutectic high-entropy alloy with seven components (Fe, Ni, Cr, V, Co, Mn, and Nb) was designed via the melt route guided by CALPHAD predictions. Configurational entropy estimated for the two-phase microstructure qualifies it to be referred to as a high-entropy alloy. When the Nb content exceeded 9.7 at. pct, the microstructure changed from hypoeutectic with primary face-centered cubic phase to hypereutectic with primary Laves phase.

Functionally Graded SS 316L to Ni-Based Structures Produced by 3D Plasma Metal Deposition

Abstract: In this investigation, the fabrication of functionally graded structures of SS316L to Ni-based alloys were studied, using the novel technique 3D plasma metal deposition. Two Ni-based alloys were used, a heat resistance alloy Ni80-20 and the solid-solution strengthened Ni625. Different configurations were analyzed, for the Ni80-20 a hard transition and a smooth transition with a region of 50% SS316L/50% Ni80-20. Regarding the structures with Ni625, a smooth transition configuration and variations in the heat input were applied. The effect of the process parameters on the geometry of the structures and the microstructures was studied. Microstructure examinations were carried out using optical and scanning electron microscopy. In addition, microhardness analysis were made on the interfaces. In general, the smooth transition of both systems showed a gradual change in the properties. The microstructural results for the SS316L (both systems) showed an austenite matrix with δ-phase. For the mixed zone and the Ni80-20 an austenite (γ) matrix with some M7C3 precipitates and laves phase were recognized. The as-built Ni625 microstructure was composed of an austenite (γ) matrix with secondary phases laves and δ-Ni3Nb, and precipitates M7C3. The mixed zone exhibited the same phases but with changes in the morphology.