In this study, we explain why the amount of Laves phase in Inconel 718(IN718) manufactured by selective laser melting (SLM) unexpectedly increases under extremely rapid solidification and clarify its distinct dissolution behavior during heat treatment based on a comparative analysis of laser directed energy deposition (L-DED)-built and cast IN718. Compared with the cast and L-DED, SLM has much higher cooling rate, which greatly improves the dendrite growth rate and the solute trapping during the solidification process of molten pool. Thus, more Nb atoms are trapped in the matrix, and less Nb atoms are expelled to form inter-dendritic Laves phase in SLM-built IN718. However, the volume fraction of Laves phase is the highest in SLM-built IN718, which is attributed to the lower Nb concentration in the Laves phase and few Nb-rich region. During homogenization treatment, the dissolution of Laves phase is controlled by three-dimensional diffusion process at the early stage, and then is mainly controlled by the interface reaction process. After homogenization, there are still many tiny carbides, oxides, and nitrides undissolved both along the grain boundaries and inside the grains. 

The formation and dissolution mechanisms of Laves phase in Inconel 718 fabricated by selective laser melting compared to directed energy deposition and cast

High-temperature tensile and high cycle fatigue properties of hot isostatic pressed LPBF Inconel 625 (laser powder bed fusion, HIPed LPBF 625) were investigated and then compared to conventional wrought Inconel 625. HIPed LPBF and wrought alloys show similar microstructural characteristics in average grain size, defects, fraction of special grain boundaries, and grain morphologies. However, Al2O3 and TiN phase was found (without carbides) in HIPed LPBF 625, and various stoichiometry of carbides (without Al2O3) were identified in wrought 625. The yield and tensile strengths and elongation at fracture of both alloys have similar values at room temperature. At 650 °C, elongation at fracture of HIPed LPBF 625 significantly decreases compared to that of wrought 625. HIPed LPBF 625 also has a lower high-cycle fatigue limit of 500 MPa (at 107 cycles without fracture) and shorter life in low-stress conditions compared to wrought 625. The relatively higher sulfur content in HIPed LPBF 625 (33 ppm) is suggested as the reason for lower fatigue endurance at 650 °C. The LPBF (or additive manufacturing), which uses powder feedstock, is vulnerable to impurity concentration increases, elemental contamination control is critical in the initial powder and LPBF process.

High-temperature tensile and high cycle fatigue properties of inconel 625 alloy manufactured by laser powder bed fusion

A Review Study on Thermal Stability of High Entropy Alloys: Normal/Abnormal Resistance of Grain Growth

Influence of heat treatment on Inconel 625 superalloy sheet: carbides, γ'', δ phase precipitation and tensile deformation behavior

Based on the temperature sensitivity characteristics, high-temperature tensile tests were performed at different temperatures and after different solid solution treatments to investigate the effect of Cr-rich M23C6, Nb-rich MC and the delta phase on the mechanical properties of Inconel 625. The experimental results indicated that the Cr-rich M23C6 carbides and the Nb-rich MC carbides decomposed at 700°C, which could be the reason for resulting tensile strength anomaly that was observed in a narrow temperature range from 650 to 700°C during the tensile tests at different temperatures. For the samples subjected to a prior solid solution treatment, the size of the δ phase was found to increase with the solution treatment temperature, whereas the elongation at fracture decreased.

High-temperature mechanical properties of Inconel-625: role of carbides and delta phase

As with all alloys, the grain structure of the nickel-base superalloy 625 has a significant impact on its mechanical properties. Predictability of the grain structure evolution in this material is particularly pertinent because it is prone to inter-metallic precipitate formation both during manufacture and long term or high temperature service. To this end, analysis has been performed on the grain structure of Alloy 625 aged isothermally at temperatures between 600 and 800 °C for times up to 3000 h. Fits made according to the classical Arrhenius equation describing normal grain growth yield an average value for the activation energy of a somewhat inhomogeneous grain structure above 700 °C of 108.3±6.6 kJ mol−1 and 46.6±12.2 kJ mol−1 below 650 °C. Linear extrapolation between 650 and 700 °C produces a significantly higher value of 527.7±23.1 kJ mol−1. This result is ultimately a consequence of a high driving force, solute-impeded grain boundary migration process operating within the alloy. Comparison of the high and low temperature values with the activation energy for volume self-diffusion and grain boundary diffusion identifies the latter as the principle governing mechanism for grain growth in both instances. A decrease in the value of the time exponent (n) at higher temperatures despite a reduction in solute drag is attributable to the Zener pinning imposed by grain boundary M6C and M23C6 particles identified from Transmission Electron Microscopy (TEM) and Energy Dispersive X-ray Spectroscopy (EDXS) analysis. Vickers hardness results show the dominance of intermetallic intragranular precipitates in the governance of the mechanical properties of the material with grain coarsening being accompanied by a significant increase in hardness. Furthermore, the lack of any correlation with grain growth behaviour indicates these phases have no significant effect on the grain evolution of the material.

/pdf/grain-coarsening-behaviour-of-solution-annealed-alloy-625-g3q8t5j0ub.pdf

Grain coarsening behaviour of solution annealed Alloy 625 between 600–800 °C

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Grain coarsening behaviour of solution annealed Alloy 625 between 600–800 °C