The influence of Laves phases on the room temperature tensile properties of Inconel 718 fabricated by powder feeding laser additive manufacturing

Simulative builds, typical of the tip-repair procedure, with matching compositions were deposited on an INCONEL 718 substrate using the laser additive manufacturing process. In the as-processed condition, these builds exhibit spatial heterogeneity in microstructure. Electron backscattering diffraction analyses showed highly misoriented grains in the top region of the builds compared to those of the lower region. Hardness maps indicated a 30 pct hardness increase in build regions close to the substrate over those of the top regions. Detailed multiscale characterizations, through scanning electron microscopy, electron backscattered diffraction imaging, high-resolution transmission electron microscopy, and ChemiSTEM, also showed microstructure heterogeneities within the builds in different length scales including interdendritic and interprecipitate regions. These multiscale heterogeneities were correlated to primary solidification, remelting, and solid-state precipitation kinetics of γ″ induced by solute segregation, as well as multiple heating and cooling cycles induced by the laser additive manufacturing process.

Rationalization of Microstructure Heterogeneity in INCONEL 718 Builds Made by the Direct Laser Additive Manufacturing Process

In this paper, a comparative study of high-cycle fatigue tests (T=650 °C, f=110 Hz, R=0.1, Kt=1) were carried out with wrought Inconel 718 and LAMed Inconel 718. The results show that the influences of the Laves phases on high-cycle fatigue properties are based on the applied stress amplitudes. At a low stress amplitude, most of the Laves phases held their original morphologies. The fatigue cracks stopped in front or detoured around them, which means that the unbroken Laves phases play an important role in hindering crack propagation. In this way, the high-cycle fatigue life of LAMed Inconel 718 was superior to that of wrought Inconel 718. However, at a high stress amplitude almost all of the Laves phases in the crack propagation region splintered into smaller fragments, parts of which separated from the austenite matrix. Microscopic holes or cracks were formed at the interface, which provided passages for the fatigue cracks to propagate. In this case, the Laves phases were harmful, leading to the degradation of fatigue performance in LAMed Inconel 718 compared with wrought Inconel 718.

The influence of Laves phases on the high-cycle fatigue behavior of laser additive manufactured Inconel 718

The quick-emerging paradigm of additive manufacturing technology has revealed salient advantages in enabling the tailored-design of structural components with more exceptional performances over ordinary subtractive processing routines. As a peculiar feature, sub-micro cellular structures widely exist in additively manufactured (AM) metallic materials. This phenomenon primarily appears with high-density dislocations and segregated elements or precipitates at the cellular boundaries. The discovery of novel metastable substructures in various alloys through numerous investigations has proven their substantial effects on the engineering properties of AM components. This paper reviews the most recent research momentum regarding the formation mechanisms (elemental segregation, dislocation cell and oxide inclusion), the kinetics of the size and morphology, the growth orientation and the thermodynamic stability of these cellular structures by taking AM austenitic stainless steel as an exemplary material. Another topic of concern here is the inherent correlation between the unique cellular microstructure and the corresponding mechanical properties (strength, ductility, fatigue, etc.) and corrosion responses (passivity, irradiation damage, hydrogen embrittlement, etc.) for this category of AM materials. The design, control, and optimization of cellular structures for additive manufacturing techniques are expected to inspire new strategies for advancing high-performance structural alloy development.

About metastable cellular structure in additively manufactured austenitic stainless steels

A crystal plasticity model incorporating the effects of precipitates in superalloys: Application to tensile, compressive, and cyclic deformation of Inconel 718

Gamma double prime (γ′′), precipitation was studied in Alloy 718 using isothermal and isochronal aging heat treatments applied between 943 and 1003 K. It is shown, that the coarsening behavior of γ′′ precipitates follows the coarsening kinetic predictions of the Lifshitz–Slyozov–Wagner (LSW) theory. The activation energy for γ′′ growth has been determined as equal to 272 kJ mol−1 and seems to be controlled by volume diffusion of niobium in the matrix. The energy of the γ′′/matrix interface, Γ, has been found to be 95 ± 17 mJ m−2 by assuming that the γ′′ precipitates adopt a disk shape which minimizes the total energy. This energy includes a volume distortion term calculated from the Eshelby inclusion theory and a surface component which is assumed to be isotropic. This interfacial energy is discussed and compared with the energy of γ′/matrix and γ′′/matrix interfaces in other superalloys. The constant K′′ of the LSW law time dependence has been calculated using the value of interfacial energy and the activation energy of γ′′ precipitates coarsening and is found to be in good agreement with our experimental values.

Gamma double prime precipitation kinetic in Alloy 718

The effect of thermal cycling on the creep behavior and life of the second generation Ni-based single crystal superalloy CMSX-4® has been investigated in the very high temperature/low stress domain. Repeated overheatings at 1100°C and/or at 1150°C were introduced in the course of the creep life of the alloy at 1050°C/120 MPa. The detrimental effect of the thermal cycling compared to the isothermal creep behavior of the alloy is shown: thermal cycling leads to an increased average creep strain rate and a reduced creep life. This is always the case even when compared to the isothermal creep behavior at the highest temperature of the cycle (i.e. 1150°C/120 MPa). Moreover, the detrimental effect of the thermal cycling frequency is observed: the greater frequency the larger the creep properties decay. Compared to the first generation MC2 alloy under the same thermal cycling conditions, CMSX-4® exhibits superior non-isothermal creep properties in the very high temperature domain (T > 1100°C) due to its rhenium and ' contents.

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Effect of the prior microstructure degradation on the high temperature/low stress non-isothermal creep behavior of cmsx-4® ni-based single crystal superalloy

High temperature creep and dwell-fatigue properties of the new nickel-based superalloy AD730™ have been investigated. Three microstructures have been studied in creep (850 °C and 700 °C) and dwell-fatigue (700 °C stress control with trapezoidal signals, and dwell times ranging from 1 s to 3600 s): a coarse grains microstructure, a fine grains one, and single crystalline samples. The aim of this study is to assess the influence of the grain size on creep and creep-fatigue properties. It is demonstrated that fine and coarse grains microstructures perform similarly in creep at 700 °C, showing that the creep properties at this temperature are controlled by the intragranular precipitation. Moreover, both the coarse grains and the fine grains microstructures show changes in creep deformation mechanisms depending on the applied stress in creep at 700 °C. At higher creep temperatures, the coarse grains microstructure performs better and almost no effect is observed by suppressing grain boundaries. During dwell-fatigue tests at 700 °C, a clear effect of the mechanical cycling has been evidenced on the time to failure on both the coarse and the fine grains microstructures. At high applied stresses, a beneficial effect of the cyclic unloading to the lifetime has been observed whereas at lower applied stresses, mechanical cycling is detrimental compared to the pure creep lifetime due to the development of a fatigue damage. Complex creep-fatigue interactions are hence clearly evidenced and they depend on the pure creep behavior reference.

Relationships between Microstructural Parameters and Time-Dependent Mechanical Properties of a New Nickel-Based Superalloy AD730™

Non-isothermal creep tests were performed on a first generation single crystal Ni-based superalloy. These tests consist in performing one short exposure at 1200 °C during a creep test at 1050 °C. Based on the experimental results, residual creep life after the overheating can be well predicted using a continuum damage mechanics model derived from the Rabotnov–Kachanov law. It appears clearly that the behavior of the material is conditioned either by the microstructure developed before the overheating or by the overheating duration. In fact, as a striking result, an overheating performed on a microstructure with cubic γ′ precipitates or increasing the overheating duration lead to longer non-isothermal creep life. Microstructure modifications, such as γ′ precipitates rafting and dislocation network recovery at the γ/γ′ interfaces due to the overheating, modifying the internal stress σ i of the material, prove to be deleterious to the non-isothermal creep life of the superalloy in the conditions investigated in this paper.

Antoine Organista

Papers

Gamma double prime precipitation kinetic in Alloy 718

Effect of the prior microstructure degradation on the high temperature/low stress non-isothermal creep behavior of cmsx-4® ni-based single crystal superalloy

Relationships between Microstructural Parameters and Time-Dependent Mechanical Properties of a New Nickel-Based Superalloy AD730™

On the role of the internal stress during non-isothermal creep life of a first generation nickel based single crystal superalloy