Superalloy Metallurgy a Gleeble Study of Environmental Fracture in Inconel 601 (original) (raw)
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In-situ SEM study of temperature-dependent tensile behavior of Inconel 718 superalloy
Journal of Materials Science , 2021
The effect of deformation temperature on tensile behavior of Inconel 718 alloy has been studied by a self-developed in-situ high-temperature tensile stage inside a scanning electron microscopy at a temperature range from room temperature (RT) to 750°C. The dynamic microstructure evolution and mechanical properties at different temperatures were performed and compared by the uniaxial tensile tests. The in-situ test results showed that the mechanical properties and fracture mechanisms of Inconel 718 alloy were sensitive to deformation temperatures. From RT to 650°C, the yield stress and ultimate tensile strength decrease slightly and the tensile ductility is comparable. While up to 750°C, the yield stress and ultimate tensile strength decrease significantly, the elongation and reduction of cross section also showed a significant decrease from RT to 750°C. It was found that at RT and 650°C, tensile cracks tended to initiate around the carbide particles and the triple junctions of grain boundaries, also propagated transgranularly; at 750°C, the cracks initiated at grain boundaries and propagated intergranularly. In fact, with the increase in deformation temperature, the fracture mechanism transformed from the ductile transgranular fracture to the brittle intergranular fracture.
High Temperature Deformation of Superalloy Inconel 718
Solid State Phenomena, 2012
Experimental results on hot deformation and dynamic structural processes of nickel based alloy Inconel 718 are reviewed. The focus is the analysis of dynamic precipitation processes which operate during hot deformation of these materials at elevated temperatures. Hot compression tests were performed on the solution treated precipitation hardenable nickel based superalloy Inconel 718 at 720-1150°C with a constant true strain rates of 10 -4 and 4x10 -4 s -1 . The dynamic behaviour is explained through observation of the microstructure using standard optical, scanning and transmission electron microscopy. Structural observations of solution treated Inconel 718 deformed at high temperatures, reveal non uniform deformation effects. The distribution of niobium-rich carbides were affected by localized flow within the strain range investigated at relatively low deformation temperatures 720 -850°C.
Tensile behaviour of Inconel 718 alloys under extreme conditions of temperature and strain-rate
EPJ Web of Conferences
Nickel-based superalloys are widely used in critical applications where structural components are subjected to harsh operating conditions such as elevated temperatures and high strain-rate. These alloys are also among the most hard-to-cut materials. For this reason, some critical components with complex geometrical features along with critical dimensions cannot easily manufactured by conventional technologies. A rising disruptive Additive Manufacturing (AM) technique, namely powder-based Laser Metal Deposition (LMD), is able to overcome these limitations in terms of manufacturing costs, tool wear, as well as lead time. As a consequence, the mechanical response under harsh condition of additively manufactured Nickel-based superalloys has to be accurately understood in order to guarantee the reliability of the structural parts made with them. Presently very few researches were addressed to study the dynamic tensile behaviour of Inconel 718 produced by additive manufacturing under high...
Mechanical Behavior of Inconel 625 at Elevated Temperatures
2018
Inconel 625 is a nickel-based alloy that is mainly used in high-temperature applications. Inconel 625 exhibits an unstable plastic flow at elevated temperatures characterized by serrated yielding, well-known as the Portevin-Le Chatelier effect. The evaluation of the mechanical properties of Inconel 625 at high temperatures is the aim of this work. The tensile tests were executed in temperatures ranging from room temperature to 1000 • C with strain rates of 2 × 10 −4 to 2 × 10 −3 s −1. The creep tests were executed in the temperature range of 600-700 • C and in the stress range of 500-600 MPa in a constant load mode. The optical and scanning electron microscopes were used for surface fracture observation. In the curves obtained at 200-700 • C the serrated stress-strain behavior was observed, which was related to the dynamic strain aging effect. The yield strength and the elongation values show anomalous behavior as a function of the test temperature. An intergranular cracking was observed for a specimen tensile tested at 500 • C that can be attributed to the decohesion of the carbides along the grain boundaries. The fracture surface of the specimen tensile tested at 700 • C showed the predominance of transgranular cracking with tear dimples with a parabolic shape.
Study of the Oxidation Assisted Intergranular Cracking Mechanism on a Ni-Base Superalloy
The minerals, metals & materials series, 2018
In order to collect accurate information about the widely reported Oxidation Assisted Intergranular Cracking (OAIC) mechanism of the superalloy 718 at 650°C in air environment, we investigated a new tensile test procedure which enables to have access to a quantitative semi-continuous assessment of the damaging process of the alloy 718 during a standard tensile test. Tensile tests were carried out on a solutionized and aged alloy 718 by varying the deformation mode of the alloy during the ongoing experiment. The semi-continuous quantification of the intergranular damage was performed after a fracture surface analysis of samples tested in both Dynamic Strain Ageing (DSA) domain and DSA-subdomain where Portevin-Le Chatelier (PLC) effect occurs. This innovative testing method allows to characterize the intergranular damage that occurs when the material is stressed with an unserrated DSA deformation mode as the alloy 718 is only sensitive to OAIC in this domain. In these conditions, the onset of the intergranular cracking process was found to be around 10% of total strain. An increase in the cracking kinetic when necking occurs was also noticed. A close connection between the intensity of the intergranular damage and the cumulated strain in DSA deformation mode before the onset of the damaging process was found. The gathered information represents a significant improvement in the understanding of still debated OAIC or Stress Corrosion Cracking (SCC) mechanisms.
Study of fracture mechanisms of a Ni-Base superalloy at different temperatures
AbstrAct Purpose: The Ni-base superalloy GTD-111 gains its appropriate microstructure and high temperature strength through precipitation hardening mechanism. Because of their service condition, tensile properties of the alloy have strong influence on stability and life of the blades. Design/methodology/approach: Tensile fracture mechanisms of the cast and heat treated superalloy were studied over a wide range of temperatures from 25 to 950°C with a constant strain rate of 10-4s-1. The present paper provides structural and fractography evidence by means of scanning electron microscopy at different temperatures for the superalloy GTD-111. Findings: The variation in alloy ductility was found to correlate well with the fractography results of the tensile tested specimens. Transgranular and intergranular fracture with fine dimples, cleavage facets and a combination of them were shown in the fractographs. Research limitations/implications: Although tensile properties alone are important for the alloy, it is suggested for future research to work on the simultaneous effects of tension and fatigue on the fracture mechanisms. Originality/value: It was cleared that different fracture mechanisms operate in different temperature ranges; while transgranular dimple fracture was dominant at 650°C, the dominant fracture mechanism at room temperature was intergranular.
High strain-rate behaviour of as-cast and as-build Inconel 718 alloys at elevated temperatures
Mechanics of Materials, 2021
The restoration of Inconel 718 components through conventional processes typically require cost-intensive machining operations. Moreover, the refurbishment and reuse are extremely important in structural components likely to be subjected to harsh operating conditions (dynamic loads and elevated temperatures). That is why additive manufacturing processes are rising as disruptive techniques for the restoration of Inconel 718 components. However, a limited number of studies have evaluated the mechanical properties of this nickel-base superalloy in harsh conditions. The objective of this study is to evaluate the coupled effect of strain-rates (0.001, 200 and 800 s − 1) and temperatures (20, 350 and 550 • C) on Inconel 718 additive manufactured samples produced by Direct Energy Deposition. A raw Inconel 718 material for the preparation of as-cast samples has been also investigated, and the mechanical results have been compared. The mechanical characterisation at high strain-rates has been performed through a SHTB equipped with a water-cooled induction heating system. Similar mechanical behaviour has been observed between samples realised with a power laser of 400 W and as-cast samples. A gradual decrease of the mechanical strengths is observed for increasing temperatures. The calibration of the Johnson-Cook strength model highlighted the necessity of using specific thermal softening values as a function of fixed strain-rate and temperature.
Brittle intergranular fracture of a Ni-base superalloy at high temperatures by dynamic embrittlement
Materials Science and Engineering: A, 2004
Oxygen-induced dynamic embrittlement of nickel (Ni)-base superalloys at 650 • C has been studied by static loading of notched bars in pure bending at fixed displacement on IN718 in the form of conventionally heat-treated polycrystals, polycrystals specially processed to increase the fraction of 3 boundaries, and bicrystals with a random high-angle boundary and a 5 tilt boundaries. The susceptibility to cracking is shown to depend strongly on grain-boundary character.
Materials Science and Engineering: A, 2003
Ni Á/Fe base superalloy, Inconel 718, was processed through powder metallurgy (P/M) hot isostatic pressing (HIP) route. In order to balance the strength and ductility, the HIPed material was given the standard heat treatment, viz. solution treatment at 980 8C for 1 h/water quenched (WQ) to room temperature and a two-step ageing treatment consisting of 720 8C for 8 h/furnace cooling (FC) at 55 8C h (1 to 620 8C and holding at 620 8C for 8 h before air cooling (AC) to room temperature. Optical microscopy and scanning electron microscopy (SEM) studies on the heat treated alloy have shown a homogeneous microstructure with fine grain size (25 mm) along with the presence of prior particle boundary (PPB) networks. Transmission electron microscopy (TEM) on the heat treated material has revealed the presence of oxides, MC carbides and d-precipitates at the grain boundaries and a uniform precipitation of fine gƒ and g? strengthening phases in the matrix. Tensile and stress rupture tests were performed on the heat treated material. While the yield strength (YS) and ultimate tensile strength (UTS) of the HIPed and heat treated alloy at room temperature and 650 8C were comparable to those of conventionally processed wrought IN 718, its ductility was lower. The stress rupture life of the HIPed alloy improved marginally due to heat treatment and met the minimum specification requirement of life hours but the rupture ductility was found to be inferior to that of the wrought material. The fractography of the failed samples has revealed the transgranular ductile mode of fracture in the as-solution treated alloy, while intergranular mode of failure with the decohesion of PPBs occurred more predominantly in the aged condition. This change of fracture mode with ageing treatment shows the ductility dependence on the relative strength of the matrix and PPBs. The TEM studies on the deformed alloy have revealed that the brittle oxides and carbides at the prior particle boundaries coupled with the fine gƒ and g?-precipitates in the matrix are responsible for low ductility at 650 8C. The investigations of the present study have led to better understanding of the structure Á/property relationships in HIP'/heat treated alloy 718 and suggest that the standard heat treatment recommended for wrought IN 718 is not suitable for HIPed alloy and has to be modified to realise optimum properties.
Journal of Materials Engineering and Performance, 2019
55Ni-17Cr-12Fe-9Mo-2Nb-1.5Al alloy is a nickel-based superalloy (Russian designation is XH55MбЮ or KhN55MBYu, XH55) without any equivalent in American/European alloy designation. It is used in cryogenic engine of satellite launch vehicles application in two different heat-treated conditions: (1) standard aged (STA) at 730°C/15 h+650°C/10 h and (2) STA+BC (brazing cycle) treatment carried out in vacuum at 1030°C with holding time of 30 min. Due to the braze cycle adopted for manufacturing, it is essential to study the deterioration in mechanical properties, if any. Hence, the present work is carried out to understand the material behavior in tensile mode (25, 425, 575, 700 and 900°C) for XH55 alloy in STA condition and STA+BC conditions, compared with corresponding microstructural analysis, morphology and composition using microscopy at various length scales. The tensile stress-strain curve shows characteristic sudden drops in stress with respect to strain, attributed to dynamic strain aging at different temperatures for both STA and STA+BC conditions. In STA condition, the yield strength of the material decreased with increase in temperature. In STA+BC condition, the yield strength decreased up to 425°C, increased up to 700°C as the material was subjected to artificial aging during testing and finally decreased at 900°C. Marginal deterioration in mechanical properties have been observed due to the braze cycle adopted against STA condition.