Fatigue Test of the Inconel Alloy 718 Under Three Point Bending Load at Low Frequency
Inconel alloy 718 is a high-strength, corrosion-resistant, and hardenable Ni–Cr alloy with good tensile, fatigue, creep, and rupture strength which has resulted in its usage in a wide range of applications. It is used at temperatures ranging from −250 up
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stract Inconel alloy 718 is a high-strength, corrosion-resistant, and hardenable Ni–Cr alloy with good tensile, fatigue, creep, and rupture strength which has resulted in its usage in a wide range of applications. It is used at temperatures ranging from −250 up to 705 °C. The main areas of application are the aircraft industry, as well as the space industry. Turbine-engine components experience significant fluctuations in stress and temperature during their repeated takeoff-cruise-landing cycles. These cycles can result in localized, small, plastic strains. Thus low-cycle, low-frequency fatigue is of interest to engine design. Engine vibrations and airflow between the stages of the turbine can also result in high-cycle fatigue with rapid cycle accumulation in airfoils at much higher frequencies, in the kHz range. This article deals with fatigue test of the IN 718 alloy under three point bending load at low frequency. Most of the fatigue tests for this alloy have been previously done under rotation bending loading, with the aim of comparing the results achieved in fatigue crack initiation and crack propagation. SEM microstructure and fractography analyses have also been carried out in this regard. Keywords Inconel alloy 718 SEM fractography
Fatigue test Three point bending S-N curve
J. Belan (&) L. Hurtalová A. Vaško E. Tillová M. Chalupová Faculty of Mechanical Engineering, Department of Materials Engineering, University of Žilina, Univerzitná 8215/1, 01026 Žilina, Slovakia e-mail: [email protected] L. Hurtalová e-mail: [email protected] A. Vaško e-mail: [email protected] E. Tillová e-mail: [email protected] M. Chalupová e-mail: [email protected] © Springer Science+Business Media Singapore 2017 A. Öchsner and H. Altenbach (eds.), Properties and Characterization of Modern Materials, Advanced Structured Materials 33, DOI 10.1007/978-981-10-1602-8_7
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1 Introduction This alloy was developed in the 1960s when the former Fe-based superalloys evolved towards Ni-based superalloys. Alloy 718 contains both Fe and Ni alloyed with some Al and Ti, though the most important addition is the refractory element Nb. The strength of the alloy 718 comes from coherent solid-state precipitates, which are for a small part c′-Ni3Al but mostly c″-Ni3Nb precipitates [1–6]. The major strengthening phases in the alloy 718 are the c″ and c′ phases which produce coherency strains in the c matrix. The c″ phase is considered to be the main strengthening phase and has a DO22 BCT crystal structure, while the c′ phase is a FCC ordered phase with a Ll2 crystal structure. The strengthening phases c″ and c′ contain 3 atoms of Ni, while the c″ phase is richer in Nb, while the c′ phase is richer in Al. The c″ and c′ phases have unique morphologies. The c′ phase precipitates as round particles with size less than 200 Å, and continues to be round in shape when it coalesces at higher temperatures. The c″ phase rather takes the shape of a disk with a length of 5
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