Abstract
The effects of the γ″-Ni3Nb phase on fatigue behavior of nickel-based 718 superalloys with standard heat treatment, hot isostatic pressing + solution treatment + aging, and hot isostatic pressing + direct aging were investigated by scanning electron microscope, transmission electron microscopy, and fatigue experiments. The standard heat treatment, hot isostatic pressing + solution treatment + aging, and hot isostatic pressing + direct aging resulted in the formation of more and smaller γ″ phases in the matrix in the nickel-based 718 superalloys. However, the grain boundaries of the hot isostatic pressing + direct aging sample showed many relatively coarse disk-like γ″ phases with major axes of ~80 nm and minor axes of ~40 nm. The hot isostatic pressing + direct aging sample with a stress amplitude of 380 MPa showed the longest high cycle fatigue life of 5.16 × 105 cycles. Laves phases and carbide inclusions were observed in the crack initiation zone, and the cracks propagated along the acicular δ phases in the nickel-based 718 superalloys. The precipitation of fine γ″ phases in the matrix and relatively coarse γ″ phases in the grain boundaries of the hot isostatic pressing + direct aging sample can hinder the movement of dislocation.
Highlights
Nickel-based 718 superalloys, which have excellent mechanical properties, high temperature oxidation resistance, and fatigue resistance at elevated temperatures, have been used extensively in aerospace, gas turbine engines, and power generation for decades [1,2,3,4,5,6]
Fatigue damage readily occurred in the turbine disc, which was subjected to the combined effects of mechanical stress and high temperature while in service [8]
The polycrystalline nickel-based superalloy had long fatigue cycle life of ~1.5 × 104 cycles since casting pore and acicular δ phases were eliminated by hot isostatic pressing [25]
Summary
Nickel-based 718 superalloys, which have excellent mechanical properties, high temperature oxidation resistance, and fatigue resistance at elevated temperatures, have been used extensively in aerospace, gas turbine engines, and power generation for decades [1,2,3,4,5,6]. Among all the properties of superalloys, fatigue is one of the most important, often limiting the overall service life The microstructures, such as segregation, δ phases, γ’ phase, γ” phase, and carbide particles, have a significant effect on fatigue behavior of nickel-based 718 superalloys [10,11,12,13]. The polycrystalline nickel-based superalloy had long fatigue cycle life of ~1.5 × 104 cycles since casting pore and acicular δ phases were eliminated by hot isostatic pressing [25]. The effects of the γ” phases on the fatigue behavior of the nickel-based 718 superalloys after different heat treatments were systematically investigated in this paper
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