Rotary bending fatigue properties of Inconel 718 alloys by ultrasonic nanocrystal surface modi fi cation technique

This study investigates the influence of ultrasonic nanocrystal surface modification (UNSM) technique on fatigue properties of SAE AMS 5662 (solution treatment) of Inconel 718 alloys. The fatigue properties of the specimens were investigated using a rotary bending fatigue tester. Results revealed that the UNSM-treated specimens showed longer fatigue life in comparison with those of the untreated specimens. The improvement in fatigue life of the UNSM-treated specimens is attributed mainly to the induced compressive residual stress, increased hardness, reduced roughness and refined grains at the top surface. Fractured surfaces were analysed using a scanning electron microscopy (SEM) in order to give insight into the effectiveness of UNSM technique on fracture mechanisms and fatigue life.


Introduction
Inconel 718 is a precipitation hardened, nickel-based superalloy that is used extensively in the aircraft engine industry.Rotor components such as compressor disks and blades, turbine disks and blades, power drive-shafts are typical applications that exploit the excellent cyclic fatigue resistance, high tensile strength, fracture toughness and oxidation resistance the alloy provides up to maximum useful service temperature of about 649 °C [1].Turbine blades, however, do not have the same strength requirements imposed on them as do turbine disks, but must be able to withstand much higher temperatures.High creep resistance and a correspondingly coarse grain size are, therefore, of primary importance in turbine blade applications [2].There are various methods of improving the Inconel 718 alloys fatigue performance by surface modification treatment.Shot peening (SP), low-plasticity burnishing (LPB), deep rolling (DR), and laser shock peening (LSP) are the most common methods [3,4].Regardless of the method, the fatigue performance of the metal is improved by inducing a greater hardness and compressive residual stress in the surface and subsurface of the material, while reducing the subsurface grain size.This study investigates the ability of ultrasonic nanocrystal surface modification (UNSM), a novel surface treatment technology, to induce similar changes in the surface and subsurface of the annealed Inconel 718 alloy to improve fatigue performance.The detailed description of this UNSM technology was given in elsewhere [5][6][7].
2 Specimen and experiment method

Specimen and configuration
The material used in this study is the annealed Inconel 718 alloy.Table 1 list the chemical composition (in wt %) according as SAE AMS 5662.Table 2 the treatment process of the annealed Inconel 718 alloy.Fig. 1 shows the shape and dimension of the rotary bending fatigue test (RFT, 53 Hz) specimen, used in this study [8,9].Table 3 lists the UNSM treatment conditions.

Observation of subsurface layers
Microstructural study of the test specimens were characterized using a scanning electron microscope (SEM; Jeol, Japan), an electron backscatter diffraction (EBSD; MIRA II LMH; Tescan, Brno, Czech).

Hardness test and Roughness test
Micro-Vickers hardness tester (MVK-E3, Mitutoyo) was used for hardness test and the average of 3 test results on each depth was determined, indentation load was 300 g and dwell time was 10 sec.Surface roughness and 3D profiles of the specimens were measured using a surface profilometer (VK-X100, Keyence).

Fatigue test
A fatigue testing machine (YRB200, Yamamoto) used in this test is a cantilever-type rotary bending fatigue testing machine (RFT, 53 Hz).The test was conducted at about 3,150 rpm (53 Hz) after mounting test specimens in order to prevent eccentricity in no-load state.The stress ratio R of rotary bending fatigue test was R = −1 and the test was conducted at the room temperature.

Fracture surface
Fractured test specimens were observed using Scanning Electron Microscope (S-4700, Hitach) in order to examine crack behavior and reasons of fatigue crack initiation in test specimens of before and after UNSM.   3 Results and Discussion

Observation of subsurface layers
Fig. 2 shows the cross-sectional SEM images of the annealed Inconel 718 specimens before and after UNSM treatment, at ×1,000 magnification.This indicates that the surface plastic deformation was introduced by UNSM technology which is capable of smoothing the majority of the original micro cracks or notches remained by machining and polishing.The SEM images show that the deformation depth is ∼50 µm.However, some of the grain features is not be well revealed by the SEM imaging.Fig. 3 shows the cross-sectional EBSD (electron backscatter diffraction) observation of the annealed Inconel 718 alloy.The nanolayer of the UNSM treated specimen (Fig. 3b) is deeper (∼100 µm) and the shape of grain appears smaller than that of the untreated sample.

Hardness and roughness tests
Fig. 4 shows of hardness variation to the direction of depth.Surface hardness of UNSM treated specimen is maximum 34 % increased comparing to that of untreated specimen.Hardness results showed that the UNSM technology can increase the hardness at the top surface, which decreased gradually with increasing the depth from the top surface.Fig. 5 shows the 3D images and surface roughness profiles of the untreated and UNSM-treated specimens.The measurement results revealed that the surface roughness was found to be R a = 0.226 and R a = 0.091 for the untreated and UNSM-treated specimens, respectively.

Fatigue test (rotary bending fatigue, 53 Hz)
Fig. 6 shows the S-N curve of RFT of the annealed 718 Inconel alloys where the fatigue strength of the UNSM-treated specimen is much larger than that of untreated specimen.UNSM effect was even greater in the long life region of 10 6 cycles or more.As a result of the fatigue crack retardation resulting from the UNSM treatment, the high-cycle fatigue endurance at 10 7 cycles was improved by as much as 42 % (UNSM 15/60) for the annealed Inconel 718 alloys.

Fracture surface observation
Fig. 7 shows a SEM photo of UNSM treated specimen by rotary bending fatigue testing at stress of σ α = 900 MPa and cycle of N f = 3.73 × 10 4 .For most of the fatigue test the fish-eye crack was not observed in the specimens.As shown in Fig. 7a, there were 6 surface crack initiation cites which were responsible for the fracture.Fig. 7b shows the cite A where the crack was initiated.This kind of crack initiation cite can be found at low cycle fatigue.Fig. 8 shows a SEM photo of the untreated specimen by rotary bending fatigue testing at stress of σ α = 700 MPa and cycle of N f = 8.76 × 10 4 .For most of the fatigue test the fish-eye crack was not observed in the specimens.As shown in Fig. 8a, there were 8 surface crack initiation cites which were responsible for the fracture.Fig. 8b shows the cite B where the crack was initiated.
Fig. 9 shows a SEM photo of UNSM treated specimen by rotary bending fatigue testing at stress of σ α = 700 MPa and cycle of N f = 2.11×10 8 .Fig. 9a shows the combination of surface-originating crack and fish-eye crack.Also, a twin fish-eye crack was found in Fig. 9c.
RBF tests at a stress of σ α = 700 MPa, the crack initiated on the untreated and UNSM-treated specimens at a cycle of 2.11×10 8 and 8.76 × 10 4 , respectively, which may be attributed to the effects of UNSM technology such as grain refinement, compressive residual stress etc.It was found that the UNSM technology can extend the fatigue life of Inconel 718 alloy to high-cycle fatigue.

Conclusions
The following conclusions can be drawn from this study: -A nanostructured layer with a thickness of about 100 µm was produced in Inconel 718 alloy by UNSM technology.
-The surface hardness of Inconel 718 alloy increased by about 34 % by UNSM technology.
-The surface roughness of the untreated and UNSM-treated specimens was found to be Ra 0.226 and Ra 0.091 µm, respectively.-The fatigue lifetime of Inconel 718 alloy increased by about 42 % by UNSM technology.
-Combination of surface-originating crack and fish-eye crack was observed.

Acknowledgment
This study was supported by the Nuclear R&D Program through the Ministry of Science, ICT and Future Planning (NRF-2014M2A8A2074099).

Fig. 3
Fig. 3 Cross-sectional EBSD analysis of a untreated of the annealed Inconel 718 alloy b UNSM treated of it

Fig. 4
Fig. 4 Variation of micro-Vickers hardness from surface to depth

Table 1
Chemical compositions of an Inconel 718 alloy

Table 2
Treatment process of the annealed Inconel 718 alloy

Table 3
UNSM treatment conditions