Increase in Strength and Fretting Resistance of Alloy 718 Using the Surface Modification Process

Auezhan Amanov,Rakhmatjon Umarov,Tileubay Amanov

doi:10.3390/ma11081366

Abstract

This work comparatively investigated the strength (hardness, yield strength, dynamic elastic modulus, and surface residual stress), fretting failure, and corrosion resistance of the as-received and treated Ni-based superalloy Alloy 718. The goal of the current research is to improve the hardness, fretting wear, and corrosion resistances of Alloy 718 through the ultrasonic nanocrystal surface modification (UNSM) process with the aim of extending the lifespan of aircraft and nuclear components made of Alloy 718. The experimental results revealed that the surface hardness increased by about 32%, the fretting wear resistance increased by about 14%, and the corrosion resistance increased by about 18% after UNSM process. In addition, the UNSM process induced a tremendous high compressive surface residual stress of about −1324 MPa that led to an increase in yield strength and dynamic Young’s modulus by about 14 and 9%, respectively. Grain size refinement up to ~50 nm after the UNSM process is found to be responsible for the increase in surface hardness as well. The depth of the effective layer generated by the UNSM process was about 20 µm. It was concluded that the UNSM process played a vital role in increasing the strength and enhancing the corrosion and fretting resistances of Alloy 718.

Highlights

Alloy 718 is mainly used in aerospace and nuclear applications in the range of operating temperature of 650–675 ◦ C because of relatively excellent mechanical properties, fusion weldability, good, excellent oxidation resistance, corrosion resistance, and creep at elevated temperatures [1].the relatively poor fretting wear resistance of Alloy 718 may restrict its use without any surface engineering [2]
The Hall-Petch expression explains the enhanced surface hardness of the ultrasonic nanocrystal surface modification (UNSM)-treated specimen, in which the surface hardness can be controlled by refining the coarse grain size [28]
It is well established that the UNSM process generates a nanocrystal layer up to a certain depth from the top surface, along with refined nano-grain size down to ~50 nm [29]

Summary

Introduction

Alloy 718 is mainly used in aerospace and nuclear applications in the range of operating temperature of 650–675 ◦ C because of relatively excellent mechanical properties, fusion weldability, good, excellent oxidation resistance, corrosion resistance, and creep at elevated temperatures [1].the relatively poor fretting wear resistance of Alloy 718 may restrict its use without any surface engineering [2]. Alloy 718 is mainly used in aerospace and nuclear applications in the range of operating temperature of 650–675 ◦ C because of relatively excellent mechanical properties, fusion weldability, good, excellent oxidation resistance, corrosion resistance, and creep at elevated temperatures [1]. Alloy 718 usually exhibits poor wear resistance, resulting in shortening the service life of components owing to the relatively low hardness in spite of its excellent properties such as corrosion, erosion, oxidation, etc. Alloy 718 can be hardened by the presence of precipitation of γ0 (Ni3 (Al,Ti)) and γ” (Ni3 Nb) phases within a (face centered cubic) FCC structure. The latter phase can be transferred into Ni3 Nb—δ one at such a high temperature, where a softening can take place [5]. It is well established that Alloy 718 can be directly used for various structural applications with no heat treatment thanks to the standard precipitation treatment leading to an increase in strength [9,10]

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Conclusion