The Effect of Hydrogen on the Nanoindentation Behavior of Heat Treated 718 Alloy

Gaute Stenerud,Roy Johnsen,Jim Stian Olsen,Iman Taji,Afrooz Barnoush,Tarlan Hajilou

doi:10.3390/met10111451

Abstract

In this study, the effect of precipitates on the surface mechanical properties in the presence of hydrogen (H) is investigated by in situ electrochemical nanoindentation. The nickel superalloy 718 is subjected to three different heat treatments, leading to different sizes of the precipitates: (i) solution annealing (SA) to eliminate all precipitates, (ii) the as-received (AR) sample with fine, dispersed precipitates, and (iii) the over-aged (OA) specimen with coarser precipitates. The nanoindentation is performed using a conical tip, and a new method of reverse imaging is employed to calculate the nano-hardness. The results show that the hardness of the SA sample is significantly affected by H diffusion. However, it could be recovered by removing the H from its matrix by applying an anodic potential. Since the precipitates in the OA and AR samples are different, they are influenced by H differently. The hardness increase for the OA sample is more significant in −1200mV, while for the AR specimen, the H is more effective in −1500mV. In addition, the pop-in load is reduced when the samples are exposed to cathodic charging, and it cannot be fully recovered by switching to an anodic potential.

Highlights

Nanoindentation is a widely recognized technique that can probe a material’s properties at the sub-micron scale
A conical tip was used for nanoindentation to rule out this intrinsic anisotropic effect
The hardness is calculated by H = PAmax where Pmax is the maximum load in c the load function

Summary

Introduction

Nanoindentation is a widely recognized technique that can probe a material’s properties at the sub-micron scale. To determine the origin of damage or a failure in the structural components, it is necessary to distinguish the exact role of any microstructural parameter. It would be more complicated to understand the failure mechanism, when a complex microstructure is subjected to an environmental factor like hydrogen (H). Nanoindentation, in the nano-meter scale, makes it possible to resolve specific parameters influencing the mechanical properties. Ni-based super alloys gain their excellent mechanical properties from the nano-sized dispersed precipitates of γ0 and γ00 , which are embedded in the γ matrix. The γ00 precipitates (Ni3 (Nb)) have a tetragonal (DO22 ) crystal microstructure, which can transform to a more stable δ phase (DOa ) under thermomechanical processes

Methods

Results

Conclusion