Short-Term Creep Behavior of an Additive Manufactured Non-Weldable Nickel-Base Superalloy Evaluated by Slow Strain Rate Testing

Jinghao Xu,Ru Lin Peng,Johan J Moverare,Hans Gruber,Dunyong Deng

doi:10.2139/ssrn.3427550

Abstract

Additive manufacturing (AM) of high γ' strengthened Nickel-base superalloys, such as IN738LC, is of high interest for applications in hot section components for gas turbines. The creep property acts as the critical indicator of component performance under load at elevated temperature. However, it has been widely suggested that the suitable service condition of AM processed IN738LC is not yet fully clear. In order to evaluate the short-term creep behavior, slow strain rate tensile (SSRT) tests were performed. IN738LC bars were built by laser powder-bed-fusion (L-PBF) and then subjected to hot isostatic pressing (HIP) followed by the standard two-step heat treatment. The samples were subjected to SSRT testing at 850 oC under strain rates of 1×10-5/s, 1×10-6/s, and 1×10-7/s. In this research, the underlying creep deformation mechanism of AM processed IN738LC is investigated using the serial sectioning technique, electron backscatter diffraction (EBSD), transmission electron microscopy (TEM). On the creep mechanism of AM polycrystalline IN738LC, grain boundary sliding is predominant. However, due to the interlock feature of grain boundaries in AM processed IN738LC, the grain structure retains its integrity after deformation. The dislocation motion acts as the major accommodation process of grain boundary sliding. Dislocations bypass the γ' precipitates by Orowan looping and wavy slip. The rearrangement of screw dislocations is responsible for the formation of subgrains within the grain interior. This research elucidates the short-creep behavior of AM processed IN738LC. It also shed new light on the creep deformation mechanism of additive manufactured γ' strengthened polycrystalline Nickel-base superalloys.

Highlights

Nickel-base superalloys have been one of the most important alloy systems owing to its significant role in fuel-efficient turbine engines [1]
Where b 1⁄4 0.9 by assuming 90% mechanical deformation work was converted to heat, r is the density of the specimen and Cp is the specific heat capacity, taken as r 1⁄4 7840 kg/m3 and Cp 1⁄4 675 J/k/kg for IN738LC superalloy at 1200 K [33]
By using the slow strain rate tensile testing approach with different strain rates corresponding to 10À5/s to 10À7/s, we systematically investigated the short-term creep behavior as well as creep deformation mechanism of a laser powder-bed-fused IN738LC superalloy

Summary

Introduction

Nickel-base superalloys have been one of the most important alloy systems owing to its significant role in fuel-efficient turbine engines [1]. Through the past two decades, metallic additive manufacturing (AM), known as three-dimensional (3D) printing technique has been developed rapidly [2] This novel process promotes localized melting and solidification of a power bed to build up a part in the manner of layer by layer. It provides advanced design flexibility for net-shaped components, e.g. turbine blades with complicated internal cooling channels. Since each component in a turbine engine has its own critical service environment and been adopted to the corresponding mature material systems, and the high-temperature mechanical behavior has attracted plenty of investigations on those conventionally processed Nickel-base superalloys. Tailored compositions have been developed to improve the critical hightemperature mechanical properties

Results

Discussion

Conclusion