A method for enhancing ductility of a polycrystalline Ni-based superalloy K417G

Abstract

In this work, the effects of hot isostatic pressing and heat treatment (solution and double aging) on the high-temperature tensile properties of a nickel-based polycrystalline superalloy K417G were investigated. The experimental results indicate that following the hot isostatic pressing and heat treatment, the porosity of the alloy decreases from 0.072% (in as-cast state) to 0.043%. The volume fraction of γ' phase increases from 43.28% to 56.54%, and the shape tends to be more cubic. The plasticity of the superalloy at 900 °C increases from 5.1% in as-cast state to 9.8% followed HIP and HT, but the tensile strength remains unchanged. Large size micropores are present in the as-cast sample and cracks sprout and expand at the micropores. After hot isostatic pressing and heat treatment, micropores are effectively eliminated, preventing them from becoming sources of cracks. This delay in the emergence of cracks results in enhanced plasticity. In addition, dislocations in the specimens after hot isostatic pressing and heat treatment bypasses the γ' phase through the Orowan mechanism, leading to a further increase in plasticity.

1. Introduction

Since the mid-20th century, cast superalloys have been rapidly developed in the aerospace field, and they are mainly used in hot-end components of aeroengines, such as turbine blades, turbine disks, guide vanes, and combustion chambers [1-3]. K417G superalloys not only have the advantages of low density, good plasticity, and high medium-temperature strength, but also are inexpensive and have good stability. The as-cast structure of the alloy consists of the γ phase as the matrix, within which the (γ+γ') eutectic and carbides are uniformly dispersed. Additionally, numerous γ' strengthening phases are present within this microstructure [4, 5]. With an increase in the size of as-cast K417G superalloy components, the microscopic porosities and shrinkage cavities caused by solidification and shrinkage increase, thereby increasing the starting points for crack initiation and greatly reduces the performance of the parts [6-8]. In addition, the smaller volume fraction and coarser shape of the γ′ phase in cast superalloys weaken the strengthening effect of the γ′ phase and has an adverse effect on the mechanical properties.

Hot isostatic pressing (HIP) technology, which combines high-temperature and isostatic pressing, is an effective method for eliminating microscopic holes and loose defects inside castings. Through the combined action of high temperature and high pressure, the castings are subjected to the same pressure to achieve densification [9-11]. Roncery et al. [12] reported that appropriate HIP and heat treatment can effectively reduce the number of micropores in a material and refine the γ′ phase in an alloy, thereby improving its durable creep. Zheng et al. [10] reported that micropores in investment castings are usually located at the solidification front of the γ/γ′ eutectics, with sizes ranging from a few microns to tens of microns. Micropores are closed through the coalescence of fine γ' phase during HIP, and the diffusion of solute into the porosities induces the formation of γ' phase. However, the slow cooling rate after HIP may lead to the formation of coarse γ' precipitates, which can be refined by subsequent heat treatment.