Evaluation of Hot Workability of Powder Metallurgy Ni-Based Superalloy with Different Initial Microstructures

Masaya Higashi,Naoya Kanno

doi:10.1007/s11661-020-06085-5

Abstract

The effect of the initial microstructure on the hot workability of a powder metallurgy Ni-based superalloy was investigated in the high-temperature range of 950 °C to 1180 °C and strain rate range of 0.001 to 1.0 s−1. Six samples with different initial microstructures were fabricated by various hot isostatic pressing (HIP) conditions and subsequent treatments such as hot extrusion. The coarse-grained samples exhibited low hot workability regardless of the deformation conditions. In contrast, the hot workability of the fine-grained samples significantly varied depending on the deformation conditions. The hot workability exhibited a peak at the sub-solvus temperature of ~ 1100 °C and decreased at temperatures higher and lower than this temperature. In addition, the hot workability decreased monotonically with increasing the strain rate. The prior particle boundaries (PPBs) acted as cavity nucleation sites and crack paths, especially at lower temperatures and higher strain rates, resulting in early fracture and low hot workability. With decreasing the grain size, the hot workability at the peak temperature improved. The extruded sample with the smallest grain size exhibited the best hot workability, owing to the avoidance of PPB fracture and the acceleration of dynamic recrystallization.

Highlights

TURBINE disks are one of the aero-engine components exposed to extreme environments, and polycrystalline Ni-based superalloys that exhibit balanced mechanical properties, such as high creep resistance, high tensile strength, and low cycle fatigue at temperatures of 700 °C and higher are commonly used to produce these disks
The average grain sizes were 6.8, 6.5, and 8.2. These results indicate that the hot isostatic pressing (HIP) pressure has little effect on the grain size when it lies between 98 and 196 MPa, but the long holding time leads to a slight grain growth between 4 and 24 hours
The effects of the deformation condition and initial microstructure on the hot workability were investigated in detail using a conventional powder metallurgy (P/M) disk superalloy

Summary

Introduction

TURBINE disks are one of the aero-engine components exposed to extreme environments, and polycrystalline Ni-based superalloys that exhibit balanced mechanical properties, such as high creep resistance, high tensile strength, and low cycle fatigue at temperatures of 700 °C and higher are commonly used to produce these disks. The superalloys used for turbine disks have been processed by the cast and wrought (C&W) route.[1] With an increase in the demand for higher-temperature resistant alloys for aero-engines, higher-strength disk superalloys have been developed by increasing alloying element concentrations and c¢ volume fractions.[2,3] In order to form such high-strength superalloys into complex shapes, it is necessary to improve the processing technique as well as the alloy development toward the practical application.

Results

Discussion

Conclusion