Enhancing the High-Temperature Strength of a Co-Base Superalloy by Optimizing the γ/γ′ Microstructure

Abstract

Compositionally complex polycrystalline γ/γ′ CoNi-base superalloys, such as CoWAlloy2 (Co41-Ni32-Cr12-Al9-W5-Ti0.3-Ta0.2-Si0.4-Hf0.1-C-B-Zr) are interesting candidates for new high-temperature materials. To maximize their high-temperature strength, the γ/γ′ microstructure has to be optimized by adjusting the multi-step heat treatments. Various microstructures after different heat treatments were analyzed by scanning and transmission electron microscopy and especially in-situ small-angle neutron scattering during heat treatment experiments. The corresponding mechanical properties were determined by compression tests and hardness measurements. From this, an optimum γ′ precipitate size was determined that is adjusted mainly in the first precipitation heat treatment step. This is discussed on the basis of the theory of shearing of γ′ precipitates by weak and strong pair-couplings of dislocations. A second age hardening step leads to a further increase in the γ′ volume fraction above 70% and the formation of tertiary γ′ precipitates in the γ channels, resulting in an increased hardness and yield strength. A comparison between two different three-step heat treatments revealed an increase in strength of 75 MPa for the optimized heat treatment.

High-Energy Computed Tomography as a Prospective Tool for In Situ Monitoring of Mass Transfer Processes inside High-Pressure Reactors—A Case Study on Ammonothermal Bulk Crystal Growth of Nitrides including GaN

“…Data on the elemental composition and density of the alloys were taken from manufacturer datasheets [ 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 ]. Average values were used if ranges were given in the datasheets.…”

Deformation Mechanisms in Ni-Based Superalloys at Room and Elevated Temperatures Studied by In Situ Neutron Diffraction and Electron Microscopy

“…These neutron methods enable the identification of the (1) present phases, their lattice parameters and volume fractions [11,12], (2) morphology and volume fraction of nano-sized precipitates [13,14], and (3) spatial representation of features such as cracks or air channels [15]. Neutrons facilitate the investigation of real bulk samples, typically several cubic centimeters, because of their high penetration depth and spot size [10,11,16]. In situ ND tensile tests are a well-established method to correlate the macroscopic material behavior to the deformation behavior in the crystallographically differently oriented grains under various loading conditions or elevated temperatures [17][18][19][20][21].…”

Microstructural Changes in Ni-Al-Cr-Based Heat-Resistant Alloy with Re Addition

“…The level of the requirements for these materials is constantly growing due to the new problems that arise when creating new equipment, particularly when operating under extreme conditions, such as high velocities, high temperatures, etc. The search for metallic materials that can operate in these extreme conditions is constantly ongoing [4]. One of the promising directions is the development of superalloys containing intermetallic phases.…”

“…One of the promising directions is the development of superalloys containing intermetallic phases. Some examples of these superalloys are those based on a mixture of γ -and γ-phases [1][2][3][4][5][6], where the γ-phase is a disordered fcc solid solution based on, e.g., nickel and aluminum, while the γ -phase (in this case, Ni 3 Al) is an ordered structure with an L1 2 superstructure [7][8][9]. Currently, superalloys are often created on the basis of an alloy of nickel and aluminum alloyed with various refractory elements.…”