Advanced Engineering Alloys: Nickel Superalloys

Ni-based superalloys are widely deployed as structural materials in extreme operating environments of high temperature (700-1000°C) oxidizing conditions. Typical applications for these include the high-pressure stages of gas-turbines in aerospace and engine exhaust systems in high performance vehicles. While superalloy development in the 20th century has raised the operational limits of these materials impressively, systematic methods for designing new alloys rather than empirical means are yet to be fully exploited. The role of the different alloying elements and their optimum levels is further complicated at interfaces where oxidation effects can dramatically alter both alloy microstructures and chemistries. APT is one of the few techniques which can explore the atomic-scale chemistry of these highly complicated alloys.

 

Nickel Superalloy partitioning case study: Effects of Si and Re alloying on the thermal–mechanical fatigue behaviour of a new single crystal superalloy

 TMF behaviour of a new single crystal material for land-based turbines, known as STAL-15, is studied in detail. It has been designed particularly with the aim of matching the mechanical behaviour of current standard alloys but with significantly superior oxidation/corrosion behaviour, by alloying with moderate amounts of Re and Si. Nevertheless their role and alterations they might cause in thermal-mechanical fatigue behaviour were previously not understood. Atom probe tomography was crucial in providing an understanding of  segregation and phase partitioning behavious of Re and Si in STAL 15, as shown in the figure below. The full article can be found here.

 

 

 

Nickel Superalloy oxidation case study: Oxidation behaviour of a next generation Mn containing polycrystalline Ni-based superalloy

A prototype next-generation superalloy containing 1 at.% Mn was oxidised in air at 800 °C for 100 h, and compared with a commercial Ni-based superalloy. The oxide scale consisted of a multi-phase layered structure measured by Atom Probe Tomography as uppermost NiCr2Mn2O4, followed by an inhomogeneous mix of Cr2O3, spinel MnCr2O4 and rutile (Ti,Cr)O2. The Mn did not form a homogeneous, surface passivating oxide layer. Despite this, the alloy showed a 3 × reduction of oxide thickness compared to a commercial polycrystalline Ni-based superalloy.

No other technique could have given the 3D distribution of oxides at the atomic scale. We  were able to use iit to successfully prove that Mn alloying was beneficial, but not in the expected way. Full journal article is available (open access)  here.

 

 

Ni Superalloy Grain Boundary Characterisation Case Study: On the effect of boron on Grain boundary character of a new polycrystalline superalloy

The role of boron in conferring the grain boundary character in a new polycrystalline superalloy suitable for power generation applications is considered. One boron-free and three boron-containing variants are studied using a suite of high resolution characterisation techniques including atom probe tomography (APT), high resolution secondary ion mass spectroscopy (SIMS) and transmission electron microscopy (TEM). The primary effect of boron addition is the suppression of Cr-rich M23C6 carbide and the formation instead of the Cr-rich M5B3 boride. The SIMS analysis indicates that the boride particles are distributed fairly uniformly along the grain boundaries, of length up to 500 nm along the grain boundary. The substantial majority of the boron added resides in the form of these M5B3 borides; some boron segregation is found at the γ′/M5B3 interfaces but interfaces of other forms – such as γ/γ′, γ/M5B3, γ/MC and γ′/MC – show no significant segregation. Creep testing indicates that the optimum boron content in this alloy is 0.05 at.%.

Atom probe tomography was crucial as one of the very few techniques in the world which could prove the nature and segregation of the B, C and particles at the grain boundary helping to elucidate the so-called "boron effect". Full journal article is available (open access)  here.

 

 

Contacts: Dr Stella Pedrazzini, Prof. Michael Moody