Directionally Solidified Materials:
Nickel-base Superalloys for
Gas Turbines

From the first forged turbine blades made of iron base alloys to the present nickel base single-grain turbine blades and vanes manufactured by directional solidification, an enormous amount of research has been directed to attaining the hottest possible combustion chamber temperatures in jet engines. Temperature has been increased by about 15 K each year for the last two decades, improving the thermodynamic efficiency of the engines. The more recent developments concern the manufacturing of single-grain parts made of nickel base superalloys with large amount of the γ′ hardening phase.This paper first presents the directional solidification process used to produce single-grain parts, the formation of as-cast microstructures and the defects that can arise during solidification. In the second part the thermal treatments that are applied to the nickel base superalloys in order to enhance their mechanical properties are detailed. The effect of crystallographic orientation and of the γ/γ′ microstructure on the mechanical properties is briefly presented, as well as the. microstructural changes that can possibly arise during service.

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Cutting Force Modeling When Milling Nickel-Base Superalloys

ASME 2010 International Manufacturing Science and Engineering Conference, Volume 2 ◽

10.1115/msec2010-34325 ◽

2010 ◽

Cited By ~ 5

Author(s):

Andrew J. Henderson ◽

Cristina Bunget ◽

Thomas R. Kurfess

Keyword(s):

Tool Wear ◽

Cutting Forces ◽

Gas Turbines ◽

Elevated Temperatures ◽

Base Alloy ◽

Turbine Blades ◽

Subsurface Damage ◽

Machining Process ◽

Nickel Base Superalloys ◽

Nickel Base

Superalloys are a relatively new class of materials that exhibit high mechanical strength, ductility, creep resistance at high operating temperatures, high fatigue strength, and typically superior resistance to corrosion and oxidation even at elevated temperatures. These properties make superalloys ideal for applications in aircraft, cryogenic tanks, submarines, nuclear reactors, and petrochemical equipment. In the aerospace industry, the most commonly used superalloy is the nickel-base alloy and it accounts for 30–50% of the total material required in the manufacturing of the aircraft engine. It is used for rotating parts of gas turbines such as blades and disks, engine mounts, turbine casings and components for rocket motors and pumps. To make full use of nickel-base superalloys, a machining process must be developed that is capable of controlling and identifying tool wear, and identifying the onset of subsurface damage and controlling its formation during processing. To accomplish this, a model relating process characteristics and cutting parameters need to be developed. Due to high tool wear, the cutting forces increase drastically during machining, thus making impossible to estimate the forces with existing models. This research proposes an update to the specific cutting forces model taking into consideration rapid tool wear. As milling is the most common machining processes used to cut superalloys (e.g., turbine blades), it is specifically targeted by this research. Experiments were conducted under different cutting conditions to observe the cutting characteristics of nickel-base superalloys. Empirical observations were used to formulate updated coefficients. Later this model will be applied for real-time control of the process results, such as geometry, tool wear and subsurface damage, and also for estimation and control of other quantities such as force, deflection, surface quality and energy consumed. This will provide new insights into machining these advanced alloys.

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Modeling of Unidirectional Growth in a Single Crystal Turbine Blade Casting

Materials Science Forum ◽

10.4028/www.scientific.net/msf.508.111 ◽

2006 ◽

Vol 508 ◽

pp. 111-116 ◽

Cited By ~ 5

Author(s):

Qing Yan Xu ◽

Bai Cheng Liu ◽

Zuo Jian Liang ◽

Jia Rong Li ◽

Shi Zhong Liu ◽

...

Keyword(s):

Single Crystal ◽

Directional Solidification ◽

Turbine Blade ◽

Defect Formation ◽

Turbine Blades ◽

Solidification Process ◽

Nickel Base Superalloy ◽

Transversal Section ◽

Nickel Base ◽

Blade Casting

Single crystal superalloy turbine blade are widely used in aero-engineering. However, there are often grain defects occurring during the fabrication of blade by casting. It is important to study the formation of microstructure related defects in turbine blades. Single crystal blade sample castings of a nickel-base superalloy were produced at different withdrawal rates by the directional solidification process and investment casting. There was a difference between the microstructure morphology at the top part of the turbine blade sample castings and the one at the bottom. Higher withdrawal rates led to more differences in the microstructure and a higher probability of crystallographic defect formation such as high angle boundaries at locations with an abrupt change of the transversal section area. To further investigate the formation of grain defects, a numerical simulation technique was used to predict the crystallographic defects occurring during directional solidification. The simulation results agreed with the experimental ones.

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Mo Fiber Reinforced NiAl Composites Produced by Directional Solidification – Process, Microstructure and Mechanical Properties

MRS Proceedings ◽

10.1557/opl.2011.44 ◽

2011 ◽

Vol 1295 ◽

Author(s):

L. Hu ◽

S. Bogner ◽

W. Hu ◽

A. Bührig-Polaczek ◽

G. Gottstein

Keyword(s):

Electron Microscopy ◽

Mechanical Properties ◽

Directional Solidification ◽

Gas Turbines ◽

Eutectic Reaction ◽

Solidification Process ◽

Tensile Tests ◽

Microstructure And Mechanical Properties ◽

Structural Applications ◽

Nial Matrix

ABSTRACTComposites with a eutectic composition NiAl-9at.%Mo were produced by controlled directional solidification (DS) so that refractory metallic Mo fibers were precipitated and aligned in the NiAl matrix parallel to the solidification direction through the eutectic reaction. Such NiAl composites can be used for structural applications at high temperatures (> 1000 °C), for example as blade material for modern gas turbines. The microstructure of the composites was examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The interface fine structure between Mo fiber and NiAl matrix was studied by high resolution TEM (HRTEM). Mechanical properties were measured by tensile tests at 700 °C and 1100 °C. Accordingly, a correlation of the DS parameters, microstructure and mechanical properties was established.

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ChemInform Abstract: THE MECHANICAL PROPERTIES OF SURFACE OXIDES ON NICKEL-BASE SUPERALLOYS. I. OXIDATION

Chemischer Informationsdienst ◽

10.1002/chin.197837033 ◽

1978 ◽

Vol 9 (37) ◽

Author(s):

J. B. JOHNSON ◽

J. R. NICHOLLS ◽

R. C. HURST ◽

P. HANCOCK

Keyword(s):

Mechanical Properties ◽

Surface Oxides ◽

Nickel Base Superalloys ◽

Nickel Base

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Advancements in Fast Epitaxial High-Temperature Brazing of Single-Crystalline Nickel-Base Superalloys

Volume 4: Cycle Innovations; Industrial and Cogeneration; Manufacturing Materials and Metallurgy; Marine ◽

10.1115/gt2009-59264 ◽

2009 ◽

Author(s):

Britta Laux ◽

Sebastian Piegert ◽

Joachim Ro¨sler

Keyword(s):

High Temperature ◽

Gas Turbines ◽

Base Material ◽

Secondary Phases ◽

Single Crystalline ◽

Thermodynamic Simulations ◽

Nickel Base Superalloys ◽

Stray Grains ◽

Nickel Base ◽

Nickel Manganese

High temperature diffusion brazing is a very important technology for filling cracks in components from single-crystalline nickel-base superalloys as used in aircraft engines and stationary gas turbines: alloys, which are similar to the base material, are enhanced by a fast diffusing melting-point depressant (MPD) like boron or silicon, which causes solidification by diffusing into the base material. Generally, epitaxial solidification of single-crystalline materials can be achieved by use of conventional braze alloys, however, very long hold times are necessary to provide a complete diffusion of the MPD out of the braze gap. If the temperature is lowered before diffusion is completed, brittle secondary phases precipitate, which serve as nucleation sites for stray grains and, therefore, lead to deteriorating mechanical properties. It was demonstrated in earlier works that nickel-manganese-based braze alloys are appropriate systems for the braze repair of particularly wide gaps in the range of more than 200 μm, which allow a significant shortening of the required hold times. This is caused by the complete solubility of manganese in nickel: epitaxial solidification can be controlled by cooling in addition to diffusion. In this work, it will be shown that the nickel-manganese-based systems can be enhanced by chromium and aluminium, which is with regard to high-temperature applications a very important aspect. Furthermore, it will be demonstrated that silicon, which could be identified as appropriate secondary MPD in recent works, can be replaced by titanium, as this element has additionally a γ′ stabilizing effect. Several braze alloys containing nickel, manganese, chromium, aluminium and titanium will be presented. Previously, the influence of the above mentioned elements on the nickel-manganese-based systems will be visualized by thermodynamic simulations. Afterwards, different compositions in combination with a heat treatment, which is typical for nickel-base superalloys, will be discussed: a microstructure, which is very similar to that within the base material can be presented.

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Study on the Formation of Stray Grains during Directional Solidification of Nickel-Based Superalloys

Materials Science Forum ◽

10.4028/www.scientific.net/msf.879.1582 ◽

2016 ◽

Vol 879 ◽

pp. 1582-1587 ◽

Cited By ~ 3

Author(s):

Maria Rita Ridolfi ◽

Oriana Tassa ◽

Giovanni de Rosa

Keyword(s):

Directional Solidification ◽

Gas Turbines ◽

Complex Geometry ◽

Turbine Blades ◽

Columnar Dendrite ◽

Affecting Factors ◽

Stray Grains ◽

Geometrical Factors ◽

Stray Grain ◽

The Impact

Ni-based superalloy single-crystal turbine blades are widely used in gas turbines for aircraft propulsion and power generation as they can be subjected to high service temperature and show high mechanical properties due to the almost total elimination of grain boundaries. Particularly in presence of complex geometry shapes, rare grains nucleating apart from the primary grain, become a serious problem in directional solidification, when characterized by high-angle boundaries with the primary grain, extremely brittle due the elevated amount of highly segregating elements and the absence of grain boundary strengthening elements. It is of fundamental importance analyzing the physical mechanisms of formation of stray grains, to understand which thermo-physical and geometrical factors highly influence their formation and to find possible ways to reduce the impact of the problem. In this paper, constrained dendrite growth and heterogeneous grain nucleation theories have been used to model the formation of stray grains in directional solidification of Ni-base superalloys. The study allows to derive the preferred locations of stray grains formation and the role played by the most affecting factors: (i) geometrical: angle of primary grain dendrites with withdrawal direction and orientation of the primary grain with respect to the side walls, responsible for the formation of volumes where the stray grain undercooling is lower than the undercooling of the columnar dendrite tip; (ii) process and alloy: thermal gradient ahead to the solidification front and alloy composition, influencing the columnar dendrite tip undercooling; (iii) wettability of foreign substrates, on which the stray grain undercooling strongly depends.

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A Life Prediction Model for Single-Crystal Nickel-Base Alloys Under Low-Cycle Fatigue

Volume 10B: Structures and Dynamics ◽

10.1115/gt2020-14250 ◽

2020 ◽

Author(s):

Firat Irmak ◽

Navindra Wijeyeratne ◽

Taejun Yun ◽

Ali Gordon

Keyword(s):

Single Crystal ◽

Gas Turbine ◽

Life Prediction ◽

Low Cycle Fatigue ◽

Turbine Blades ◽

Deformation Model ◽

Cycle Fatigue ◽

Nickel Base Superalloys ◽

Nickel Base ◽

Prediction Approach

Abstract In the development and assessment of critical gas turbine components, simulations have a crucial role. An accurate life prediction approach is needed to estimate lifespan of these components. Nickel base superalloys remain the material of choice for gas turbine blades in the energy industry. These blades are required to withstand both fatigue and creep at extreme temperatures during their usage time. Nickel-base superalloys present an excellent heat resistance at high temperatures. Presence of chromium in the chemical composition makes these alloys highly resistant to corrosion, which is critical for turbine blades. This study presents a flexible approach to combine creep and fatigue damages for a single crystal Nickel-base superalloy. Stress and strain states are used to compute life calculations, which makes this approach applicable for component level. The cumulative damage approach is utilized in this study, where dominant damage modes are capturing primary microstructural mechanism associated with failure. The total damage is divided into two distinctive modules: fatigue and creep. Flexibility is imparted to the model through its ability to emphasize the dominant damage mechanism which may vary among alloys. Fatigue module is governed by a modified version of Coffin-Manson and Basquin model, which captures the orientation dependence of the candidate material. Additionally, Robinson’s creep rupture model is applied to predict creep damage in this study. A novel crystal visco-plasticity (CVP) model is used to simulate deformation of the alloy under several different types of loading. This model has capability to illustrate the temperature-, rate-, orientation-, and history-dependence of the material. A user defined material (usermat) is created to be used in ANSYS APDL 19.0, where the CVP model is applied by User Programmable Feature (UPF). This deformation model is constructed of a flow rule and internal state variables, where the kinematic hardening phenomena is captured by back stress. Octahedral, cubic and cross slip systems are included to perform simulations in different orientations. An implicit integration process that uses Newton-Raphson iteration scheme is utilized to calculate the desired solutions. Several tensile, low-cycle fatigue (LCF) and creep experiments were conducted to inform modeling parameters for the life prediction and the CVP models.

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