Currently nickel-base single crystal (SX) superalloys are considered for the manufacture of critical components such as turbine blades, vanes etc., for aircraft engines as well as land-based power generation applications. Microstructure and high temperature mechanical properties are the major factors controlling the performance of SX superalloys. Rafting is an important phenomenon in these alloys which occurs during high temperature creep. It is essential to understand the rafting mechanism, and its characteristics on high temperature properties before considering the advanced applications. In this review article, the thermodynamic driving force for rafting with and without stress is explained. The nature and influence of rafting on creep properties including pre-rafted conditions are discussed. In addition, the effect of stress state on γ/γ′ rafting, kinetics and morphological evolution are discussed with the recent experimental results.

M. Kamaraj

Department of Metallurgical and Materials Engineering

Aircraft engines

Components

Creep

Crystal growth

Crystal microstructure

High temperature properties

Mechanical properties

Morphology

Nickel alloys

Reaction kinetics

Single crystals

Thermodynamics

Channel width

HIGH TEMPERATURE MECHANICAL PROPERTIES

MULTIAXIAL STRESS STATES

PARABOLIC GROWTH

RAFTING

SINGLE CRYSTAL NICKEL BASE SUPERALLOYS

THERMODYNAMIC DRIVING FORCE

superalloys

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IIT Madras

Rafting in single crystal nickel-base superalloys - An overview

Sadhana - Academy Proceedings in Engineering Sciences

Nickel based single crystal super alloys are widely used as aircraft engine blades and are still in the process of development to improve the high temperature capabilities. The typical microstructure of these alloys consists of gamma phase as the matrix strengthened by the gamma prime precipitates. In recent times, addition of refractory metals like Re are tried to improve the creep strength. The first generation of these alloys does not contain any such additions. The second generation of the alloys contains around 3–6% Re. These additions and modifications have been found to improve the temperature capability of the CMSX, though there are limitations in their addition. In this article the microstructural developments, the metallurgical and high temperature behavior of the first and second generation NiSX family subjected to stress annealing, are discussed from engineering point of view. Based on experimental findings reported in literature, deformation maps illustrating the different mechanisms over a wide range of temperature and stress levels for the first and second generation alloys are presented. © 2017, The Indian Institute of Metals - IIM.

Transactions of the Indian Institute of Metals

First Report on the Deformation Mechanism Mapping of First and Second Generation Ni-Based Single Crystal Super Alloys

In the present paper we investigate the shear creep behavior of the single crystal super alloy CMSX-4 at temperatures between 950 and 1100°C and shear stresses ranging from 80 to 155 MPa. A double shear creep test technique is used to study the shear creep behavior of four specific macroscopic crystallographic shear systems defined by a specific crystallographic shear plane and a specific crystallographic shear direction (systems investigated: {001}<110>, {100}<010>, {011}<01-1>, and {111}<01-1>). The shear creep behavior is analyzed in terms of the shape of individual creep curves and in terms of the stress and the temperature dependence of the secondary shear creep rate. Individual creep curves are generally characterized by a pronounced primary creep range where creep rates decrease by up to three orders of magnitude. A sharp creep rate minimum is not observed. The secondary creep range starts at shear stresses of the order of 0.02 and is followed by a secondary creep range which extends over shear strain ranges of the order of 0.1. No pronounced increase of shear creep rate in the later stages of creep is observed. Norton plots yield power law stress exponents ranging from 5.5 to 9.7. The temperature dependence of the secondary creep rate is of an Arrhenius type and apparent activation energies between 549. and 690 kJ/mol were found. There is a clear influence of crystallography on shear creep rates, which vary between different macroscopic crystallographic shear systems; this effect decreases with increasing temperature. The shear creep results obtained in the present study are discussed in the light of results from uniaxial testing and in the light of underlying microscopic deformation processes.

Materialwissenschaft und Werkstofftechnik

Shear creep deformation of the super alloy single crystal cmsx-4 at high temperatures and low stresses

Diffusion in Solids

Scripta Materialia

Rafting in monocrystalline nickel-base superalloys induced by shot peening

Materials Science and Technology

Directional coarsening ofγ′ phase in single crystal nickel based superalloys during tensile creep

Acta Materialia

On the kinetics of rafting in CMSX-4 superalloy single crystals

Journal	Sadhana - Academy Proceedings in Engineering Sciences
Publisher	Indian Academy of Sciences
ISSN	02562499
Open Access	No