stefano˜gialanella alessio˜malandruccolo aerospace alloys

Topics in Mining, Metallurgy and Materials Engineering Series Editor: Carlos P. Bergmann

Stefano GialanellaAlessio Malandruccolo

Aerospace Alloys

Topics in Mining, Metallurgy and MaterialsEngineering

Series editor

Carlos P. Bergmann, Federal University of Rio Grande do Sul, Porto Alegre,Rio Grande do Sul, Brazil

“Topics in Mining, Metallurgy andMaterials Engineering”welcomes manuscripts inthese three main focus areas: Extractive Metallurgy/Mineral Technology;Manufacturing Processes, and Materials Science and Technology. Manuscriptsshould present scientific solutions for technological problems. The three focusareas have a vertically lined multidisciplinarity, starting from mineral assets, theirextraction and processing, their transformation into materials useful for the society,and their interaction with the environment.

More information about this series at http://www.springer.com/series/11054

http://www.springer.com/series/11054

Stefano Gialanella • Alessio Malandruccolo

Aerospace Alloys

Stefano GialanellaIndustrial Engineering DepartmentUniversity of TrentoTrento, Italy

Alessio MalandruccoloMetallurgy Industrial ConsultantBolzano, Italy

ISSN 2364-3293 ISSN 2364-3307 (electronic)Topics in Mining, Metallurgy and Materials EngineeringISBN 978-3-030-24439-2 ISBN 978-3-030-24440-8 (eBook)https://doi.org/10.1007/978-3-030-24440-8

© Springer Nature Switzerland AG 2020This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or partof the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations,recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission orinformation storage and retrieval, electronic adaptation, computer software, or by similar or dissimilarmethodology now known or hereafter developed.The use of general descriptive names, registered names, trademarks, service marks, etc. in thispublication does not imply, even in the absence of a specific statement, that such names are exemptfrom the relevant protective laws and regulations and therefore free for general use.The publisher, the authors, and the editors are safe to assume that the advice and information in this bookare believed to be true and accurate at the date of publication. Neither the publisher nor the authors or theeditors give a warranty, express or implied, with respect to the material contained herein or for any errorsor omissions that may have been made. The publisher remains neutral with regard to jurisdictional claimsin published maps and institutional affiliations.

This Springer imprint is published by the registered company Springer Nature Switzerland AG.The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

https://doi.org/10.1007/978-3-030-24440-8

To my son Giulio Carlo(SG)To my friends and to all those who supportedme(AM)

Preface

The aerospace field is traditionally strategic for the development of novel materialsneeded for the specific requirements of aircraft structures and propulsion systems. Alarge number of advanced materials and relevant processing routes have beenimplemented first for aerospace needs. On many occasions, aerospace materialsand technologies have subsequently been transferred successfully to a wider rangeof applications, like chemical industry, energy production, nuclear plants, terrestrialand marine transportation, and many more.

The aim of this book is to present an overview of the main classes of metallicmaterials, like light alloys (Al-, Mg-, and Ti-based, including Ti-aluminides), steels,superalloys, refractory alloys, oxide dispersion strengthened alloys, ordered inter-metallics, shape memory alloys, and related systems, e.g., laminate composites andcoatings.

The basic concepts concerning the structure of aircrafts and the working princi-ples of gas turbine engines are recalled to provide the reader with indications on thespecific needs and possible issues in this field, so that the adoption of given materialssystems and relevant processing routes are better understood.

The content of the book is organized according to the main classes of materialsthat are of actual and potential interest for aerospace applications. In each chapter,materials properties and relevant technological aspects, particularly as concernsprocessing, are presented.

The motivations that have guided the research efforts for the development of newmaterials systems have changed over the comparatively short life of aeronautics.Nowadays, energy-saving and environmental issues are paramount: these standpoints and their role in materials development are also highlighted.

A deliberate attempt, pursued in writing Aerospace Alloys, has been to include anupdated bibliography and indication of further readings, both important in a contin-uously evolving field. A list of general references concerning materials science andengineering and physical metallurgy is included among the “Further Readings” listsat the end of each chapter. In this way, the fundamental aspects of these disciplines,

vii

which may contribute to a better understanding of the book content, can easily berecovered.

We believe the book can be used in advanced undergraduate courses in materialsand mechanical engineering, considering that the starting point for writing this bookhas been a collection of lecture notes of courses that SG ran over several years at theUniversity of Trento and the University of Florence in Italy. However, we wouldexpect that the subject and relevant materials information may also be of interest toresearchers working in R&D industrial laboratories, considering that in this perspec-tive, we have included compositional tables, some in the appendices, so that thetextbook turns out be reasonably self-consistent. We believe that the mixed aca-demic (SG’s) and industrial (AM’s) background may provide original and comple-mentary stand points on the different subjects, attracting the interest of researchersand professionals working in the different but still strongly interacting developmentlaboratories in the academia and industry.

We conclude this preface with the acknowledgments. In the first place, we wishto thank all the students (AM has been one of them at some stage!), who during thelecture classes placed questions and contributed to a stimulating discussion, pro-moting in this way the improvement and correction of the lecture notes. We alsothank our colleagues for stimulating discussions and suggestions on several subjectdealt in the book.

Trento, Italy Stefano GialanellaBolzano, Italy Alessio Malandruccolo

viii Preface

Contents

1 A Brief Introduction to Aerospace Applications . . . . . . . . . . . . . . . . 11.1 Historical Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Novel Design Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81.3 Aerospace-Related Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

2 Gas Turbine Aero-Engines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172.2 Types of Aircraft Engines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182.3 Compressors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262.4 Combustors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 282.5 Turbines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312.6 State of the Art and Future Trends . . . . . . . . . . . . . . . . . . . . . . . 33References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

3 Alloys for Aircraft Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 413.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 413.2 Aluminum Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

3.2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 423.2.2 Cast Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 513.2.3 Wrought Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 533.2.4 Aluminum-Lithium Alloys . . . . . . . . . . . . . . . . . . . . . . . 733.2.5 Aluminum Alloys in Aircraft Structures: Present State

and Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 773.3 Magnesium Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

3.3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 873.3.2 Aerospace Magnesium Alloys: Metallurgy, Main

Designation System, and Tempers . . . . . . . . . . . . . . . . . . 953.3.3 Magnesium Alloy Processing . . . . . . . . . . . . . . . . . . . . . 98

ix

3.3.4 Development of Aerospace Magnesium Alloys . . . . . . . . 993.3.5 Current Aerospace Applications . . . . . . . . . . . . . . . . . . . 106

3.4 Composite Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1103.4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1103.4.2 Fiber-Metal Laminates: Development and Designation . . . 1123.4.3 Main Production Steps and Resulting Characteristics . . . . 1133.4.4 Mechanical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . 1183.4.5 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127

4 Titanium and Titanium Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1294.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1294.2 Physical Metallurgy Aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . 1364.3 Main Groups of the Titanium Alloys . . . . . . . . . . . . . . . . . . . . . 145

4.3.1 Conventional Titanium Alloys . . . . . . . . . . . . . . . . . . . . 1464.3.2 Advanced Titanium Alloys . . . . . . . . . . . . . . . . . . . . . . . 164

4.4 Titanium Processing Technologies . . . . . . . . . . . . . . . . . . . . . . . 1694.5 Defects in Titanium Alloys and Diagnostics . . . . . . . . . . . . . . . . 1794.6 Main Aerospace Applications of Titanium Alloys . . . . . . . . . . . . 182References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187

Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189

5 Steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1915.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1915.2 Iron and Steel Metallurgical Base Concepts . . . . . . . . . . . . . . . . 193

5.2.1 Annealing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1935.2.2 Quenching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1945.2.3 Tempering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1975.2.4 Age Hardening . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1995.2.5 Steel Designation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199

5.3 Standard and Special Production Techniques . . . . . . . . . . . . . . . 2005.3.1 Arc Melting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2015.3.2 Vacuum Induction Melting (VIM) . . . . . . . . . . . . . . . . . . 2035.3.3 Argon Oxygen Decarburization (AOD) . . . . . . . . . . . . . . 2035.3.4 Vacuum Oxygen Decarburization (VOD and VODC) . . . . 2075.3.5 Electroslag Remelting (ESR) . . . . . . . . . . . . . . . . . . . . . 2095.3.6 Vacuum Arc Remelting (VAR) . . . . . . . . . . . . . . . . . . . . 212

5.4 Steels for Aerospace Applications . . . . . . . . . . . . . . . . . . . . . . . 2165.4.1 Maraging Steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2165.4.2 AerMet Steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2265.4.3 Stainless Steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 230

5.5 Steel-Based Aerospace Components . . . . . . . . . . . . . . . . . . . . . . 2525.5.1 Bearings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2535.5.2 Brake Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2545.5.3 Gas Turbine Jet Engine Parts . . . . . . . . . . . . . . . . . . . . . 2585.5.4 Gearboxes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 258

x Contents

5.5.5 Undercarriage Applications . . . . . . . . . . . . . . . . . . . . . . . 2605.5.6 Wings and Engine Pylons . . . . . . . . . . . . . . . . . . . . . . . . 2605.5.7 Aerospace Applications of Steels: An Overview . . . . . . . . 261


6 Superalloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2676.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2676.2 Cobalt-Based Superalloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274

6.2.1 Main Constituent Elements and Relevant StrengtheningMechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 282

6.2.2 Latest Developments of Co Superalloys . . . . . . . . . . . . . . 2866.3 Iron-Based Superalloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 289

6.3.1 Strengthening Mechanisms and Classificationof Iron-Based Superalloys . . . . . . . . . . . . . . . . . . . . . . . 290

6.3.2 Alloy 718 and Properties of Iron-Based Superalloys . . . . . 2976.4 Nickel-Based Superalloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299

6.4.1 Relevant Metallurgical Issues . . . . . . . . . . . . . . . . . . . . . 3116.4.2 Selection Criteria for Alloying Elements

in Nickel-Based Superalloys: The γ Matrix . . . . . . . . . . . 3216.4.3 Selection Criteria for Alloying Elements

in Nickel-Based Superalloys: The γ0 Precipitates . . . . . . . 3316.5 Single-Crystal and Directionally Solidified Superalloys . . . . . . . . 343

6.5.1 Properties of DS and SC Superalloys . . . . . . . . . . . . . . . . 3456.5.2 Principles and Technological Aspects of Directional

Solidification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3546.6 Heat Treatments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3646.7 Applications of Superalloys in the Aerospace Field . . . . . . . . . . . 378References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383

Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 386

7 Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3877.1 General Aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3877.2 Coating Deposition Techniques for Gas Turbine Engines . . . . . . . 388

7.2.1 Thermal Spraying . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3907.2.2 Physical Vapor Deposition . . . . . . . . . . . . . . . . . . . . . . . 3967.2.3 Chemical Vapor Deposition . . . . . . . . . . . . . . . . . . . . . . 398

7.3 Main Coating Systems in Gas Turbine Engines . . . . . . . . . . . . . . 4017.3.1 Coatings Resistant to High-Temperature Corrosion . . . . . 4027.3.2 Thermal Barrier Coatings . . . . . . . . . . . . . . . . . . . . . . . . 4097.3.3 Failure Mechanisms of Corrosion Resistant

and Thermal Barrier Coatings . . . . . . . . . . . . . . . . . . . . . 4197.3.4 Clearance Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4247.3.5 Erosion- and Fretting-Resistant Coatings . . . . . . . . . . . . . 428


Contents xi

8 Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4398.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 439

8.1.1 General Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4448.1.2 Pitting Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4458.1.3 Crevice Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4488.1.4 Selective Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4498.1.5 Galvanic Corrosion (Two-Metal Corrosion) . . . . . . . . . . . 4518.1.6 Mechanochemical Corrosion . . . . . . . . . . . . . . . . . . . . . . 4518.1.7 Hydrogen Embrittlement . . . . . . . . . . . . . . . . . . . . . . . . 456

8.2 High-Temperature Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . 4598.2.1 High-Temperature Oxidation: General Thermodynamic

and Kinetic Aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4598.2.2 Alloy Oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4678.2.3 Surface Nanocrystallization . . . . . . . . . . . . . . . . . . . . . . 4728.2.4 Gas Phase Corrosion: Sulfur-Induced Corrosive

Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4788.2.5 Hot Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4808.2.6 Hot Corrosion Testing . . . . . . . . . . . . . . . . . . . . . . . . . . 492


9 Other Interesting Alloys for Aerospace and Related Applications . . . 5019.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5019.2 Refractory Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 502

9.2.1 General Aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5029.2.2 Refractory Metal Alloys . . . . . . . . . . . . . . . . . . . . . . . . . 5049.2.3 Other Refractory Alloys: Chromium and the Platinum

Group Metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5099.3 ODS: Oxide Dispersion Strengthened Alloys . . . . . . . . . . . . . . . 5119.4 Intermetallic Compounds and Ordered Alloys . . . . . . . . . . . . . . . 5209.5 Shape Memory Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 526

9.5.1 Main Phenomenological Aspects and RelevantMechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 526

9.5.2 Aerospace Applications of Shape Memory Alloys . . . . . . 531References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 537

Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 545

Appendices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 547

xii Contents

Abbreviations

ACARE Advisory Council for Aviation Research and Innovation in EuropeAISI American Iron and Steel InstituteALPS Advanced low-pressure systemsAOD Argon oxygen decarburizationAPB Antiphase boundaryAPS Air plasma sprayingASM American Society of MaterialsASTM American Society for Testing and MaterialsATC Atmosphere and temperature controlled plasma sprayingAUW All-up weightBC Before Christbcc Body centered cubicbct Body-centered tetragonalBVI Blade-vortex interactionCBN Cubic boron nitrideCCT Continuous cooling transformationCEVAM Consumable electrode arc meltingCEVAR Consumable electrode vacuum arc remeltingCF Casting factorCFRP Carbon fiber reinforced polymerCGDS Columnar-grained directionally solidifiedCHM Cold heart meltingCMAS Calcium-magnesium-aluminosilicateCP Commercially pureCROR Counter-rotating open rotorCTE Coefficient of thermal expansionCTOL Conventional take-off and landingCV Carrier variantCVD Chemical vapor depositionDAS Dendrite arm spacingDB Diffusion bonding

xiii

DBT Ductile to brittle transitionDBTT Ductile to brittle transition temperatureDC Direct currentD-Gun Detonation gunDIN Deutsche Institut für NormungDMHT Dual microstructure heat treatmentDOS Density of statesDRX Direct recrystallizationDS Directionally solidified or directional solidificationEAF Electric arc furnaceEB Electron beamEBCHR Electron beam cold heart remeltingEBM Electron beam meltingEB-PVD Electron beam physical vapor depositionEDXS Energy-dispersive X-ray spectroscopyEFTA European Free Trade AssociationELI Extra low interstitialEN European Norms (Euronorm)ESR Electroslag remeltingESRR Electroslag rapid remeltingETEM Environmental transmission electron microscopyF/M Ferritic martensiticF1 Formula 1fcc Face-centered cubicFML Fiber-metal laminateGAR Grain aspect ratioGB Grain boundaryGE General ElectricGFRP Glass fiber reinforced polymerGPZ Guinier-Preston zoneGT Gas turbineGTE Gas turbine engineGTEs Gas turbine enginesHAZ Heat-altered zone or heat-affected zonehBN Hexagonal boron nitridehcp Hexagonal close packedHCP High-pressure compressorHDI High density inclusionHE Hydrogen embrittlementHEAC Hydrogen environmental assisted crackingHEDE Hydrogen-enhanced decohesionHELP Hydrogen-enhanced localized plasticityHID Hard interstitial defectsHIP Hot isostatic pressing

xiv Abbreviations

HSLA High-strength low-alloy steelsHTHC High-temperature hot corrosionHTLA High-temperature low activityHVOF High-velocity oxy-fuel flame sprayingIADS International Alloy Designation SystemIHAC Internal hydrogen-assisted crackingIUPAC International Union of Pure and Applied ChemistryLBW Laser beam weldingLCF Low-cycle fatigueLDI Low-density inclusionsLPPS Low-pressure plasma sprayingLPSO Long-period stacking orderedLPT Low-pressure turbineLTHA Low-temperature high activityLTHC Low-temperature hot corrosionMA Mechanical alloyingMISS-NC Mechanically induced surface self-nanocrystallizationMs Martensite startNDT Nondestructive testingNEO Near Earth objectsNEP Nuclear electric propulsionNS NanostructuredNTP Nuclear thermal propulsionODS Oxide dispersion strengthenedOEW Operating empty weightOPR Overall pressure ratioOTB Oxygen top and bottom blowingOWSME One-way shape memory effectPAM Plasma arc meltingPCA Process control agentPFZ Precipitation-free zone or precipitate-free zonePGM Platinum group metalPH Precipitation hardening or precipitation hardenablePHSS Precipitation hardening stainless steels or precipitation hardenable

stainless steelsPM Powder metallurgyPS Plasma sprayingPSZ Partially stabilized zirconiaPTC Plasma transferred arcPVD Physical vapor depositionR&D Research and developmentRA Reduced activationRRB Reconfigurable rotor blade

Abbreviations xv

RS Rapid solidificationSAE Society of Automotive EngineersSAP Sinter-aluminum pulverSC Single crystalSCC Stress corrosion crackingSE SuperelasticitySEM Scanning electron microscopySFC Specific fuel consumptionSFE Stacking fault energySFH Simulated flying hoursSHE Standard hydrogen electrodeSIM Stress-induced martensiteSMA Shape memory alloySMAT Surface mechanical attrition treatmentSME Shape memory effectSNC Surface nanocrystallizationSPF Superplastic formingSRZ Secondary reaction zoneSSCP Stress-strain calculation programSTOVL Short take-off and vertical landingTBC Thermal barrier coatingTBCs Thermal barrier coatingsTBO Time between overhaulTCP Tetragonally close packedTD ThoriatedTEM Transmission electron microscopyTET Turbine entry temperatureTGO Thermally grown oxideTISS-NC Thermally induced surface self-nanocrystallizationTPS Thermal protection systemTTT Time-temperature-transformationTWSME Two-way shape memory effectUNFCCC United Nations Framework Convention on Climate ChangeUNS Unified Numbering SystemUSD US dollarUT Ultrasonic testingVAFN Variable area fan nozzleVAR Vacuum arc remeltingVARTM Vacuum assisted resin transfer moldingVARTMFML Vacuum-assisted resin transfer molding fiber-metal laminateVARTMPCL Vacuum-assisted resin transfer molding-infused plasma coated

laminatesVGC Variable geometry chevron

xvi Abbreviations

VIM Vacuum induction meltingVOCD Vacuum oxygen converter decarburizationVOD Vacuum oxygen decarburizationVPS Vacuum plasma sprayingVSTOL Vertical and/or short take-off and landingXRD X-ray diffractionYSZ Yttria-stabilized zirconia

Abbreviations xvii

About the Author

Stefano Gialanella is an Associate Professor in materials science and technology atthe Department of Industrial Engineering, University of Trento, Italy. He is respon-sible for delivering courses to undergraduate students in industrial engineering,Master’s students in materials engineering and cultural heritage, and PhD studentsin materials, mechatronics, and system engineering. In addition, he has givenlectures and courses in materials science at the University of Florence, Universityof Pisa, Politecnico di Milano, and Universitat Autonoma de Barcelona (Spain). Dr.Gialanella’s research interests include intermetallics, oxidation of structural alloys,nondestructive testing of power plant components, shape-memory alloys, tribologywith particular reference to automotive brake systems, and environmental monitor-ing of cultural heritage with particular reference to phase transformations andrelevant structural and microstructural aspects. Based on this activity, he hascoauthored more than 130 scientific papers in international refereed journals. He isalso a member of the editorial boards ofMaterials Science Foundations and Journalof Metallurgy.

Alessio Malandruccolo is a plant metallurgist at an Italian steel and nickel alloycompany, a position held since the time he received his Master degree in MaterialsScience and Engineering at the University of Trento. He is currently involved inR&D, quality control and improvement, failure analysis, and revamping and designof new installations for production of new alloys. He has also worked as a teachingassistant in Materials Technology and Applied Chemistry in the Master course inArchitecture and Building Engineering at the University of Trento (Italy), collabo-rating also with the Faculty of Science and Technology of the University of Bolzano(Italy), with lectures and seminars regarding his professional sector. He developedpart of his research experience and skills at the European Synchrotron RadiationFacility – ESRF (Grenoble, F) – and at the Diamond Light Source (Didcot, UK),focusing mainly on X-ray diffraction applied to inorganic materials characterization.He cultivates a personal interest in product design and perception, user-orienteddesign, and cognitive sciences.

xix

Chapter 1A Brief Introduction to AerospaceApplications

1.1 Historical Background

Aerospace applications represent an extremely challenging field, for the develop-ment of new materials, and for the improvement of the existing ones, with theimportant additional requirement of complying with highest safety standards, par-ticularly as concerns civil and commercial aviation. A successful design andmanufacturing of components, relevant performances, certification, maintenance,and safe operation are all directly influenced by the properties of materials used forengines and airframe structures. Therefore, mechanical strength, creep, corrosion,and oxidation resistance must be optimized for any specific application in this field(Mouritz 2012). Process technologies for materials and production of componentsare also subjected to a continuous evolution, with important reciprocal interaction.

Aerospace materials technology proceed very rapidly, in order to keep up with themomentum of changes in the air transportation that over the years have been drivenby different forces. Nowadays, compositional refining and novel processing routesattain better properties in already existing material systems so that more demandingservicing conditions and higher safety standards can be met. The development ofnew materials and processing technologies, needed for the new engines, airframes,and control systems, not only resulted in improved aircraft performances but wasalso transferred and applied in other technological fields (King et al. 2009), asrecalled in the next chapters.

The first engine-operated aircraft was the Flyer I (1903, Fig. 1.1), built by Wilburand Orville Wright. In order to achieve an adequate rigidity, the wing structure wasrealized using a triangulated frame (see Fig. 1.2).

The triangulated structure of the wing provides several advantages, in addition torigidity, especially interesting for the aircraft:

• High maneuverability.• Low weight as compared to bulk structures.

© Springer Nature Switzerland AG 2020S. Gialanella, A. Malandruccolo, Aerospace Alloys, Topics in Mining, Metallurgyand Materials Engineering, https://doi.org/10.1007/978-3-030-24440-8_1

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• Possibility to swap from tension to compression of the bracing wires to sustain thespars in all operational conditions.

At the time the Flyer I was built, no specific materials were available for theconstruction of an aircraft, mostly due to the absence of an industrial field dedicatedto these applications. The materials used for the fabrication of the Flyer I were:

• Aluminum. An aspect not always mentioned is that the engine of the Flyer Iairplane, to meet the weight reduction requirements, was made of cast aluminum.

• Cotton fabric used as coating for the wing surface.• Steel used for the critical structural parts, like the bracing wires (i.e., tension-

compression wires) and the wing struts.

Fig. 1.1 The Flyer I airplane of the Wright brothers (NASA 2003)

Fig. 1.2 Schematic of triangulated structures under two different loading regimes. The letter Cindicates compressive load, the letter T indicates tensile load, the number 0 indicates the load-freecondition, and P is the overall load acting on the structure. (Redrawn from Curtis 1997)

2 1 A Brief Introduction to Aerospace Applications

• Wood for the fabrication of the ash (inner part the wing), the ribs, the spars, andthe propeller.

The maximum speed of the Flyer I was 56 km/h, due to the limitations caused bythe aerodynamic drag associated with the shape of the aircraft, in particular with thestructure of the double-wing with bracing wires (see Fig. 1.3). To overcome the drageffect of the double-wing surface and relevant wiring, needed to achieve a suitablerigidity of the structure, as a possible approach to reduce this effect, a cantilever wingwas developed.

A sort of intermediate step toward this innovative structure can be found in theBlériot monoplane (1909). The Blériot monoplane had a single wing, although notfully self-sustaining, so that steel wires were needed to ensure the rigidity of thestructure (see Fig. 1.4). This airplane was the first to feature a particularly rigidfuselage structure, a choice made to improve further its stability and maneuverabil-ity. The airframe was made of tension and compression wires (see Fig. 1.5) with asimilar geometry to the wings of the Flyer I (see Fig. 1.3). The maximum speed ofthis aircraft was still below 100 km/h, due to the poor aerodynamics of the wholestructure.

The situation was improved with the introduction of real cantilever wing struc-tures. The main difference with respect to past geometries was that this new design

Fig. 1.3 Schematic of the overall structure of the Flyer I wing and detail of the wing rib. (Redrawnfrom Curtis 1997)

1.1 Historical Background 3

was based on a self-standing structure, not requiring bracing wires and parts otherthan the wing itself, to guarantee an adequate rigidity. An example, probably thefirst, certainly the most popular, of the application of cantilever wings was theFokker DR 1 (1917) (Fig. 1.6). The main feature of this kind of wing is its thicknessthat is the key factor as concerns its strength and rigidity. The internal structure of thewing, characterized by composite ribs, is sufficient to avoid any additional externalstructure (e.g., bracing wires). Thanks to the cantilever wing, the maximum speedincreased up to 160 km/h, still maintaining substantially the same materials as theFlyer I. The situation evolved with the availability of more performing pistonengines and, subsequently, toward the end of World War II, with the developmentof gas turbine engines, affording even higher speed and in principle capable toovercome the speed of sound.

This evolution required, in the first place, new wing geometries and, conse-quently, the selection of new materials.

The higher velocity had another important consequence: the overheating of theaircraft skin, particularly at the wing leading edge and other frontal parts of the

Fig. 1.4 1909 Blériot monoplane and overall schematic of the structure. (Redrawn from Curtis1997)

Fig. 1.5 Schematic of the Blériot’s fuselage. (Redrawn from Curtis 1997)


aircraft in direct contact with the static air film that forms along the splitting line ofthe air flux. Therefore, the materials traditionally used for aircraft skin, especiallyfabric and wood, turned out to be absolutely inadequate to face the temperaturereached under the new operational conditions.

The search for new materials was also encouraged by the contemporary shortageof wood, a factor that turned to be critical, particularly in the late years of World WarII, so that alternative structural materials were increasingly needed. The solution tothese two problems was the production of metallic aircrafts, like the Junkers J-1(1915). This airplane was fully built using steel: quite an important demonstration ofwhat might have been achieved with metal in aircraft structures. On the other hand,the exceedingly high density of the ferrous alloy, notwithstanding the adopteddesign solutions, rendered the airplane not very practical and flexible in operationalmaneuvering. Therefore, few years later (1917), the availability of a novel aluminumalloy was very much welcome, and a full-aluminum airplane was produced: theJunkers J-4. No pure aluminum but an Al-Cu alloy was used (see Sect. 3.2). Thealloy had been developed a few years earlier by the German metallurgist AlfredWilm, who could only confirm empirically the hardening effect of thermal treat-ments, without having the possibility of a direct observation of the precipitatesresponsible for this improvement in the mechanical properties (see Sect. 3.2.3.3).It can be noted that this was quite a fast technological transfer, from materialsresearch to product development, of particular specific interest, considering the

Fig. 1.6 The cantilever wing of the Fokker DR 1 (Redrawn from Curtis 1997)


importance that Al-alloys were bound to have in the aerospace field, still lasting untilthe present times. These two cases are particularly important, since they havedemonstrated for the first time the possibilities of metallic materials in the fabricationof aircrafts: airframe, fuselage, and engines. This was certainly an enabling factor forthe development of commercial aviation that in fact, in between World War I andWorld War II, started its history. In this regard, a year that is worth mentioning is1933. In February the first Boeing 247, a full-metal airliner, capable to carry tenpassengers, took off for the first time, being regarded as the first modern transportairliner. Although on board comfort was very much limited by structural restrictions,the orders started to arrive soon, so numerous that the company was not able to keepup with the pace. The situation was better exploited by Douglas Aircraft, whichlaunched in the second half of the same year 1933 the 12-passenger DC-1 airliner,more spacious and comfortable than the Boeing 247. Other DCs (DC stays forDouglas Commercial) were delivered in the following years: DC-2 (1934, 14 pas-sengers) and DC-3 (1935, 21 passengers). The DC-3, thanks to its flight perfor-mances and payload capabilities, gained enormous success among the starting upairline companies that also thanks to this airplane, realized the possibility of makingprofit out of air transportation.

Although, particularly in the war years, the technological advancements in themilitary field were transferred to civilian air transportation, in some cases, the needswere rather divergent. This is the case of the maximum operational speed that withthe advent of gas turbine engines disclosed the possibility to overcome the intrinsiclimit for a propeller aircraft, i.e., the speed of sound. The supersonic flight implies achange both in the profile and in the dimension of the wings, in order to reduce airdrag, being the lift anyway guaranteed by the higher accessible speed. A goodexample of this new philosophy was the Lockheed F-104 “Starfighter” (seeFig. 1.7) capable to reach a maximum speed of Mach 2.2. Shorter and thinnerwings require a change not only in their design but also in the selected materials.Titanium alloys for the fabrication of structural components became an obviouschoice. The development of aircraft structures, considering the interplay with thedevelopment of new materials, is actually quite a complex process, since a greatnumber of variables must be considered. When increasing the maximum speed, theskin temperature becomes an issue. In this regard, the application of titanium alloysreached a particularly impressive level in the Lockheed YF 12 A and its successor,the Lockheed SR-71 “Blackbird” (see Fig. 1.8), in which not only structural parts butalso the skin and control surfaces were made of newly developed β-Ti-alloys (seeChap. 4 and Sect. 4.3.1.4).

The adoption of titanium alloys also for the fabrication of the skin of the SR-71was a necessary choice to stand the peak temperatures at its highest speed of Mach3.35. It is worthwhile to note that conventional aluminum alloys would not have hada sufficient strength to resist above Mach 2.0 and steel would be too heavy. On thisoccasion too, the experience and knowledge developed for a military aircraft wereimportant also for the design and its implementation in supersonic civil aircrafts. TheConcorde structure and skin were made of aluminum alloys (see Chap. 3), suitablefor a peak speed not exceeding Mach 2.


Fig. 1.7 Lockheed F-104 Starfighter. (Friend and Sefic 1972, NASA 2017, reproduced withpermission of the Publisher)

Fig. 1.8 Lockheed SR-71 “Blackbird.” (NASA 2008, reproduced with permission of thePublisher)


If higher temperatures of the aircraft skin are to be stood, considering, forinstance, the conditions faced in atmosphere reentry after orbital flight, like in caseof the Space Shuttle, a composite, reusable, ceramic-based protection system isneeded, made of tiles of zirconia-glassy silica felt and, for the hottest parts,carbon-carbon composites. Thanks to this protective shield, surface temperaturesin excess of 1300 �C do not provoke any harm to the underlying Al-alloy frame,unless faults in the protection system would intervene, as occurred in case of a tragicaccident (Savino et al. 2005). The engine temperature is another parameter that hasbeen sensitively influenced by the selected materials properties. The trend in theoperational temperatures and capabilities of aircraft engines has always been increas-ing, starting from the in-line piston engines of the early propeller-operated aircrafts,reaching material temperature in the 200 �C range, going through the radial high-efficiency engines, with peak temperatures of 500 �C, arriving to the extremethermal challenge of the newly developed gas turbine engines. The hottest structuralparts, i.e., the high-pressure stage turbine blades, nowadays made of nickel-basedsuperalloys (see Sect. 6.4), are operated at gas temperatures above the materialcapability, i.e., its incipient melting point. It is only thanks to the specific micro-structure of the components, i.e., single crystal, and optimized cooling design, i.e.,hollow blades, that these conditions can be safely sustained.

1.2 Novel Design Criteria

In recent years, the development of aircraft structures has become increasinglycomplex in terms of metrics and design drives, with respect to the early times,when technical performances were by far the dominant aspects. Nonetheless, as inthe past, contemporary materials are still a decisive enabling factor, since onlysuitable materials properties will render new design requirements feasible. A remark-able evolution has occurred in the selection criteria that, depending on the specificaircraft structure, will anyway be a trade-off choice.

The improvements achieved in aircraft structures have been continuously accom-panied by a growing concern for safety. This aspect becomes increasingly importantwith the development of civil aviation, where the protection of passengers’ lives isnot only an ethical issue but also an economic parameter, considering the impact ofinsurance costs on the business plan budget of any company, not to mention theindirect costs that air accidents may provoke. This aspect appeared soon dramaticallyevident, for the accidents occurred at two de Havilland Comet aircrafts duringtake-off at the airport of Ciampino in Rome (Italy) in 1954. The de HavillandComet (1952, see Fig. 1.9) was a commercial plane derived from a military cargothat originally had no windows in the fuselage that were present instead in the case ofthe commercial version.

The reason for the failure, occurring under the effect of pressurization-depressurization cycles, was an inappropriate design of the window frames, whosecorners acted as initiation spots for the fracture, propagating to the rest of the


fuselage. This failure could not occur in the original military model since nowindows were there in the fuselage. The disasters blocked the fast expansion ofBritish Airways that at the time was gaining a leading position in civil aero-transportation. Important consequence of this sort of structural failures was theadoption of two general design criteria for aircraft structures: safe life and fail safe,which can be regarded as the main guidelines for the development of passengerairliners and in general of all aircrafts for commercial transportation. Safe lifecriterion means the possibility to predict the useful lifetime of structures, compo-nents, and systems of an aircraft, in order to schedule maintenance, to proceed withlimited repairs, and to replace heavily damaged parts. As to the fail safe criterion, it isparamount in aircraft structures since it should allow the airplane to retain sufficientoperational capabilities, notwithstanding any possible failure, allowing, for instance,emergency landing procedures.

Looking at the present and past use of aluminum alloys in aircraft structures, itsounds strange that after the Comet disasters, Al-Zn alloys (see Fig. 1.10 and alsoChap. 3) were banned, since they were considered the main responsible for thestructural failure. Once the real reasons for the accidents were made clear, the useand consequent development of these alloys was resumed and is continuing until

Fig. 1.9 RCAF de Havilland Comet. (a) Internal view of the aircraft showing the rupture aroundthe window; (b) external view showing the same rupture in the skin of the fuselage (see arrows forthe indication of the position of the window and the crack nearby); (c) schematic of the airplane(Darling 2001; The National Archives 2019)

1.2 Novel Design Criteria 9

present, involving also other systems like: Al-Li and Al-Sc alloys, the latest familiesof Al-alloys to be developed.

The success of aluminum in aerospace applications is due to the excellent specificmaterial properties, considering its comparatively low density, also as concerns itsalloys. For this same reason, magnesium alloys are attracting renovated interest, afterhaving had an important role particularly in military aircraft soon after the end ofWorld War II and a decline afterward, for their limitations (see Sect. 3.3) but also forthe strong competition by Al-alloys. The absolute leading role of Al-alloys in aircraftindustry has been “spoiled” by a real breakthrough brought about by the latestBoeing airliner: the 787 Dreamliner (see Fig. 1.11). The whole project has beendeveloped on the basis of completely new approaches, redesigning all parts andsystems, according to new standards. One of the most innovative and revolutionaryaspects regards the massive use of structural composites, adopted also for thefabrication of large single-piece components, like the wing box and the forwardfuselage piece including the cockpit, and the barrel section. These are polymermatrix composites with reinforcing carbon fiber felts, manufactured directly withthe component shape, thus requiring particularly large infrastructures.

Interestingly the extended use of structural composites in the Dreamliner, sum-ming up to 50% of the operating empty weight (OEW, see Fig. 1.11), has occurred inassociation with a further increase in the percentage of titanium alloys. Thesematerials had already been extensively used in the previous Boeing airliner, theB777 for structural components and other parts, previously largely made of steel (seeTable 1.1).

Fig. 1.10 Evolution of the yield strength of various Al-alloys as a function of the 1st year they wereused in relevant commercial aircrafts. (Redrawn from Starke and Staley 1996)


This choice is coherent with the deliberate strategy, which drove the design ofthese more recent airliners, for an extreme weight reduction. This is of coursegenerally important for aircraft structures. As concerns composites, also in thepast, their early use in aeronautics turned out to be decisive on some notoriousoccasions. The vertical takeoff option of the Sea Harrier could only be implementedonce some structural parts were made using polymer matrix composites, as illus-trated in Fig. 1.12, with a consequent decisive weight reduction. However, thechoice of lighter materials is nowadays driven also by the need to reduce fuelconsumption, with relevant benefits not only on budget savings but also environ-mental sustainability. For a large airliner, a weight reduction of 1000 kg reduces thefuel consumption by 1.1–1.5% (King et al. 2009). This can be achieved by anintelligent combination of the different classes of available materials, as shown bythe data in Table 1.2 pertaining to popular aircrafts.

Fig. 1.11 Materials used in Boeing 787 Dreamliner. (Redrawn from Boeing 2013)

Table 1.1 Titanium alloys used on the Boeing 777

Alloy Condition UTS [MPa] Forms

Commercially pure Annealed 345–550 Bar, plate, sheet

Ti-3Al-2.5 V Cold worked and stress relieved 860 Hydraulic tubing

Annealed 690 Honeycomb core

Ti-6Al-4 V Annealed 895 All forms

β-annealed 895 Forgings

Solution treated and annealed 1100 Fasteners

Ti-10V-2Fe-3Al Solution treated and annealed 1190 Forgings

Ti-15V-3Cr-3Al-3Sn Solution treated and annealed 1035 Sheet

Solution treated and annealed 1140 Castings

Ti-3Al-8V-6Cr-4Mo-4Zr Cold drawn and aged 1240–1450 Springs

Beta-21S Solution treated and annealed �860 Nacelle area

1.2 Novel Design Criteria 11

As it is for other transportation fields, also aviation companies have to considertheir own economical sustainability and competitiveness. A successful aerospaceproduct should be valuable for the customers, with a low environmental impact, andstill retain elevated safety standards. The cost for the end user, the passenger in caseof civil aviation, results from a number of parameters, including initial cost and costof ownership. The total cost of ownership includes several terms, according to thescheme in Fig. 1.13. It is worth saying that a more efficient management affords notonly money savings to the companies but also a reduction of the impact of aviationon the environment and, thereby, on human health. Better designed and lighteraircraft structures will have better aerodynamic performances, lower exhaust, andnoise emissions. Of course, for these latter aspects, an improved efficiency of theengines is paramount. This is the reason why also for the engines, a continuoussearch for new materials and part design have been pursued, as reflected by theevolution over the years of the turbine inlet temperature, even named turbine entrytemperature (TET).

Fig. 1.12 Sea Harrier and some parts replaced by structural composites to achieve an adequateweight reduction. Values among brackets refer to the weight saving obtained by means ofcomposite materials. (Redrawn from Calvert 1990)

Table 1.2 Weight percentages of structural materials used in the airframes of civilian and militaryaircrafts (Mouritz 2012)

AircraftAluminumalloys Composites Steel Titanium

Othermaterials

Boeing 737 81% 3% 6% 3% 7%

Airbus A330-340 68% 17% 9% 4% 2%

Airbus A380 61% 25% 10% 4%

McDonnell Douglas F-18Hornet

51% 9% 16% 13% 11%

Lockheed F-22 Raptor 11% 35% 5% 33% 16%


stefano˜gialanella alessio˜malandruccolo aerospace alloys

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