Fundamentals of Creep in Metals and Alloys

Michael E. Kassner

• Publisher: Butterworth-Heinemann
• Year: 2015
• Edition: 3
• eBook ISBN:
  9780080994321
• Topic: Technology & Engineering
• Subtopic: Mechanical Engineering
• Index: Technology & Engineering

Although the present edition of Fundamentals of Creep in Metals and Alloys remains broadly up to date for metals, there are a range of improvements and updates that are either desirable, or required, in order to ensure that the book continues to meet the needs of researchers and scholars in the general area of creep plasticity. Besides updating the areas currently covered in the second edition with recent advances, the third edition will broaden its scope beyond metals and alloys to include ceramics, covalent solids, minerals and polymers, thus addressing the fundamentals of creep in all basic classes of materials. - Numerous line drawings with consistent format and units allow easy comparison of the behavior of a very wide range of materials - Transmission electron micrographs provide direct insight into the basic microstructure of metals deforming at high temperatures - Extensive literature review of about 1000 references provides an excellent overview of the field

Chapter 1

Fundamentals of Creep in Materials

Abstract

Creep of materials is classically associated with time-dependent plasticity under a fixed stress at an elevated temperature, often greater than roughly 0.5 T_m, where T_m is the absolute melting temperature. The plasticity under these conditions is described for constant stress and constant strain-rate conditions. Several aspects of the curve require explanation. First, three regions are delineated: Stage I, or primary creep, which denotes that portion where the creep rate (plastic strain rate), ɛ˙ = dɛ/dt is changing with increasing plastic strain or time. The primary creep rate decreases with increasing strain, but with some types of creep, such as solute drag with “3-power creep,” an “inverted” primary occurs where the strain rate increases with strain. Analogously, under constant strain rate conditions, the metal hardens, resulting in increasing flow stresses. Often, in pure metals, the strain rate decreases or the stress increases to a value that is constant over a range of strain. The phenomenon is termed Stage II, secondary, or steady-state creep. Eventually, cavitation and/or cracking increases the apparent strain rate or decreases the flow stress. This regime is termed Stage III, or tertiary, creep and leads to fracture. Sometimes, Stage I leads directly to Stage III and an “inflection” is observed. Thus, care must sometimes be exercised in concluding a mechanical steady state.

Keywords

Creep; Plasticity; Strain rate; Stress

Contents

1. Introduction 1

1.1 Description of Creep 1

1.2 Objectives 6

1. Introduction

1.1. Description of Creep

Creep of materials is classically associated with time-dependent plasticity under a fixed stress at an elevated temperature, often greater than roughly 0.5 T_m, where T_m is the absolute melting temperature. The plasticity under these conditions is described in Figure 1 for constant stress (a) and constant strain rate (b) conditions. Several aspects of the curve in Figure 1 require explanation. First, three regions are delineated: Stage I, or primary creep, which denotes that portion where (in (a)) the creep rate (plastic strain rate), ε˙=ⅆε/ⅆt

is changing with increasing plastic strain or time. In Figure 1(a), the primary creep rate decreases with increasing strain, but with some types of creep, such as solute drag with “3-power creep,” an “inverted” primary occurs where the strain rate increases with strain. Analogously, in (b), under constant strain rate conditions, the metal hardens, resulting in increasing flow stresses. Often, in pure metals, the strain rate decreases or the stress increases to a value that is constant over a range of strain. The phenomenon is termed Stage II, secondary, or steady-state (SS) creep. Eventually, cavitation and/or cracking increases the apparent strain rate or decreases the flow stress. This regime is termed Stage III, or tertiary, creep and leads to fracture. Sometimes, Stage I leads directly to Stage III and an “inflection” is observed. Thus, care must sometimes be exercised in concluding a mechanical SS.

The term “creep” as applied to plasticity of materials likely arose from the observation that at modest and constant stress, at or even below the macroscopic yield stress of the metal (at a “conventional” strain rate), plastic deformation occurs over time as described in Figure 1(a). This is in contrast with the general observation, such as at ambient tempera...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Preface
• List of Symbols and Abbreviations
• Chapter 1. Fundamentals of Creep in Materials
• Chapter 2. Five-Power-Law Creep
• Chapter 3. Diffusional Creep
• Chapter 4. Harper-Dorn Creep
• Chapter 5. The 3-Power-Law Viscous Glide Creep
• Chapter 6. Superplasticity
• Chapter 7. Recrystallization
• Chapter 8. Creep Behavior of Particle-Strengthened Alloys
• Chapter 9. Creep of Intermetallics
• Chapter 10. Creep Fracture
• Chapter 11. γ/γ′ Nickel-Based Superalloys
• Chapter 12. Creep in Amorphous Metals
• Chapter 13. Low-Temperature Creep Plasticity
• References
• Index