Observations, theories, and predictions of high-temperature creep behavior
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I. INTRODUCTION
AT temperatures above about half the absolute melting point (Tm), most metals and alloys exhibit normal creep curves, i.e., following the initial strain on loading, the creep rate decays during the primary stage, reaching a minimum or secondary value before accelerating during the tertiary stage that leads to fracture. Despite this complex curve shape, the creep properties of materials are usually described by reference to the dependences of the minimum creep rate (˙ m) on stress (), temperature (T ), and grain diameter (d ) using power-law equations of the form ˙ m ⫽ A n(1/d )m exp ⫺ Qc /RT
[1]
where A, n, m, and R are constants, and Qc is the activation energy for creep. The fact that n, m, and Qc vary depending on the test conditions imposed is then explained by assuming that different mechanisms, each associated with different vales of n, m, and Qc, control the creep characteristics displayed within different stress/temperature regimes. Pure metals are usually considered to show regimes with n ⬵ one at low stresses and n ⬵ 4 at higher stress levels, with n increasing rapidly in the so-called “power-law breakdown” range, as illustrated by results obtained for aluminum[1–7] in Figure 1. When n ⱖ 4, creep is known to occur by diffusioncontrolled generation and movement of lattice dislocations, but no general agreement has been reached on the precise mechanisms involved. Controversy also continues[8–15] over whether creep in the n ⬵ one regime takes place by diffusional creep mechanisms that do not require dislocation movement (i.e., Nabarro-Herring[16,17] or Coble creep[18]) or
B. WILSHIRE, Professor, is with the Department of Materials Engineering, University of Wales Swansea, Swansea SA2 8PP, United Kingdom. This article is based on a presentation made in the workshop entitled “Mechanisms of Elevated Temperature Plasticity and Fracture,” which was held June 27–29, 2001, in San Diego, CA, concurrent with the 2001 Joint Applied Mechanics and Materials Summer Conference. The workshop was sponsored by Basic Energy Sciences of the United States Department of Energy. METALLURGICAL AND MATERIALS TRANSACTIONS A
by dislocation processes (often referred to as Harper– Dorn creep[2]). The mechanisms governing creep of pure metals, therefore, remain the subject of unresolved debate, so it is hardly surprising that mechanism identification has proved to be an even more intractable problem with particle-hardened alloys. Yet, the phenomenon of creep has been studied for almost a century, and power-law approaches have been widely adopted to quantify creep behavior patterns for over half a century. For this reason, it seems timely to re-evaluate the key observations providing the foundations for powerlaw theories, focusing on information obtained for (a) aluminum and copper, the pure metals most often selected for mechanistic studies; and (b) 0.5Cr0.5Mo0.25V ferritic steel, a particle-hardened alloy for which extensive data sets are available.
II. CREEP OF ALUMINUM The creep properties of pure aluminum a
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