Theoretical and practical implications of creep curve shape analyses for 7010 and 7075
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I. INTRODUCTION
ALTHOUGH the primary and tertiary stages are the distinctive features of normal creep curves, the creep properties of metals and alloys are usually described only through mea# surements of the minimum or secondary creep rate (m) . With this approach, the resulting data sets are# frequently discussed by reference to the dependences of em on stress () and temperature (T), using power-law expressions[1] of the general form # [1] em A1s>E2 n exp 1Qc>RT2 where A is a constant, E is the Young’s modulus at the creep temperature, n is the stress exponent, and Qc is the activation energy for creep (in units of J mol1, when the gas constant, R 8.314 J K1 mol1). It is then widely assumed that different creep mechanisms become dominant in different stresstemperature regimes, causing variations in the values of parameters such as n. For instance, with pure metals, decreases from n 5 at high stresses toward n 1 at low stresses have been taken as evidence for a transition from dislocation to diffusional creep processes, a view contested by other researchers.[2] Further debate then continues over the mechanisms involved with the large decreases from n 5 toward n 1 as the applied stress is reduced with alloys strengthened by dispersions of fine precipitates or insoluble particles.[3] Power-law relationships are also commonly used to represent stress rupture# data, because the time to fracture (tf) is often related to m as[4] # [2] m tf constant (M) # with this inverse dependence of tf on m signifying that creep failure is strain controlled. Yet, despite their widespread adoption, power-law approaches have not led to theories capable of predicting the properties exhibited by creepresistant alloys under the conditions encountered during
H. BURT, Senior Research Officer, and B. WILSHIRE, Professor and EngD/IGDS Director, are with the Materials Research Centre, School of Engineering, University of Wales, Swansea, SA2 8PP, United Kingdom. Contact e-mail: [email protected] Manuscript submitted July 15, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
prolonged service exposure. Consequently, to provide the long-term data required for high-temperature engineering design, extensive test programs are still currently needed,[5] generally analyzed using the empirical parametric methods devised in the 1950s.[6,7] In contrast, adopting the Evans Wilshire approach, termed the Projection Concept,[8,9] the creep and creep fracture properties for stress-temperature conditions giving lives up to 100,000 hours have been predicted accurately by analyses of creep curves lasting only a few thousand hours. The predictive capabilities of the methodology have been demonstrated recently for two precipitation-hardened aluminum alloys, 2124 and 2419.[10] With these alloys, decreases ranging from n 30 at 373 K toward n 5 at 463 K were observed, with differences in behavior attributed to the loss of creep strength due to precipitate coarsening being more severe at 427 K and above with 2124. Yet, while
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