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TALAT Lecture 1253: Creep

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This lecture constitutes an introduction to creep and to the creep response of aluminium and its alloys. It provides basic information on creep and its mechanisms; it gives a description of the more extensively used mathematical relations among creep variables (time, stress and temperature); it illustrates the creep response of pure Aluminium and of Al-Mg alloys; it provides a synthesis of the information available in the literature on the creep behaviour of a number of new alloys and composites in the form of a series of figures elaborated on the basis of the data reported in same sources. Basic knowledge of physics and chemistry and some familiarity with TALAT lectures 1201 through 1205 is assumed.

Text of TALAT Lecture 1253: Creep

  • 1.TALAT Lecture 1253 Creep of Aluminium and Aluminium Alloys 26 pages, 20 FiguresAdvanced Level IPrepared by S.Spigarelli INFM/Department of Mechanics, University of Ancona, Ancona, Italy Objectives:This chapter constitutes an introduction to creep and to the creep response of Aluminium and its alloys. Its major goals are: - to provide basic information on creep and its mechanisms - to give a description of the more extensively used mathematical relations among creep variables (time, stress and temperature) - to illustrate the creep response of pure Aluminium and of Al-Mg alloys - to provide a synthesis of the information available in the literature on the creep behaviour of a number of new alloys and composites in the form of a series of figures elaborated on the basis of the data reported in same sources.Prerequisites:Familiarity with the contents of Lectures 1201 through 1205. Date of Issue: 1999 EAA- European Aluminium Association

2. 1253Creep of Aluminium and Aluminium Alloys Contents 1253 Creep of Aluminium and Aluminium Alloys ________________________2 1253.01 Introduction to Creep ____________________________________________ 3 1253.02 The Creep Curve _________________________________________________ 3 1253.03 Secondary or minimum creep rate, time to rupture _____________________ 51253.03.01Diffusion and creep________________________________________________ 6 1253.04 Parametric Approach for Time-to-Rupture Prediction ___________________ 8 1253.05 Creep mechanisms: pure metals (class M) and solid solutions (class A) ______ 9 1253.06 Methods to Improve the Creep Strength of a Metallic Material___________ 12 1253.07 Creep Rupture _________________________________________________ 14 1253.08 The Creep Response of Pure Aluminium ____________________________ 15 1253.09 The Creep Response of Al-Mg Solid-solution Alloys __________________ 16 1253.10 The Creep Response of High Strength Aluminium Alloys_______________ 17 1253.11 Creep-resistant Al-base Materials: High-strength Powder Metallurgy-Dispersion-strengthened Aluminium Alloys __________________________________ 19 1253.12 Creep-resistant Al-base Materials: Composites _______________________ 23 1253.13 Final comments on the effectiveness and common features of creep- resistantaluminium alloys _______________________________________________________ 24 1253.14 Literature_____________________________________________________ 24 1253.15 List of Figures __________________________________________________ 26 TALAT 12532 3. 1253.01Introduction to Creep The term creep indicates the permanent, time-dependent deformation that occurs when a material is exposed to high temperature under a constant applied stress (or load). Creep is observed in all metals, provided that the operating temperature (T) exceeds 0.3-0.5 TM, where TM is the absolute melting temperature. The following table lists the lower temperature limit under which creep phenomena can be neglected (pure metals). It can easily be observed that, due to its low melting temperature, Aluminium is subject to creep even at relatively low temperatures (i.e. 200-300C). Table I : Temperature limit above which creep is a limiting factor in design (pure metals and heat-resistant alloys). Aluminium T > 0.54 TMTitaniumT > 0.30 TMLow alloyed steel T > 0.36 TMAustenitic stainless steels T > 0.49 TMSuperalloys T > 0.56 TM Creep is investigated by tension, compression or even torsion tests under constant stress or, more frequently, constant load. While constant-stress data provide a better understanding of the basic creep mechanisms, constant-load tests give more useful information from an engineering point of view (for instance, the weight of a self- loading component operating at high temperature, remains unchanged throughout its life). Temperature must obviously be maintained constant during each test, and elongation should be continuously recorded. 1253.02 The Creep CurveFigure 1253.02.01 shows a schematic representation of a creep curve as obtained during a constant-load test. After an instantaneous deformation upon loading ( 0), the strain increases monotonically with time up to rupture (R). The overall deformation after loading can thus be written in the generic form: = 0 + ( ,T , t )The creep curve, i.e. the (,T,t) component of total strain, consists of three regions:A primary stage, or transient creep during which, in general, the strain rate & ( = t ) decreases with time; in this stage, deformation becomes more and more difficult as strain increases, i.e. the material experiences strain hardening.& & A secondary stage, or steady state in which the strain rate is constant ( = SS ). The occurrence of a constant strain-rate regime is generally explained in terms of a balance between strain hardening and structure recovery (a softening process determined by the high temperature).TALAT 1253 3 4. A tertiary stage during which strain rate increases with time, leading to the fracture of the component. The increases in creep rate during this stage result from a number of different damaging phenomena, such as cavities nucleation and growth, cracks formation, and extensive macroscopic necking. All these mechanisms produce a reduction in cross-sectional area that, under constant load, produces a continuous increase in applied stress and ultimately fracture. Different equations have been proposed to describe the creep curve [1]; among the soundest equations, one has the form: = 0 + P [1 exp( t / t P )] + SS t + T [exp(t / tT ) 1]&(1)where: 0 is the instantaneous elasto-plastic deformation upon loading, notassociated with creep tis time (t=0 at loading) P, tP are two stress- and temperature-dependent parameters associated withthe magnitude and duration of primary creep & t is the term for secondary creep SS T, tTare two stress- and temperature-dependent parameters associated with the magnitude and duration of tertiary creep.Figure 1253.02.01 represents the creep curve in its more general form; yet, frequently one or two stages of the curve may be absent. For example, for some materials primary and secondary creep can be neglected, since the whole creep curve is dominated by an extensive tertiary stage. Even more frequently, secondary creep is extremely short and very difficult to identify, and in this case a minimum creep-rate value ( m ) rather than a steady-state value is commonly considered.& TALAT 12534 5. 1253.03Secondary or minimum creep rate, time to ruptureThe analysis of the creep curve permits to identify the two parameters used in the engineering practice to evaluate the creep behaviour of a material. Time to rupture is perhaps the most useful from an engineering point of view, and is particularly important when creep life is the dominating concern (for example in turbine blades and rocket motor nozzles). The second parameter is steady-state or minimum creep rate. This parameter is considered when rupture, due to the low operating stress, will presumably not occur during the life of the component. In this case, in designing a component, the magnitude of the secondary creep rate can be used to evaluate the steady accumulation of strain with time in service and, with some caution, even to extrapolate the creep life of the component.The magnitude of the minimum or steady-state creep rate is influenced by both temperature and applied stress. In particular, an increase in applied stress will correspond to a large increase in steady-state creep rate. The dependence of the steady- state creep rate on applied stress at constant temperature is commonly described by the power law equation: SS = B n & (2)where B is a temperature-dependent material parameter and n, for pure metals, is 4-5. A plot of the logarithm of strain rate versus a logarithm of applied stress at different temperatures thus, results in a series of straight lines with slope n, Figure 1253.03.01 (a). When the applied stress is particularly low, a transition to a different creep regime with n=1 is frequently observed; this transition is associated with a variation of the creep mechanism from dislocation creep (n=4-5) to purely diffusive creep (n=1). The variation of the operating temperature also produces a significant change in the magnitude of the steady-state creep rate. This behaviour, at a given stress, is described by an equation in the Arrhenius form: SS = C exp ( Q / RT ) & (3)where C is a stress-dependent material parameter, R is the gas constant, and Q (activation energy for creep) for pure metals coincides with the activation energy for diffusion (Q0). When the natural logarithm of the strain rate at different stresses is reported as a function of 1/T, a family of straight lines with slope Q/R is obtained, Figure 1253.03.01 (b). TALAT 1253 5 6. Equations 2 and 3 can be combined, giving: SS = A n exp ( Q / RT ) & (4)where A is a material parameter. While in pure metals n does not depend on temperature, and Q does not depend on stress, in many engineering alloys a complex dependence of Q on applied stress, and a high or very high (7-20) value of the temperature-dependent stress exponent n are frequently observed. 1253.03.01 Diffusion and creepDiffusion in a solid occurs because atoms can move from one atomic site to another. In particular, the atoms vibrate about their mean position with a frequency while, at temperature T, the average total energy for each atom is 3kT (where k is the Boltzmann constant). Since 3kT is an average value, it is easy to find atoms with higher or lower energy in any instant; thus, according to the statistical mechanical theory, the probability p that an atoms has an energy >q is:qp = exp kT Based on this expression, it can be demonstrated that, for an alloy A+B, the flow of atoms per unit time (J) between two different adjacent planes with different concentrations of the A element obeys the relationship (Ficks first law of diff

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