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Creep Properties of an Extruded Copper–8% Chromium–4% Niobium Alloy

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Creep Properties of an Extruded Copper–8% Chromium–4% Niobium Alloy Materials Science and Engineering A369 (2004) 101–111 Creep properties of an extruded copper–8% chromium–4% niobium alloy M.W. Decker, J.R. Groza, J.C. Gibeling∗ Department of Chemical Engineering and Materials Science, University of California, One Shields Avenue, Davis, CA 95616-5294, USA Received 23 June 2003; received in revised form 20 October 2003 Abstract Constant-stress creep experiments were conducted on extruded Cu–8 Cr–4 Nb (GRCop-84) from 0.57 to 0.79 Tm, with observed strain rates spanning over four orders of magnitude. GRCop-84 is composed of approximately 14 vol.% Cr2Nb distributed in sizes ranging from approximately 30 nm to 0.5 ␮m in a matrix of pure copper with a grain size of approximately 1–3 ␮m. This high-strength alloy exhibits creep curves with a standard pure metal-type appearance and true failure strains typically in excess of 10%. The Monkman–Grant relationship is obeyed with a slope of −1.08. Computation of activation energies between neighboring isotherms yields an average value of 287 ± 14 kJ/mol. Despite apparent power law creep behavior within an isotherm, a master plot of temperature compensated strain rate (ε˙/exp[−Q/RT]) versus temperature compensated stress (σ/G) demonstrates that most of the data are above power law breakdown, consistent with σ/G>˜ 10−3.A hyperbolic sine model facilitates interpolation on this master plot to stresses and/or temperatures where experimental data do not exist. Neither the threshold stress model nor the dislocation detachment model applies to GRCop-84 under these creep conditions. © 2003 Elsevier B.V. All rights reserved. Keywords: Power Law Breakdown; Creep; Copper; Particle-strengthened; Cu–Cr–Nb Alloy 1. Introduction Nb [1]. It is made by extruding argon atomized powders [1]. The particulate reinforcements in this alloy are Cr2Nb that Many engineering applications require the use of creep occur as approximately 30–500 nm diameter particles [2–4]. resistant materials. Of particular concern in this work is the The copper grain size is reported to be between 1 and 3 ␮m creep performance of a particle-reinforced copper alloy be- in the as-extruded material [1,3]. ing considered for the combustion chamber liner and nozzle GRCop-84 is a difficult material to categorize as being of the next generation space shuttle main engine (SSME). either dispersion strengthened (DS), precipitation hardened These components will be actively cooled, requiring high (PH), or a metal matrix composite (MMC). It is unlike tra- thermal conductivity and good creep resistance. This appli- ditional PH alloys in that the particles form during rapid cation requires a relatively pure matrix with the addition of solidification rather than by precipitating from a solid solu- non-shearable particles to improve creep strength to suitable tion. GRCop-84 is similar to a MMC in that it has a rela- levels. tively high volume fraction reinforcement with the largest One material that has been proposed for this application reinforcement size similar to the largest grain size. These is a particle-reinforced alloy developed by NASA Glenn particles are too large to interact with moving dislocations Research Center at Lewis Field (NASA GRC), known as and probably improve strength by a load transfer mecha- GRCop-84. This alloy has a fine-grained, pure copper matrix nism. The alloy is also similar to DS materials since Cr2Nb with approximately 14 vol.% particulate obtained through is also present as small (few tens of nanometers) intragran- the addition of 8 atomic percent Cr and 4 atomic percent ular particulate. The creep behavior of DS and MMC materials is poorly ∗ characterized by the power law model relationship that is Corresponding author. Tel.: +1-530-752-7037; fax: +1-530-752-9554. normally used to represent the dependence of creep strain E-mail address: [email protected] (J.C. Gibeling). rate on stress in pure metals and some metal alloys [5]. The 0921-5093/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2003.10.306 102 M.W. Decker et al. / Materials Science and Engineering A369 (2004) 101–111 stress exponents in particle-strengthened materials are of- [6]. This detachment stress is the mechanical stress required ten very large (20–100 [6,7]) and the activation energies are to release a dislocation from the particle/matrix interface, significantly larger than those for matrix self-diffusion [6]. after it has climbed over the particle. Since most engineer- These limitations of the power law model have led to nu- ing materials have service temperatures above zero Kelvin, merous efforts to formulate improved deformation behavior creep can occur with thermal assistance for applied stresses models in DS and MMC materials. Most of these models are less than the detachment stress. Rösler and Arzt demonstrate phenomenological in nature, but a few have a fundamental that localized climb can be stabilized if the dislocation line basis. While a single unifying model of creep behavior in energy is relaxed at the particle/matrix interface [16]. The DS and MMC materials does not exist, two prominent mod- magnitude of the detachment stress is proportional to the ex- els, the first phenomenological and the second fundamental, tent of relaxation described by the dimensionless relaxation are considered in this paper and they are discussed below. parameter k, as defined in their paper: 2 σd = σOr 1 − k (2) 2. Threshold stress model where σOr is the Orowan stress and is computed by: The basis of the threshold stress model is that the applied MGb r σOr = 0.84 √ ln (3) stress is reduced by the so-called threshold stress, resulting π(2λ − 2r) 1 − ν b in an effective stress that drives dislocations over particles. A form of this model was proposed by Barrett [8] and is where r is the average particle radius, 2λ is the interparticle used in this paper written as: spacing, M is the Taylor factor, ν is Poisson’s ratio, b is the σ − σ n magnitude of the Burgers vector, and other terms are as pre- ε˙ = A × o G (1) viously defined. The relaxation parameter can, in principal, vary from 0 to 1, but as a practical matter it is usually in the where ε˙ is the observed steady-state strain rate, A the ma- range 0.77 ≤ k ≤ 0.94 [6]. The creep rate is given by: terial constant, σ is the applied stress, σo is the threshold 2r σ 3/2 stress, G is the temperature-dependent matrix shear modu- Gb 3/2 ε˙ = ε˙o exp − (1 − k) 1 − (4) lus, and n is the stress exponent. This model has been used kBT σd to describe a number of materials regardless of whether the creep data exhibit curvature on a strain rate–stress plot, indi- where ε˙o = 6DVλρ/b is a reference strain rate, ρ is the cating the presence of a threshold (e.g., Nardone and Strife dislocation density, DV is the volume diffusion coefficient, [9]). Often investigators invoke this model as a way to re- kB is Boltzmann’s constant, and all other coefficients are as duce the apparent values of the stress exponent and acti- previously defined. In their paper, RA provide expressions vation energy to magnitudes that can be rationalized. The to compute σ/σd and k from the apparent (curve fit) stress threshold stress model has been applied to a wide variety exponent (napp), the apparent creep activation energy (Qapp), of materials [7], notably rapidly solidified aluminum alloys and the activation energy for vacancy diffusion in the pure [10], oxide DS nickel alloys [11,12], particle strengthened matrix material (Qc). intermetallics [13], and powder processed aluminum alloys The RA model has three significant shortcomings. Dislo- [14]. cation density is assumed constant and often must be made The threshold stress model has at least three notable short- unrealistically large to correlate model predictions to exper- comings. While this model does effectively describe some imental data. In addition, both the interaction parameter and creep data, it does not have a satisfactory theoretical basis. the detachment stress depend sensitively on the difference As suggested in Eq. (1), the threshold stress should depend of (Qapp − Qc) used to compute σ/σd. on temperature in proportion to (shear) modulus. In fact, The RA creep equation only applies for stresses less than some experimental results demonstrate a much stronger de- the detachment stress. As such, the detachment stress repre- pendence [7,15]. Reinforcements apparently do not cause sents an upper limit for the creep stress, whereas the thresh- the threshold stress observed in MMCs, but the threshold old stress represents a lower limit above which creep can stress scales with volume fraction of reinforcement [7,15]. occur. The detachment stress and threshold stress should not In these materials, the subtractive threshold stress may re- be equated unless a transition between these two mecha- late to the proportioning of load between the matrix and the nisms exists. particles. 3.1. Objectives 3. Dislocation detachment stress model The objective of this work is to characterize the ten- sile creep properties for extruded GRCop-84 tested under Rösler and Arzt (RA) have proposed a model of high tem- isothermal, isostress conditions from 773 to 1073 K. A sim- perature creep in DS materials based on a detachment stress ple power law model, as well as the two models presented in M.W. Decker et al. / Materials Science and Engineering A369 (2004) 101–111 103 detail above are applied to gain understanding of deforma- then cutting the “dogbone” profile, and finally stacking sev- tion mechanisms in this material. In addition, we provide a eral specimens together to drill holes in each end.
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