Creep Deformation of Dispersion-Strengthened Copper S.E. BROYLES, K.R. ANDERSON, J.R. GROZA and J.C. GIBELING The creep behavior of an internally oxidized, A12Oa dispersion-strengthened copper alloy, GlidCop Al-15, has been investigated in the temperature range of 745 to 994 K. The results exhibit a high apparent stress exponent (10 to 21) and a high apparent activation energy for creep (253.3 kJ/mole). To describe the creep behavior of this alloy, the Rrsler-Arzt model for attractive particle/dislocation interaction is applied. The results are in good agreement with the model when account is taken of the effects of the fine elongated grains and heavily dislocated structures revealed through transmission electron microscopy. The analysis demonstrates that the dislocation/particle interaction is of moderate strength in this alloy, consistent with the observation that the particle/matrix interface is partially coherent. In addition, the analysis reveals that the choice of mechanism and corresponding activation energy for vacancy diffusion has only a small effect on the calculated model parameters. It is argued that the weak dependence of subgrain size on stress demonstrates that creep deformation is particle controlled, rather than subgrain size controlled. In addition, the poorly developed subgrain structure and high dislocation densities are attributed to the presence of the fine oxide particles. Finally, the dependence of rupture time on stress is shown to be consistent with a description of creep fracture based on diffusive cavity growth with continuous nucleation. I. INTRODUCTION THE development of future space and energy technol- ogies, such as aerospace propulsion systems and fusion power plants, will require continuing improvements in the performance of materials that can resist high temperatures. One approach to meet this need is to develop the structural capabilities of high melting point materials such as ceram- ics, refractory metals, and intermetallic compounds that re- sist high operating temperatures in a static fashion. As an alternative, a variety of current and proposed engineering applications are based on the concept of active cooling in order to extend the operating temperatures of structural ma- terials. 11~1In this approach, heat is extracted from a source, through the component, and dissipated through a cooling medium on the opposite side. The heat flux through these components is usually quite high and may fluctuate during a service cycle, thereby causing failure by thermal-mechan- ical fatigue. Thus, active cooling requires a structural ma- terial that has a high thermal conductivity as well as adequate creep and fatigue resistance at elevated tempera- tures. Because of its excellent thermal properties, copper has been proposed as a material for actively cooled compo- nents. For elevated temperature strength, dispersion strengthened (DS) copper alloys are particularly attractive due to their excellent combination of thermal and electrical conductivities, strength retention, and microstructural sta- bility.t5.6] The superior elevated temperature strength of var- ious DS alloys has motivated a great deal of study of their S.E. BROYLES, formerly Graduate Research Assistant, Division of Materials Science and Engineering, Department of Chemical Engineering and Materials Science, is Product Development Engineer, W.L. Gore and Associates, Inc., Flagstaff, AZ 86002. K.R. ANDERSON, Graduate Research Assistant, J.R. GROZA, Associate Professor, and J.C. GIBEL1NG, Professor, are with the Division of Materials Science and Engineering, Department of Chemical Engineering and Materials Science, University of Califomia, Davis, CA 95616-5294. Manuscript submitted November 15, 1994. creep behavior. These materials are characterized by high apparent stress exponents and high apparent activation en- ergies for creep. The mechanisms responsible for the im- provement of creep resistance are now reasonably well understood, as discussed by Arzt in a recent review.t7] How- ever, relatively little of this previous work has involved studies of DS copper alloys. Recently, Arzt and co-workers have proposed a model for the creep of dispersion-strengthened alloys based on an attractive particle/dislocation interaction.17,s,91 In the Rrsler- Arzt model, the rate-controlling step in the creep process is the thermally activated detachment of a dislocation from the departure side of a particle. The time for glide between particles is assumed to be insignificant. In addition, climb over particles is assumed to occur easily and is not the rate- controlling factor. The attraction between dislocations and particles controls the creep rate by the introduction of a substantial athermal detachment stress. The strength of this interaction can be described by the interaction parameter k and is related to the detachment stress, o'o, by the expres- sion tro = trot ~/1 - k 2 [11 where trot is the Orowan stress and is calculated asV01 ~ro, = 0.84 ~'(2A - 2r)(1 - v) ~a In [2] where r is the average particle radius, 2A is the interparticle spacing, G is the temperature-dependent shear modulus, M is the Taylor factor, v is Poisson's ratio, and b is the Burg- ers vector. The interaction parameter has a maximum value of k = 1, when the line energy of the dislocation is not relaxed at the particle/matrix interface and there is no at- tractive interaction. This is the case for coherent particles with low interfacial energies. [sl Although a strong parti- cle/dislocation attraction is described by small values of k, calculations by Arzt and Wilkinson [91 indicate that creep behavior can be controlled by dislocation detachment with METALLURGICALAND MATERIALSTRANSACTIONS A VOLUME 27A, MAY 199(~-1217