Strength and microstructure of powder metallurgy processed restacked Cu-Nb microcomposites
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INTRODUCTION
T H E multifilamentary Cu-Nb microcomposites combine high strengths with high electrical conductivit i e s , [1,2,3~ which make them attractive for applications in high field magnet technology and heat exchangers, r4,Sj These composites normally contain less than 20 vol pet Nb. Due to very limited mutual solubility of Cu and Nb, the Cu matrix remains pure during the processing of the composite and maintains its high conductivity. The key in achieving high strengths for Cu-Nb composites is the attainment of submicron filamentary structure. ~1,2,3[Two alternatives, in situ casting and powder metallurgy (PM), may be used to produce the initial ingot for the fabrication of the Cu-Nb composites. Both processes are successful in producing pure Nb inclusions (precipitates in the in situ process and powder particles in PM) embedded in a pure Cu matrix. The ingots are then deformed to high areal reductions (R = initial cross section area/ final cross section area, R = A o / A ) by techniques such as extrusion and wire drawing to produce the multifilamentary microcomposites. To fabricate high-strength composites with substantial diameters larger than 1 mm, restacking techniques are normally u s e d . [6,7,8] The tensile strength of the restacked Cu-Nb composites shows a different dependence of ultimate tensile strength (UTS) vs deformation than that of the not-restacked in situ composites, t9,1~ The most distinct difference is that for the restacked PM composites, strengthening reaches a saturated value in the final stages of areal reduction, but the not-restacked in situ Cu-Nb composites exhibit exponential increases fL2,3i with increased deformation. S. POURRAHIMI, Staff Member, and S. FONER, Associate Director, are with the Francis Bitter National Magnet Laboratory, Massachusetts Institute of Technology, Cambridge, MA 02139. H. NAYEB-HASHEMI, Associate Professor, is with the Department of Mechanical Engineering, Northeastern University, Boston, MA 02115. Manuscript submitted March 12, 1991. METALLURGICAL TRANSACTIONS A
Various existing models for the strengthening mechanisms of Cu-Nb c o m p o s i t e s II-3'll] do not explain the behavior of the PM-processed composites. The model presented here examines various effects of extensive deformation and accounts for their contribution to the overall strength. Understanding of the strengthening mechanism of our microcomposites required a detailed analysis of filament elongation, filament morphology, dislocation density, grain size, and substructure. The results of transmission electron microscopy (TEM), scanning electron microscopy (SEM), and optical metallography are presented which complement our arguments for the strengthening mechanism. Our model was in agreement with the experimental measurements of yield strength of a typical PM-processed Cu-18 vol pet Nb composite. Heat treatments at various temperatures have adverse effects on the tensile strength of a typical Cu-Nb microcomposite. [9'121Spheroidization of Nb filaments [~2,13Jand annealing of the Cu m
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