Pergamon 0956-7151(94)00323-8 Acta metall, mater. Vol. 43, No. 3, pp. 877 891, 1995 Copyright @ 1995 Elsevier Science Ltd Printed in Great Britain, All rights reserved 0956-7151/95 $9.50 + 0.00 OVERVIEW NO. 119 SUPERPLASTICITY IN POWDER METALLURGY ALUMINUM ALLOYS AND COMPOSITES R. S. MISHRA t, T. R. BIELER 2 and A. K. MUKHERJEE3t LDefence Metallurgical Research Laboratory, Hyderabad-500 158, India, 2Department of Materials Science and Mechanics, Michigan State University, East Lansing, MI 48824, U.S.A. and 3Department of Mechanical, Aeronautical and Materials Engineering, University of California, Davis, CA 95616, U.S.A. (Received 19 January 1994; in revised form 20 July 1994) Abstract Superplasticity in powder metallurgy aluminum alloys and composites has been reviewed through a detailed analysis. The stress strain curves can be put into four categories: a classical well-behaved type, continuous strain hardening type, continuous strain softening type and a complex type. The origin of these different types of stress strain curves is discussed. The microstructural features of the processed material and the role of strain have been reviewed. The role of increasing misorientation of low angle boundaries to high angle boundaries by lattice dislocation absorption is examined. Threshold stresses have been determined and analyzed. The parametric dependencies for superplastic flow in modified conventional aluminum alloys, mechanically alloyed alloys and aluminum alloy matrix composites is determined to elucidate the superplastic mechanism at high strain rates. The role of incipient melting has been analyzed. A stress exponent of 2, an activation energy equal to that for grain boundary diffusion and a grain size dependence of 2 generally describes superplastic flow in modified conventional aluminum alloys and mechanically alloyed alloys. The present results agree well with the predictions of grain boundary sliding models. This suggests that the mechanism of high strain rate superplasticity in the above-mentioned alloys is similar to conventional superplasticity. The shift of optimum superplastic strain rates to higher values is a consequence of microstructural refinement. The parametric dependencies for superplasticity in aluminum alloy matrix composites, however, is different. A true activation energy of 313kJmol ~ best describes the composites having SiC reinforcements. The role of shape of the reinforcement (particle or whisker) and processing history is addressed. The analysis suggests that the mechanism for superplasticity in composites is interface diffusion controlled grain boundary sliding. 1. INTRODUCTION Powder metallurgy (PM) aluminum alloys have attracted considerable interest in the last decade. This includes modified conventional aluminum alloys, mechanically alloyed aluminum alloys and aluminum alloy matrix composites. The modification of conven- tional aluminum alloys by Zr, Cr, Mn additions leads to a fine grained microstructure since the intermetallic phases of Zr, Cr, Mn pin the grain boundaries. The fine grained materials are more amenable to the superplastic forming. This also causes the technologi- cally important result that the optimum strain rates at which the alloys are superplastic shift to higher strain rates, which is desirable for superplastic form- ing operations. Table 1 gives a summary of results on superplastic behavior in modified conventional aluminum alloys [1 14]. The stress strain curves illus- trated in Fig. 1 and tabulated in Table 1 indicate that strain hardening, strain softening and/or combi- nation of both occurs during superplastic defor- +To whom all correspondence should be addressed. mation. For one exceptional alloy, the optimum strain rate is 3 s ~ [14], and a few have an optimum strain rate near 10 -~ s ~[5, 9-11]. This is considerably higher than the optimum strain rates observed in conventional aluminum alloys of ~10 410 3s-J I151. The strain rate is usually represented in the generalized form [16, 17] = A K T ~ (1) where b is Burgers vector, D the relevant diffusivity, E the Young's modulus, k the Boltzmann constant, T the absolute test temperature, d the grain size, p the grain size exponent, a the flow stress, a 0 the threshold stress, n the stress exponent and A a geometrical constant. Table 1 shows that the appar- ent stress exponent n is generally between 2 and 3. (The strain rate sensitivity m = 1In is also frequently used to describe rate dependence in the form of a = Ki", but we will use n in this paper.) For slip accommodated grain boundary sliding, a stress expo- nent of 2 is expected [15]. For conditions where n > 2, 877