Some observations on the high-temperature creep behavior of 6061 Al-SiC composites
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Some Observations on the High-Temperature Creep Behavior of 6061 AI-SiC Composites R.S. MISHRA and A.B. PANDEY Aluminum alloy matrix-SiC composites have shown promising room-temperature properties. These composites are currently being used in structural aerospace applications. The SiC-reinforced composites are different from other dispersion-strengthened alloys, as Orowan strengthening does not seem to be applicable for the former. Nardone and Prewo [1] have shown that the calculated Orowan stress is far lower than the observed strength because the expected interparticle spacing for composites is quite large (of the order of a few microns). The understanding of high-temperature creep behavior of these composites, however, is rather poor, although some studies have been carded out. The steady-state creep deformation of Al-matrix composites has been described as (a) a simple power-law creep by Nieh and co-workers, [2,3] ~ = Aor" exp ( - Q / R T ) , where ~ is the steady-state strain rate, or the applied stress, Q the activation energy, T the temperature, R the gas constant, n the stress exponent, and A a constant; (b) power-law creep with threshold stress by Nardone and S t r i f e , I4] ~ oc ( o r - Or0)n' where o'0 is the threshold stress; and (c) an exponential law by Morimoto et al.,[5] = B exp (Cor), where B and C are constants at a given temperature. The above analyses were based on the creep data of 2124 and 6061 alloy composites. The creep curves of these two alloy composites show that while the 6061 alloy
matrix composites studied by Nieh and co-workers (2,3] show a well-defined steady-state regime, the same is absent in 2124 alloy composites. [4j In such a case, the minimum creep rate data of 2124 alloy are unlikely to represent steady-state behavior. Further, the data of Nardone and Strife [4] on 2124 alloy composites have been criticized by Nieh et aL p] on three accounts: (a) the steady state is not well defined, (b) the n values obtained by stress-change tests are suspect for such composites because of limited tensile ductility, and (c) the test duration at low strain rate is so short that creep strain is very small. On the other hand, the stress exponent of 20.5 and an activation energy of 390 kJ/mole for 6061 A1SiCw, as observed by Nieh [2] (which is by far the most comprehensive study), does not have any theoretical or phenomenological explanation. The purpose of this paper is to present the results obtained by reanalysis of the reported data on 6061 A1-SiC composites and provide a single model which can explain all the reported data. In our analysis, we have assumed that (a) a threshold stress exists at all test temperatures, (b) the stress exponent is equal to 8, as shown by Sherby et al.,[6] and (c) the activation energy for the steady-state creep corresponds with the activation energy for self-diffusion of aluminum. The individual set of data was plotted as ~1/8 vs or on linear scales to obtain a value for threshold stress (Figure 1). The observed linearity validates the assumed stress exponent
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