Threshold stress and load transfer effects in creep of an Al-8.5Fe-1.3V-1.7Si alloy reinforced with silicon carbide particulates an Al-8.5Fe-1.3V-1.7Si-15SiC<SUB align=right>p composite

Abstract

The creep behaviour of an Al-8.5Fe-1.3V-1.7Si-15SiCp composite (the subscript p stands for particulates) in two temperature intervals, namely 623 to 723 K and 773 to 823 K is investigated using the isothermal constant stress creep test technique. The measured minimum creep strain rates cover six to seven orders of magnitude. Occasionally, this creep behaviour is compared with that of an unreinforced (matrix) Al-8.5Fe-1.3V-1.7Si (8009Al type) alloy. For the temperature interval 623 to 723 K the true threshold creep behaviour is characteristic. The true threshold stress σTH decreases with increasing temperature more strongly than the shear modulus and is higher in the composite than in the matrix alloy. With the strong temperature dependence of the true threshold stress, extremely high values of the apparent activation energy and the apparent stress exponent are associated. However, the minimum creep strain rate is matrix lattice diffusion controlled and depends on the fifth power of the effective stress σe = σ - σTH (σ is the applied stress). The creep behaviour is interpreted in terms of athermal detachment of dislocations from fine incoherent Al12(Fe,V)3Si phase particles in the composite matrix. At temperatures ranging from 773 to 823 K the true threshold stress is absent. The apparent stress exponent increases with increasing applied stress, the apparent activation energy decreases with both increasing applied stress and temperature. The true activation energy of creep is higher than the activation enthalpy of the matrix lattice diffusion. The creep is interpreted in terms of thermally activated detachment of dislocations from fine Al12(Fe,V)3Si phase particles. The load transfer effect does not play any role in creep either at temperatures 623-723 K or at temperatures 773-823 K.