Creep behaviour of a 2124 Al alloy reinforced by 20 vol.% silicon carbide particulates

Abstract The creep in a 2124 Al alloy reinforced by 20 vol.% silicon carbide particulates -2124 Al -20 SiC p particulate composite-is investigated in a temperature interval ranging from 623 to 748 K and at applied shear stresses ranging from 2.5 to 40.0 MPa, depending on the testing temperature. The double shear creep test technique is used. It is shown that the composite exhibits the threshold creep behaviour at least up to a temperature of 698 K. Up to this temperature, the true activation energy of creep is very close to the activation enthalpy of matrix lattice diffusion and the value of the true applied stress exponent of minimum creep strain rate is almost exactly equal to five. The threshold stress decreases linearly with increasing temperature and, if extrapolated to temperatures above 700 K, it disappears at about 735 K. At 748 K, and even at 723 K, the apparent applied stress exponent of minimum creep strain rate increases with applied stress and depends slightly on temperature. But, most probably, the creep strain rate remains to be matrix lattice diffusion controlled. It is assumed that the origin of threshold stress is closely related to its temperature dependence and to its disappearance at a relatively well defined high creep testing temperature. An attempt is made to account for the origin of threshold stress, accepting the concept of load transfer to particulates and the long range internal back stresses generated in creep in the composite matrix, which reduce the load transfer to a value approximately independent of applied stress at any given temperature, except at high temperatures where the internal back stress is very close to the applied stress. Under the latter conditions the threshold stress can be expected to disappear

The role of matrix microstructure in the creep behaviour of discontinuous fiber-reinforced AZ 91 magnesium alloy

Constant stress tensile creep tests were conducted to failure at temperatures of 423 and 473 K on an AZ 91 (Mg–9wt.%Al–1wt.%Zn) alloy reinforced with 20 vol.% Al2O3 short fibres and on an unreinforced AZ 91 matrix alloy. The creep resistance of the reinforced material was considerably improved by comparison with the matrix alloy. A microstructural investigation revealed that the most frequent morphology of the ?-phase precipitates in the composite is continuous Mg17Al12 platelets. Detailed investigations using transmission electron microscopy indicate that the matrix microstructure has no significant influence on the creep properties of these two materials. This result confirms the proposal that the creep strengthening of the composite is controlled by an effective load transfer between the matrix and the fibres