Creep and microstructure in discontinuous metal matrix composites and their matrices

High temperature deformation in discontinuous SiC (SiC particulate or SiC whisker) reinforced powder metallurgy 2124 Al composites was investigated for the first time over seven orders of magnitude of strain rate in a temperature range of 618 K-678 K. In order to provide the basis for comparison, the creep behavior of the matrix material--PM 2124 Al alloy was also studied. The experimental results show that the strengthening effect of SiC reinforcement is significant only at low stress range which corresponds to a strain rate lower that 10{dollar}sp{lcub}-5{rcub}{dollar} to 10{dollar}sp{lcub}-4{rcub}{dollar}sec{dollar}sp{lcub}-1{rcub}{dollar}. In high stress range, the strength of SiCp-2124 Al composites is getting close to that of unreinforced 2124 Al alloy. In the stress range used in the present investigation, SiCw-2124 Al is always stronger than 2124 Al alloy, but due to the different curvatures of the stress-strain rate plots for the materials, it can be expected that the strength of SiCw-2124 Al composite could be the same as that of its matrix material--PM 2124 Al alloy at very high stress levels which correspond to a strain rate higher than 10{dollar}sp{lcub}-1{rcub}{dollar}sec{dollar}sp{lcub}-1{rcub}{dollar}. The apparent stress exponent and apparent of activation energy for the materials tested in the present investigation are not constant, rather they are a function of applied stress. It is demonstrated that with an assumption of existence of a threshold stress for creep the abnormal creep behavior of discontinuous SiC-2124 Al composites can be characterized by a modified power law equation. In order to clarify the creep mechanism and the origin of the threshold stress in the materials, transmission electron microscopy (TEM) observations on creep microstructure in 2124 Al specimens were conducted. It is shown that subgrain structures are developed during the deformation and the average size of the subgrains decreases with increasing applied stress. The observation indicates that the rate-controlling process of creep in the materials is high-temperature dislocation climb. This conclusion is consistent with that inferred from the mechanical tests. Other two important features revealed by TEM approaches are (i) the presence of a non-uniform distribution of ultrafine oxide particles (average particle size of 8.8 nm) in the material and (ii) the extensive attractive interaction between the oxide particles and dislocations. This attractive interaction may provide an explanation for the origin of the threshold stress for creep in the materials used in the present investigation.