Creep behaviour of unreinforced and short fibre reinforced AlSi12CuMgNi piston alloy

The isothermal creep resistance of AlSi12CuMgNi alloys produced by squeeze casting, unreinforced and reinforced with 10, 15 and 20 vol% of short alumina fibres is investigated by means of long-term tensile creep tests at 300 degrees C. Dislocation creep-mechanisms associated with a Norton exponent n = 3 are attributed to both the unreinforced and the short fibre reinforced materials (SFRMs) in the stress range between 10 and 50% of the yield strength at 300 degrees C. The embedding of short fibres reduces the creep rate of the alloy by more than one order of magnitude. The SFRM with 15 vol% of reinforcement is the most creep resistant, while the 20 vol% SFRM is less creep resistant than the 10 and 15 vol% SFRMs due to its higher defect density and larger interface area. Load changes are imposed during creep tests to study the influence of periods of overloading. Returning to the initial load causes the stationary creep rates of the SFRMs to be further reduced down to one third of their initial values. Furthermore, a small increase in the creep exponent n of the SFRMs is observed after an overloading cycle. The increase of the connectivity of the Si/Al2O3-short fibre network in the direction of loading enhances the load transfer effect from the matrix to the reinforcement decreasing thus the external stress acting on the matrix. The yielding of highly stressed zones in the matrix around the ceramic reinforcement during the overload is able to produce a more uniform distribution of internal stresses when returned to a lower stress level. Both effects are considered to be responsible for the observed reduction of the stationary creep rate. The unexpected low creep resistance of the 20 vol% SFRM in comparison with the two other composites can be explained by the larger number of micro-structural defects (<= 1 vol% porosity) found in this material after the infiltration process. Although the small portion of these defects does not affect the high temperature tensile properties of the composite, it accelerates the diffusion controlled creep mechanisms. The larger interface area present in the materials with higher reinforcement volume fraction increases the diffusivity as well. The connectivity of the Si/Al2O3-short fibre (SF) network is higher for the 20 vol% SFRM than for the other SFRMs already before creep exposure. Thus the training effect by overloading periods is less efficient in the 20 vol% SFRM than in SFRMs with less fibre content. Those effects are counteracted by the fact that the higher the reinforcement volume fraction the smaller the load acting on the matrix. Yet for the investigated 20 vol% SFRM samples, the negative contributions prevail resulting in a lower creep resistance than for the other two composites. (c) 2006 Elsevier B.V. All rights reserved.