A study on the fracture toughness of Al-based MMCs containing different volume fractions Al2O3

Abstract

The need for lightweight, high performance and wear resistant structural materials to satisfy the demands of aerospace, automotive and consumer-related industries has been the trigger for the development and emergence of metal matrix composites (MMCs) [1]. Starting with aluminum alloys as an attractive base material, incorporation of discontinuous ceramic reinforcements offers improvement in elastic modulus, wear resistance, strength and control of physical properties such as density and coefficient of thermal expansion, thereby providing improved mechanical performance [2–13]. At the same time the receptivity to common metallurgical processing and characterization techniques is maintained. The primary disadvantage of MMCs is their tendency to brittle behavior [14–24], which manifests itself in a low fracture toughness. The true relations between the microstructural parameters and the fracture toughness values are generally not clear both because of the many factors involved (see Fig. 1) and the difficulty of obtaining reliable fracture toughness values. In order to extend the knowledge of the effects of microstructural parameters on fracture toughness, we studied the effects of the volume fraction Al2O3 on the fracture toughness of a precipitation hardening AA6061 aluminum alloy matrix using three different test standards, ASTM E399, El290 and E1737 (see [25] for further details). Five material grades were produced via a powder metallurgical route yielding circular tablets, suitable for making standard disc-shaped compact tension specimens. The morphology of the metallic and ceramic powders is shown in Fig. 2a and b respectively. The Al2O3 powder was much finer (particle size 98.5%) and macroscopically uniform MMC material. Typical process parameters are given in Table I. A more detailed description of the production process is given elsewhere [26]. Although the precipitation kinetics of MMCs are known to be slightly different from those of their base alloys [27, 28], the standard thermal conditions for a T6 treatment in AA6061 (solutionizing for 2 h at 530 ◦C and artificial aging for 8 h at 175 ◦C) were chosen. The microstructure of the samples was studied using scanning electron microscopy (SEM) and examples are shown in Fig. 3. Due to the aluminum powder being much coarser than the ceramic powder the microstructure consisted of large aluminum-rich zones surrounded by an MMC network containing an estimated volume fraction ceramic particles of up to 75%. The degree of network formation and the particle density in this network increased with increasing overall Al2O3 concentration. Even for extremely high volume fraction Al2O3 particles in this network particle wetting was found to be good and no interfacial porosity was observed. It should be pointed out that this spatial distribution of the reinforcing particles deviates from that in commercial MMC material produced from equal-sized ceramic and metallic powders [25]. This inverse structure of the MMC must be kept in mind when comparing the test results of these materials with fracture toughness values reported in literature. Preparation of samples for fracture toughness testing meeting the criteria of a straight pre-fatigue crack with a sharp tip [25] turned out to be difficult to achieve. The best results were obtained by pre-cracking the sample at a frequency of 35 Hz using a constant Kmax (10 MPa · m1/2). The R-ratio was increased stepwise during the test to flatten the crack front [29]. By performing continuous compliance measurements during pre-cracking the desired initial crack length could be obtained in 95% of the samples, but the usual crack front curvature and obliqueness [30] could not be avoided