Selected mechanical properties of composites based on aluminium alloys reinforced Al2O3 particles
* Paweł Kurtyka,** Marek Faryna, * Stanisław Wierzbiński * Akademia Pedagogiczna, Instytut Techniki, ul. Podchorążych 2, 30-048 Kraków ** Uniwersytet Jagielloński, Środowiskowe Laboratorium Analiz Fizykochemicznych i Badań Strukturalnych, ul. Ingardena 3, 30-060 Kraków
Annals 2 No. 4, 2002 pages 185-190
DOI:
keywords:
abstract Metal Matrix Composites (MMCs) based on precipitate strengthening aluminium alloys attract attention of industry because of advantageous strength to density ratio, stability of properties at elevated temperatures as well as appropriate wear resistance in exploitation conditions. Particulate reinforced aluminium alloy matrix composites require deformation processed to shape elements and improve properties. However, the standard processing parameters used for the conventional aluminium alloys cannot be directly adopted for the respective composites, as the mechanical characteristics of the composites are considerably different from that of the corresponding matrix material. The composites based on AA6xxx and AA7xxx alloys reinforced with Al2O3 particles in the amount of 10 and 20% of average size 10 and 25 µm were subjected to compression tests in a wide temperature range 293÷873 K at deformation strain rate 10-3÷10-5 s-1. Based on the compression tests results, the ranges of temperatures and deformation rates, at which plastic deformation occurs in the investigated alloys, have been established together with strength properties at elevated temperatures. Observation of surface reliefs was conducted on an electron-scanning microscope Philips XL-30. Figures 1a, b show the distribution of Al2O3 reinforced particles of the composite in initial state, after squeeze casting and extrusion, the diameter of the reinforced particles 10 µm. In the photos there can be observed near homogeneity distribution of the reinforcement particle in two directions. Figure 2 shows the mechanical characteristics of strain of the composite AA6xxx- Al2O3 and AA7xxx-Al2O3 in the system σ -ε-T reinforced of the particles of the size 10 and 25 µm, respectively. The compression tests were performed on the samples in the initial state (prior to the solution and ageing). The deformation rates and size of reinforcing articles are given in the Figure. The observed change in the value of the flow stress with temperature indicates the possibility of the occurrence of competing processes - recovery and dynamic recrystallization. Figure 3 shows the influence of temperature and strain rate on the values of yield points of investigated composites, and Figure 4 dependence of work hardening intensity on the stress for these composites deformed at constant temperature and different strain rate as well as at constant strain rate and different temperatures. It has been observed that in the composites with volume fraction of reinforced particles 10%, relations Θ = f(σ ) show very small deviation from the rectilinear course, while for the volume fraction of reinforced particles 20% two ranges of these relation were observed. Examples of surface reliefs after compression tests at elevated temperature are shown in Figs 5a, b. For both 10% (Fig. 5a) and 20% (Fig. 5b) volume fraction of reinforced particles the degree and the temperature of strain were 0.18 and 573 K, respectively. It has been observed that for the composite with larger particles the failure process was more intensively than for 10 µm particles. On the basis of the conducted investigations to established that addition of Al2O3 particles in amount up to 20 wt.% to AA6xxx and AA7xxx aluminium alloy considerably increases the mechanical properties of the composite, including the yield point in relation to the matrix alloy. The investigations have revealed the possibility - within limited range of temperature and strain rate - of the formation of AA6xxx-Al2O3 and AA7xxx-Al2O3 composites with the reinforcement particles 10 and 25 µm. Additional increase of mechanical properties of composite is possible after supersaturation and ageing process.