Intercrystalline fracture surface obtained with strain rate 2.5·10 –5 s –1 and temperature 400 °C 

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... larger dispersed particles, too. They ranged from 85 nm to 1 μ m in size, making up about 30 % of dispersed amount, observed by Scanning Electron Microscopy (SEM) and metallography. The Al 4 C 3 and Al 2 O 3 particles are on grain boundaries as well as inside in Al grains. The observed microstructure is fine, even, with grains less than 1 μ m, elongated in the direction of extrusion, Fig. 2. Test pieces 3 mm in diameter and 15 mm gauge length had been machined for tensile test. They were positioned in longitudinal direction, in respect to the direction of extrusion. For the evaluation of strain and fracture mechanisms SEM examined fracture surfaces. The results are summarised in Fig. 3, where the yield strength R p 0.2 and the reduction of area Z are plotted in dependence on the test temperature and strain rate applied. Note that by temperature 400 °C and strain rate 10 –1 s –1 occur rapid increase in value Z but the value of yield strength don’t remark the substantial changes. Corresponding results were obtained for UTS and elongation. For high strain rate ε & = 10 –1 s –1 and temperature 20 °C the fracture surface is presented in Fig. 4. For low strain rates ε & = 2.5·10 –5 s –1 it is in Fig. 5. There are no significant differences between them. Both are ductile, transcrystalline fractures with dimples. The dimples are shallow with a characteristic dimension 0.45 μ m. Fracture surfaces fractured at strain rate ε & =2.5·10 –5 s –1 and temperature 300 °C showed underdeveloped intercrystalline facets. They are underdeveloped because at the end of fracture, a ductile fracture takes place. At the strain rate ε & = 10 –1 s –1 the fracture is transcrystalline with dimples. The dimples are deeper and larger then those at 20 °C. The characteristic dimple diameter is around 0.6 μ m. At low strain rate ε & = 2.5·10 –5 s –1 and temperature 400 °C the fracture took place at Z = 8 %. Typical micro facets of the fracture are shown in Fig. 6. Prevailingly, developed intercrystalline facets are present, with dimensions corresponding to the fine grain size, and great angle disorientation. There are small parts of fracture showing crests of ductile facets. For the strain rate ε & = 10 –1 s –1 the fracturing ended at a reduction of area Z = 64 %. The fracture is ductile transcrystalline with developed deep dimples Fig. 7. The characteristic dimple dimension is 0.65 μ m. The dependence the middle dimples diameter of temperature for strain rate 10 –1 s –1 is seen on Fig. 8. The analysis of fracture at 400 °C and strain rate ε & = 2.5·10 –5 s –1 has shown intercrystalline fractures, as the result of damage to grain boundaries formed by interactions of dislocations with dispersed particles on grain boundaries. However, the fine particles on grain boundaries ...