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Änderungen des Gefüges von Al‐Al4C3‐Werkstoffen durch Verformungsverfestigung und thermische Entfestigung
Using a Depth Sensing Indentation technique (DSI), the Martens hardness HM, indentation modulus EIT and deformation work W for Al matrix and Al 4 C 3 particles have been measured. It has been shown that the hardness of Al 4 C 3 particles is 5–7 times higher than the hardness of matrix. The change in fracture for the Al-12Al 4 C 3 system was investigated and analysed at different temperatures and strain rates. With increasing tensile load, local cracks are formed by rupture of large particles and by decohesion of smaller particles from the matrix. Further increase of load leads to the crack growth by coalescence of cavities.
In the presented work the change in fracture for the Al-12Al4C3 system was investigated and analysed at temperatures from 20 to 400 °C and strain rates from 2.5·10 -5 to 10-1 s-1. At room temperature 20 °C, during tensile testing at strain rates in the tested region, the strain is first controlled by work hardening, expressed by the exponent n. In the second generally smaller part the deformation is limited by local straining and forming the neck. There is a marked decrease of plastic properties for the strain rate ε& = 2.5·10 -5 s-1 with the growth of temperature in the investigated region. It is explained by changes in the micromechanism of deformation and fracture. Fracture surface shows the transition from ductile fracture with dimples at 20 °C, to intercrystalline fractures, with the growth of the temperature, an indication of exhausted grain boundary plasticity. The intercrystalline fracture initiation tends, it is supposed, to be localized in triple points. At temperatures 400 °C, and ε& = 10 -1 s-1 there is a marked growth of plastic properties. The first part of the strain characterized by work hardening is very short. After the short growth of the stress to maximum, a deformation mechanism, showing presence of thermally and mechanically activated dynamic recovery processes takes place. The strain is this way uniform all over the body of the test piece. The fracture process ends with the increase of cavities and transcrystalline fracture with deep pimples.
In the present work, the change in fracture for the Al–12Al4C3 system was investigated and analyzed at temperatures from 20 to 400 °C and strain rates from 2.5×10−5 to 10−1 s−1. At room temperature during tensile testing, the strain is controlled by dislocation movement and reactions. For high temperatures in the investigated region, the principal mechanism keying the strain is the presence of dynamic recovery processes. Strain rate influences on fracture are analyzed.
In the presented work the change in fracture for the Al-Al4C3 system was investigated and analyzed at temperatures from 20 to 450°C and strain rates from 2.5 10−5 to 10−1 s−1. At room temperatures during tensile testing the strain is controlled by dislocation movement and reactions. The first part of the strain is characterized by work hardening expressed by the exponent “n”, the second part by the local strain in the neck. For high temperatures in the investigated region the principal mechanism keying the strain is the presence of dynamic recovery processes. Strain rate influences on fracture are analyzed.
Die Festigkeit und Streckgrenze eines teilchenverfestigten Werkstoffes setzt sich additiv aus Einzelanteilen, nämlich der kritischen Schubspannung des Matrixmetalles, einem durch vorhandene Versetzungen verursachten Spannungsanteil, der Spannungserhöhung durch die Größe der Subkornstrukturen, sowie einem Spannungsanteil, verursacht durch die Dispersoidteilchen, zusammen. Jeder dieser Einzelanteile ist aus gemessenen Gefügeparametern getrennt berechenbar. Bei den Untersuchungen an Al-Al4C3-Werkstoffen mit verschiedenem Dispersoidanteil ergab sich eine überraschend gute Übereinstimmung der aus unabhängigen Messungen abgeleiteten Streckgrenze mit der tatsächlich an den Werkstoffen gefundenen. Auch die Abhängigkeit der Streckgrenze von der Strangpreßtemperatur ließ sich erklären und durch die gute Übereinstimmung von errechneten und gemessenen Streckgrenzen die Brauchbarkeit der Vorstellungen über den Mechanismus der Dispersionshärtung beweisen.
Dispersion Strengthening Mechanism of Al with Al4C3
Tensile strength and yield strength of dispersion hardened materials consist of different additive shares: critical shear stress of the matrix metal, effect of the dislocation, strength increase attributable to the sub grain structure size, and finally the share of the dispersoid particles. Each share can be separately calculated from measured microstructural parameters. Investigations of Al-Al4C3 materials with different dispersoid content showed surprisingly close coincidence of the calculated and the experimentally determined yield strength, and also the correlation between yield strength and extrusion temperature could be explained, which results strongly support the theories about the mechanism of dispersion hardening.
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