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The corrosion resistance of 7475 aluminium alloy processed by hydrostatic extrusion

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High strength 7xxx aluminium alloys are commonly used in aircraft structure applications. Conventionally, they are precipitation strengthened which makes them the strongest among aluminium alloys. The grain refinement down to nanometre scale offers an additional possibility to improve their mechanical properties as the yield stress is a linear function of inverse square root of grain size (Hall-Petch relationship). The efficiency of such an approach was already confirmed in a number of studies [1, 2]. The size of grains can be efficiently refined down to nanoscale via a number of processing routes, among others utilizing severe plastic deformation. Such a processing enables micrograined aggregates to be transformed into nanograined via accumulation and rearrangement of defects, primarily dislocations, generated during the deformation. Experimental observations showed, that for such a transformation to take place, a large degree of plastic deformation is required, which usually exceeds the maximum equivalent strain achievable in simple plastic forming methods [3, 4]. As a result, special deformation methods have been developed, e.g. equal channel angular pressing and high pressure torsion. Also, it was recently demonstrated that hydrostatic extrusion (HE) can be used to efficiently refine the grain size in a number of metals and alloys [5, 6]. In comparison to the other SPD methods, HE usually requires a significantly smaller total strain. This is primarily due to the high strain rates, which frequently exceed 102 s-1 and reduce the extrusion time to seconds. As a result, the work associated with plastic deformation is more efficiently transformed into the energy of the accumulated defects. It was already established that nanograin refinement in SPD metals results in a high strength not possible to obtain by other strengthening mechanisms. However, it should be noted that wider industrial applications of these materials are res[...]

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