High Pressure Torsion Deformation And Strengthening Models Of Aluminium Alloys

High pressure torsion (HPT) is a severe plastic deformation (SPD) procedure with the ability to refine the grain size in polycrystalline materials down to micrometer and even to nanometer level. Although the grain refinement and the mechanical properties are dependent of the strain paths, there are a few descriptions of evaluating the effect on them of strain paths applied during HPT processing. Another objective of the present investigation is to analyse microstructural development and hardening during HPT, and provide a model that captures the main mechanisms for the hardening. The third objection is the effect of alloying elements on the strengthening of Al alloys under HPT.Present investigation focuses on above three aspects of 1050, two 2XXX and four 5XXX aluminium alloys processed by HPT at room temperature. Microhardnes testing was performed to evaluate the strength and work hardening of the alloys. Experiments by means of optical microscopy (OPM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC) were also carried out to provide the relevant information for the microstructure evlotuion and precipitate behavior of the alloys processed by HPT. The primary innovation points and conclusions are:(1) Three strain paths including monotonic HPT (m-HPT), cyclic HPT (c-HPT) and single reversal HPT (sr-HPT) were employed to strengthen the 1050 alloys. The results show the microhardness is lower and there is less grain refinement in the central regions of the disks in the initial stages of torsional straining but the microstructures become reasonably homogeneous across the disks at high imposed strains. Hardening is lower for c-HPT as compared to m-HPT. The extent of strengthening produced by m-HPT mainly depends on total strain/turns, whereas the extent by c-HPT does on not noly total strain but also strain per cyclic deformation. The sr-HPT initially reduces hardness drastically, and that decrease is most marked for the centre region. As strains by sr-HPT proceed, the hardness increases again.(2) The hardening behaviours and microstructure evolution of commercially pure aluminium during HPT processing were interpreted in terms of the density of geometrically necessary dislocations (GND) and statistically stored dislocations (SSD). GND dominate the strengthening in the centre of disk. Instead, with increasing the distance from the centre of disk, the SSD dominate strengthening. The density increase of SSD together with GND is responsible for the rapid rise of hardness in m-HPT. The softening on c/sr-HPT was attributed to the change in the GND density. A model has been presented:σy＝σ0+σgb＋M(?)The model describes quantitatively the experimental results in m/c-HPT and explains quanlitativly the hardening and softening in sr-HPT. The model indicated that the strength/ hardness is predominantly due to GND and SSD, with grain refinement providing less than 10% of the strengthening effect.(3) Two 2XXX aluminium alloys were selected for elucidating the relationship between heat treatments as well as HPT processings and the final hardness of the alloys. The strengthening mechanisms in the ageing of 2XXX alloys were interpreted. During HPT, the strength/hardness depends on the strengthening of Cu-Mg co-clusters and strengthening of dislocation density. The influence of quench in liquid nitrogen on strengthening of ageing at room temperature was interpreted in terms of the relationship between the amounts of retained co-clusters (ηcl)and HPT strain, and the strengthening model was further developed.(4) The effect of alloying on strengthening by strain paths was studied. Mg addition strongly improves the strengthening effect produced by HPT, where the increment of hardness initially rapidly increases with increasing the distance from the centre of disks and then slowly approaches stable values. It was observed that near the centre regions of hardness incremental curves of 1050 and 5XXX Al alloys processed by c/sr-HPT have the slopes with opposite signs and explanations for them were given. It was found that the characteristic structure of Al-1Mg-0.4Cu alloys developed during m-HPT consists of bands of elongated (sub)grains with high angles of misorientation, in contrast, the TEM micrographs and slected SEAD (Selected area electron diffraction) patterns for the same alloy after various reversal HPT processing steps showed that grains are nearly equiaxed with relatively low grain boundary misorientations.