Deformation Microstructure And Mechanical Properties Of Fine-Grained Aluminium Produced By Spark Plasma Sintering

Fine-grained metals have attracted much interest due to the possibility to obtainhigh mechanical strengths, although in general such metals only have a limited ductility.Studies of the deformation mechanisms in fine-grained metals are therefore important,and can also help fill a gap in knowledge between nano-grained metals andconventional coarse-grained metals, which is an area of both scientific and industrialinterest.In the present work, the deformation mechanism of fine-grained metals has beenstudied, primarily using Al as an example system. Efforts have been focused oninvestigations of deformation microstructure evolution, and on the relationship betweenmicrostructure and mechanical properties. For such an investigation it is very importantto use a starting material with a simple starting microstructure. For this purpose thespark plasma sintering (SPS) technique has been used. By careful selection of powdercharacteristics and processing conditions, samples of fully dense, fine-grained Al withaverage grain sizes ranging from5.2m to0.8m, in a fully recrystallized condition,with equiaxed grains and a random texture, have been produced. The microstructureevolution during compression of the fine-grained Al SPS-samples up to a strain of0.3has been systematically investigated using a range of techniques, including ECC, EBSDand TEM. Detailed investigations of the dislocation content of dislocation boundarieshave also been carried out using a weak-beam technique, and the observations havebeen interpreted via a slip system analysis including Schmid factor calculations, use ofthe Frank formula and the Bishop-Hill crystal plasticity model. Based on the abovestudies, relationships between the microstructure and mechanical properties have beenestablished.The main conclusions arising from these studies are as follows:(1) The formation ofdeformation microstructure depends on grain size. It is found that as the grain sizedecreases, the deformation microstructure changes from one containing well definedfeatures to one with less-well defined features. Additionally as the grain size decreasesbelow1m, only few dislocation boundaries are formed.(2) The relationship betweenmicrostructure and grain orientation changes with decreasing grain size, showing an expansion of grain orientations with type3microstructure from [111] corner toward the[100]-[110] line.(3) The above grain size effect is attributed to different slip systemactivity in fine-grained Al compared to coarse-grained Al. By investigating the Burgersvector of dislocations in the dislocation boundaries, it is found that more slip systemsare activated in fine-grained Al, and that slip systems with the highest Schmid factor areless dominant in the dislocation boundaries formed during deformation. The use of aBishop-Hill model is found to give better predictions compared to a Schmid factoranalysis.(4) The mechanical properties of fine-grained Al show a transition behavior asthe average grain size decreases, and this transition can be related to the differentmicrostructure evolution behavior. For samples with grain size larger than1.3m,dislocation boundaries are stored in each grain during deformation, causingwork-hardening after yielding. However, for samples with grain size below0.8m, fewdislocation boundaries are stored in each grain during deformation, causing a lack ofwork-hardening after yielding.(5) For samples showing work-hardening after yielding,the relationship between microstructure and flow stress has been established. By a linearcombination of oxide particle hardening, forest dislocation hardening and grainboundary hardening, the flow stress can be well predicted by choosing a criticalmisorientation angle of2~3oto distinguish forest dislocation hardening and grainboundary hardening.