Synthesis and mechanical properties of two phase nanostructured aluminum based composites.

摘要

Nanostructured materials (<100 nm) exhibit novel and superior mechanical properties in comparison to their coarse grained counterparts. However the associated deformation mechanisms are poorly understood. Synthesizing bulk nanocrystalline materials to measure the meaningful/reasonable mechanical properties is still a grand challenge. Although there exist several experimental/theoretical studies on mechanical behavior of single phase materials, studies on the effect of a second phase (soft/hard) on the mechanical behavior of nanocrystalline materials are very limited. Therefore, the thrust of the current work is to synthesize bulk nanostructured two phase materials and to establish the influence of a second phase (soft/hard) on the mechanical properties of two phase materials benchmarked against the corresponding single phase material and to identify the governing mechanics of plasticity at the nano scale.;Nanocrystalline aluminum was synthesized using ball milling at room temperature. The resultant powder material was consolidated to the bulk form using warm compaction and argon atmosphere and consolidation using high pressure torsion. The samples after high pressure torsion exhibited high end mechanical properties. The hardness of the nanostructured aluminum (of grain size 32 nm) was as high as 1200 MPa which is 6 times harder than its coarse grained counterpart.;Nanocrystalline Al-W composites with varying compositions were synthesized. With the increased addition of W, the hardness of these nanocomposites was increased. This hardness trend followed the behavior predicted by the rule of mixtures based on the volume fractions of Al and W. With the addition of 4 atomic % of W, the strength of the nanocrystalline aluminum was elevated by 70%.;Nanocrystalline Al-Pb composites were synthesized by two routes. In the first route, the room temperature ball milled samples were compacted at 573 K in an argon atmosphere. In the second route, the alloys were consolidated in situ during ball milling using a combination of milling at cryogenic temperature and milling at room temperature. Irrespective of the processing sequence employed in the current study, the minute additions of Pb to the nanocrystalline aluminum decreased its strength drastically beyond the projections made by the rule of mixtures. The Pb segregated to the grain boundaries of nanocrystalline aluminum appeared to be making the difference.;In situ consolidated nanocrystalline Al-0.7%Pb composite was subjected to high pressure torsion at room temperature. Interestingly, the additional straining caused by the high pressure torsion further weakened the material by 25%. The mean grain size of the nanocrystalline aluminum was the same before and after the HPT. The mechanism for this abnormal behavior is yet to be known.;The creep properties of nanostructured aluminum, synthesized using the sequential combination of ball milling at room temperature and high pressure torsion, were evaluated using the impression creep testing. The measured stress exponent values do not correspond to the Coble creep mechanism. However the activation energy measured was that of grain boundary diffusion in aluminum.