Study On Twin Mechanism Of Severe Plastic Deformation Copper Alloy

Mechanical twining is one of deformation mechanisms of nanocrystalline materials. Twin boundary (TB) with unique coherency not only can block dislocation slip to improve strength, but also can propogate dislocation to improve plasticity. Therfore, some certain desity of twins can improve both the strength and ductility of materials. As known everyone, deformation twinning is determined by intrinsic structural characteristics of materials (i.e. stacking fault energy (SFE), grain size) and extrinsic deformation conditions (i.e. strain rate, temperature). For the latter case, the common knowledge on twinning is that low temperature and high strain rate could promote the occurance of twinning; for the former case, however, there still exist some controversy, for example, grain size effect on twinning. Moreover, the study on TB migration and propagation of dislocations are only limited to molecular dynamics (MD) simulations, experimental evidences are in need.The main objective of this dissertation is to analyze the influences of SFE and grain size on deformation twinning, characterize the reaction between TB and dislocations/partials via high resolution transmission electron microscope (HREM). Cu-30wt%Zn and Cu-Al (99.99wt%Cu, Cu-0.86wt%Al, Cu-2.2wt%Al) were selected as model materials. High pressure torsion (HPT), equal channel pressing (ECAP) followed by cold rolling were used to achieve a wide range of grain size from about 5 nm to 500 nm. The detailed experimental results are as follow:With decreasing the grain size down to 20 nm, twin thickness of Cu-30wt%Zn gradually reduces to one (111) atomic plane space (stacking fault); while stacking faults exist within grains with the grain sizes of the whole range. These results indicate that twinning in ultra-fine alloys with low SFE is operated via partial dislocation (stacking fault) emission from grain boundaries.For SFE effect on twinning of Cu-Al alloys, dislocation slip is the only mechanism in 99.99wt%Cu while both slip and twinning operate in Cu-0.86wt%Al and Cu-2.2wt%Al during deformation. Furthermore, with decreasing SFE, grain refinement is more pronounced and twinning plays a more significant role in refinement.The HREM observations on the TB migration revealed that Shockley partial gliding at TBs causes the migration of TBs. What’s more, the interaction between extrinsic dislocation and TBs leads to the formation of sessile Frank dislocation, and the incoherent TBs with steps can act as dislocation sources. The above reactions between dislocations/partials and TBs could improve ductility and strength of the materials.