טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentHai Shimon
SubjectA Computational and Theoretical Investigation of Deformation
Twinning at Aluminum Crack Tips
DepartmentDepartment of Mechanical Engineering
Supervisor Mr. Ellad Tadmor


Abstract

Deformation twinning is an important mechanism for plastic deformation in hcp metals. It is less common in fcc materials, but is still important particularly at low temperature. Recent experimental evidence has shown that even fcc materials that are not normally associated with deformation twinning, such as aluminum, will twin given a sufficiently high stress concentration such as at a crack tip.

       In this research we study the atomic structures that form at crack tips in aluminum single crystals during loading. The simulations were carried out using the quasicontinuum method - a mixed continuum and atomistic approach. A variety of loading modes and orientations were examined. It was found that for certain combination of loading mode and orientation, deformation twinning does occur at the crack tip in agreement with experimental observation.

       In an attempt to rationalize the results, a nucleation criterion for deformation twinning at crack tips based on Peierls concept was formulated. The deformation-twinning criterion identifies a new material parameter which we name the unstable twinning energy in analogy to Rice’s unstable stacking energy .We find that the applied stress intensity factor required for the nucleation of a deformation twin near a crack tip is proportional to . It is suggested that this parameter control deformation twinning in materials rather than the “twinning stress” parameter that is normally given.

         Using the analytic criterion we examine the tendency for deformation twinning for different kinds of fcc metals. We find that the tendency for twinning is proportional to  where  is the stacking fault energy. The predictions of the criterion were compared against numerical simulations for a variety of  fcc materials for different orientations and loading modes and found to be in good agreement.