In this study, the deformation mechanisms of bulk metallic glass (BMG) foam with around 49% porosity were investigated by using X-Ray Computed Tomography (CT) technique. X-Ray CT experiments were conducted at ESRF Synchrotron Source, Grenoble, France where the in-situ compression testing is possible. The collected CT images were digitally enhanced and the volumetric strain of the foam cells was calculated. The visual analysis of the foam cells was compared with a finite element model to determine whether the foam exhibited any pressure sensitivity.
The sample experiences uniform local deformation under compression until a few geometric instabilities in the foam induce local cell bending, followed by multiple shear band formation and localized fracture. Contrary to other reports [1, 2] that suggest plastic buckling in a single narrow crushing band, the majority of the strain the 49% porous sample experiences is from a percolating diffuse brittle crushing along shear bands of the sample. Reports of highly porous samples show a more ductile behavior in individual ligaments because of multiple shear band formation, but commonly fail along a relatively narrow (crushing) band. In moderate porosity foams, this ductility of thin ligaments was rarely observed to occur locally, but before a single crushing band can form, shear bands within highly stressed solid regions propagate until fracture with a similar behavior of a solid sample with very little porosity. However, once the crack encounters another bubble while propagating, it is temporarily blunted by the formation of more shear bands. During this phase, the energy of the system is relieved and redirected to different parts of the foam. Overall, there is a percolating failure of the foam due to relatively rare membrane bending followed by local membrane bursting and cracking into highly stressed nearby solid portions. By considering this failure mechanism, metallic glass foams with moderate porosity understandably contain deformation characteristics of solid BMG brittle failure and the plasticity of thin plates/BMG foams with high porosity.
Finite element modeling of a 2D cross-section shows that the foams exhibit less overall pressure sensitivity compared to a solid sample. FEM results showed that for pressure insensitive foam material, plastic hinge formation has preceded thin ligament buckling. By increasing the pressure sensitivity index (friction angle), thin ligament buckling is delayed. The FEM results corroborate with the observed cell ligament failure at about 45° from of the ligament axis. It is speculated that the dominated failure mode is by initiation and propagation of shear bands that eventually nucleate mode II cracks.
|Commitee:||Bastawros, Ashraf, Gray, Joe, Kessler, Michael|
|School:||Iowa State University|
|Department:||Materials Science and Engineering|
|School Location:||United States -- Iowa|
|Source:||MAI 47/05M, Masters Abstracts International|
|Keywords:||Amorphous metallic glass, Foams, Palladium alloys, Tomography|
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