Engineering topics
Cissé, Cheikh
Publications and source records attributed to Cissé, Cheikh.
Transformation-induced fracture toughening in CuAlBe shape memory alloys: A phase-field study
Stress-induced transformation ahead of a crack tip can relax the stress concentration and produce fracture toughness in shape memory alloys (SMAs). In this manuscript, we utilize a nonisothermal phase-field model (PFM) to study the martensitic transformation induced crack tip toughening in CuAlBe SMA. The force-displacement curve, transformation zone and high stress zone in single crystalline samples show high dependency on the grain orientation with respect to the crack alignment. Comparison between isothermal and non-isothermal simulations reveals that the transformation-induced self-heating decreases the toughening capability by increasing the critical transformation stress. Investigating the high stress zones in CuAlBe and the nontransforming CuAl alloy shows that the toughening is obtained by redistributing the stress concentration far from the crack tip, as the high stress zone follows the tip of the transformation zone. Increasing the acuity of the crack tip is found to generate more symmetric martensite wings on both side of the crack axis and a more localized high stress zone. The polycrystal specimen displays higher toughening due to the internal constraints related to the presence of various grains with difference orientations. Depending on the orientation of the grain inclosing the crack tip, the toughening can be lower or higher. Finally, coincidence of the crack tip with a triple junction is found to improve the toughening behavior.
On the elastocaloric effect in CuAlBe shape memory alloys: A quantitative phase-field modeling approach
We report the reversible stress-induced phase transformation in shape memory alloys (SMAs) is a dissipative process during which heat is absorbed or released. The inherent temperature variations inside the material has an elastocaloric effect (eCE) with appealing applications in solid-state cooling technology such as compact and efficient on-board refrigeration system for eletronic devices. In this manuscript, we conduct the first study of eCE of CuAlBe SMAs utilizing phase-field modeling. For an applied stress of 500 MPa, the results for polycrystalline Cu-Al11-2Be (at. %) show a minimum adiabatic unloading temperature change of -10 K over a pseudoelastic window of 40 K. In the absence of plastic deformation, the material demonstrates good reproducibility of the eCE over a few loading–unloading cycles. The presence of plastic deformation is found to cause functional fatigue that deteriorates the cooling capacity; however, the coefficient of performance only decreases from 9.04 to 8.03, which is still a very good value. These results place CuAlBe as a frontrunner SMA for solid-state cooling compared to the expensive NiTi.
A phase-field model for non-isothermal phase transformation and plasticity in polycrystalline yttria-stabilized tetragonal zirconia
Here we propose an elastoplastic phase-field (PF) model to investigate the mechanics of tetragonal-to-monoclinic phase transformation (TMPT) and elastoplastic deformation of polycrystalline yttria-stabilized tetragonal zirconia (YSTZ). A Landau polynomial with non-vanishing chemical energy at the equilibrium temperature is introduced to account for the actual formation energies of the phases. The effects of different grain orientations, latent heat, and temperature on TMPT and deformation mechanisms are considered. The suppressive transformation effects of the grain boundaries (GBs) is modeled using an inhomogeneous kinetic coefficient in the bulk and GBs. The simulation results for single crystals demonstrate the capability of the model to reproduce the orientation-dependent compressive deformation of YSTZ similar to atomistic simulations and micropillar experiments. The single crystal with [100] crystallographic orientation along the loading direction (SC[100]) displays both TMPT and plasticity, SC[101] experiences only phase transformation, while SC[001] undergoes only plastic yielding. The TMPT induced by compressive loading exhibits shape memory effect (SME) below the equilibrium transformation temperature and pseudoelasticity above it, while the critical transformation stress increases with increasing loading temperature. The irrecoverable plastic strain is found to trap a part of the monoclinic phase, which prevents a complete reverse transformation. The polycrystalline cases also display SME and PE at low and high temperatures, respectively. Due to the orientation differences between grains and the stress concentrations at geometric nonlinearities, plastic deformation occurs in polycrystalline YSTZ for an applied load less than the yield stress. The results suggest a possible limitation of plasticity and an improvement of the shape recovery of YSTZ if one can control the orientation of the grains and/or increase the density of stacking faults at the GBs during material processing.