Papers by Keyword: Deformation Twinning

Abstract: Dynamic mechanical properties and microstructures of pure iron and Fe-30Mn-3Si-4Al TWIP (TWinning Induced Plasticity) steel were conducted by SHPB (Split-Hopkinson Pressure Bar), OM (Optical Microscopy) and TEM (Transmission Electron Microscope), at the strain rate ranging from 10² to 10⁵ s^-1 and at room temperature. The effect of high strain rate on the mechanical responses of pure iron and Fe-30Mn-3Si-4Al TWIP steel belonging to BCC (Body Centered Cubic) and FCC (Face Centered Cubic) structures respectively was evaluated. The comparison of deformation mechanism was analyzed between them and it concluded that dislocation gliding is a major deformation mechanism in pure iron with BCC structure and deformation twinning plays a significant role in Fe-30Mn-3Si-4Al TWIP steel with FCC structure.

Abstract: A coupled model based on crystal plasticity and phase field theories that express both plastic anisotropy of HCP metals and expansion/shrinkage of twin-bands is proposed in the present study. In this model, the difference of the hardening rate in each slip system is expressed by changing their dislocation mobility as a numerical parameter defined in the crystal plasticity framework. The stress calculated via crystal plasticity analysis becomes to the driving force of multi-phase filed equations that express the evolution of twin bands of several variants, which include both the growth and shrinkage. Solving this equation set, the rate of twinning/detwinning and the mirror-transformed crystal basis in the twinned/detwinned phase are obtained and then, crystal plasticity analysis is carried out again. Using the present model, a uniaxial cyclic loading simulation along [0001] direction on the specimen including two variants of twin-bands is carried out by means of finite element method (FEM). The results show that the detwinning stress decreases with increase of the pre-tensioned strain. This is caused by a residual compression stress resulting from the twin shearing that occurs in the vicinity of two twin boundaries approaching each other.

Abstract: Multiscale simulations using the quasicontinuum (QC) method with the embedded-atom method (EAM) potential are performed to investigate the process of nanocontact including sliding and subsequent withdrawal between Ni tip and Au substrate. The multiscale model reveals that deformation twinning in Au substrate is induced not only by the sheer stress but also by the adhesive stress. Combining with the generalized planar fault energy (GPF) curve of Au, the underlying formation mechanism of deformation twinning is studied in detail. During the withdrawal process, the dislocation degeneration and the vacancy evolution are observed.

Abstract: Rolling texture evolution of pure nickel, and nickel – cobalt alloys containing 20wt.%, 40wt.%, 60wt.% cobalt content has been studied to very large true strain (ε ~ 4). The texture evolution in pure nickel and Ni-20Co was very similar, and resulted in typical Cu-type rolling texture. Microstructural analyses showed that the deformation was mostly slip dominated up to 95% beyond which it shear bands. Deformation twinning was a major deformation mechanism up to 50% reduction, and at higher strains, microstructure showed extensive shear banding. The evolution of final Goss texture in low SFE Ni-Co alloys could be explained based on the twin fraction and shear band volumes which showed grains preferably oriented towards Goss.

Abstract: Titanium alloys typically exhibit a limited ductility (typically 20%) and little strain-hardening. An alloy design with new concept was conducted aiming at improving both ductility and strain hardening while keeping the mechanical resistance at an excellent level. An experimental validation was illustrated with the Ti-12(wt.%)Mo alloy, exhibiting true stress - true strain values at necking, of about 1000MPa and 0.38, respectively, with a large strain hardening rate close to the theoretical limit. In order to clarify the origin of this outstanding combination of mechanical properties, detailed microstructural investigation and phase evolution analysis were conducted by means of in-situ synchrotron XRD, in-situ light microscopy, EBSD mapping and TEM microstructural analysis. In the deformed material, combined Twinning Induced Plasticity (TWIP) and Transformation Induced Plasticity (TRIP) effects are observed. Primary strain/stress induced phase transformations (β->ω and β->α’’) and primary mechanical twinning ({332}<113> and {112}<111>) are simultaneously activated in the β matrix. Secondary martensitic phase transformation and secondary mechanical twinning are then triggered in the twinned β zones. The {332}<113> twinning and the subsequent secondary mechanisms are shown to be dominant at the early stage deformation process. The evolution of the deformation microstructure results in a high strain hardening rate (~2GPa) bringing both a high tensile strength and a large uniform elongation.

Abstract: Effect of cold rolling on the microstructure and mechanical properties of a Fe-23Mn-0.3C-1.5Al (in wt. %) TWIP steel with an initial grain size of 24 μm was studied. Extensive deformation twinning occurred upon reduction by rolling. The volume fraction of the deformation twins attained about 0.2 at a reduction of 20%. Then, the intensity of deformation twinning gradually decreased with increasing the total rolling reduction. The average twin thickness of about 20 nm remained unchanged, although the distance between twins progressively reduced with increasing strain. The deformation banding was observed after a reduction of 60%. The thickness and volume fraction of microshear bands increased with increasing rolling reduction. The cold rolling led to significant strengthening of the steel that is accompanied by a drop on ductility. The yield stress (YS) increased from 235 MPa in the initial state to 1400 MPa after cold rolling with a reduction of 80%, whereas the elongation to failure decreased from 96% to 4%, respectively.

Abstract: The deformation behaviour of binary Ti-Al model alloys has been observed in respect to nucleation, propagation and growth of twins, specifically the interaction of slip and {1 0 -1 2} tensile twins. The area fraction of twins increases by trend from Ti-0Al to Ti-4Al but significantly reduces to Ti-6Al. The lattice strain evolution during compression was measured using in-situ neutron diffraction and decreases from Ti-0Al to Ti-4Al but increases to Ti-6Al. The twin activity in hcp Ti behaves as expected from fcc materials until 4 wt.% Al concentration, but it is believed that short or long range ordering reduces the twin activity in Ti-6Al.

Abstract: A large-scale molecular dynamics simulation was used to investigate the propagation of cracks in three dimensional samples of nanocrystalline copper, with average grain sizes ranging from 5.34 to 14.8 nm and temperatures ranging from 1K to 500 K. It was shown that intragranular fracture can proceed inside the grain at low temperature, and plastic deformation around the tip of the crack is accommodated by dislocation nucleation/emission; indeed, both fully extended dislocation and deformation twinning were visible around the tip of the crack during fracture. In addition, due to a higher concentration of stress in front of the crack at a relative lower temperature, it was found that twinning deformation is easier to nucleate from the tip of the crack. These results also showed that the decreasing grain size below a critical value exhibits a reverse Hall-Petch relationship due to the enhancing grain boundary mediation, and high temperature is better for propagating ductile cracks.

Abstract: EBSD investigation of texture and microstructure evolution during a complete thermomechanical treatment of commercially pure titanium (HCP-Ti) is presented. Titanium was cold rolled to reach various degrees of thickness reduction: 20%, 40% and 60%. Next, annealing in air atmosphere was conducted at different conditions to achieve the recrystallized state. EBSD topological maps were measured on RD-TD and RD-ND surface of each sample. Strong heterogeneity of deformed titanium microstructures is described with focus on the important role of twinning mechanisms. Texture evolution in investigated titanium appears to be limited, especially in recrystallized state. However some subtle mechanisms are discussed.

Abstract: In this work, the conventional rolling (CR) and warm rolling (WR) have been carried out with an ultra purified ferritic stainless steel. After different rolling processes and subsequent recrystallization annealing, different recrystallized microstructure characteristics had been obtained. It was observed that as compared to the conventional process, the warm rolling and subsequent recrystallization annealing could obviously refine the recrystallized microstructure of the hot band. The ductile-to-brittle transition for the ultra purified ferritic stainless steel can be closely related to the occurrence of deformation twinning. The refined microstructure can decrease the temperature at which twinning occurs and, consequently, the ductile-brittle transition temperature for the ultra purified ferritic stainless steel was lowered to be below -40°C, and its notch toughness was significantly improved when the fine grain route can be applied.