Papers by Keyword: Deformation Twinning

Abstract: Experiments show that high purity titanium (HP-Ti) samples have been successfully processed by equal channel angular pressing (ECAP) using a 120 degree die and a relatively slow ram speed at cryogenic temperature when the HP-Ti rods were trapped with 6061 Al alloy tubes. Optical microscopy (OM) and transmission electron microscopy (TEM) were utilized to investigate the deformed microstructure of the material. Typical microstructures of shear bands and deformation twinning were found in the deformed microstructure of ECAPed HP-Ti. Furthermore, the SAED pattern analysis of the twinning structures revealed that the deformation twinning occurred on {112} planes. Keywords: High purity titanium; ECAP; TEM; Deformation twinning; Cryogenic temperature.

Abstract: In the plastic deformation of hexagonal metals, deformation twinning plays an important role as well as slip deformation. Therefore, a modelling of deformation twinning is essential in the crystal plasticity modeling. In this study, a model considering the volume fraction of deformation twinning is presented in the framework of crystal plasticity, and it is combined with a finite element-based homogenization scheme to represent the polycrystalline behavior. The presented model is adopted to a sheet necking formulation. Plastic flow behaviors under several strain paths are evaluated using the present framework, and the effect of volume fraction of deformation twinning on the formability of hexagonal metal is discussed.

Abstract: The mechanical properties of Fe-28%Mn-1.5%Al and Fe-0.6%C-18%Mn-1.5%Al-0.07%Nb (all in wt.%) steels subjected to hot plate rolling at a temperature of 1423 K with a total reduction of 60% were studied. The steels exhibited quite different mechanical properties in spite of almost the same original microstructures and similar stacking fault energies. The yield strength and total elongation of the Fe-28%Mn-1.5%Al steel are about 260 MPa and 45%, respectively, whereas those properties in the Fe-0.6%C-18%Mn-1.5%Al-0.07%Nb steel comprise 350 MPa and 53%, respectively. The tensile flow stress vs strain curves of the hot rolled steel samples can be described by Ludwigson-type relations with parameters being dependent on the strengthening mechanisms. Frequent deformation twinning in the Fe-0.6%C-18%Mn-1.5%Al-0.07%Nb steel promoted the strain hardening and improved the strength and ductility.

Abstract: Recently, it was reported that Cu-Al alloys of low stacking fault energy (SFE) processed by severe plastic deformation show excellent tensile properties due to TWIP (Twinning induced Plasticity) phenomenon. In this study, Cu-15at% Al sheets were heavily processed by conventional multi-pass cold rolling up to 90% in reduction in thickness without annealing. In order to reveal the change in mechanical properties and the microstructure evolution, tensile test, hardness test, optical microscopy (OM) and electron backscattering diffraction (EBSD) analysis were performed. Deformation twinning due to low SFE is observed even in the case of low reduction in thickness. As the reduction increases, grains are refined by intersections of shear bands. It is found that the balance of strength and elongation of the processed sheets is comparable to those by severe plastic deformation followed by annealing in literature.

Abstract: We have investigated {332}<113> twinning and detwinning mechanisms in β-Ti alloys. Microstructure-twinning relations were evaluated in a β-Ti-15Mo (wt.%) alloy by statistical analysis of the evolving twin structure upon deformation by in-situ SEM testing and electron backscattering diffraction (EBSD). We find that most of the primary twins (~80%) correspond to the higher stressed variant and follow Schmid’s law with respect to the macroscopic stress. Detwinning mechanism was evaluated in a multilayered β-Ti-10Mo-xFe (x: 1-3 wt.%) by EBSD. We find that the detwinning process consists of two independent events that occur at two different microstructural regions, i.e. twin tips located at grain interiors and grain boundaries. Both detwinning modes can be explained from a thermodynamic standpoint where the boundary dissociation processes minimize the boundary free energy.

Abstract: A crystal plasticity analysis of polycrystalline pure magnesium is conducted to investigate deformation twinning behavior at the crystal grain scale. A dominant factor in the onset of deformation twinning is the resolved shear stress on a twinning system. More than one twin system may simultaneously be activated in a crystal grain, resulting from inhomogeneous stress distribution caused by constraints imposed by neighboring grains. In this study, a pure magnesium polycrystal is modeled using a fine finite element mesh and analyzed using the crystal plasticity model involving deformation twinning. The evolution of deformation twinning at the crystalline scale is numerically investigated, and the present approach demonstrates that two or more twinning systems are be activated in a single crystal grain because of the strong inhomogeneity in the grain.

Abstract: The deformation microstructures and mechanical properties of an austenitic stainless steel subjected to warm plate rolling were studied. The warm rolling was carried out at 300°C to different total true strains of 0.5, 1, 2 or 3. The structural changes during warm rolling were characterized by the elongation of original grains towards the rolling direction and the development of spatial network of strain-induced high-angle boundaries leading to the evolution of ultrafine-grained microstructure at sufficiently large strains. The grain refinement was assisted by the development of deformation twinning. After straining to 3, the transverse grain size decreased down to 220 nm in the warm rolled samples. The warm plate rolling resulted in significant strengthening. The microhardness increased from 2910 MPa to 4192 MPa with increase in the total strain from 0.5 to 3. Correspondingly, the yield strength approached 1005 MPa after warm rolling to a total strain of 3.

Abstract: In this paper, the uniaxial compression of Mg, Ti, Zr and Co single crystals along the direction is performed by molecular dynamics (MD) to investigate the elastic-to-plastic transition in these hexagonal close-packed (hcp) metals. Two deformation twinning modes are observed in these simulations, including the twinning in Ti, Zr and Co and the [0001] twinning in Mg. The underlying atomistic mechanisms of these twinning modes are analyzed in detail.

Abstract: The influence of thermo-mechanical treatment consisting of cold rolling followed by recrystallization annealing on the grain size and mechanical properties of a high-Mn TWIP steel was studied. An Fe-23Mn-0.3C-1.5Al TWIP steel (wt. %) was subjected to extensive cold rolling with a reduction of 80% (true strain of ∼1.6) and then annealed in the temperature interval ranging from 400 to 900 °C during 20 minutes. Recovery processes took place below 500 °C, partial recrystallization was evident at ~550°C and fully recrystallized structure evolved after annealing at 600 °C and higher. The static recovery resulted in a slight decrease in the yield strength from 1400 MPa to 1250 MPa and the ultimate tensile strength from 1540 MPa to 1400 MPa whereas the total elongation of 4% did not changed. The recrystallization development led to a drastic drop of strength and an increase in ductility. The yield strength of 225 MPa, the ultimate tensile strength of 700 MPa and the total elongation of 79% was obtained after annealing at 900 °C. Correspondingly, the grain size increased from 0.2 μm to 6.2 μm with increase in anneal temperature from 550 to 900°C.

Abstract: An understanding of the plastic deformation behavior of Ti6Al4V (Ti64) is of great interest because it is used in aerospace applications due to its high specific strength. In addition, Ti alloys have limited slip systems due to hexagonal crystal structure; hence twinning plays an important role in plastic deformation. The present work focuses upon the grain size effect on plastic deformation behaviour of Ti64. Various microstructures with different grain size were developed via annealing of Ti64 alloy in α-β phase regime (825°C and 850°C) for 4 hours followed by air cooling. The deformation behavior of these samples was investigated at various deformation temperature and strain rate conditions. Detailed microstructure studies showed that (i) smaller grains undergoes twinning only at low temperature and high strain rate, (ii) large grain samples undergo twinning at all temperatures & strain rates, though the extent of twinning varied.