Papers by Keyword: Deformation Twinning

Abstract: In this study, a biaxial tensile test of cruciform specimens containing centre notch was conducted in order to clarify the deformation behavior near the crack tip at the early stages of crack initiation when objected to a biaxial stress state. Results show that the hardness and stress value within the deformed zone increased with increase in the loading ratio. Observation of the microstructure reveals that the deformation is dominated by basal slip under equal biaxial tensile loading. The asymmetrical biaxial tensile loading generates deformation twins near the crack tip. These results indicate that existing deformation twins contribute to higher hardness, and there is obvious anisotropism in the vicinity of crack tip under asymmetrical biaxial tensile loading.

Abstract: The effect of different hydrostatic extrusion ratios on the microstructure and mechanical properties of the ZK60 magnesium alloys were investigated. The results showed that, the major deformation mechanism of the alloy is twinning at room temperature, which resulted in that the tensile strengthen and hardness of the extruded alloy improved greatly. With deformation ratio increasing, the ultimate tensile strengthen and hardness are linearly increased, with the functions of Y= 4.2X+358.3 and Y=2.3X +73.69, respectively. And the maximum tensile strength and hardness of the extruded alloy are 383 MPa and 87HB, respectively. But the elongation decreases obviously, the minimum decreasing degree is 50%. With the deformation ratio increasing, the tendency of elongation rate increased as an “M” model.

Abstract: This paper summarizes a recent review about the brass-type texture and its deviation from the copper-type texture by the present author and R.K. Ray – with somewhat sharpened conclusions.

Abstract: The morphology of deformation twinning, which influences a brittle fracture at low temperatures, was investigated in Fe-8mass%Al. Tensile tests were performed at 129K and room temperature. The specimen tested at room temperature showed yielding and kept deformed by usual slip while the specimen tested at 129K fractured in a brittle manner in an elastic regime with a number of straight markings due to deformation twinning. Detail analysis of those deformation twins suggests that the collision of deformation twinning is the initiation site of the brittle fracture.

Abstract: The increased use of metallic biomaterials in contact with blood e.g. for application as coronary stents is steadily resulting in the development of new biomaterials. Conventional bare-metal stents made by stainless steel were reported on adverse reactions against human body and are gradually replaced by coated stainless steel. The new generation of stent requires fundamental improvements at the materials point of view. Although titanium and classical Ti-alloys display superior biocompatibility compared to other metallic materials (stainless steels, Co-Cr), the major drawback of their relatively low ductility (typically 15%-25% of elongation) seriously limits their applications as cardiovascular stents, where large ductility is basically required during the stent deployment procedure and long-term service. In this paper, new titanium alloys with high ductility, a binary Ti-12Mo (wt%) and a ternary Ti-9Mo-6W (wt%) were designed by using a chemical formulation strategy based on the electronic design method called “the d-electron alloy design method”. Both alloys were synthesized and thermo-mechanically treated into beta-metastable state. In tensile tests, both alloys exhibited outstanding ductility of 43% and 46% in total elongation at room temperature, which is almost two times greater than the normal value shown with classical titanium alloys. Optical microscopy and detailed TEM observations on the deformed specimens revealed a complex deformation mechanism, involving {332}<113> mechanical twinning, stress induced plate shaped omega phase and stress induced martensitic (SIM) transformation β-α’’.

Abstract: In the paper the concept of structure refinement due to mechanical twinning is discussed. It is postulated that the process of structure refinement may occur when dominant crystal shear proceeds across twin-matrix interface (Mode 2). Contrary, if the crystal shear proceeds parallel to the interface (Mode 1) no condition for structure refinement is satisfied. The conditions of the structure refinement and no structure refinement are shown taking example of tensile Cu-8%at. Al single crystals of two orientations [1 4 5] and [1 1 2] tested in liquid nitrogen. The tensile characteristics are therefore divided into stages associated with the Mode 1 and Mode 2, which correspond to the fact whether refinement of single crystal structure is present or not, as it is proved by the EBSD analysis. The performed analysis showed that structure refinement consists of formation of regions of new orientations, where the most common feature is the II order twinning (the case [1 4 5]) supported by other regions of specific orientations necessary to accommodate mainly the transfer of crystal twin shear across the twin-matrix interface. Moreover, if the II order twinning plays the dominating function (the case [1 1 2]) higher order twins are to bring into operation to assure further ductility of a deformed sample.

Abstract: Ultrafine-grained (UFG) pure Cu and Cu–Zn alloys samples were prepared using cold-rolling under liquid nitrogen temperature to investigate the influence of stacking fault energy (SFE) on the mechanical properties and microstructure. The tensile tests were performed at room temperature with the strain rate of 10-4/s, and the strain hardening rate (SHR) was computed from the engineering stress-strain curves. A decrease in SFE simultaneously improves strength and ductility. The average grain sizes, microstrain, dislocation density and twin density were examined using X-ray diffraction (XRD). X-ray diffraction measurements indicate that grain size decreased and microstrain, dislocation and twin densities increased with decreasing SFE.

Abstract: Deformation behavior and structural changes were studied in a 304-type austenitic stainless steel subjected to large strain multiple forging at an ambient temperature. The number of forging passes was 10, leading to the total cumulative strain of 4.0. The yield stress rapidly increased to about 1000 MPa after the first forging pass and then gradually approached a saturation level of about 2000 MPa in large strains. The grain/subgrain size decreased to about 50 nm at total strain of about 2. This grain/subgrain size reduced a little upon further processing; and comprised 35 nm after a total strain of 4.0. The fast kinetics for grain refinement was associated with deformation twinning and strain-induced martensitic transformation. The both of them resulted in fast grain subdivision at relatively small strains.

Abstract: Within the past decade, it has been shown that twinning in α, β, and α + β titanium alloys can occur at speeds much lower than the speed of sound by many orders of magnitude. This is related to the twinning deformation mechanisms controlled by the diffusion of oxygen as compared to simply a shear process. Very recent developments, such as strain-rate effects on twinning, support a recent hypothesis that the twinning in these materials is controlled by a slow diffusion process, resulting in time-dependent twinning. These recent developments, along with the ramifications of the findings will be outlined in this article.

Abstract: Development of microstructure in chips during machining of Grade 2 titanium at different cutting speeds has been investigated. The morphology of the chip changes from continuous chip to irregular and regular segmented chip with increasing cutting speed. The deformation in continuous and segmented chips is characterized as continuous and localized shear respectively. The deformation mechanism in the irregular segmented chip is the dislocation slip in the continuous region and twinning around the localized shear. Deformation twinning was observed inside the segment between the shear bands in the regular segmented chip. These deformation twins are responsible for the hardening inside the segment.