Papers by Keyword: Twin

Abstract: The effect of annealing treatment on the mechanical properties and microstructure of cold-rolled Cu-20% Zn alloys was investigated in this work. Mechanical properties changed dramatically with the increase of temperature. According to the microhardness test, it can roughly concluded that 150 is the optimal annealing temperature for deformation, at which a uniform elongation increased from 1.4658% before annealing to about 6.89%, and the elongation to failure increased from 7.426% to 16.81% with the same strength almost retained. The changes of microstructure during the annealing process are mainly distributed to the improvement of mechanical properties.

Abstract: Dynamic recrystallization (DRX) behavior in a coarse columnar-grained Cu-0.65Sn-0.025P and Cu-0.025P (mass%) alloys were systematically investigated by compression tests at temperatures between 1073 K and 1253 K and at true strain rates from 2 x 10^-4 s^-1 to 2 x 10^-1 s^-1 in vacuum. As a model sample, an orientation-controlled bicrystal having [0 1 twist 18^o boundary was prepared and also hot deformed. Appearance of the peaks stress, where DRX onsets, was much delayed in Cu-Sn-P alloy compared with that in Cu-0.025P alloy. The onset of DRX was, therefore, obviously impeded by the small addition of Sn to Cu-P. While nucleation of new grains took place almost at random in Cu-Sn-P when strain rate was high enough, it tended to appear more preferentially at grain boundary with decreasing strain rate and with increasing temperature. The most of new grains were annealing twins formed behind the migrating grain boundary. Because grain boundary migration took place more extensively with increasing temperature and with decreasing strain rate, the preferential nucleation at grain boundary became more significant.

Abstract: Zn and Al are major alloying elements of Mg alloys. Main slip system of Mg is a basal slip and the CRSS increases with Zn or Al content. According to von-Mises criterion, five kinds of independent slip systems are required for uniform deformation, so it is necessary to activate non-basal slip systems to show good ductility. However, it has not become clear the effect of Zn or Al for non-basal slip systems yet. To investigate deformation behavior of magnesium crystal by non-basal slip, Mg-Zn and Mg-Al single crystals were stretched in the [110] direction and Mg-Zn single crystals were compressed in the [0001] direction. {112}<23> second order pyramidal slip was activated in Mg-0.1at%Zn and Mg-0.5at%Al. On the other hand, {101} twin was mainly activated in Mg-1.0at%Al alloy. Yield stress due to the pyramidal slip of magnesium decreased by 0.1at%Zn addition, however they increased with addition of aluminum..

Abstract: Tensile and compressive tests were performed on extruded Mg-10Gd-2Y-0.5Zr (mass fraction, %) alloy specimens with different tilt angles relative to extrusion direction. The microstructures were examined by X-ray diffraction (XRD), optical microscopy and scanning electron microscopy (SEM). Calculations of the orientation factors for basal slip were done for that existed texture. The results show that the alloy doesn’t show tensile-compressive yield strength asymmetry, and the highest flow stress is appeared along the extrusion direction. Meanwhile the extruded textures are much more randomized. The reason may be that the addition of rare-earth is benefit to activate the non-basal slips, especially pyramidal <c+a>slip. The reduction of c/a ratio is helpful to enhance the symmetry of the Mg crystal, which decreases the critical resolved shear stress of <c+a> slip.

Abstract: Microstructures of electron beam welded joints for TA15 titanium alloy with different hydrogen contents were observed and analyzed by SEM and TEM. And the influence of hydrogen on microstructure of the joints was investigated. The results show that the microstructure of the weld metal is lamellar α+β phase after hydrogen charging. In the range of hydrogen contents discussed in this study (from 0 to 0.101 wt%), With the increase of hydrogen content, there is little change in the appearance of the microstructure of the weld metal. The presence of hydrogen can promote the growth of twins in electron beam welded joints. With the increase of hydrogen content, the number of twins is increased. When hydrogen content reaches to a certainty level, hydrides are found in TA15 electron beam welded joints.

Abstract: In Fe₃Ga single crystals with the D0₃ structure, three types of pseudoelasticities based on dislocation motion, martensitic transformation and twinning take place depending on the heat treatment, the loading axis and the stress sense. In this paper, we report the detail of the transformation and twinning pseudoelasticities in the crystals focusing on the crystallography and the temperature dependence. In particular, the driving force for the twinning pseudoelasticity was discussed, focusing on the atomic arrangement. In Fe₃Ga single crystals homogenized or solutionized in the α disordered region, the martensites with the 14M structure, containing numerous stacking faults were stress-induced during loading, while they disappeared during unloading by the reverse transformation, resulting in the transformation pseudoelasticity with small stress-strain hysteresis. In contrast, twinning pseudoelasticity caused by twinning and untwinning of 2.2^T-type pseudo-twins appeared in the well-ordered D0₃ crystals, accompanied by a serrated flow in the stress-strain curve. The contribution of the twinning pseudoelasticity to strain recovery became significant at low temperatures at which the dislocation motion was difficult. It should be noted that the formation of the pseudo-twins could be regarded as a certain displacive phase transformation since the crystal structure of the twins became orthorhombic due to the twin shear without atomic shuffling. The free energy difference between the D0₃ matrix and the pseudo-twins resulted in the twinning pseudoelasticity. Moreover, the pseudo-twins were transformed into the perfect twins by annealing at 300 °C where the atomic shuffling could occur. The perfect twins remained even after complete unloading due to their low driving force for the pseudoelasticity.

Abstract: Sacking fault energy (SFE) is the key role in solving this problem of getting high strength and expected ductility simultaneously. This work adds Al as the procedure of decreasing SFE in Cu face-centered cubic. It is an economic and effective method to counterpart Cold-rolling at liquid nitrogen temperature to get high density deformation twin and ultrafine-grains size. After undergoing tensile and X-ray diffraction tests, Cu-4.5 wt.% Al plays the best performance on both strength and ductility. Thus there exist the optimal SFE of Cu-Al alloys which get both high strength and expected ductility simultaneously.

Abstract: The tensile deformation microstructures of an extruded AZ31 Mg alloy were examined at temperatures ranging from room temperature to 250°C over a strain rate range from 10-4 s-1 to 10-2 s-1. It is found that the strain rate has an enhanced effect on the tensile flow behavior of AZ31 Mg alloy with increasing temperature, which is closely related to the changes of deformation microstructures. The tensile deformation of AZ31 Mg alloy is mainly accommodated by twinning and slipping at room temperature and 100°C, and the amount of deformation twins reduces with increasing temperature and decreasing strain rate. However, discontinuous dynamic recrystallization (DRX) occurs primarily at grain boundaries, and nearly no deformation twins form, as the temperature is as high as 250°C. With decreasing strain rate, more significant DRX takes place with an increasing DRX grain size. The tensile deformation of AZ31 Mg alloy at 250°C is thus primarily controlled by slipping and DRX.

Abstract: The nature of arrested cracks in run-out fatigue tests of a type 316L austenitic stainless steel with electropolished surfaces has been investigated. The fatigue limit was determined in rotating-bending by means of the staircase method to be 302 ± 5 MPa. Arrested crack nuclei were shown to arrest at coherent deformation twins, developed by fatigue.

Abstract: The thermal stability of ultrafine-grained (UFG) microstructure in face centered cubic metals processed by severe plastic deformation (SPD) was studied. The influence of the SPD procedure on the stability was investigated for Cu samples processed by Equal-Channel Angular Pressing (ECAP), High-Pressure Torsion (HPT), Multi-Directional Forging and Twist Extrusion at room temperature (RT). It is found that HPT results in the lowest thermal stability due to the very high dislocation density. Furthermore, the effect of the low stacking fault energy of Ag on the stability is also investigated. It is revealed that the UFG microstructure produced in Ag by ECAP is recovered and recrystallized during storage at room temperature. The driving force for this unusual recovery and recrystallization is the high dislocation density developed during ECAP due to the high degree of dislocation dissociation caused by the very low stacking fault energy of Ag.