Papers by Keyword: Twinning

Abstract: Cyclic impact of external parameters on the structure, such as the daily change of temperature, change of operating parameters (pressure, temperature, volumes of loaded raw material) may lead to the brittle fracture of the equipment material with fast propagation of the main crack. In view of this, there is a need to use the nondestructive test methods which allow determining the critical level of damage accumulation. The article considers the pattern of change in properties as a result of low-cyclic loading by studying the parameters of acoustic signal. Changes in acoustic emission (AE) parameters were measured in the process of uniaxial tension of specimens with different degrees of accumulated damage. Upon processing of these AE parameters, stages of damage accumulation in cyclic fatigue were determined. Thus, is was determined that acoustic-emission diagnostics is sensitive to the said changes and may be used for assessment of the damage degree of 09G2S steel specimens.

Abstract: The tensile deformation behaviour of magnesium alloy AE44 (Mg-4Al-4RE) under strain rates ranging from 10^-6 to 10^-1 s^-1 has been investigated. Present study shows that the deformation mode begins with the activation of elastic (Stage 1), followed by <a> basal slip and twinning (Stage 2), <a> prismatic slip (Stage 3) and finally to <c+a> pyramidal slip (Stage 4). The commencement of these deformation mechanisms results in four distinct stages of strain hardening in the stress-strain curve. In this work, the four stages of deformation behaviour are modelled, and an empirical equation is proposed to predict the entire stress-strain curve. Overall, the model predictions are in good agreement with the experimental data. This study on the decomposition of stress-strain curve into four stages provides insights into the contribution of individual deformation mechanism to the overall deformation behaviour and opens a new way to assess mechanical properties of die-cast magnesium alloys.

Abstract: In order to study the plastic deformation mechanism of AZ31 magnesium alloy, in situ texture measurement during uniaxial tensile deformation is conducted by using neutron diffraction. The specimen is prepared from a rolled sheet so that the deformation axis is parallel to the rolling direction. By increasing strain, the alignment of <10-10> along the tensile axis is strengthened, which is due to the activation of the prism slip system. The basal pole concentration at the prior sheet normal direction is slightly decreased by the deformation and the new texture component is formed at the transvers direction. This can be understood by activation of the {10-12} tension twinning. These results indicate that the tension twinning plays an important role even when the tensile deformation is applied parallel to the basal plane.

Abstract: Two-phase materials, such as α+β Titanium (Ti) alloys, are technologically important. A number of factors can affect deformation behavior, including the interaction stresses between phases, the crystallographic relationships between phases, and the morphology. As a result, the deformation mechanisms of two-phase alloys may be different from the individual single-phase materials. For example, twinning may not occur in a single phase material if the grain size is very small but twinning can occur in a very fine grained alloy if the second phase contributes to the interfacial stresses due to elastic interactions. Interaction stresses can result from the difference in the elastic properties of the two phases. In particular, these elastic interaction stresses can be quantified by the finite element method (FEM). In this paper recent developments regarding two-phase deformation mechanisms will be reviewed and the ramifications on mechanical behavior in regard to two-phase Ti alloys in particular and on two-phase metallic materials in general will be outlined.

Abstract: Deformation twinning behaviors have been studied in high Ni alloys, Alloy 28 or Sanicro 28 and Alloy 625 at RT and at cryogenic temperature. The microstructures were evaluated using SEM-EBSD. Some constitutive approach has also been used in the discussion on the deformation twinning in high Ni-alloys. The results show that deformation twinning can occur in high Ni alloys depending on the chemical composition, strain range and stress conditions. TWIP can occur in the Ni based superalloy Alloy 625 at cryogenic temperature, which increases both strength and ductility. This is the first report of this phenomena for this alloy. Deformation twinning in high Ni alloys occurs heterogeneously in the material, depending on crystallographic parameters such as grain orientation and Schmid factor. Formation of deformation twins can lead to high texture in the material, which will contribute to the increase of strength. The mechanisms for the formation of deformation twins in high Ni alloys have been discussed.

Abstract: A textured Mg-Al-Zn alloy rolled plate was solution treated and aged at 320 oC for 2 h and 116 h, respectively. Afterwards, the three conditions were compressed at room temperature along the transverse direction to activate {110} twinning. Ageing treatments were observed to strengthen the alloy in relation to the solution-treated condition. This has been mainly attributed to the restricted lateral growth of twins in the presence of particles and thus to the additional stress required for twin growth. Accordingly, a slightly reduced twin volume fraction, but an increased number of smaller twins was observed after compression in the aged samples.

Abstract: A temperature dependence of the tensile mechanical properties, microstructure and fracture mechanism of high-nitrogen Fe-(19-23)Cr-(17-21)Mn-(0-3)V-(0.1-0.3)C-(0.5-0.9)N vanadium-free and vanadium-containing steels was investigated. For all steels, the 0.2% offset yield strength and strain-hardening drastically increase with a decrease in test temperature. This is associated with high interstitial solid solution strengthening of the steels and more pronounced twinning and stacking-fault formation during straining below room temperature. For the vanadium-free steel, a ductile-to-brittle transition was evaluated: at 77K specimens destroy by cleavage mechanism while at room temperature steels show ductile fracture. Vanadium-alloying provides a particle strengthening of the steels and, at the same time, reduce solid-solution strengthening. Increase of vanadium concentration fully or partially suppress brittle fracture of the steels at 77K. Particle strengthening changes interstitial solid-solution effect, dislocation arrangement and slip/twinning relation in vanadium-containing high-nitrogen steels compared to vanadium-free one.

Abstract: Variety of interatomic potentials for magnesium can be found in the literature. Result of computer simulations can be slightly different depending on used potential. Particularly, twin boundary structure with the lowest energy can be different in a frame of different models. Comparison of several popular embedded-atom method potentials is provided. It is shown that either reflection or glide structure of twin boundary has the lowest energy for different potentials.

Abstract: The present study shows that warmly forged and low-temperature annealed twinning induced plasticity (TWIP) steel exhibited very high dislocation density and apparent yield-point phenomenon in addition to very high yield strength. The initial density of dislocations significantly affected the evolution of dislocations during the subsequent tensile deformation. Original high dense dislocations prompted the rapid increase of dislocations, and intensified the complexity of dislocation configurations. All these effects made the twinning deformation weakened but the dislocation deformation enhanced, leading to increased strength but decreased plasticity.

Abstract: Deformation characteristics of brass alloy are still under discussion, particularly concerning the critical level of when the change of deformation mechanism occurs. Previous research showed that the addition of Mn on brass alloys resulted in grain refinement and mechanical properties alteration. However, the effects of Mn on the deformation characteristic of brass alloys have not been investigated. In this research, Cu-Zn-xMn alloys were manufactured by gravity casting process using pure Cu and Zn ingots, as well as Mn chips as the feeding materials. Mn addition was varied to 1.26, 3.48, and 5.83wt.%. As-cast samples were homogenized at 800 °C for 2 h in a muffle furnace. The samples were then cold-rolled with the level of deformation of 20, 40, and 70 %. Samples characterization includes chemical composition analysis, microstructure observation, tensile and hardness testing. The results showed that addition of Mn for 5.83 wt.% and above created β’ phase, which is richer in Mn compare to that in the matrix. This phase segregated in the grain and along the grain boundary with irregular forms. Significant increase in hardness, yield and tensile strengths was observed with addition of Mn. The maximum elongation was achieved by addition of 3.48 wt.% Mn, while further addition tended to decrease it. At 20% deformation, slip dominated and its density reduced with addition of Mn. When the deformation level increased to 40%, twinning replaced slip as the predominant mechanism. Twinning density is slightly increase with the presence of Mn. Further deformation at 70% produced shear bands and flattened the β’ phase. Greater Mn content led to formation of more shear band.