Papers by Keyword: Twinning

Abstract: Magnesium (Mg) alloy sheets are expected to contribute to the lightweighting of structural components, owing to their inherent benefits of low density and high specific strength. However, the limited room-temperature press formability exhibited in Mg alloy sheets remains a barrier to their expanded use. A significant factor contributing to the limited formability is the strong basal texture. To improve the room-temperature press formability, ZX series Mg alloy sheets that weakened the basal texture have recently been developed. In our previous study [Hama et al., Mater. Res. Proc., 28(2003), 711-716], cup drawability of a Mg-1.5mass%Zn-0.1mass%Ca (ZX10Mg) alloy sheet was investigated at room temperature. The obtained cup exhibited that the cup height and thickness strains differed significantly in the circumferential direction of the cup. However, a more detailed discussion on the mechanisms of this anisotropic deformation was hampered by a reliance solely on experimental observations. Therefore, in this study, crystal plasticity finite-element simulations of cup drawing of the ZX10Mg alloy sheet were performed. The simulation results qualitatively reproduced macroscopic and microscopic deformation behaviors during cup drawing. Numerical studies showed that the anisotropic deformation during drawing was primarily induced by the texture of the material, suggesting that anisotropic deformation is inevitable unless the anisotropic c-axes distribution remains in the initial texture.

Abstract: The design flexibility afforded by additive manufacturing, commonly known as 3D printing, is broadening the industrial applications of high-entropy alloys (HEAs). The 3D-printed CrMnFeCoNi HEA (or Cantor alloy) exhibits a unique combination of strength and ductility, attributed to its multifaceted deformation mechanisms. While the deformation behavior of this alloy under monotonic loading has been extensively studied, its cyclic plasticity, which is crucial for fatigue performance, remains a relatively underexplored area. To address this gap, the current work investigates the deformation microstructure of a CrMnFeCoNi HEA fabricated using laser-beam powder bed fusion. Electron backscatter diffraction (EBSD) is employed to characterize the surface microstructural changes. The results reveal the simultaneous activation of multiple slip systems in the region near the fatigue crack, which induces grain rotation. Additionally, the activation of twinning-induced plasticity plays a significant role in accommodating the cyclic plastic strain.

Abstract: Magnesium alloys are promising for bioabsorbable stents due to their biocompatibility and degradability. Unlike conventional stainless steel stents that remain in the body and may cause complications, magnesium stents gradually degrade, reducing risks like restenosis and thrombosis. However, magnesium has low corrosion resistance, and its corrosion resistance needs to be improved. The crystal structure is one factor affecting the corrosion properties of metallic materials. Several studies have been conducted on the relationship between crystal structure and corrosion properties to improve magnesium's corrosion resistance. It is essential to elucidate the relationship between crystallographic factors and corrosion mechanisms, in the case of stents, plastic deformation during expansion results in the formation of fine crystal grains and twinning deformation. Therefore, the purpose of this study is to investigate the influence of refined grains and twinning on the corrosion properties of magnesium. Hot rolling and compression are used to refine the crystal grains and form twinning in experiments. The crystal structure can be observed by optical microscopy and SEM-EBSD. Following the evaluation of the crystal structure, immersion tests in brine are conducted to measure the mass loss and observe the corrosion behaviour.

Abstract: Intensive global research is focused on advanced conductive materials to meet the electrical requirements of the telecommunication and power industry. The primary aim is to enhance electrical conductivity, resulting of improved current-carrying capacity and reduced energy loss during transmission. Copper and its composites are vital for power transmission and telecommunications due to their electrical, thermal, and mechanical qualities. However, current methods have drawbacks, such as compromised conductivity with alloying. Graphene, an extraordinary carbon allotrope with exceptional properties and high conductivity, offers promising opportunities for the development of superior materials; such as graphene-incorporated copper (GrCu). The incorporation of graphene into copper wire holds significant potential for various industries, including electronics, energy transmission, and telecommunications, where high conductivity and reliability are paramount. This study investigates GrCu characteristics through mixing graphene and copper, vacuum melting, fine copper wire drawing, and GrCu coaxial cable manufacturing. Graphene infusion enhances conductivity and mechanical properties, altering microstructure and inducing twin boundaries in copper grains. Graphene's disruption during wire drawing triggers this effect, elevating wire conductivity to 103.5% by IACS. GrCu coaxial cable demonstrates performance coherence with HFSS simulation up to 6 GHz. Graphene's inclusion offers tailored material properties. Ongoing research promises further optimization and advancement of graphene-copper composites, paving the way for novel technological progress.

Abstract: A comparative study of the structure and properties of two biodegradable Fe – 27Mn and Fe – 27Mn – C alloys for biomedical use after equal channel angular pressing (ECAP) has been carried out. It is noted that addition of carbon in the alloy leads to a change in the mechanism of plastic deformation from the formation of martensite to deformation twinning in austenite. ECAP improves the strength characteristics of the alloys under study and the corrosion rate by refining the structure and increasing the dislocation density. The presence of a partially twinned structure in the Fe – 27Mn – C alloy results in a lower corrosion rate despite a stronger refinement of the alloy structure after ECAP.

Abstract: The goal of this study is to make selective etch possible for the next generation of MEMS(microelectromechanical systems) devices that are composed Ni-Mn-Ga and silicon layers. Due tothe large magnetic-field-induced strains of Ni-Mn-Ga, sensing and actuating components can be fab-ricated in the Ni-Mn-Ga layers. Other functional components can be manufactured in the silicon layer.Single crystalline Ni-Mn-Ga alloys that are grown by using the Bridgman vertical growth techniquehave so far obtained the largest magnetic field-induced strain (MFIS), a magnetic shape memory(MSM) effect. Similar to silicon wafers, Ni-Mn-Ga wafers are also sliced from crystal-oriented singlecrystalline ingots. To fabricate hybrid MEMS devices such as micromanipulators and robots, lab-on-chip containing micropump manifolds and valves, or vibration energy harvesters, the fabricationprocesses used for MEMS devices will be also used to fabricate components in the Ni-Mn-Ga layer ofthe hybrid MEMS devices. One of the most important processes for MEMS fabrication is the structur-ing of materials by chemical etching. The main goal of this study is to obtain evidence that the etchantetches silicon but not Ni-Mn-Ga and to identify an etchant that etches Ni-Mn-Ga but not silicon. Thepresent paper reports on a novel experiment in dissolving Ni-Mn-Ga alloys. An etchant compositionof 69% HNO3, 98% H2SO4, and CuSO4•5H2O is proposed for dissolving Ni-Mn-Ga alloys and thevariation in the dissolution rate by adjusting the concentrations of HNO3 and ultrapure water (UPW)is demonstrated. This etchant was demonstrated to etch Ni-Mn-Ga but not silicon. The HF+HNO3acidic solution commonly used for etching silicon does not dissolve Ni-Mn-Ga alloys.

Abstract: Magnesium (Mg) alloy sheets are expected to be used as light-weight materials for structural components because of their low density and high specific strength. However, their press formability at room temperature is poor due to the strong crystal anisotropy of the hexagonal structure and the strong basal texture especially observed in AZ series rolled Mg alloy sheets. Recently, ZX series Mg alloy sheets have been developed that weaken the basal texture, thus improving press formability at room temperature. Although the plastic deformation behavior of ZX series Mg alloy sheets should be different notably from that of AZ series Mg alloy sheets, it is not substantially understood yet. In the present study, the work-hardening behavior of a rolled Mg-1.5mass%Zn-0.1mass%Ca (ZX10Mg) alloy sheet under monotonic and reverse loadings was investigated in detail experimentally. The microstructures of prestrained samples were also measured by means of EBSD measurements. Under monotonic tension, the stress in the rolling direction is higher than that in the transverse direction. A plateau region appears only in the transverse direction. Under monotonic compression, plateau regions appear in both the rolling and transverse directions. The in-plane anisotropy is less pronounced than that under tension. Under reverse loading from compression to tension, a sigmoidal curve appears during tension regardless of the loading direction. The sigmoidal trend depends strongly on the loading direction. The mechanisms that induce the abovementioned anisotropic deformation are discussed in terms of the difference in twinning and detwinning activities.

Abstract: This research was conducted to investigate the effect of thermomechanical process on microstructures and mechanical properties of Cu-28Zn-2Al alloys. Thermomechanical process was carried out by cold rolling process with 70% thickness reduction and followed by annealing process with variation temperature of 400°C, 500°C and 600°C. The result show that the β phase and shear bands are found in the samples. Further, cold rolling process can increase hardness of Cu-28Zn-2Al alloys from 100 to 172 VHN. The heat treatment with annealing process at 400°C tend to decrease the tensile strength of cold rolled samples from 695 to 472 MPa and more decreased until 422 MPa at 600°C. In contrast, annealing process at 400°C tend to increase the elongation from 10 to 28% and more increased up to 56% at 600°C. This phenomenon prove that the annealing process will increase ductility of cold rolled samples of Cu-28Zn-2Al alloys.

Abstract: Hot and cold deformation behavious and microstructure evolutions of Mn18Cr18N were investigated by thermo-mechanical modeling experiments and microstructure analysis. The results show that hot deformation flow stress curves characterized by the same work hardening and subsequent stress softening varied with temperatures and strain rates for both of as-cast and as-forged samples. And flow stresses are sensitive to strain rate. At strain rates lower than 0.01s^-1, the flow stresses are lower, and microstructure evolution controlled by dislocation mechanism dynamic recrystallization; At strain rates higher than 0.1s^-1, the flow stresses are higher, and microstructure evolution controlled by twinning mechanism dynamic recrystallization. But the dynamic recrystallzed fraction of the as-cast sample was much less than that of the as-forged sample. For cold deformation, the simple uniaxial tensile sample shows that the monotone increasing flow stress curve and monotone decreasing work-hanrdening rate. However, for the uniaxial and biaxial compression-tensile samples with different previous compression, the subsequent tensile yield stress, the maximum tensile stress, the reduction of cross sectional area and the elongation have extremums respectively at the previous compressive deformation of about 25%-30%. Microstructure evolution mechanisms during cold deformation were planar slipping and twinning.

Abstract: Fatigue tests were conducted under several stress ratios, including negative maximum stress to elucidate the fatigue crack initiation mechanism of a magnesium alloy, AZ31. The specimen surface near the crack initiation site was analyzed by EBSD. On the basis of the results of EBSD analysis, it is concluded for an alternating cyclic stress condition (fully reversed cyclic stress) that fatigue cracks formed from grains where both the grain size and Schmid factor of the basal slip system are large, and that the crack initiation mechanism is based on irreversible slipping and unrelated to twinning. Under compression-compression fatigue test (R=10), cracks were formed along boundary of grains with large Schmid factor and misfit of both side grain are large. At the tip of the initiated crack, twin bands were observed.