Formation Of Deformation Twins Research Articles

Formation and annihilation of stressed deformation twins in magnesium

The mechanical response of polycrystalline materials to an externally applied load and their in-service performance depend on the local load partitioning among the constituent crystals. In hexagonal close-packed polycrystals such load partitioning is significantly affected by deformation twinning. Here we report in-situ compression-tension experiments conducted on magnesium specimens to measure the evolution of grain resolved tensorial stresses and formation and annihilation of twins. More than 13000 grains and 1300 twin-parent pairs are studied individually using three-dimensional synchrotron X-ray diffraction. It is shown that at the early stages of plasticity, the axial stress in twins is higher than that of parents, yet twins relax with further loading. While a sign reversal is observed for the resolved shear stress (RSS) acting on the twin habit plane in the parent, the sign of RSS within the majority of twins stays unchanged until twin annihilation during the load reversal. The variations of measured average stresses across parents and twins are also investigated.

Analysis of dislocation density in strain-hardened alloy 690 using scanning transmission electron microscopy and its effect on the PWSCC growth behavior

The dislocation density in strain-hardened Alloy 690 was analyzed using scanning transmission electron microscopy (STEM) to study the relationship between the local plastic strain and susceptibility to primary water stress corrosion cracking (PWSCC) in nuclear power plants. The test material was cold-rolled at various thickness reduction ratios from 10% to 40% to simulate the strain-hardening condition of plant components. The dislocation densities were measured at grain boundaries (GB) and in grain interiors of strain-hardened specimens from STEM images. The dislocation density in the grain interior monotonically increased as the strain-hardening proceeded, while the dislocation density at the GB increased with strain-hardening up to 20% but slightly decreases upon further deformation to 40%. The decreased dislocation density at the GB was attributed to the formation of deformation twins. After the PWSCC growth test of strain-hardened Alloy 690, the fraction of intergranular (IG) fracture was obtained from fractography. In contrast to the change in the dislocation density with strain-hardening, the fraction of IG fracture increased remarkably when strain-hardened over 20%. From the results, it was suggested that the PWSCC growth behavior of strain-hardened Alloy 690 not only depends on the dislocation density, but also on the microstructural defects at the GB.

Sluggish hydrogen diffusion and hydrogen decreasing stacking fault energy in a high-entropy alloy

Hydrogen diffusion and its interaction with dislocations play an important role in hydrogen embrittlement, however, such a process in multiple-principal high entropy alloys (HEAs) is still elusive. Here, first-principles calculations were performed to investigate the solution and diffusion of hydrogen and its effect on the stacking fault energy of FeCoNiCrMn. It is shown that the unique lattice distortion in HEAs causes a wide distribution of local hydrogen solution energy, and the trapping of hydrogen in low energy sites increases diffusion barriers. The zigzag path and asymmetry of forward and backward diffusion result in the sluggish diffusion of hydrogen. Furthermore, hydrogen reduces unstable and stable stacking fault energies, originated from the transfer of electron between hydrogen and metal atoms, which promotes formation of deformation twins. This provides a theoretical guidance for designing novel engineering materials with optimal combination of their mechanical properties and hydrogen embrittlement resistance.

Unveiling the atomic-scale origins of high damage tolerance of single-crystal high entropy alloys

High entropy alloys (HEAs) exhibit an unusual combination of high fracture strength and ductility. However, atomic mechanisms responsible for crack propagation in HEAs are still not clear, which limits further improving the damage tolerance. Here we investigate effect of crystal orientation on the crack-tip behaviors in single-crystal HEA CrMnFeCoNi using atomic simulations to explore fracture micromechanism. The formation of deformation twinning and activation of multislip systems are observed during the propagation crack with the $(001)\ensuremath{\langle}110\ensuremath{\rangle}$ orientation, consistent with the previous experiments. Under the $(\overline{1}10)\ensuremath{\langle}110\ensuremath{\rangle}$ orientation, the amorphous region takes place throughout the crack growth, and is difficult to occur in traditional metal materials. Dissimilarly, for the $(1\overline{1}\overline{1})\ensuremath{\langle}110\ensuremath{\rangle}$ orientation, the blunting and slip bands occur at the front of the crack tip by switching the slip mode from the planar to wavy slip, observed in recent transmission electron microscopy experiments. The chemical disorder leads to the obvious fluctuation of flow stress, but hardly affects the deformation mechanism at the crack tip. Compared to traditional metals and alloys, the high local stress concentration induced by coupling effect of severe lattice distortion and tension strain leads to the structure transformation from crystallization to amorphization at the crack tip in HEA. While the presented atomic simulations and the associated conclusions are based on CrMnFeCoNi HEA, it is believed that the current deformation mechanism at crack tip could also be applied to other face-centered-cubic HEA.

Revisiting the role of prestrain history in the mechanical properties of ultrafine-grained CoCrFeMnNi high-entropy alloy

The yield strength of face-centered cubic (FCC) alloy is always insufficient for applications. In this work, different prestrain histories were imposed to improve the yield strength and strain-hardening capability of an ultrafine-grained (UFG) CoCrFeMnNi high-entropy alloy (HEA). In contrast to the specimens prestrained at 293 K, the specimens prestrained at 77 K possess higher yield strength, elongation, and strain-hardening capability, which were intensely related to the formation of deformation twins. The cryogenic strengthening magnitude is found to be strongly associated with the grain size, but slightly affected by dislocations. By modulating the prestrain history, an ultrahigh yield strength of 1.84 GPa and a considerable uniform elongation of 13% were achieved at 77 K in the CoCrFeMnNi HEA. Hence, imposing prestrain on HEAs at 77 K could be an efficient strategy to harmonize the mechanical properties of the FCC HEAs, which would enrich the application of HEAs in cryogenic fields.

Investigations of high-temperature tensile properties of Zn–25Sn–x(0.1–0.2)Cu–y(0.01–0.02)Ti high-temperature Pb-free solders

The use of Pb-containing solders in electronic products has been restricted due to their harm to both human health and the environment. Although the Sn–37Pb solder has been well replaced by Sn–3.0Ag–0.5Cu or other Pb-free solders, there is no drop in replacement for high-temperature Pb-free solders. In this study, the microstructure and high-temperature tensile properties of the Zn–25Sn–x(0.1–0.2)Cu–y(0.01–0.02)Ti high-temperature Pb-free solders were investigated. The design of the moderate alloy composition prevented undesirable Cu- and Ti-containing intermetallic compound formation that may cause alloy embrittlement. The solders exhibit superior tensile strength and competitive elongation compared with the conventional high-Pb solders and the other potential candidates. The Zn–25Sn–xCu–yTi solders can be strengthened in terms of the tensile strength enhancement without a loss of ductility under both room-temperature and high-temperature testing conditions. The minor Cu and Ti elements served as heterogeneous nucleation sites for inducing the microstructure refinement of the primary (Zn) phase. The Cu-in-Zn solid solution phenomenon as well as the formation of deformation twins during the high-temperature tensile test also contributed to the superior tensile properties of the designed alloys.

Simultaneous twinning and microband formation under dynamic compression in a high entropy alloy with a complex energetic landscape

High entropy alloys (HEAs) have been the subject of significant research in recent years due in part to their excellent mechanical properties and unique microstructure. Chemical disorder in these alloys is thought to lead to a complex energetic environment that affects the nucleation and movement of dislocations. In this study the development of deformation substructures is assessed in the Cantor HEA (CoCrFeMnNi) due to quasistatic and dynamic compression. Scanning and transmission electron microscopy (SEM/TEM) techniques are used to image and quantify dislocation density. When increasing the strain rate from 10−3 s−1 to 5000 s−1 we observe an increase in the overall dislocation density which leads to the formation of deformation twins. Further at a rate of 8000 s−1, both deformation twins and microbands become operative plasticity modes, which are usually associated with different extremes of stacking fault energy. To relate this plastic response to chemical disorder, atomistic calculations are used to compute the generalized stacking fault energy curve and approximate the temperature dependence of the intrinsic stacking fault energy for the Cantor alloy, which reveals significant local variation in these critical energetic parameters associated with dislocation and twinning behaviors.

Effect of gaseous hydrogen embrittlement on the mechanical properties of additively manufactured CrMnFeCoNi high-entropy alloy strengthened by in-situ formed oxide

Effect of gaseous hydrogen charging on the mechanical properties of additively-manufactured CrMnFeCoNi high-entropy alloy (HEA) strengthened by in-situ formed oxide was investigated. The results showed that SLM-built HEA exhibited yield strength of ∼729.6 MPa and has an excellent hydrogen embrittlement resistance due to high-hydrogen solubility and easy deformation twin formation.

On the effect of aluminum and chromium on the deformation twinning of body-centered cubic iron

This paper reports on investigations on the effect of aluminum and chromium on the formation of deformation twins in body-centered cubic iron. For this purpose, impact loading experiments (ε = 104 s−1) were carried out at room temperature on samples of iron and the alloys Fe-10at.-% Al and Fe-10at.-% Cr. Furthermore, the grain size was varied in order to consider the effect of the alloying elements for both fine-grained and coarse-grained microstructures. The investigations on twin formation were carried out by electron backscatter diffraction (EBSD) and by quantitative image processing using optical microscopy. The observations show a comparable twin portion in iron and Fe 10at.-% Cr after impact loading for both grain sizes. In the alloy Fe-10at.-% Al the portion of twins is significantly increased for fine-grain and coarse-grain state. In order to clarify the possible reason, X-ray diffraction analyses (XRD) were additionally carried out. These show a shift in the reflexes which can be attributed to an elastic distortion of the lattice. This lattice distortion due to the formation of solid solution correlates very well with the determined twin fractions.

Hot compression deformation behavior of Mg–Y–Zn alloys containing LPSO phase

The effect of the volume fraction of the long periodic stacking ordered (LPSO) phase on elevated temperature compressive deformation behavior is investigated using four types of Mg–Y–Zn alloys (Mg-0.4Y-0.2Zn, Mg–2Y–1Zn, Mg–5Y-2.5Zn, Mg–9Y–6Zn, where the numbers indicate at.%) produced by casting. The LPSO phase volume fraction is found to increase with increases in the concentrations of Y and Zn. The results obtained from compression tests at elevated temperatures reveal that the responses of these alloys to compression behavior, i.e., the stress vs. strain behavior, are changed by variations in the test conditions, such as the temperature and strain rate. The flow stress of the dilute solid-solution Mg-0.4Y-0.2Zn alloy containing a few volume proportions of the LPSO phase exhibits a small strain rate dependence on flow stress. In contrast, the flow stress clearly decreases with lower strain rate and higher temperature in the other three Mg–Y–Zn alloys, irrespective of the LPSO volume fraction. Observations of the deformed microstructures indicate that deformation twins are formed in the dilute alloy; however, the other three alloys show different microstructural features. Alternative for deformation twin formation, deformation kink bands are produced in these alloys; some of the existences of the LPSO phase (at least several dozens percent) are necessary to induce deformation kink band formation and to control the rate-controlling mechanism. Normalized plots obtained from previous hot compression testing together with the present results suggest that the volume fraction of the LPSO phase is unlikely to affect compressive deformation behavior.

Extrinsic size dependent plastic deformability of ZnS micropillars

ZnS has been widely used as an infrared optical material due to its superb optical properties, but its mechanical properties are less well understood. In this study, we investigated the size-dependent mechanical behavior of vacuum hot-pressed ZnS micropillars with a diameter of 3, 1.5, and 0.5 μm by in-situ microcompression tests in a scanning electron microscope. A clear size effect was observed. The 3 μm micropillars showed the highest variability of stress-strain behavior. Whereas the 0.5 μm pillars exhibited higher flow stress and improved strain-hardening capability to a compressive strain of ~10%. Post-mortem microscopy analyses coupled with ASTAR crystal orientation mapping suggest that plastic deformation is accommodated by stress-induced wurtzite-to-sphalerite phase transformation and the formation of abundant stacking faults and deformation twins. Meanwhile, the substantial strain hardening derives from preexisting stacking faults, twin boundaries, and grain boundaries that hinder the propagation of partial dislocations. This study provides a fresh perspective on the design of ceramic materials with high strength and plastic flow ability.

Simultaneous Twinning and Microband Formation Under Dynamic Compression in a High Entropy Alloy with a Complex Energetic Landscape

High entropy alloys (HEAs) have been the subject of significant research in recent years due in part to their excellent mechanical properties and unique microstructure. Chemical disorder in these alloys is thought to lead to a complex energetic environment that affects the nucleation and movement of dislocations. In this study we assess the development of deformation substructures in the Cantor HEA (CoCrFeMnNi) due to quasistatic and dynamic compression. Scanning and transmission electron microscopy (SEM/TEM) techniques are used to image and quantify dislocation density. When increasing the strain rate from 10-3 s-1 to 5000 s-1 we observe an increase in the overall dislocation density which leads to the formation of deformation twins. Further at a rate of 8000 s-1, both deformation twins and microbands become operative plasticity modes, which are usually associated with different extremes of stacking fault energy. To relate this plastic response to chemical disorder, atomistic calculations are used to compute the generalized stacking fault energy curve and approximate the temperature dependence of the intrinsic stacking fault energy for the Cantor alloy, which reveals significant local variation in these critical energetic parameters associated with dislocation and twinning behaviors.

Microstructure and mechanical properties of CoCrFeMnNi high entropy alloy with ultrasonic nanocrystal surface modification process

In this study, mechanical properties improvement of equiatomic CoCrFeMnNi treated with an ultrasonic nanocrystal surface modification (UNSM) was studied. The applied UNSM treatment with static loads of 10 N, 20 N, and 60 N provided a severe plastic deformation, which produced a gradient structure. The near-surface area exhibited a high number of dislocation densities and deformation twin interaction, leading to a surface strengthening and hardness improvement of up to 112% than the deformation-free interior region. Increment of dislocation densities and deformation twin formation on the surface also enhanced the yield and ultimate tensile strength of the UNSM-treated specimens. Furthermore, the combination of hard nanocrystallites layer on the surface and ductile coarse grain in the specimen interior as a result of the UNSM treatment successfully maintained the strength–ductility balance of the CoCrFeMnNi.

The synergistic effect of deformation twins and polycrystalline structure on strain hardening in a high-SFE Fe-Mn-Al-C austenitic cast steel in compression

The deformation microstructures of the Fe-31.6Mn-8.8Al-1.38C alloy with high stacking fault energy (SFE) of ~ 82.3 mJ/m2 were studied during compression. During the whole compression, typical planar dislocation configurations were observed including Taylor lattices, domain boundaries and micro-bands due to the glide plane softening effect. Besides, some special deformed structures were observed including the deformation twins and a kind of polycrystalline structure. In the present steel, the formation of deformation twins was related to the ultra-high applied stress and the critical twinning stress (σT). Moreover, dislocation cell blocks were formed due to dislocation cross slip inside the polycrystalline structure. The results show a synergistic effect of deformation twins and polycrystalline structure on strain hardening, which restricted the deformation and hindered the dislocation motion, and finally enhanced the strain hardening.

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Abstract The plasticity of high Si steels is improved remarkably by adding Cr in high purity steels. The factors which contribute to this phenomenon were investigated. At the normal level of purity, the plasticity of Fe–Si alloys is deteriorated by Cr addition. However, in high purity steels, the plasticity drastically improves when Cr is added. When the influence of microstructure was removed, the improvement of plasticity by adding Cr to high purity Fe–Si alloys was attributed to the suppression of deformation twinning formation. The microstructural improvement was explained by grain refinement and by formation of subgrains during hot rolling. The properties of these steels are expected to be applied to new functional steels.

Bauschinger Effect of Mn18Cr18N Austenitic Stainless Steel

The experimental study of Bauschinger effect in Mn18Cr18N austenitic stainless steel was presented by compression-tensile cyclic loading tests with the prestrains ranging from 0.005 to 0.1, which was illustrated utilizing stress-strain curves and analysed by TEM images from aspects of microstructural mechanisms. Moreover, the Bauschinger effect and its associated roundness phenomenon in reverse flow curve with respect to different cycles and cyclic strain amplitudes were evaluated in a quantitative manner. The experimental results indicate that Bauschinger effect is apparent during the test. At smaller cyclic strain amplitude, intergranular backstress is the main source of Bauschinger effect. With further increasing of cycles, dislocation density increases and dislocation movement is hindered in the reverse deformation. Therefore, Bauschinger effect is weakened to some extent. At large cyclic strain amplitude, backstress originating from the dislocation pile-up at grain boundaries and the continuous formation of deformation twins dominate the Bauschinger effect. In addition, the backstress results in the roundness of reverse curve during cyclic loading. The larger value of Δep*, the more obvious the roundness of the reverse curve, and the more significant the Bauschinger effect.

Deformation twinning in equiaxed-grained Fe-6.5 wt.%Si alloy after rotary swaging

Tensile behavior of an equiaxed-grained Fe-6.5 wt.%Si alloy, which was deformed into Φ6 mm bar by hot rotary swaging, was investigated at various temperatures (300–400 °C) and stretching rates (0.42–1 mm/min). The results revealed an enhancement in the intermediate-temperature tensile ductility after heat treatments. Deformation twinning was found in the equiaxed-grained Fe-6.5 wt.%Si bars during the tensile test, and heat treatments can enhance the deformation twinning. More twins can be observed in the necking areas than other regions. The high Schmid factor values above 0.4 after heat treatments demonstrated that deformation twinning can easily occur in the equiaxed-grained Fe-6.5 wt.%Si alloy. Higher deformation temperatures, higher strain rates, and larger degree of order suppressed the formation of deformation twinning, while the grain sizes had little effect on the deformation twinning. The twinning stress of the Fe-6.5 wt.%Si alloy increased with the increasing grain size, which did not agree with the Hall–Petch type relationship. The deformation twinning resulted in the improved ductility of the Fe-6.5 wt.%Si alloy.

Microstructural Characterization, Functional Behavior and Deformation Mechanism of Ni>sub>54Mn <sub>25</sub>Ga <sub>21-x</sub>Dy <sub>x</sub> High Temperature Shape Memory Alloys with Low Thermal Hysteresis

Potential applications at elevated temperatures require shape memory alloys (SMAs) to possess high transformation temperature, excellent shape recovery ability and thermal/functional stability. The present study investigated the influences of Dy addition on the microstructural characterization, functional behavior and possible deformation mechanisms in high temperature Ni54Mn25Ga21 SMAs. The results indicated that adequate Dy addition contributed to a desirable tradeoff among compressive strength, ductility and functional performance. Ni54Mn25Ga20.9Dy0.1 alloy showed the highest shape memory recoverable strain due to the improvement of mechanical properties and formation of small Dy-rich precipitates. Simultaneously, an obvious pseudoelasticity was observed over a wide temperature range with a maximum recoverable strain of 3.74% at 320°C in this alloy. All the Dy-containing alloys exhibited high thermal cyclic stability and quite low thermal hysteresis. However, it was revealed that a complicated and abnormal relationship between thermal hysteresis and the middle eigenvalue λ2 existed for the investigated alloys with a lattice transition from cubic to tetragonal. Calculated results based on energy minimization theory revealed that both the transformation strain and critical stress for inducing martensitic transformation varied with crystallographic orientations. The detwinning of pre-existing (112) compound twins and formation of (112) deformation twins were the dominant deformation mechanisms of martensite, which were highly compatible with the macroscopical recoverable strain. The present study provided valuable insights for developing high-performance multifunctional shape memory alloys for high temperature applications.

Weldability of cast CoCrFeMnNi high-entropy alloys using various filler metals for cryogenic applications

This study investigates the effect of stainless steel (STS) 308 L and high-entropy alloy (HEA) filler metals on gas tungsten arc (GTA) welding. The weldability of the cast CoCrFeMnNi HEAs was determined based on the microstructural and mechanical properties of the welds. The cast HEA exhibited larger dendrite packets than the weld metals (WMs). The hardness in the WM was superior compared with that in the base metal (BM). The WM using STS 308 L exhibited a fully face-centred cubic (FCC) structure with no indication of δ-ferrite and lower hardness than that using HEA filler. The GTA welds using both fillers showed tensile properties comparable to the cast BM at 298 K, and the tensile fracture of the transverse welds occurred near the cast BM. The cryogenic tensile properties in the GTA welds were superior compared with the room-temperature property due to the significant formation of deformation twins and high dislocation density at 77 K. This was probably due to the decrease in the stacking fault energy at the cryogenic temperature compared with that at the room temperature. Therefore, it is possible to apply commercial STS 308 L filler metal for the CoCrFeMnNi HEA in cryogenic applications.

Microstructural evolution and mechanical properties of deep cryogenic treated Cu–Al–Si alloy fabricated by Cold Metal Transfer (CMT) process

Cryogenic treatment is an effective route to obtain fine grained microstructure in materials. In this work, the effect of deep cryogenic treatment (DCT) on the microstructural evolution and mechanical properties of a copper-aluminum-silicon (Cu–Al–Si) alloy deposited by cold metal transfer (CMT) technology was investigated. Copper-rich Cu–Al–Si alloy with ~8.3% aluminum content was deposited on T2 copper substrate using an innovative dual wire feed system based on CMT technology. The deposited alloy was subjected to DCT for varying time duration (0, 6, 12 and 24 h). Evaluation of microhardness and tensile properties showed that the deep cryogenic treatment enhanced these properties. Microhardness increased about 10% with the increase in the DCT time duration. Under tensile loading conditions, the DCT conducted for 12 h provided the best properties, with an increase in both the tensile yield and ultimate strengths (~20% increase). Microstructural evolution and grain orientation were investigated using Electron Back Scattered Diffraction (EBSD). Microstructural studies revealed the formation of subgrains, fine grains and deformation twins due to DCT. Significant grain refinement was achieved due to the increase in the formation of high angle grain boundaries. It is summarized that the deep cryogenic treatment induces microstructural evolution: (i) grain refinement and (ii) texture randomization, and improves the mechanical properties of Cu–Al–Si alloy.