Volume Fraction Of Deformation Twins Research Articles

Electroplastic Effects on the Mechanical Responses and Deformation Mechanisms of AZ31 Mg Foils.

Electrical-assisted (EA) forming technology is a promising technology to improve the formability of hard-deformable materials, such as Mg alloys. Herein, EA micro tensile tests and various microstructure characterizations were conducted to study the electroplastic effect (EPE) and size effect on the mechanical responses, deformation mechanisms, and fracture characteristics of AZ31 Mg foils. With the assistance of electric currents, the ductility of the foils was significantly improved, the size effects caused by grain size and sample thickness were weakened, and the sigmoidal shape of the flow stress curves during the early deformation stage became less obvious. The EBSD characterization results showed that the shape change of the flow stress curves was due to the EPE suppressing the activation of extension twinning at the early deformation stage, especially for the coarse grain samples. The suppression of extension twinning resulted in a quick increase in flow stress due to the dislocation-dominant work hardening, and the increased flow stress eventually promoted extensive deformation twins at large deformation. Thus, as the sample strained to 10% tensile deformation, the EA-tested samples showed a larger volume fraction of deformation twins than the non-EA samples. The reference orientation deviation analysis verified that the deformation twins in the EA samples were formed in the large deformation stage. Combined with the fractography, the EPE also improved the ductility by suppressing the expansion of cleavage surfaces.

Enhancing low-cycle fatigue life of commercially-pure Ti by deformation at cryogenic temperature

In this study, the low-cycle fatigue behavior of a cryogenic-rolled commercially pure titanium alloy was investigated, and compared with those of undeformed and room-temperature rolled ones. The amounts of deformation twins increased to >69.8% after cryogenic temperature rolling, and to 27.5% after room temperature rolling. Strain-controlled low-cycle fatigue tests were performed at total strain amplitudes 0.4%≤Δεt2≤ 1.2%. The Coffin-Manson and hysteresis energy-based models confirmed that low-cycle fatigue resistance was remarkably improved with increasing the volume fraction of deformation twins by pre-deformation. As the proportion of deformation twins in the microstructure increased, the hysteresis loop area decreased, the striation spacing decreased, and the severity of crack-deflection behavior increased. At low Δεt2 = 0.4%, dislocation recovery was suppressed in the pre-deformed microstructure, so cyclic behavior was stable. However, at Δεt/2 ≥ 0.8%, the recovery became the predominant mechanism, so well-defined cell structures formed and cyclic softening occurred. The smaller dislocation cells formed in RTR and CTR were considered to cause more severe crack arrest than in AR.

Effects of Silicon on the Microstructure and Mechanical Properties of 15–15Ti Stainless Steel

Effects of silicon on the microstructure and mechanical properties of 20% cold-worked 15–15Ti austenitic stainless steels are systemically investigated by uniaxial tensile tests, scanning and transmission electron microscope observations, and strength calculations. The results reveal that a large number of deformation twins are formed in the 20% cold-worked steels with various silicon additions. The volume fraction of deformation twins and the density of dislocations increase with silicon content, while the twin thickness slightly decreases. A better strength–ductility combination is achieved by silicon addition, since the yield strength of the steel with 2% silicon is 61 MPa higher than that of the steel with 1% silicon, while their uniform elongations are almost both equal to 16%. The yield strength of the 15–15Ti stainless steels is predominantly contributed by the solid solution, dislocation and deformation twin strengthening effects, which can be enhanced by silicon addition.

Evolution of deformation twins with strain rate in a medium-manganese wear-resistant steel Fe–8Mn–1C–1.2Cr–0.2V

Microstructure evolutions of the medium-manganese wear-resistant steel Fe–8Mn–1C–1.2Cr–0.2V (in wt.%) with stacking-fault energy of 22 mJ m−2 during deformation at strain rate ranging of 10−2–1 s−1 were analyzed by means of X-ray diffraction, field emission scanning electron microscopy and high-resolution transmission electron microscopy. The results indicate that the twinning-induced plasticity effect is the main strengthening mechanism of the studied steel, whilst the transformation-induced plasticity effect only occurs at high strain rate. With an increase in strain rate, volume fraction of the deformation twins, in particular that of the secondary twins, increases significantly along with decreasing average size. When applied strain rate is higher than 10−1 s−1, the parallel deformation twins are turned into a crossing morphology, and the original straight twin boundaries exhibit a ladder feature, which is attributed to the interactions between regular dislocations and twin dislocations at the twin boundary. The critical strain, a key indicator of the initiation of deformation twin, decreases with increasing strain rate. In addition, the ductility and strength of medium-manganese wear-resistant steel Fe–8Mn–1C–1.2Cr–0.2V are mainly determined by the shape and volume fraction of deformation twins.

The effect of precipitates on voiding, twinning, and fracture behaviors in Mg alloys

The effect of precipitates on fracture behaviors was comparatively investigated among three kinds of Mg alloys with different precipitates, i.e., Mg-Gd alloy with prismatic plate-shaped precipitates, Mg-Zn alloy with [0001]α rod-shaped precipitates, and Mg-Gd-Zn-Zr alloy with basal plate-shaped precipitates. By comparing the fracture behaviors of the alloys before and after aging treatment, it was evident that the presence of precipitates greatly promoted the formation of microvoids that were initiated at primary intermetallic particles. Voiding was the most accelerated in the Mg-Gd alloy, where the precipitate-dislocation interactions are the strongest. While in the Mg-Gd-Zn-Zr alloy with the weakest precipitate hardening, the precipitate-facilitated voiding was the least significant. The precipitate-dependent voiding suppressed the twinning behaviors, causing the volume fraction of deformation twins decreased in turn from Mg-Gd-Zn-Zr to Mg-Zn, and finally to Mg-Gd alloys. The tensile ductility of present Mg alloys approximately scaled with the volume fraction of deformation twins, and was highly sensitive to the precipitates. The fracture scenario of present Mg alloys was proposed that the voiding suppressed twinning and the competition between voiding and twinning can be mediated by precipitates. In the Mg-Gd-Zn-Zr alloy with limited voiding, the twinning dominated the deformation process and concomitantly resulted in a great ductility. The formation of microvoids or the fracture of primary intermetallic particles was quantitatively analyzed by applying a Weibull model, where both the alloy strength and the volume fraction of primary intermetallic particles were considered to rationalize the remarkable difference in fracture behaviors among the present three Mg alloys. Furthermore, the coupling contribution of precipitates and twins to the work hardening was modelled in the aged Mg-Gd-Zn-Zr alloy that displayed a large room temperature tensile ductility of ~ 13.5%.

Cryorolling impacts on microstructure and mechanical properties of AISI 316 LN austenitic stainless steel

Microstructure evolution and mechanical properties of AISI 316 LN austenitic stainless steel (SS) after cryorolling with different strains were investigated by means of optical, scanning and transmission electron microscopy, X-ray diffractometer, microhardness tester, and tensile testing system. The deformation-induced martensite transition and the deformation microstructure occurred during cryorolling process were always composed of high-density dislocations, deformation twins, and deformation-induced martensites. Following the strain, the dislocation density in deformation microstructure approached saturation state and the volume fraction of deformation twins combined with deformation-induced martensites increased significantly. At the 70% strain, original austenite was transformed into martensite completely. Further increasing the strain to 90% would refine the martensitic lamellae to nanoscale. The deformation degree also led to remarkable increase of the strength and hardness of the cryorolled SS, and drastic reductions of the elongation. Due to the cryorolling, the tensile fracture morphology changed from typical ductile rupture to a mixture of quasi-cleavage and ductile fracture.

Fracture behavior of an austenitic stainless steel with nanoscale deformation twins

A bulk nanotwinned austenitic stainless steel containing a large volume fraction of deformation twins was produced by dynamic plastic deformation. The tensile tests and J-integral fracture toughness measurements indicate that this steel exhibits a combination of high strength (σys=920MPa) and considerable fracture toughness (KJC>126MPam1/2). The fracture is associated with localized shear band induced destruction of the nanotwinned structure and with micro-void development at the generated nanoscale large angle grain boundaries within the shear bands, which consumes large plastic energy and contributes the fracture resistance.

Equal-channel angular pressing and annealing of a twinning-induced plasticity steel: Microstructure, texture, and mechanical properties

In this work, a high-manganese Fe–23Mn–1.5Al–0.3C Twinning-Induced Plasticity (TWIP) steel was subjected to plastic shear deformation using Equal-Channel Angular Pressing (ECAP) at 300 °C following route BC and additional annealing. The microstructure evolution during both deformation by ECAP and subsequent annealing was investigated and correlated with the mechanical properties. The successive grain refinement during ECAP was promoted by two parallel mechanisms, namely dislocation driven grain fragmentation and twin fragmentation, and accounted for the ultra-high strength. In addition, due to the relatively low volume fraction of deformation twins after ECAP at 300 °C, further contribution of deformation twinning during room temperature deformation allowed additional work-hardening capacity and elongation. During subsequent recovery annealing the ultra-fine grains and deformation twins were thermally stable, which supported retainment of the high yield strength along with regained uniform elongation. For the first time, the texture evolution during ECAP and during the following heat treatment was analyzed. After 1, 2, and 4 ECAP passes a transition texture with the characteristic texture components of both high- and low-SFE materials developed. During the following heat treatment the texture evolution proceeded similar to that observed in the same material after cold rolling. Retaining of the ECAP texture components due to oriented nucleation at grain boundaries and triple junctions as well as annealing twinning accounted for the formation of a weak, retained ECAP texture after recrystallization.

Mechanical Properties Anisotropy of Isothermally Forged and Precipitation Hardened Inconel 718 Disk

The present work describes the tensile and cyclic flow behavior of the as-received disk of Inconel 718 in solution treated and precipitation hardened condition at different locations and orientations. The disk shows moderately high values of anisotropy index indicating significant difference in uniform true strain along radial and tangential orientations. The tensile true stress–plastic strain curves exhibit two slopes defined by Ludwigson relation [\( \sigma = K_{1} \varepsilon^{{n_{1} }} + \exp (K_{2} + n_{2} \varepsilon ) \)]. The low-strain regime during tensile test is associated with low-strain localization between broad annealing twins and slips, while high-strain regime is related to the presence of large volume fraction of deformation twins and high-strain localization between narrow deformation twins. It appears that both the γ′ and γ″ play a critical role during low deformation regime while the role of γ″ precipitates becomes significant in high-strain regime. The stabilized cyclic true stress–plastic strain curves follow Ludwik relationship (σ = Ke n ) similar to that of high-strain regime of two-slope tensile curves. The true stress–strain curves show softening during cyclic test in comparison to that of monotonic condition and are independent of sample orientations and locations. The lower degree of cyclic softening associated with radial-oriented sample can be attributed to the alignment of δ-phase precipitates normal to the loading direction. The low ductility and low work-hardening exponent of radial-oriented sample in web region have been explained based on the dislocation storage capacity and dynamic recovery coefficient using Kock–Mecking–Estrin analysis.

Effect of Temperature on Mechanical Behaviour of High Manganese TWIP Steel

The aim of the present study was to investigate the role of deformation temperature on the active deformation mechanisms in a 0.6C-18Mn-1.5Al (wt%) TWIP steel. The tensile testing was performed at different temperatures, ranging from ambient to 400°C at a constant strain rate of 10-3 s-1. The microstructure characterization was carried out using a scanning electron microscopy. The deformation temperature revealed a significant effect on the active deformation mechanisms (i.e. slip versus twinning), resulting in different microstructure evolution and mechanical properties. At the room temperature, the mechanical twinning was the dominant deformation mechanism, enhancing both the strength and ductility. Dynamic strain aging (DSA) effect was observed at different deformation temperatures, though it was more pronounced at higher temperatures. The volume fraction of deformation twins significantly reduced with an increase in the deformation temperature, deteriorating the mechanical behavior. There was a transition temperature (~300°C), above which the mechanical twinning was hardly observed in the microstructure even at fracture, resulting in low ductility and strength. The current observation can be explained through the change in the stacking fault energy with the deformation temperature.

Effects of deformation on hydrogen absorption and desorption properties of titanium

In this study, we analyzed the effects of deformation on hydrogen absorption and desorption properties of titanium to improve such properties. Hydrogen was introduced into commercially pure (99.5%) titanium by the electrochemical method. The amount and existing state of hydrogen were examined using hydrogen desorption curves obtained by thermal desorption spectroscopy. Hydrogen absorption was promoted by applying tensile deformation prior to charging, which leads to hydride formation within a short charging time. The amount of hydrogen absorbed decreased when the volume fraction of deformation twins exceeded about 0.2. It was considered that hydrogen was mainly trapped by dislocations forming hydride while a large fraction of deformation twins hindered dislocation motion, thus reducing dislocation density leading to a decrease in the amount of absorbed hydrogen. Almost half the charged hydrogen was released when in-plane compressive stress was applied to a charged plate specimen at room temperature due to hydride decomposition under compressive stress.

Dislocation slip and deformation twinning interplay during high temperature deformation in γ-TiAl base intermetallics

Interrupted compression creep tests were performed at 973 K and 350 MPa to study strain dependent microstructural changes in near-γ equiaxed Ti–48Al–2Cr–2Nb–B alloy. Dislocation densities and volume fraction of deformation twins were measured in the range of strain where sharp creep rate minima occur. The steep decrease of creep rate during initial 1% of strain is associated with a considerable increase in density of ordinary dislocations and superdislocations. Twinning processes are rather limited during this primary stage. On the other hand, in the strain range of minimum creep rate the volume fraction of deformation twins grows due to the increasing number of twinned grains and decreasing spacing between individual twins. Based on these results it is suggested that, in the pure- and near-γ TiAl equiaxed grain microstructures, the increase of creep rate after minimum and its new gradual saturation reflects a contribution of deformation twins to the creep strain accumulation kinetics. The contribution is twofold: (i) deformation twins relax incompatibility stresses set in during primary creep; and (ii) twinning supplies up to 30% of the overall strain in advanced stages of creep.

High strain rate deformation of Mo and Mo-33re by shock loading: Part I. Substructure development

The details of substructure development in Mo and Mo-33Re deformed at ultrahigh strain rates by shock loading have been studied quantitatively with optical, electron optical, and X-ray diffraction techniques. The finite rates of dislocation generation, apparently limited to rates of the order of 10 21 m−2 s−1, cause shock-induced dislocation densities to decrease at short shock pulse durations for a constant shock pulse amplitude. Similarly, the volume fraction of deformation twins in Mo-33Re also decreases at short pulse durations. Twin thicknesses were found to be 32 nm and 200 nm for Mo-33Re and Mo, respectively, with these thicknesses independent of pulse duration. Measurements of the dislocation loop densities support the concept of loop formation by a dislocation mechanism, rather than through the agglomeration of shock-induced excess point defects.

Cu-Ti合金の極低温における双晶変形

Polycrystalline specimens with a modulated structure of a Cu-4 mass%Ti alloy were stretched in the temperature range from 4 to 70 K and at 293 K, and resulting microstructure were observed. Relationships between deformation twinning and serrated stress-strain curves were studied. Results obtained were summarized as follows:(1) Serrated deformation or sharp load drops on the load-elongation curves were observed during stretching at temperatures below about 40 K. They occurred at stress levels lower than that corresponding to the proportional limit. The number of load drops increased with increasing stress level or strain and also increased with decreasing deformation temperature. No indications of serrated deformation were observed in stretching at 68 K and 293 K. (2) Deformation twins were observed in specimens stretched up to certain stress levels at all the test temperatures. Volume fraction of deformation twins increased with increasing stress level or strain. It increased with decreasing deformation temperature for the same amount of strain. (3) Sharp load drops detected at stress levels below the proportional limit are due not to “discontinuous slip” not to “thermal instability” but to the deformation twinning. The twinning stress can be given by detecting the first small load drop. The observed twinning stress has a tendency to increase with decreasing deformation temperature at the cryogenic temperature range. This temperature dependence of the twinning stress is explainable in terms of temperature dependence of the stacking-fault energy, which can be estimated from the electron concentration of this alloy.