Formation Of Mechanical Twins Research Articles

Evaluation of hydrogen embrittlement susceptibility of underwater laser direct metal deposited 316L stainless steel

Underwater laser direct metal deposition (UDMD) shows great application potential in underwater high-performance repair. The susceptibility of underwater repaired components to hydrogen embrittlement (HE) is not extensively researched yet. In this study, HE in 316L stainless steel repaired by UDMD is systematically investigated and compared with in-air direct metal deposited (DMD) 316L and conventionally manufactured (CM) 316L. The experimental results show that the high cooling rates involved in UDMD contribute to a “austenite-ferrite” solidification mode. The cellular structure with a high-density of dislocations increases the tensile strength of as-built UDMD 316L. Thermal desorption spectroscopy analysis reveals that the hydrogen concentrations are similar for 316L manufactured by three processing methods, suggesting that the complex microstructural features by UDMD and DMD do not provide additional H-trapping sites. SSRT results show that the HE susceptibility is in the sequence: CM 316L (30.7%) > UDMD 316L (15.1%) > DMD 316L (9.6%). Compared with CM 316L, the stable cellular structure and higher stability towards martensitic transformation of as-built 316L are beneficial for the HE resistance. In the hydrogen-charged UDMD 316L, the hydrogen-assisted dislocation movement and mechanical twinning formation enhance the localized plasticity and induce cracking. The micron-sized oxide particles and micro-pores further contribute to the fast decohesion of interfaces in the presence of hydrogen. Those combined effects lead to the reductions in tensile strength and elongation of the hydrogen-charged UDMD 316L.

Optimization of hot-rolling parameters for a powder metallurgy Ti–47Al–2Cr–2Nb-0.2W alloy based on processing map and microstructure evolution

In this work, a two-stage rolling method with varied temperature and strain rate was proposed for powder metallurgy (PM) Ti–47Al–2Cr–2Nb-0.2W alloy sheets based on the guidance of processing map and microstructure evolution. Compared with conventional rolling at a high temperature and a low strain rate, the TiAl alloy exhibited enhanced plastic deformability through the implementation of the two-stage rolling method, resulting in a higher cumulative true strain. Microstructure analysis revealed significant grain refinement in the rolled sheet. During the first stage of hot rolling with high temperatures and low strain rates, the softening behavior caused by dynamic recovery (DRV) and dynamic recrystallization (DRX) is conducive to the plastic deformation in the second stage rolling (low temperatures and high strain rates). In addition, the formation of mechanical twins further promotes DRX behavior. The softening mechanism of α2 phase is DRV, and that of γ phase is continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX).

Creep behavior and creep-oxidation interaction of selective electron beam melted Ti–48Al–2Cr–2Nb alloy at different temperatures

In this work, creep behavior and creep-oxidation interaction behavior of Ti–48Al–2Cr–2Nb alloy fabricated by selective electron beam melting at different temperatures were investigated in detail. This alloy exhibits a near-γ microstructure composed of coarse/fine mixed equiaxed grains. Creep test results show that creep behavior of this alloy is sensitive to temperature. This is mainly manifested in that creep life reduces significantly by 84.8 % as temperature increases from 800 °C to 850 °C. During creep, discontinuous dynamic recrystallization characterized by grain nucleation and growth and continuous dynamic recrystallization characterized by strain gradient and orientation accumulation occur simultaneously. The formation of mechanical twins plays an important role in stimulating dynamic recrystallization. Meanwhile, creep deformation and fracture mechanisms of this alloy at different temperatures were also discussed. On the other hand, oxidation test results reveal that the applied tensile stress has a positive effect on the oxidation behavior of this alloy: the growth of TiO2 oxide is inhibited and the oxidation rate is slowed down. However, under creep-oxidation interaction, the intergranular cracking of the oxide film occurs along with the necking of the alloy matrix during creep.

Microstructural studies of CuCrFeNi2Mn0.5 high entropy alloy during cold rolling

A study was conducted to determine the impact of cold rolling on the evolution of the microstructure of the CuCrFeNi2Mn0.5 high entropy alloy. Cold rolling resulted in the development of crystallographic textures and microstructure evolution. It was observed that the main deformation mechanism during cold rolling of the mentioned alloy is the movement of dislocations and the formation of mechanical twins, which lead to the formation of shear bands by increasing the amount of deformation. Microstructural studies suggest that the decrease in the microhardness of cold-rolled samples is caused by the formation of strain-free grains on the shear bands. The development of strain-free grains is aided by the rotation of subgrains on the shear bands.

Microstructural and isotopic analysis of shocked monazite from the Hiawatha impact structure: development of porosity and its utility in dating impact craters

U–Pb geochronology of shocked monazite can be used to date hypervelocity impact events. Impact-induced recrystallisation and formation of mechanical twins in monazite have been shown to result in radiogenic Pb loss and thus constrain impact ages. However, little is known about the effect of porosity on the U–Pb system in shocked monazite. Here we investigate monazite in two impact melt rocks from the Hiawatha impact structure, Greenland by means of nano- and micrometre-scale techniques. Microstructural characterisation by scanning electron and transmission electron microscopy imaging and electron backscatter diffraction reveals shock recrystallisation, microtwins and the development of widespread micrometre- to nanometre-scale porosity. For the first time in shocked monazite, nanophases identified as cubic Pb, Pb3O4, and cerussite (PbCO3) were observed. We also find evidence for interaction with impact melt and fluids, with the formation of micrometre-scale melt-bearing channels, and the precipitation of the Pb-rich nanophases by dissolution–precipitation reactions involving pre-existing Pb-rich high-density clusters. To shed light on the response of monazite to shock metamorphism, high-spatial-resolution U–Pb dating by secondary ion mass spectrometry was completed. Recrystallised grains show the most advanced Pb loss, and together with porous grains yield concordia intercept ages within uncertainty of the previously established zircon U–Pb impact age attributed to the Hiawatha impact structure. Although porous grains alone yielded a less precise age, they are demonstrably useful in constraining impact ages. Observed relatively old apparent ages can be explained by significant retention of radiogenic lead in the form of widespread Pb nanophases. Lastly, we demonstrate that porous monazite is a valuable microtexture to search for when attempting to date poorly constrained impact structures, especially when shocked zircon or recrystallised monazite grains are not present.

Dislocation structure evolution during room temperature dwell loading of a Ti-6Al-4 V alloy

The current work examines the evolution of the dislocation structure, within both the α and β phases, of a Ti-6Al-4 V alloy subjected to room temperature dwell loading performed at about 96% of the macroscopic yield stress level for several durations, as part of a synchrotron in-situ deformation experiment. The obtained findings represent a vital contribution to creating a scientific basis for improving lifing predictions for aero-propulsion components. The dislocation configuration, type and density were examined post-mortem using transmission electron microscopy (TEM). Despite the relatively low applied stress level, well-developed dislocation structures were already formed within the α phase at a dwell duration of 10 min and changed only modestly up to 60 min. The plastic deformation of the above phase took place solely via dislocation slip. To facilitate a comparison between different dwell durations, the TEM investigation focused on the regions oriented for the prismatic glide that represented the main deformation mode due to the starting crystallographic texture. After deformation, the regions oriented for the prismatic slip contained planar slip bands largely consisting of dislocations of a screw type. There was rather limited interaction between different bands, as the dislocation arrays passed through the slip band intersections with only minor difficulty. The screw-type dislocations seemed to preferentially form during straining. Dislocation Burgers vectors experimentally determined within the β phase occasionally departed from the Schmid rule and such dislocations typically showed a prevalent screw component. Some deformation banding and {112} 〈111〉 − type mechanical twin formation were locally detected within the β phase.

On the orientation-dependent mechanical properties of interstitial solute-strengthened Fe49.5Mn30Co10Cr10C0.5 high entropy alloy produced by directed energy deposition

Interstitial solute-strengthened Fe49.5Mn30Cr10Co10C0.5 (at%) high entropy alloy was additively manufactured by directed energy deposition (DED) process in this work. While the as-deposited material exhibits an excellent combination of strength and ductility, the effect of anisotropy on the mechanical performance of the DED processed component was studied in detail. The ultimate tensile strength (UTS) of the horizontal tensile sample with a main fiber texture of <111> // tensile direction (TD) went up to 1 GPa while maintaining a superb failure elongation of 36%. The vertical tensile sample, with a dominant <001> // TD texture, failed at an UTS of 750 MPa with an enhanced failure elongation of 52%. Microstructural analysis of the deformed samples showed that the horizontal samples were mainly deformed via the formation of mechanical twins, whereas the twining activity was less profound in the vertical samples. Single crystal micro-pillar compression testing revealed that the deformation mechanism complies well with the Schmid’s factor. In addition, a higher critical resolved shear stress for twining compared to slip was also confirmed in the micro-pillar compression testing.

Hardening mechanism of high manganese steel during impact abrasive wear

To investigate the hardening mechanism of high manganese steel (HMS) during impact-abrasive wear (IAW), the microstructures and hardness evolution behaviours of the worn sub-surfaces of Mn13 (13.32% Mn and 0.22% Cr), Mn13-2 (13.23% Mn and 2.13% Cr), and Mn18-2 (18.35% Mn and 2.04% Cr) were investigated. The matrix hardness values of Mn13, Mn13-2, and Mn18-2 were 240.2, 256.6, and 266.5 HV, while their worn sub-surface hardness values were 670.1, 638.2, and 713.1 HV, respectively. The increase in the hardness values was attributed to the formation of ε-martensite and nano-sized mechanical twins in the worn sub-surfaces. The ε-martensite observed in the worn sub-surface of each of the three materials was similar. Therefore, the differences between the degrees of increase in the hardness values of the materials were attributed to the presence of the mechanical twins. The formation of the mechanical twins was affected by the stacking-fault energy (SFE), which increases with Mn content but decreases with Cr content. The degree of increase in the hardness increased in the following order: Mn13-2 (381.6 HV), Mn13 (429.9 HV), and Mn18-2 (446.6 HV). In summary, the degree of increase in the hardness of HMS during IAW can be improved by engineering a higher SFE for a higher number of mechanical twins.

Effects of power density on residual stress and microstructural behavior of Ti-2.5Cu alloy by laser shock peening without coating

The present study concerns the micro structural changes of Ti-2.5 Cu after LPwC at various laser intensities (i.e., ≈3 GW cm−2, 6 GW cm−2 and 9 GW cm−2). X-ray diffraction (XRD), electron backscattered diffraction (EBSD), transmission electron microscope (TEM) were used to analyze the microstructure. X-ray photoelectron spectroscopy (XPS) was used to characterize the oxidation behavior of the samples after peening. The depth profile of residual stress was obtained using the sin2ѱ X-ray diffraction technique, and the compressive residual stress increased for all the peened conditions in the surface and sub-surface regions, and the maximum obtained was −402.44 MPa at 50 μm depth for the 9 GW cm−2. Furthermore, the cross-sectional micro hardness of the peened samples improved at a depth of 50 μm with increasing laser intensity. For the highest energy, the maximum hardness at a depth of 50 μm was 439.40 HV (0.1), which was higher than the unpeened sample (315.56 HV0.1). TEM studies showed nano precipitates (50–170 nm), mechanical twins with a width ranging from (27.20–118.51 nm) and cell size dislocation structure. EBSD results conformed the high angle grain boundaries increased to 43% with the formation of mechanical twins at the higher energy.

Study on Mechanical Properties and Deformation Mechanism of TWIP Stainless Steel

In this study, based on the sensitivity of the chemical composition fluctuation to the thermodynamic parameter, which controls the level of the stacking fault energy (SFE), a series of high Cr–Mn–N twinning-induced plasticity (TWIP) stainless steels are designed by using a sublattice model, and their mechanical properties and micro deformation mechanism are analyzed The formation of mechanical twins (Mts) during the deformation makes the test steel show a perfect combination of strength and ductility after different solution treatments. Among them, after a solution treatment at 950 °C and 1050 °C, the 19Cr–0.7N and 19CrSi–0.7N samples have the maximum value with the product of the strength and plasticity reaching 60.7% and 64.6%, and 12Cr–CN has the maximum value after the solution treatment at 1200 °C, reaching 81.3%. The SFE values of the 19Cr–0.7N and 19CrSi–0.7N samples were relatively high, 48 mJ·m−2 and 45 mJ·m−2, respectively. The SFE of 12Cr–CN is 37 mJ·m−2, and the Mts grow rapidly during the deformation and maintain the highest twinning density under the same strain conditions. The characterization of the tensile samples occurs under different deformations by electron backscattered diffraction (EBSD) and transmission electron microscope (TEM). The results of the EBSD local misorientation difference angle analysis showed the Silicon element addition with a good Mts saturation rate. It is observed from the TEM that the nucleation process of the Mts with a high SFE is difficult, and the Mts emit and grow inward along the grain boundary during the tensile process and present a cross shape with the increase in strain. The contribution of the grain boundary strengthening (σ0), dislocation strengthening (σf), and twinning strengthening effect (σt) under dynamic micro-refinement to stress were calculated. It is known that under a certain amount of strain, the ratio of σt and σf changes with increasing, and when the contribution of the twinning deformation to the stress exceeds about 25%, the reinforcement of the plastic deformation is dominated by the plane of σf.

Stress corrosion cracking and precipitation strengthening mechanism in TWIP steels: progress and prospects

AbstractTwinning-induced plasticity (TWIP) steels are increasingly receiving wide attention for automotive applications due to their outstanding combination of ductility and strength, which can largely be attributed to the strain hardening effect, formation of mechanical twins during straining, and the presence of manganese (Mn) as an alloying element. However, the premature cracking and sudden failure frequently experienced by the TWIP steels under the combined action of tensile stress and corrosion environment remain a challenge for many material scientists and experts up till now. Driven by this challenge, an overview of the stress corrosion cracking (SCC) susceptibility of high-Mn TWIP steels (under the action of both mechanical loading and corrosion reaction) is presented. The SCC susceptibility of the high-Mn TWIP steels is specifically sensitive to hydrogen embrittlement, which is a major factor influencing the SCC behavior, and is a function of the hydrogen content, lattice-defect density and strength level. Besides, the corrosion susceptibility to hydrogen embrittlement may be reduced by suppressing the martensite in the TWIP steels by carbon additions. This review further discusses in detail the precipitation strengthening mechanisms as well as the corrosion behavior of TWIP steel by mechanism.

Effect of Ag Addition on the Hot Deformation, Constitutive Equations and Processing Maps of a Hot Extruded Mg-Gd-Y Alloy

Hot deformation behavior of the extruded Mg-6Gd-3Y (GW63) and Mg-6Gd-3Y-1Ag (GW63-1Ag) alloys was assessed by shear punch testing (SPT) method in the temperature range of 623 K to 723 K and shear strain rate of 1.6 × 10−2 to 1.3 × 10−1 s−1. Microstructural examinations showed that Ag addition decreased the initial grain size from 10.1 to 2.9 µm. According to the hot deformation results, hyperbolic-sine exponents (n-values) of 3.39 and 2.54 were obtained for the Ag-free and Ag-containing alloys, respectively. The corresponding activation energies were found to be 321 and 178 kJ mol−1 for these alloys. Therefore, the controlling deformation mechanisms were defined as viscous glide of dislocation for the Mg-6Gd-3Y alloy and grain boundary sliding for Mg-6Gd-3Y-1Ag. Processing maps were developed based on the dynamic material model (DMM) to determine the flow instability domains during hot deformation. Accordingly, the optimum hot working conditions were found to be in the temperature range of 655 K to 723 K and shear strain rate of 3.3 × 10−2 to 1.3 × 10−1 s−1 for the GW63-1Ag alloy, whereas the safe deformation window for the GW63 alloy was narrowed down to a temperature range of 665 K to 705 K and shear strain rate of 7.2 × 10−2 to 1.3 × 10−1 s−1. The Ag-containing alloy exhibited flow instability regions only at low temperatures and high strain rates. On the other hand, the Ag-free alloy demonstrated other instability domains comprising the formation of mechanical twins and cracking. Electron back-scattered diffraction analysis was utilized to correlate the microstructural evolution of the alloys after shear deformation to the hot working parameters. Continuous dynamic recrystallization was suggested as the main mechanism for generation of the dynamically recrystallized grains during hot deformation of both alloys.

The tension-compression asymmetry of martensite phase transformation in a metastable Fe40Co20Cr20Mn10Ni10 high-entropy alloy

The microstructural evolution of a metastable face centered cubic (FCC) Fe40Co20Cr20Mn10Ni10 high-entropy alloy (HEA) under both tension and compression is systemically investigated. The results show much higher level of martensite phase transformation from FCC structure to hexagonal closed packed (HCP) structure under compression than tension, indicating a distinct tension-compression asymmetry. The compressive tests underwent higher true stresses, which further provided stronger driving forces to trigger the phase transformation than those in tensile tests. Except for the martensite phase transformation, dislocation planar slip prevails in both tension and compression, along with the occasional formation of mechanical twins. Dislocation slip dominates the whole tensile deformation, while both dislocation motions and martensite phase transformation play critical roles in the compressive deformation. The martensite phase transformation is preferred to nucleate at grain or subgrain boundaries due to a medium stacking fault energy (SFE) of ∼20 mJ m−2. The formation of HCP phase via partial dislocation emission from low angle grain boundaries offers additional pathways for martensite phase transformation. Our study thus remarkably benefits the understanding of the de formation mechanisms of metastable HEAs.

Effect of grain size and specimen thickness on formation of mechanical twin

Effect of grain size and specimen thickness on mechanical twinning: Origin of mechanical twins was analyzed in terms of sample thickness and grain size. Mechanical twins are developed in 316SS specimens for all thicknesses (30 µm, 50 µm and 90 µm) and for grain sizes ranging from ≈2.4 µm to ≈30 µm. Rate of twin decay increases with increase in specimen thickness and can be attributed to misorientation developments. Mechanical twin formation mechanisms in austenitic stainless steel samples were understood by electron backscattered diffraction (EBSD) based acquisition and analysis of Kikuchi diffraction patterns. Subsurface twins were found to result in a contrast in IQ maps while still not being “visible” in IPF maps. After deformation, twins grow to the surface and are subsequently identified in both IQ and IPF maps.

Twin formation and its strengthening mechanism of pure titanium processed by cryogenic laser peening

The aim of this present study was to explore the effects of room temperature laser peening (RT-LP) and cryogenic laser peening (CLP) on the microstructural characteristics and microhardness of pure titanium (Ti). The in-depth microhardness of the specimens subjected to different treatments were measured. The microstructural evolution was characterized by electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM). Experimental results showed that compared with RT-LP, the microhardness improvement caused by CLP was higher. Moreover, CLP induced higher-density of mechanical twins and dislocation structures. Compared with RT-LP specimen, the proportion of the near-surface twin boundary of the CLP specimen increased by 133.3%. During the process of CLP, the ultra-low temperature can inhibit the dynamic recovery and annihilation of the dislocation structures induced by laser shock wave, thus leading to higher dislocation density. Additionally, cryogenic temperature can also suppress the motion and slip of dislocation structures, which is more conducive to the formation of mechanical twins.

The subsurface frictional hardening: A new approach to improve the high-speed wear performance of Ti-29Nb-14Ta-4.5Zr alloy against Ti-6Al-4V extra-low interstitial

In the present work, the subsurface microstructure evolutions of Ti-29Nb-14Ta-4.5Zr (TNTZ) during high speed-sliding wear against Ti-6Al-4V extra-low interstitial has been studied in detail. Its correlation with surface features characteristics is also discussed. Progressive substructure development and mechanical twin formation have been proposed to be the main subsurface deformation mechanisms. The microstructure of the subsurface deformed regions consists of both primary and secondary twins holding ∑3:{112}<111> and ∑11:{332}<113> orientation relationships even at the early stage of sliding. Surprisingly, examinations indicate that an ultra-fine grained metal mixed layer is formed underneath the wear track, the extent of which increases in relation to the sliding distance. The latter has been justified considering the progression of primary mechanisms under a high imposed strain rate and the resulting tangential force. Despite the previously reported results for the same couples, the tribological performance of the TNTZ experimented alloy is significantly improved. This is well discussed relying on the increased subsurface hardenability of the alloy under the high-speed wear regime, which effectively supports the superficial oxide layer and prevents the premature micro-crack formation.

The Role of Grain Orientation and Grain Boundary Characteristics in the Mechanical Twinning Formation in a High Manganese Twinning-Induced Plasticity Steel

In the current study, the dependence of mechanical twinning on grain orientation and grain boundary characteristics was investigated using quasi in-situ tensile testing. The grains of three main orientations (i.e., 〈111〉, 〈110〉, and 〈100〉 parallel to the tensile axis (TA)) and certain characteristics of grain boundaries (i.e., the misorientation angle and the inclination angle between the grain boundary plane normal and the TA) were examined. Among the different orientations, 〈111〉 and 〈100〉 were the most and the least favored orientations for the formation of mechanical twins, respectively. The 〈110〉 orientation was intermediate for twinning. The annealing twin boundaries appeared to be the most favorable grain boundaries for the nucleation of mechanical twinning. No dependence was found for the inclination angle of annealing twin boundaries, but the orientation of grains on either side of the annealing twin boundary exhibited a pronounced effect on the propensity for mechanical twinning. Annealing twin boundaries adjacent to high Taylor factor grains exhibited a pronounced tendency for twinning regardless of their inclination angle. In general, grain orientation has a significant influence on twinning on a specific grain boundary.

Deformation behaviour of low carbon high Mn twinning-induced plasticity steel

The present study deals with the deformation behaviour of low carbon and high manganese twinning-induced plasticity (TWIP) steel (Fe–21Mn–3Si–3Al–0.06C, wt%) through microstructural investigation. Low carbon with high manganese along with the addition of aluminium in TWIP steel results in lowering of specific weight with higher strain hardening due to the formation of mechanical twins during deformation. The full austenite phase is obtained after solution treatment and deformation twins appear and austenite grains become flattened during application of 10% to 50% cold deformation. The annealing twins are relatively coarser compared to the newly formed deformation twins. With the increasing amount of cold deformation, deformation twins and dislocation density are increased. Deformation twinning can be considered to be the dominant deformation mechanism during the course of cold rolling applied in the present study. The cold deformation results in the evolution of dislocation substructure, stacking faults, deformation twins and twin–dislocation interaction, which may be correlated with the lower stacking fault energy (∼24 mJ/m2) of the investigated steel. Excellent combination of strength and ductility has been obtained in the present TWIP steel with a small rolling reduction of 10% and 30%. With the increasing amount of cold deformation, tensile strength notably increases and maximum tensile strength is obtained at 50% cold-deformed sample along with the diminutive sacrifice of the ductility.

Hydrogen Permeation in Cold-Rolled High-Mn Twinning-Induced Plasticity Steels

Hydrogen permeation is investigated in cold-rolled Fe-0.6C-18Mn-(1.5Al) alloys. The hydrogen mobility is lower in cold-rolled alloys compared with annealed alloys. Al-containing alloy shows less deceleration of hydrogen mobility compared with the Al-free alloy. This is attributed to the reduced formation of mechanical twins and dislocations. Mechanical twins trap hydrogen strongly but are vulnerable to crack initiation; suppression of these is thought to be a major favorable influence of Al on hydrogen-induced mechanical degradation.

Microstructural evolution and mechanical behaviour of Fe-30Mn-C steels with various carbon contents

ABSTRACTA Fe-30Mn-0.6C sheet steel was decarburised and/or annealed to obtain four Fe-30Mn-C alloys with carbon contents of 0.06, 0.2, 0.4 and 0.6 wt-%. The primary deformation products were found to be mechanical twins for the 0.2C, 0.4C and 0.6C alloys and a combination of mechanical twins and ε-martensite for the 0.06C alloy. Both the ε-martensite and mechanical twin formation kinetics increased sigmoidally with true strain such that the final twin volume fraction increased with increasing alloy SFE and C content, where the latter finding disagrees with some of the accepted models for high-Mn twinning induced plasticity steels. Moreover, the activation stress for twin formation was found to increase linearly with alloy SFE, per a model previously proposed by the present authors.