Geometrically Necessary Dislocations Research Articles

In-situ EBSD study of the active slip systems and substructure evolution in a medium-entropy alloy during tensile deformation

Electron backscatter diffraction (EBSD) coupled with in-situ tensile loading is powerful for investigating the microstructural evolution of alloys. Thermo-mechanically treated f.c.c. medium-entropy alloys (MEAs) typically have high densities of annealing twin boundaries (ATBs), which can not only strengthen but also toughen the MEA via interacting with dislocations. However, the evolution of ATBs and other substructures in plastically-deformed MEA has not yet been revealed. Plastic deformation involving dislocation evolution, active slip systems, lattice rotation, boundary transformation, and grain subdivision in a polycrystalline MEA Ni41.4Co23.3Cr23.3Al3Ti3V6 was studied using in-situ EBSD. The slip was accompanied by heterogeneous lattice rotation among grains and within grains, where inhomogeneous plasticity was accommodated by geometrically-necessary dislocations (GNDs). Both GND and low-angled boundaries (LABs) densities substantially increased with progressive strain, which was mainly concentrated in sites approaching ATBs or grain boundaries (GBs). Located stress, lattice rotation, or curvature caused a loss in the coherence of ATBs, which resulted in integrity loss with increasing strain and promoted a decrease in density by 60 %. Further, lattice rotation incompatibility due to constraints from neighboring grains leads to grain fragmentation into various misorientated volumes, which were separated by LABs or high-angle boundaries (HABs). The grain orientation angle increased with progressive strain and crystallographic 〈111〉 orientation gradually spread toward a tensile direction. Slip systems with maximum Schmid factor were activated first at ε ≥ 3.9 %, which is almost the same with experimental slip traces. Both single slip and double slip occurred during plasticity, where straight slip traces tend to curve due to lattice curvature. Slip transfers are not only controlled by geometric compatibility factor, which can occur between some neighboring grains with low geometric compatibility factor but high Schmid factor.

Microstructure evolution from homogeneous as-cast state to annealed heterogeneous structure and mechanical properties of Al-Zn-Mg-Cu alloys with trace TiB2 particles

Materials with a final heterogeneous structure (HS) possess an excellent combination of strength and ductility. However, fine and homogeneous grains are desired in as-cast ingots to avoid defects. The evolution from an as-cast homogeneous microstructure to a pronounced HS owing to the trace addition of TiB2 particles was studied in Al-Zn-Mg-Cu alloys with traditional thermomechanical treatment. It is revealed that the triple junctions and over 2 μm precipitates co-located with TiB2 enhance the particle-stimulated nucleation of recrystallization. The dislocation density difference between the recrystallized and recovered grains is further enhanced by cold rolling. A pronounced HS with alternating soft and hard domains accompanied by multimodal precipitates is modulated, realizing a synergy of yield strength as 632.4 MPa and elongation as 8.8%. It is confirmed that the HS, rather than precipitates, is the primary source of geometrically necessary dislocations (GNDs), leading to the synergy of strength and ductility. Atomistic simulations on the deformation behavior of HS were used to elucidate the strain partition and role of GNDs on mechanical properties. Our results provide a convenient route to fabricate heterogeneous Al-Zn-Mg-Cu alloy by trace addition of ceramic particles in ingots casting instead of elaborated controlling of deformation processing.

Phase reversion mediated the dual heterogeneity of grain size and dislocation density in an equiatomic CrCoNi medium-entropy alloy

An ultra-high strain rate (104 s-1) dynamic plastic deformation treatment at liquid nitrogen temperature (LNT-DPD) followed by annealing is carried out to obtain dual heterogeneity of grain size and dislocation density in an equiatomic CrCoNi medium entropy alloy (MEA). Such extreme loading conditions resulted in extensive phase transformation in this MEA. Subsequent annealing at 650°C for 1 h further induced reverse phase transformation and partial recrystallization, forming a complex heterogeneous microstructure characterized by nested trimodal grain sizes and partitioned dislocation density. A superior yield strength of ∼800MPa and a good ductility of ∼40% were simultaneously achieved in this heterogeneous alloy. In order to reveal the effects of grain size and dislocation density distributions on the mechanical property improvements, the underlying deformation mechanisms were systematically discussed. High density of geometrically necessary dislocations (GNDs) would be induced in complex heterogeneous structures during tensile deformation due to strain gradients or partitioning between different regions, which would lead to additional strengthening and work hardening. These results provide a novel approach to overcome the strength-ductility trade-off dilemma for FCC-structured MEAs.

Enhanced strength-ductility synergy in a gradient hetero-structured CrCoNi medium-entropy alloy

A CrCoNi medium-entropy alloy (MEA) with gradient hetero-structure (GHS) composing of gradient nanostructured surface layer and hetero-structured matrix was prepared via a two-step process: pre-cold rolling (CR) deformation and subsequent annealing followed by surface mechanical grinding treatment (SMGT) and secondary annealing. The GHS CrCoNi exhibits an excellent synergy of strength and ductility, as evidenced by a yield strength of approximately 1.2 GPa and a uniform elongation of about 20%. The GHS displays a pronounced hetero-deformation induced (HDI) hardening effect, whereby the central layer (CL) and surface layer (SL) undergo local strain hardening in a mutually compatible manner. This process facilitates the multiplication and accumulation of geometrically necessary dislocations (GNDs) at intra- and inter-layer hetero-interfaces and also activates the deformation faulting and twinning in both CL and SL, thus enhancing the overall mechanical response. The interlayer compatible deformation observed in the GHS significantly bolsters the sustainability of strain hardening at the microscopic scale. This effect is of significant importance in redistributing stress and strain more rationally across the entire GHS sample, effectively reducing the risk of localized deformation failures. This study offers insight into the correlation between macroscopic strain hardening and micromechanical behavior in gradient-/hetero-structured materials.

Optimal Arrangements and Local Anisotropy of {100} Guinier-Preston (GP) Zones by Parametric Dislocation Dynamics (PDD) Simulations.

Stress-oriented precipitation and the resulting mechanical anisotropy have been widely studied over the decades. However, the local anisotropy of precipitates with specific orientations has been less thoroughly investigated. This study models the interaction between an edge dislocation source and {100} variants of Guinier-Preston (GP) zones in Al-Cu alloys using the parametric dislocation dynamics (PDD) method. Concentric geometrically necessary dislocation (GND) loops were employed to construct a line integral model for thin platelets. The simulations, conducted with our self-developed code based on Green's function method and Eshelby inclusion theory revealed distinct strengthening behavior along the strong and weak directions for 60° GP zones, demonstrating anisotropic strengthening from the perspective of elastic interactions. Furthermore, the optimal inclined arrangement of the GP zone array was determined through elastic energy calculations, and these results were corroborated by TEM observations.

Influence of annealing on microstructures and mechanical properties of laser powder bed fusion and wire arc directed energy deposition additively manufactured 316L

The annealing response of 316L stainless steel manufactured with both laser powder bed fusion (PBF-LB) and wire-arc directed energy deposition (DED-Arc) was investigated in the context of microstructural evolution and mechanical properties. Because additive manufacturing (AM) comprises several technologies that differ in heat source, feedstock, and scan strategy, among other build variables, the final products can experience a range of thermal histories and defect (dislocation) populations. These unique thermal histories can subsequently influence as-built properties and annealing response of AM materials, most notably those manufactured with different AM deposition processes. The PBF-LB process (with higher cooling rates) yielded finer austenite microstructures having more lattice strain, higher dislocation densities, and higher yield strength values than the DED-Arc 316L process (with lower cooling rates). Electron backscattered diffraction (EBSD) data of the kernel average misorientation (KAM), boundary density, and geometrically necessary dislocation (GND) density support the differences in lattice strain. As a result, the PBF-LB 316L exhibited more recovery and recrystallization after annealing at elevated temperatures above 873 K based on changes to yield strength, work hardening behavior, and microstructure evolution. The DED-Arc 316L exhibited a δ-ferrite/austenite microstructure with microsegregation that influenced deformation mechanisms active after annealing. Tensile data, deformed microstructures and misorientation histograms from EBSD showed a decreasing amount of deformation twinning when comparing the as-built condition to annealed conditions (up to 1473 K/1h) of PBF-LB 316L. The opposite trend was noted in DED-Arc 316L. The behavior was interpreted to be due to the differences in chemical segregation during solidification and the effects of heat treatment on chemical homogenization and local stacking fault energy.

Stress relaxation aging of 7050 aluminum alloy under elastic and plastic loadings: A perspective from the geometrically necessary dislocations

Aerospace integral panels with large curvature often experience not only elastic loading but also plastic loading during creep age forming. However, the stress relaxation aging behaviors and mechanisms under loading stresses ranging from elastic to plastic regions, especially from the perspective of geometrically necessary dislocations (GNDs), have not been fully explored. The stress relaxation aging (SRA) behavior of 7050 aluminum alloy under elastic (200/250/300 MPa) and plastic loadings (350/400 MPa) are studied by uniaxial SRA, hardness and tensile tests, and corresponding mechanisms are clarified by Electron Back Scatter Diffraction (EBSD) tests and transmission electron microscopy (TEM) observations. Regardless of the loading stress, the stress relaxation of 7050 alloy shows a typical two-stage behavior (rapid stress reduction and stable stress relaxation rate). The second stage of stress relaxation under elastic loading shows an abnormal stress rebound, while the stress continuously decreases under plastic loading, and hardness and yield strength at 350 MPa are lower than that at 200 MPa during SRA. These results are attributed to strong dislocation pinning from small and dense precipitates at elastic loading and dislocations shearing large and sparse precipitates at plastic loading, respectively. Furthermore, both the stress reduction and its ratio increase with the increase of initial stress. GNDs contribute to the formation of low-angle grain boundaries (LAGBs) and accumulate at the LAGBs intersection to coordinate grain orientation for creep deformation. Compared to elastic loading, plastic loading promotes global growth of intragranular precipitates by elastic stress field and local heterogeneous nucleation and coarsening of precipitates at subgrain boundaries by more GNDs.

Interplay between deformation and fracture mechanism in gradient nanostructured austenitic stainless steel

The plastic deformation parameters are crucial for the grain size of gradient nanostructured (GNS) materials, and the multiscale grain structure within the GNS layer has a significant impact on fracture behavior. In this study, the detailed depth-dependent microstructures of GNS310S stainless steel fabricated by surface mechanical rolling treatment (SMRT) were investigated, and the relationship between the geometrically necessary dislocation (GND) density and the strain gradient was established through the local misorientation obtained by electron backscatter diffraction. The critical deformation parameters (strain gradient, shear strain, and strain rate) of SMRT-induced grain refinement at different scales were quantitatively evaluated. The tensile results show the yield strength and tensile strength of GNS310S are improved by 103 % and 23.1 % respectively compared to CG310S. Additionally, we elucidate the underlying tensile fracture mechanism of GNS310S stainless steel through detailed microstructural analysis. Due to the poor ductility of the near-equiaxed nanograins, cracking occurred on the surface of the SMRT specimen at low strain during the tensile testing. These cracks evolved into competitive shear offsets and multi-cracks with increased strain on the specimen surface, eventually leading to a hybrid fracture mode. In contrast, the subsurface nanotwinned lamellae layer displayed good ductility during tension, which can accommodate the deformation through lamellae widening and forming secondary twinning. This nanotwinned lamellae layer can suppress the inward propagation of surface cracks when the strain was not large. This work provides new insights into the fabricating parameters of the GNS stainless steel, as well as the deformation and fracture mechanisms.

Effects of stirring friction on the microstructure, texture and mechanical properties of 7055 aluminum alloy

Aluminum alloys are widely used in aerospace, transportation, and many other fields due to their excellent properties such as lightweight, ultra-strength, and high toughness. This paper analyzed the influence mechanism of friction stir on the microstructure, texture, and mechanical properties to realize the strength and toughness of 7055 aluminum alloys and optimize their deformation process. The results show that the aluminum alloys change from uneven banded grains to uniform and fine equiaxed grains after friction stir processing, and the grain size is refined from 14 μm to 0.71 μm. The high-angle grain boundary and texture intensity present an initial increase but a subsequent decrease due to the combined effect of recrystallization and high-temperature softening caused by the heat input of friction stir processing. The total texture intensity and volume fraction of the friction stir processed aluminum alloys decrease significantly, and the intensity of RtG {110} <110>, A {110} <111>, E {110} <111>, and F {111} <112> texture significantly decrease or disappear, while the Brass (B) {110} <112> texture increases. The yield strength of FSP aluminum alloys decreases with increasing heat input (w/v), which mainly depends on the weakening of the texture strengthening effect. However, the increase of geometrically necessary dislocations (GND) caused by grain boundary sliding leads to the significant improvement of the deformation hardening ability and plasticity of the material. In addition, the effect of texture on the plasticity of aluminum alloys was calculated by using Schmid law. It is found that the equivalent slip factor (ESF) and maximum Schmid factor (μmax) of A, E, and Cu textures are equal to 0. The small ESF and μmax of the S texture are not conducive to the plastic deformation property of materials, while the 1000 × 70 sample with more ESF (8) and μmax (0.41) of the Cube texture ({001} <100>) shows better plasticity.

Grain gradient in the heat-affected zone acquires an advancement in the ductility of flash-butt welded IN718 alloy

The microstructure evolution and mechanical properties are investigated systematically on Flash-butt welded IN718 alloy joint after annealing at 980 °C and subsequent aging. The yield strength (YS) is increased from 667 MPa to 1175 MPa and the ultimate tensile strength (UTS) is increased from unprocessed 980 MPa to 1412 MPa. A grain gradient distribution is achieved in the heat-affected zone (HAZ) with the grain size gradually increasing from the weld seam (WS) to the matrix after heat treatment, and the ductility of the welded sample is higher than that of the matrix without any loss of strength. The contribution mechanism of grain gradient to ductility is investigated by varying the fraction of the HAZ. The results demonstrate that the ductility of the welded sample higher than that of the matrix is because of the co-deformation between the HAZ and the matrix, which results in the effectively dynamic accumulation of geometrically necessary dislocations (GNDs) from gradient transition zones into the matrix. Therefore, premature stress concentration is prevented and additional ductility is obtained for welded samples. Increasing the fraction of HAZ effectively enhances local strain within coordinated deformation zones, leading to improved ductility.

A mechanism on multiscale heterogenous deformation of nickel-based single crystal superalloys unraveled by time-of-flight neutron diffraction technology

Time-of-flight (TOF) neutron diffraction technology is a powerful tool for acquiring rich diffraction information on the multiscale microstructures and stresses of nickel-based single crystal superalloys. In this study, the tensile deformation behaviors of DD426 superalloy at 760 °C were investigated by TOF neutron diffraction. Combined with electron backscatter diffraction technology, the multiscale deformation heterogeneity between γ/γ' phases and dendritic/interdendritic regions was simultaneously characterized for the first time. Our experiments revealed a significant misorientation difference and the existence of a long-range stress field in the adjacent area between the dendritic and interdendritic regions after tensile deformation at 760 °C. The misorientation angles of crystal planes along the loading direction between the dendritic and interdendritic regions in the deformed alloy was approximately 1.6 ± 0.6° This misorientation can be attributed to the geometrically necessary dislocations (GNDs) concentrated in the adjacent area between the two regions with densities of approximately 1014 m-2. In addition, the lattice strains of both γ' and γ showed a compressive–tensile transition from the dendritic to interdendritic regions, balanced with a long-range stress field. The pile-up GNDs in the adjacent area between the two regions contributed to the existence of long-range stress fields. This study highlights that TOF neutron diffraction technology may reveal well the heterogeneous deformation mechanisms via the formation of both GNDs and pile-up in the adjacent interdendritic/dendritic area of single crystal superalloys.

Mechanism of Fatigue-Life Extension Due to Dynamic Strain Aging in Low-Carbon Steel at High Temperature.

An enhancement in fatigue life for ferrite-pearlite low-carbon steel (LCS) at high temperature (HT) has been discovered, where it increased from 190,873 cycles at room temperature (RT) to 10,000,000 cycles at 400 °C under the same stress conditions. To understand the mechanism behind this phenomenon, the evolution of microstructure and dislocation density during fatigue tests was comprehensively investigated. High-power X-ray diffraction (XRD) was employed to analyze the evolution of total dislocation density, while Electron Backscatter Diffraction (EBSD) and High-Resolution EBSD (HR-EBSD) were conducted to reveal the evolutions of kernel average misorientation (KAM), geometrically necessary dislocations (GND) and elastic strains. Results indicate that the enhancement was attributed to the dynamic strain aging (DSA) effect above the upper temperature limit, where serration and jerky flow disappeared but hindrance of dislocations persisted. Due to the DSA effect, periods of increase and decrease in the total dislocations were observed during HT fatigue tests, and the fraction of screw dislocations increased continuously, caused by viscous movement of the screw dislocations. Furthermore, the increased fraction of screw dislocations resulted in a lower energy configuration, reducing slip traces on sample surfaces and preventing fatigue-crack initiation.

Exploring ambient and elevated temperature fretting wear behavior of laser-cladding (FeCrCoNi)84AlxNby high-entropy alloy coatings

In this work, (FeCrCoNi)84AlxNby (x = 12, 8, 4 and y = 4, 8, 12 (at. %)) high-entropy alloy (HEA) coatings were successfully deposited on 316 stainless steel via laser cladding (LC). FCC/BCC + Laves structure was introduced into FeCrCoNi HEA system by designing different Al/Nb ratios, aiming to improve the fretting wear resistance. The phase constituents, microstructure, and fretting wear resistance of (FeCrCoNi)84AlxNby HEA coatings at ambient temperature and 285 °C were respectively analyzed. Results confirmed that the microstructure of three HEA coatings displayed the interdendritic (IR) - dendritic (DR) structure. With the Al/Nb ratio decreased from 3/1 to 1/3, the phase constituents of HEA coatings transformed from the BCC solution to FCC solution and Laves phase, and the content of the Laves phase gradually increased. Moreover, the average grain size decreased from 16.9 μm to 4.6 μm, while the average geometrically necessary dislocations (GNDs) density increased from 1.842 × 1010 m−2 to 2.821 × 1010 m−2. The microhardness of the HEA coatings was improved from 210 HV0.2 to 361–647 HV0.2 compared with that of the substrate. The decrement of Al/Nb ratio improved the fretting wear resistance of HEA coatings both at ambient temperature and 285 °C, especially at 285 °C. The wear volume decreased from 0.8 × 105 μm3 to 0.39 × 105 μm3 and the fretting wear mechanism changed from severe fatigue wear to slight abrasive wear at 285 °C. In the Al4Nb12 HEA coating, the soft structure (FCC) and the hard structure (Laves) were combined into a wear-resistant skeleton, and the Laves structure was massively distributed along the FCC phase to prevent the FCC phase from further slipping in the fretting wear process, which in turn minimizes the fretting damage. The present work provides a theoretical basis for the related research on the fretting wear characteristics of laser cladding HEA coatings.

Effect of heterogeneous microstructures on mechanical properties of thin gradient nanotwinned copper foils

Strengthening of metals generally weakens ductility, but gradient strengthening is an exception. Heterogeneous microstructures, characterized by variations in grain size and twin spacing (structural gradients), were incorporated into 20-μm thin nanotwinned copper (NT-Cu) foils to enhance their mechanical properties. Four distinct heterogeneous NT-Cu samples, including two-layer (2L) structures and three-layer (3L) structures, were produced through rotary electroplating. The mechanical properties, work hardening rates, and microstructural features of these samples were systematically analyzed and compared. The results showed that an increase of 9.5 %, 6.4 %, and 20.1 % in ultimate tensile strength, yield strength, and work hardening rate, respectively, could be achieved in 3L structures, and the elongation was maintained at 5.5 %. The significant structural gradients and two interfaces of 3L structures were confirmed to stimulate the production of geometrically necessary dislocations (GNDs), which increased strength and work hardening rate. Two interfaces of 3L structures also contributed to the efficient moderation of plastic strains and thus improved the ductility. This study significantly enhances the strength and ductility of thin electroplated NT-Cu foils by introducing heterogeneous microstructures, promoting the application of thin NT-Cu films in electric vehicles, batteries, and the semiconductor industry.

Effect of grain size on tensile behavior and the underlying deformation modes in a Mg-5Y sheet

This work aims to further understand the grain size effects on individual slip/twining activities at grain scale and their correlation with the tensile behavior, for a Mg-5Y sheet with mean grain size range of 3–130 μm, based on slip trace/electron backscatter diffraction (EBSD) analysis. The correlation between tensile yield strength and the corresponding grain size fitted well with the Hall-Petch relationship, i.e., the k value was 191 MPa·μm1/2, with an R2 of 0.96. According to the identified 1004 sets of slip traces in 2477 grains, basal <a> slip was always dominant and tended to increase (from 47.2% to 71.5%) with grain size increased from 3 to 85 μm. Meanwhile, the percentage of pyramidal II <c+a> slip exhibited a marked decline from 34.5% to 8.9%, while that of prismatic <a> and pyramidal I <c+a> slip changed slightly. The decreased pyramidal II <c+a> slip activity was consistent with the decreased tensile yield strength as grain size increased. The twinning activity was limited and insensitive to grain size. In addition, the geometrically necessary dislocation (GND) density was increased with increasing grain size. Based on the slip activity statistics, the critical resolved shear stress (CRSS) ratios were estimated, which exhibited a significant grain size effect. Specifically, CRSSpris/CRSSbas showed negative grain size dependence, while the opposite was true for CRSSpyrIICA/CRSSbas. This study provides valuable grain-scale experimental evidence for the grain size effect on individual slip activity and CRSS ratios, which contributes to a better understanding of the Hall-Petch relationship in Mg-Y alloy.

Effects of plasma electrolytic polishing on the surface and mechanical properties of Ti-6.5Al-3.5Mo-1.5Zr-0.3Si specimen fabricated by laser powder bed fusion

The high roughness of titanium alloy specimen fabricated by laser powder bed fusion (LPBF) may limit the application of LPBF technology. Plasma electrolytic polishing (PEP) can effectively reduce the surface roughness of metal samples. In this study, an orthogonal test was designed to investigate the effects of polishing time, voltage and electrolyte temperature on the reduction of roughness and wall thickness of Ti-6.5Al-3.5Mo-1.5Zr-0.3Si specimens fabricated by LPBF. The surface roughness of the specimen decreased from Ra = 49.1 μm to Ra = 9.1 μm and the thickness decreased from 1.72 mm to 1.38 mm after the PEP. Due to the recoil pressure induced by rupture of vapor-gaseous envelope (VGE) and the annealing effect caused by electron impact, the width of α/α' grains on the surface of TC11 specimen decreased from 0.51 μm to 0.46 μm; the mean value of geometrically necessary dislocations (GND) increased by 9.8 %; the proportion of high angle grain boundaries (HAGBs) increased from 75.2 % to 80.8 %; the proportion of twin grain boundary length increased from 2.9 % to 11.9 %, and the activated twin variant type increased from 2 to 4. The number of pores in the samples reduced from 85 to 2 after PEP, which resulted in the improvement of tensile and fatigue properties. The ultimate strength and elongation of the tensile specimens increased by 6.2 % and 53.5 % respectively. The infinite fatigue life maximum stress of the high cycle fatigue specimen increased by 62.5 %. Another reason for the improvement of mechanical properties may be the self-healing of microcracks and the electroplasticity in the specimens.

Grain size dependence of twin nucleation in magnesium alloys

Twinning plays a prominent role in the plastic deformation of low-symmetry crystals such as magnesium alloys. However, the capture of twin nucleation has been challenging since the process is rapid and difficult to predict, making it impossible to observe directly. Because of this, the understanding of the fundamental twinning mechanism through experiments and simulation techniques incorporating twinning has progressed slowly. In this study, a quantitative analysis integrating an in-situ three-point bending experiment and crystal plasticity modeling was conducted on the twin nucleation in AZ31 magnesium alloys with different grain sizes. The threshold for triggering twin nucleation has been quantified. The prediction ability of three widely used criteria for twin nucleation, i.e., geometrically necessary dislocation (GND) density, von Mises stress, and stored energy density, is evaluated. The results suggest that only the stored energy density is able to capture the twin nucleation location accurately in all the considered grain sizes. The critical value exhibits an exponential reduction with the increase in grain size. Based on the quantitative analysis, the mechanism of the grain size effect on twin nucleation is proposed.

Effect of hydrogenation on fatigue crack growth resistance of diffusion bonded TC4 titanium alloy laminates

A method was proposed to improve the fatigue crack growth (FCG) resistance of TC4 (Ti6Al4V) titanium alloy laminate by hydrogenation/low-temperature diffusion bonding/dehydrogenation (H-DB-DH) combined process. Effects of hydrogen contents and different dehydrogenation temperatures on FCG behaviors and tensile properties as well as microstructures were investigated. Residual hydrogen contents beneath fatigue fracture surfaces were measured to determine the safe dehydrogenation temperature. The hydrogen content of 0.3 wt% and dehydrogenation temperature of 780 °C are optimal to produce TC4 laminate with a 9.87 % improvement of FCG life compared with the diffusion bonded one without hydrogenation. FCG rate decrease results from the comprehensive effect of recrystallization volume fraction increase, grain refinement, mean geometrically necessary dislocations (GNDs) density decrease and texture intensity weakness as well as texture diversity improvement. This paper provides theoretical support for fatigue property improvement of titanium alloy by hydrogen diffusion bonding process.

Insights into the microstructural evolution and strengthening mechanisms of friction stir spot welded advanced high strength ultrafine bainitic steel

Unlocking the full potential of advanced high-strength bainitic steels in spot-welding is a pivotal endeavor for harnessing their extraordinary mechanical properties in the automotive industry. This study suggests a shift in paradigm to address the inferior fusion weldability of this type of steel because of the formation of coarse martensite within the welded zone of weldments obtained by liquid-state welding, through the strategy of refining the martensite within the weld zone using the solid-state friction stir spot welding (FSSW) process. This paper delves into an in-depth investigation of the microstructural evolution and load-bearing capacity of a Fe-0.25C-3Cr-1.63Mn-1.5Si (wt%) ultrafine bainitic steel, having a remarkable tensile strength of 1.7 GPa, during the intriguing process of FSSW at rotational speeds of 600 to 2000 rpm. The research uncovers that the stir zone (SZ) undergoes dynamic recrystallization, resulting in a significantly refined microstructure. Regardless of the rotational speed employed, the high hardenability of the steel and rapid cooling during the process inevitably results in martensite formation within this region. However, a comprehensive microstructural characterization conducted by the electron backscatter diffraction (EBSD) analysis proves that the different thermal cycles induced by different rotational speeds lead into various prior austenite grain sizes, martensitic/bainitic packets and blocks attributes, and density of high-angle grain boundaries (HAGBs). Through the evaluation of strengthening mechanisms in different weld zones, the study sheds light on the key factors influencing strength. Block boundaries emerge as the primary contributor to SZ hardening, surpassing the role of geometrically necessary dislocations (GNDs) and solid solution strengthening. The load-bearing capacity observed during the tensile-shear test is governed by a delicate interplay between bonding width and crystallographic features (block size and HAGBs density) of the martensite formed within the SZ. The highest peak load was achieved under optimal welding conditions (rotational speed of 1000 rpm), which enabled the production of a sufficiently large bonding width, providing ample bonding area for load-bearing, along with an abundance of blocks and HAGBs to effectively impeding crack propagation.

Better mechanical properties of SAF2507 duplex stainless steel formed by cold rolling and normalizing

The SAF2507 duplex stainless steel with the heterogeneous structure is produced through cold rolling (ε = 90%) and subsequent 1-min normalizing at 1273 K. Compared with the corresponding initial samples, the grain sizes of ferrite and austenite phases in the 90%-1273 sample are refined by 90%, which led to an increase in the number of grain or phase boundaries between the two phases. The yield strength (YS) of the 90%-1273 sample increased to 715 MPa from 541 MPa, while the uniform elongation of 24.8% is slightly smaller than that of the coarse-grained sample. The higher YS is mainly contributed by the strengthening of grain refinement and dislocation strengthening by incorporating geometrically necessary dislocations (GNDs). During tensile deformation, the accumulation of GNDs in the heterostructured sample continuously generates hetero-deformation-induced (HDI) stress. The HDI stress plays a positive role in the 90%-1273 sample showing an ultimate tensile strength of 939 MPa. Additionally, the amount of twin boundaries in the 90%-1273 sample rose from 4.5% to 35% during tensile deformation, which indicated that the twinning induced plasticity effect was helpful in retaining the ductility of the 90%-1273 sample. This work provides strategies for optimizing the mechanical properties of duplex stainless steels and contributes to understanding the essence of a superior combination of strength and ductility of heterostructured materials.