Fraction Of Low Angle Grain Boundaries Research Articles

Effect of individual and synergistic alloying of In and Ni on microstructure, phase stability and thermal properties of lead-free Sn-0.7Cu solder

ABSTRACT The study explores the individual and synergistic effect of alloying indium (In) and nickel (Ni) with Sn-0.7 wt.% Cu solder on microstructure and thermal properties. Solders consist of β-Sn phase matrix and Cu6Sn5 intermetallic phase. In dissolves within β-Sn, while Ni substitutes Cu atoms of Cu6Sn5 forming (Cu,Ni)6Sn5. Alloying either In or Ni with Sn-0.7Cu lowers solidus and liquidus temperatures of the alloys, with a greater reduction in the case of In. Synergistic alloying of In and Ni with Sn-0.7Cu leads to a widening of 93%. Smaller fraction and localised presence of Cu6Sn5 in In-containing alloys resulted in dual-peaked grain size distribution. In the case of Ni, a fraction of (Cu,Ni)6Sn5 increased, generating more interfaces for nucleating β-Sn grains and eventually causing grain refinement. An increment in the fraction of low-angle grain boundaries (LAGBs) was observed in both alloying cases. The synergistic effect of In and Ni yields a more uniform grain size and lesser LAGBs.

Corrosion of 316L stainless steel produced by laser powder bed fusion and powder metallurgy in pressurized water reactor primary coolant

Corrosion mechanism of 316 L stainless steel produced by laser powder bed fusion-hot isostatic pressing (LPBF-HIP) and powder metallurgy-hot isostatic pressing (PM-HIP) is studied with in-situ electrochemical impedance measurements coupled to detailed oxide film characterization. Quantitative analysis of impedance spectra using the Mixed-Conduction Model and estimation of local kinetic and transport parameters by interpretation of in-depth elemental composition profiles indicated lower corrosion and oxidation rates of LPBF-HIP and PM-HIP materials in comparison to conventional wrought 316 L. This owes to a higher fraction of low-angle grain boundaries, smaller grain size, the presence of nano-sized oxide particles and elevated Cr and Ni contents.

Effect of Mn addition in W-Ni-Cu heavy alloy processed through hot press sintering techniques on microstructure, microtexture and grain boundary character evolution

In this current study, the impact of manganese addition to 93 W-4.9Ni-2.1Cu-xMn (where x = 1, 2, 3, and 4 Wt%) on the microstructure, texture development, and grain boundary character distribution of tungsten heavy alloy (WHAs) was examined. The samples were produced through a vacuum hot press sintering, with parameters 50 MPa pressure, 10 °C/min heating rate, and 1450 °C sintering temperature for 20 min holding duration. To study the influence of manganese on the texture development and microstructure in conventional WHAs, electron backscatter diffraction (EBSD) was employed. The important findings from the EBSD analysis reveal that fibre texture (001) 〈111〉 and a higher fraction of low-angle grain boundaries in the Mn unalloyed compact resulted from extensive atomic jumps between the particles at elevated temperatures. Further texture analysis on the Mn-added compacts reveals that manganese oxide (MnO) formation during liquid phase sintering led to lower densification and randomized weak fibre texture development in manganese-alloyed compacts. In addition, the texture degradation in Mn-added compacts is mainly caused by recrystallization followed by sub-structural recovery during liquid phase sintering. This study indicates that increasing manganese content in WHAs stoichiometry led to a heterogeneous distribution of binder matrix (NiCu) in the sintered compact. As a result, W-4.9Ni-2.1Cu-4Mn alloy revealed decreased relative density (90.70%), microhardness (389.5 HV0.5), and electrical conductivity (15.74% of IACS).

Influences of Si-Ti combined alloying on microstructures and properties of laser welded socket joints of molybdenum

This study selected silicon (Si) and titanium (Ti) as alloying elements to explore influences of the Si:Ti mass ratio on performance of molybdenum bead on plate laser welded joints. When the Si:Ti ratio was 1:1, the silicides exhibited a uniform continuous distribution, and significantly increased room temperature tensile strength of molybdenum bead on plate from 48 MPa to 231.6 MPa. In addition, Si-Ti combined alloying was applied to Mo tube-end plug socket joints with the optimal Si:Ti ratio. Results showed that Si and Ti were uniformly distributed in the weld of laser welded socket joints. Addition of Si and Ti in the weld increased the fraction of low-angle grain boundaries (LAGBs), which was conducive to improving the bonding strength of grain boundaries. The second-phase particles Mo3Si and Ti5Si3 were observed in the weld after adding Si and Ti, which played a role in second-phase strengthening. The room-temperature tensile strength of laser welded socket joints of Mo with the Si:Ti mass ratio of 1:1 reached 301 MPa, and the joint showed mixed fractures. The fractures in the FZ exhibited morphologies of cleavage fractures, while those in the heat affected zone (HAZ) showed mixed morphologies of intergranular and cleavage fractures. Finally, the results of high-temperature nanoindentation creep testing showed that after applying a load of 8 mN for 600 s at 400℃, the creep displacement of the LW-socket joint was measured at 20.28 nm, whereas the LW-SiTi socket joint exhibited a creep displacement of only 6.63 nm. The research findings of this paper provide references for alloying methods in laser welding of molybdenum and molybdenum alloys.

Investigation of Microstructures and Tensile Properties of 316L Stainless Steel Fabricated via Laser Powder Bed Fusion.

In this study, a thorough investigation of the microstructures and tensile properties of 316L stainless steel fabricated via laser powder bed fusion (L-PBF) was done. 316L stainless steel specimens with two different thicknesses of 1.5 mm and 4.0 mm fabricated under similar conditions were utilized. Microstructural characterization was performed using optical microscopy (OM) and scanning electron microscopy (SEM) equipped with electron backscatter diffraction (EBSD). Melt pools and cellular structures were observed using OM, whereas EBSD was utilized to obtain the grain size, grain boundary characteristics, and crystallographic texture. The 1.5 mm thick sample demonstrated a yield strength (YS) of 538.42 MPa, ultimate tensile strength (UTS) of 606.47 MPa, and elongation to failure of 69.88%, whereas the 4.0 mm thick sample had a YS of 551.21 MPa, UTS of 619.58 MPa, and elongation to failure of 73.66%. These results demonstrated a slight decrease in mechanical properties with decreasing thickness, with a 2.4% reduction in YS, 2.1% reduction in UTS, and 5.8% reduction in elongation to failure. In addition to other microstructural features, the cellular structures were observed to be the major contributors to the high mechanical properties. Using the inverse pole figure (IPF) maps, both thicknesses depicted a crystallographic texture of {001} <101> in their as-built state. However, when subjected to tensile loads, texture transitions to {111} <001> and {111} <011> were observed for the 1.5 mm and 4.0 mm samples, respectively. Additionally, EBSD analysis revealed the pre-existence of high-density dislocation networks and a high fraction of low-angle grain boundaries. Interestingly, twinning was observed, suggesting that the plastic deformation occurred through dislocation gliding and deformation twinning.

Refining the mechanistic understanding of microstructural decay during rolling contact fatigue in 52100 bearing steel tempered at high temperature

Subsurface rolling contact fatigue (RCF) failure occurs beneath heavily loaded hard contacts like gears, bearings, and cams. This study investigates microstructural decay beneath a RCF-tested surface in AISI/SAE 52100 bearing steel tempered at 240 ℃. RCF tests were conducted at 100 ℃ with a maximum Hertzian contact pressure of 4 GPa for four stress cycles. Microstructural characterization utilized scanning electron microscopy, electron backscatter diffraction, transmission Kikuchi diffraction, and transmission electron microscopy. Due to high tempering temperature, white etching bands (WEBs) were observed without preceding dark etching regions. The microstructural decay sequence involved: (1) formation of elongated ferrite and ferrite microbands, (2) complete dissolution of tempered carbides and partial dissolution of residual cementite, (3) formation of WEBs composed of nano-sized ferrite grains (100–300 nm) transformed from ferrite microbands, and (4) appearance of lenticular carbides. Within the WEBs, most nano-sized grains had high-angle grain boundaries, while the fraction of low-angle grain boundaries increased in later stages of RCF. Lenticular carbides formed alongside elongated ferrite and coalesced nano-sized ferritic grains.

Microstructure analysis of a CoCrFeNi high-entropy alloy after compressive deformation

A sharp increase in the dislocation density and in the fraction of low-angle grain boundaries (LAGBs) has often been observed during the early-stage deformation of high-entropy alloys. To study the underlying reasons for this behavior, the microstructure of plastic deformation applying at a low compressive strain of 3.3% was analyzed in the CoCrFeNi high-entropy alloy. This deformation results in a relatively higher dislocation density in the CoCrFeNi alloy in comparison to that in pure Nickel. A significantly increased dislocation density was observed near grain boundaries that is collaborated by a low value of the anisotropy factor (Az ∼ 2.37) in CoCrFeNi alloys offering favorable conditions for dislocation generation. Moreover, the low anisotropy factor in CoCrFeNi alloys appears to be caused by their strong chemical heterogeneity. The relatively easy dislocation generation provides an important feature of dislocation interactions in the CoCrFeNi alloy. Local high hardness and high Young's modulus values were observed at newly-formed LAGBs. The evolution of LAGBs strongly depends on the grain orientation and the internal strain, and LAGBs are gradually formed by the accumulation and self-organization into LAGBs at a surprisingly low strain of about 2.61%. The formation of these boundaries is intrinsically promoted by the low stacking fault energy of the CoCrFeNi alloy. An easy dislocation generation and a low strain of LAGB formation result in a high density of retained dislocations, giving rise to the observed mechanical performance of the material.

Study on strain distribution and texture evolution of Fe-0.18C-4Al-7Mn steel under different strain rate conditions

The objective of this study is to examine the effect of strain rate on the distribution of strain and evolution of texture components in thermomechanically processed intercritically annealed (at 750 °C for 1 hr.) Fe-0.18C-4Al-7Mn (wt.%) steel. Room temperature tensile tests are performed on the annealed sample at strain rates of 1.33 × 10−4, 6.65 × 10−4 and 13.3 × 10−4 s−1. The electron backscatter diffraction (EBSD) analysis reveals that there is a notable decrease in the fraction of low-angle grain boundaries (LAGBs) from 85 to 28 % as the strain rate is increased from 1.33 × 10−4 to 13.3 × 10−4 s−1. The sample, subjected to a strain rate of 1.33 × 10−4 s−1, exhibits the highest fraction of deformed grains and the lowest fraction of recrystallized grains. As the strain rate increases, the higher fraction of recrystallized grain remains undeformed, leading to lower strain intensity. The transformation-induced plasticity (TRIP) effect induces a highertransformation ratio of austenite to martensite at lowstrain rates. An intense α-fiber with strong components, primarily Ms Brass, Goss/Brass twin, and Goss is observed at low strain rates.

Overcoming the trade-off between strength and ductility in austenitic stainless steel using a high-density pulsed electric current

This paper presents a method to overcome the trade-off between the strength and ductility in metals and clarifies its mechanism via a crystal structure analysis on type 316 austenitic stainless steel subjected to high-density pulsed electric current (HDPEC) treatment. Because of the HDPEC treatment, the fractions of low-angle grain boundaries (LAGBs) and high-angle grain boundaries (HAGBs) increased, while those of ∑ 3 twin boundaries (TBs) decreased significantly. An increase in the number of HAGBs and the formation of new LAGBs, which leads to achieving refined grains, was achieved by virtue of the change of misorientation angles and dislocation formation. Furthermore, the dissolution of Cr-rich precipitates occurred in the grain surroundings, which led to an increase in ductility of the material. The dislocation density, measured by the mean kernel average misorientation values, decreased because of the dislocation motion induced by HDPEC. The strengthening of the material was attributed to the grain refinement, and the increased ductility was attributed to the decreased dislocation density and decreased content of Cr-rich precipitates, which prevented precipitation hardening. The combination of these advantages contributed to overcoming the trade-off between strength and ductility.

Densification, Microstructure and Anisotropic Corrosion Behavior of Al-Mg-Mn-Sc-Er-Zr Alloy Processed by Selective Laser Melting

High-performance additives manufactured by Al alloys provide significant potential for lightweight applications and have attracted much attention nowadays. However, there is no research on Sc, Er and Zr microalloyed Al alloys, especially concerning corrosion behavior. Herein, crack-free and dense Al-Mg-Mn-Sc-Er-Zr alloys were processed by selective laser melting (SLM). After optimizing the process parameters of SLM, the anisotropic corrosion behavior of the sample (volume energy density of 127.95 J·mm−3) was investigated by intergranular corrosion (IGC) and electrochemical measurements. The results showed that the XY plane of the as-built sample is less prone to IGC, and it also has a higher open circuit potential value of −901.54 mV, a higher polarization resistance of 2.999 × 104 Ω·cm2, a lower corrosion current of 2.512 μA·cm−2 as well as passive film with superior corrosion resistance compared to the XZ plane. According to our findings, the XY plane has superior corrosion resistance compared to the XZ plane because it has fewer primary phases of Al3(Sc, Er, Zr) and Al2MgO4, which can induce localized corrosion. Additionally, a higher fraction of low-angle grain boundaries (LAGBs) and a stronger (001) texture index along the building direction are also associated with better corrosion resistance of the XY plane.

Additive manufacturing of pure niobium by laser powder bed fusion: Microstructure, mechanical behavior and oxygen assisted embrittlement

In this study, pure niobium (Nb) was additive manufactured by laser powder bed fusion (LPBF). The effect of energy density and oxygen content on the densification behavior, microstructure and mechanical properties of the LPBF-built Nb were investigated. It was demonstrated that the density steadily increased and the pores were gradually eliminated with the increase of energy density. A maximum density of 99.7% was achieved at the energy density of 146 J/mm3. Further increase of energy density led to an intense Marangoni convection and a slightly decrease of density. The average grain size of the LPBF-built Nb was 70 μm approximately. A large fraction of low-angle grain boundaries and a high density of dislocations were observed in the LPBF-built Nb. The ultimate tensile strength (UTS) of the LPBF-built Nb increased steadily with the increase of energy density due to the increment of density. The fine grains and high-density dislocations resulted in high strength of the LPBF-built Nb. The maximum yield strength and UTS were as high as 550 MPa and 630 MPa respectively with a considerable elongation of 26%. Oxygen deteriorated the mechanical properties of the LPBF-built Nb significantly. The increase of oxygen enhanced the tensile strength of Nb and reduced its plasticity significantly. The LPBF-built Nb became brittle when the oxygen content increased from 0.05 wt % to 0.23 wt %. The elongation to fracture of the LPBF-built Nb decreased to less than 2% regardless of the input energy density. It was proposed that the interstitial oxygen atoms exert a repulsive force on the dislocation and block its movement, leading to a sharp decrease in the plasticity of LPBF-built Nb.

Detail characterization on the microstructure evolution of nickel-base alloy 600MA via surface mechanical attrition treatment

Microstructure evolution of alloy 600MA was explored after surface mechanical attrition treatment (SMAT) with different durations via scanning electronic microscopy, electron backscattered diffraction, transmission electron microscopy and transmission Kikuchi diffraction. The grain refinement and coarsening occurred gradually as strain accumulated with increasing SMAT time. During grain refinement, the initiation and development of dislocation walls or cells came up at early stage. A strip of dislocation wall led to straight element concentration just prior to the formation of ultrafine-laminated (UFL) structures containing abundant twin boundaries (TB) and low angle grain boundaries (LAGB). The UFL structures with a texture orientation were subdivided by the development of dislocation arrays into ultrafine-grained (UFG) structures with random orientations. Meanwhile, the elements were dispersedly concentrated in a few grains. During grain coarsening, the dispersed element concentration still existed. The fraction of LAGB, TB and dislocation density was significantly reduced while the fraction of high angle grain boundary (HAGB) increased along with grain growth. • The grain refinement and coarsening occurred gradually as strain accumulated with increasing SMAT time. • During grain refinement, the initiation and development of dislocation walls or cells came up at early stage. • A strip of dislocation wall led to straight element concentration just prior to the formation of ultrafine-laminated (UFL) structures containing abundant twin boundaries (TB) and low angle grain boundaries (LAGB). • The UFL structures with a texture orientation were subdivided by the development of dislocation arrays into ultrafine-grained (UFG) structures with random orientations. • During grain coarsening, the fraction of LAGB, TB and dislocation density was significantly reduced while the fraction of high angle grain boundary increased along with grain growth.

Microstructure characteristics of a René N5 Ni-based single-crystal superalloy prepared by laser-directed energy deposition

This work focused on the microstructure and mechanical properties of nickel-based single crystal (SX) superalloy René N5 prepared by laser-directed energy deposition (L-DED) using a flat-top laser beam. A comparison was conducted with the conventional directional solidification technique. Results show that the crack-free single-crystal superalloys with a fraction of low-angle grain boundary (LAGB) higher than 98.5% were obtained by epitaxial growth under different laser powers. The flat fusion line using a flat-top laser beam can suppress the formation of stray grain and high-angle grain boundaries (LAGB) during the L-DED process. Microstructure observation shows the extremely refined dendrites in L-DED SX superalloys with primary dendrite arm spacing (PDAS) values around 20–30 µm, which is significantly lower than as-cast ones (∼350 µm). PDAS increases with the increasing laser power. Segregation of refractory elements of Re, W, and Ta increases with increasing laser power, and the SX specimens with laser power of 1100 W and 1300 W have lower elemental segregation compared with as-cast specimens. The SX specimens exhibit a uniform square γ′ phase at the dendrite core, with an average size of 79–101 nm and a volume fraction of 57–68%. Besides, the carbides with discontinuous and refined shapes show a uniform distribution in the L-DED specimens in comparison with as-cast specimens. The microhardness of as-deposited specimens increases with the increasing laser power, which is higher than that of as-cast ones. This work can provide potential guidance for the microstructure control in the laser additive manufacturing of Ni-based SX superalloys and a further improvement in high-temperature mechanical performance.

Formability and micromechanisms of DP600 dual phase steels in electric-pulse triggered energetic materials forming

This paper investigates dual-phase (DP) steel’s formability and microscopic deformation mechanism under electric-pulse triggered energetic materials forming (ETEF). It provides experimental evidence and a detailed explanation for understanding the dynamic fracture mechanism and microstructure of ETEF. The present study analyzes the macroscopic fracture mechanism of ETEF specimens. Compared with quasi-static forming, the forming limits of ETEF with different strain paths (uniaxial tension, plane strain and biaxial tension) were significantly improved: the limit major strains were increased by 40.8 %, 78.2 %, and 18.2 %, respectively. Furthermore, SEM/EBSD/TEM observations revealed the relationship between the microstructure, kernel average misorientation (KAM), dislocation density, slip mechanism, and deformation twinning of DP600 steel. It was concluded that ETEF promoted the plastic flow of ferrite and martensite, reducing the void size and delaying the fracture failure of the material. High KAM increased the overall intensity of local grain deformation and reduced the strain gradient from grain interiors to grain boundaries. Under the condition of ETEF, a high fraction of low-angle grain boundaries (LAGBs) and refined grain size were obtained, and a high fraction of LAGBs enhanced the formability of DP steel. Additionally, {012}, {013} and {104} preferred planes appeared under ETEF conditions, forming a strong < 011 > preferred orientation and activating the slip mechanism of the < 100 > fiber component. Finally, TEM confirmed that coordinated deformation of the matrix led to improved formability of the DP600 material under ETEF conditions, as evidenced by the activation of the multi-slip system and deformation of the martensitic twins.

Altered microstructure characteristics and enhanced corrosion resistance of UNS S32750 duplex stainless steel via ultrasonic surface rolling process

The UNS S32750 duplex stainless steel was treated by ultrasonic surface rolling process (USRP), and effects of the plastic deformation on microstructure evolution and corrosion behavior were investigated. Results demonstrated that grain refinement, a considerable increase of the fraction of low-angle grain boundaries (LAGBs, <10°), high density of dislocations and high surface compressive residual stress were produced by the USRP. In particular, the intensity of the evolution was inhomogeneous and it was higher for austenite than ferrite phase. What’s more, an evolution toward <001> of crystallographic orientation, a decrease in intensity of texture in α phase while increase in γ phase were found for the surface deformation layer. In addition, a small amount of deformation twins in γ phase were induced on account of the USRP treatment. Taking advantages of the microstructure evolution, the corrosion potential and critical pitting potential increased, while the passive current density and metastable pitting susceptibility decreased significantly for the USRP sample. The refined grains, high-density of dislocations and high surface compressive residual stress synergistically promoted the enrichment of Cr 2 O 3 in the passive film on the deformation layer. The Cr/Fe ratio increased from 0.73 to 0.94 with the help of USRP, revealing a less defective and more stable passive film. Therefore, USRP is a promising and effective surface strengthening technique for DSS to improve the corrosion resistance in aggressive environments. • The USRP caused inhomogeneous microstructure evolution for γ and α phase of UNSS32750 DSS. • The increase of surface compressive residual stress induced by USRP was higherin γ than α phase. • The USRP decreased the metastable pitting susceptibility, while improved thecorrosion resistance of the DSS. • The ratios of Cr 2 O 3 /Cr(OH) 3 and Cr/Fe in passivefilm were increased with the assistant of USRP.

Experimental analyses and numerical modeling of the microstructure evolution of aluminum alloy using an internal state variable plasticity-based approach coupled with the effects of second phase

Continuous dynamic recrystallization (cDRX) is an effective way to refine the microstructures of materials with high stacking fault energies during their hot working. However, investigation of the influence of grain boundaries and second phases on the microstructure evolutions of these materials using conventional cDRX models is challenging. Accordingly, herein a cDRX model characterized with micron-scale second phases distributed along the grain boundaries was developed by an internal state variable plasticity-based approach. Geometrically necessary dislocation (GND) densities generated near the second phase and grain boundary were calculated by considering strain concentration in the grain, which affected the evolution of dislocation density at the subgrain boundary. The cDRX model considers an original grain as a collection of subgrains and represents the subgrains as a compound structure consisting of subgrain boundaries with relatively high dislocation densities and a subgrain interior with significantly low dislocation density. Changes in the misorientations of subgrains can be accurately determined by tracking and analyzing the evolutions of the dislocation densities of subgrain boundaries. Accuracy of the developed cDRX model was verified by comparing the hot compression data of 5052 and 2A14 Al alloys acquired using this model with the corresponding experimental results. The predicted values of variables, including flow stress, subgrain size, and the fraction of low-angle grain boundary, were in appropriate agreement with the experimental results. Effects of temperature and strain rate on the microstructure evolution of the 2A14 Al alloy were investigated. Based on the influences of strain concentration near the grain boundary and second phase on the dislocation density of the 2A14 Al alloy, the reasons for the enhancement of the occurrence of cDRX and improvement of the mechanical properties of the 2A14 Al alloy by the second phase and grain boundary are explained. The results of this study provide a direction for the subsequent improvement of the proposed cDRX model with second phases inside the grain.

Effect of ECAP Route Type on the Microstructural Evolution, Crystallographic Texture, Electrochemical Behavior and Mechanical Properties of ZK30 Biodegradable Magnesium Alloy.

In this study, billets of the ZK30 (Mg-3Zn-0.6 Zr-0.4 Mn, wt%) alloy were Equal Channel Angle Pressing (ECAP) processed for up to four passes of routes Bc (with rotating the sample 90° in the same direction between the subsequent passes), A (without sample rotation), and C (with sample rotating 180°) after each pass at a temperature of 250 °C and a ram speed of 10 mm/min using a die with an internal channel angle of 90°. The microstructural evolution and the crystallographic texture were investigated using a Scanning Electron Microscope (SEM) equipped with the Electron Back-Scatter Diffraction (EBSD) technique. Corrosion measurements were conducted in ringer lactate which is a simulated body fluid. The Vickers microhardness test and tensile tests were conducted for the alloy before and after processing. The as-annealed billets exhibited a bimodal structure as fine grains (more than 3.39 µm) coexisted with almost-equiaxed coarse grains (less than 76.73 µm); the average grain size was 26.69 µm. Further processing until four passes resulted in enhanced grain refinement and full Dynamic Recrystallization (DRX). ECAP processing through 4-Bc, 4-A, and 4-C exhibited significant reductions in grain size until they reached 1.94 µm, 2.89 µm, and 2.25 µm, respectively. Four-pass processing also resulted in the transformation of low-angle grain boundaries into high-angle grain boundaries. The previous conclusion was drawn from observing the simultaneous decrease in the fraction of low-angle grain boundaries and an increase in the fraction of high-angle grain boundaries. The pole figures revealed that 4-Bc, 4-A, and 4-C reduced the maximum texture intensity of the as-annealed billets. The potentiodynamic polarization findings revealed that route Bc is the most effective route in improving the corrosion rate, whereas the Electrochemical Impedance Spectroscopy (EIS) revealed that routes A and Bc improved the corrosion resistance with nearly identical values. Finally, 4-Bc resulted in the highest increase in Vickers hardness, yield stress, and ultimate tensile strength with values of 80.8%, 19.3%, and 44.5%, alongside a 31% improvement in ductility, all compared to the AA condition.

Liquid metal embrittlement susceptibility of zinc-coated martensitic sheet steels

This paper investigates the liquid metal embrittlement (LME) behavior of three martensitic steels (MS-1, MS-2, and MS-3), each of 1500 MPa tensile strength. Their LME response was measured via hot ductility testing at a fixed strain rate over a range of temperatures (700–950 °C). Despite having the same room temperature strength and general lath martensitic microstructure, the three MS steels exhibited different LME responses. When LME occurred, it proceeded in an intergranular manner within a limited temperature range and with the most severe response at 850 °C. The lath martensitic microstructure was investigated in detail to correlate it to the differences in the LME response. It was found that the three MS steels had different microstructural morphologies and prior austenite grain sizes (PAGS). The LME sensitivity appears to correlate with these microstructural parameters. The MS-2 steel exhibited higher LME susceptibility and possessed the smallest PAGS and a high fraction of low-angle grain boundaries. In contrast, MS-1 was immune to LME at all tested temperatures and had a large PAGS and a high fraction of large-angle grain boundaries. To better compare hot ductility results between steels, a “LME index” was formulated based upon the measured LME severity over the tested temperature range, with a large LME index indicating a severe LME response. Among the three MS steels, MS-2 had the highest LME index while MS-1 had the lowest.

Recovery and recrystallization of warm-rolled tungsten during helium thermal desorption spectroscopy annealing

Tungsten has been perceived as one of the most promising plasma-facing materials for future fusion reactors. Its behavior under helium irradiation has been extensively studied. In this work, we focus on the recovery and recrystallization of helium-implanted surface grains in warm-rolled polycrystalline tungsten. Mechanically polished specimens with and without 40 keV helium implantation were heated to 1650 °C during helium thermal desorption spectroscopy experiment. Our results suggest that recovery and recrystallization occurred via subgrain coarsening and removal of low-angle grain boundaries in as-polished specimen. Its average grain size increased from 1.70 to 8.89 μm, and its fraction of low-angle grain boundary decreased from 0.77 to 0.29. By contrast, little changes of grain size and grain boundary misorientation angle were found in helium-implanted surface grains. Such enhanced thermal stability is attributed to the pinning effect of helium bubbles on grain boundaries, and the underlying mechanism is discussed based on the proposed model of cellular microstructure.

Coupled quantitative modeling of microstructural evolution and plastic flow during continuous dynamic recrystallization

Continuous dynamic recrystallization (cDRX) dominates microstructural evolution during the hot working of metallic materials with high stacking fault energy (SFE), such as aluminum alloys. However, in reality, a lack of quantitative and visual modeling of the process hinders its widespread application in the hot working process. In this study, using a recently developed multilevel cellular automaton (MCA) that integrates the newly established cell switching rules and topology deformation technique, a novel mesoscale MCA-cDRX model was constructed to investigate the evolution of both microstructures and macroscopic mechanical response in the hot working of AA7075 aluminum alloy. By considering the evolution of dislocation density and the orientation angle of the local cells as the primary clues, the plastic flow, recrystallization kinetics, features of subgrain size and high-angle grain boundaries, and influence of initial matrix characteristics on the cDRX mechanism were analyzed. The model predictions are consistent with the experimental data. Quantitative analysis confirms that the incubation time for the initiation of subgrain formation is significantly short. The fine-grain matrix and high initial volume fraction of low-angle grain boundaries can significantly accelerate the progress of cDRX owing to a stronger accumulation of dislocations in the dislocation cell walls through the climb and cross-slip mechanisms in the deformed aluminum alloy. The subgrain size is dependent on the Zener-Hollomon parameter. The developed simulation framework offers an effective means to allow the visualization of the cDRX.