High-angle Boundaries Research Articles

Effect of growth temperature on the microstructure and properties of epitaxial MoS2 monolayers grown by metalorganic chemical vapor deposition

Metalorganic chemical vapor deposition (MOCVD) is a promising technique for wafer-scale synthesis of MoS2 monolayers for 2D field-effect transistors (2D-FETs) and related devices. Epitaxial growth of MoS2 on sapphire provides films that are crystallographically well-oriented but typically contain low-angle grain boundaries (e.g., mirror twins), voids, and other defects depending on growth conditions and substrate characteristics. In this study, we investigate microstructure, optical properties, and field-effect characteristics of wafer-scale MoS2 monolayers grown by MOCVD on c-plane sapphire over a narrow window of growth temperatures (900–1000 °C). The density of low-angle grain boundaries in the MoS2 monolayer was found to decrease dramatically from 50% areal coverage for films grown at 900 °C to 5% at 1000 °C. This decrease in low-angle grain boundary density is correlated with an increase in the room-temperature photoluminescence intensity of A excitons and a decrease in the full-width-half maximum (FWHM) of the Raman A1g peak, which are typically indicative of a general reduction in defects in MoS2. However, the best transport properties (e.g., mean field-effect mobility mFE = 17.3 cm2/V s) were obtained in MoS2 monolayers grown at an intermediate temperature of 950 °C. It was found that as the growth temperature increased, small regions bound by high-angle boundaries begin to appear within the monolayer and increase in areal coverage, from ∼2% at 900 °C to ∼5% at 950 °C to ∼10% at 1000 °C. The growth temperature of 950 °C, therefore, provides an intermediate condition where the combined effects of low-angle and high-angle boundaries are minimized. The results of this study provide guidance on MOCVD growth and characterization that can be used to further optimize the performance of MoS2 2D-FETs.

Mechanisms of formation and accumulation of misorientations in deformable metals and alloys

The formation of misoriented substructures in plastically deformable metal materials has been theoretically studied. Expressions are obtained for the intensity of accumulation of low-angle and high-angle misorientation boundaries. Within the framework of a mathematical model of shear plastic deformation and hardening, numerical calculations of the dependences of the average characteristics of a defective medium on the degree of deformation under conditions of uniaxial compression with a constant strain rate at room temperature are performed. It is shown that the intensity of generation of low-angle tilt walls depends significantly on the scenario of changes in the density of jogs on the screw segments of dislocation loops emitted by dislocation sources. The main mechanism for the formation of low-angle walls is the rearrangement of clusters of edge segments of dislocation loops into tilt dislocation walls under the influence of flows of interstitial atoms generated by moving screw segments. It is assumed that low-angle walls merge into one until the total misorientation angle of the merged walls reaches a critical value of about 10°, after which the distance between dislocations in the wall decreases to the corresponding critical value and further penetration of individual dislocations into the wall becomes impossible. The expression for the intensity of the formation of high-angle boundaries was obtained as a consequence of the continuation of the work of dislocation sources and the formation of clusters of low-angle walls, the total energy of which is higher than the energy of an equilibrium high-angle boundary at the same misorientation.

Effect of Single Crystal Growth and Solidification Grain Boundaries on Weld Solidification Cracking Behavior of CMSX-4 Superalloy

Single-crystal superalloys have been popularly employed in high-temperature parts of gas turbines, such as blades. However, the welds of such alloys are highly susceptible to solidification cracking, which limits their applicability to high-temperature turbine blades. In this study, the effects of characteristics of weld solidification on solidification cracking susceptibilities (solidification brittle temperature range, BTR) were fundamentally investigated for the CMSX-4 single-crystal superalloy. We applied a transverse-Varestraint test procedure for both the linear and oscillated arc welds by changing the weld solidification characteristics, such as the degree of single crystal growth and formation of solidification grain boundaries. The BTR for the CMSX-4 alloy is 336 K for linear welding condition, whereas the values are 434 K and 342 K for 0.6 and 1.5 Hz oscillated welds. Interestingly, the BTR continuously increases with the weld oscillation frequency. By contrast, almost no changes in the weld mushy-zone temperature range are theoretically calculated for each welding condition via the diffusion-controlled Scheil model. The mechanism underlying the increase in BTR under oscillation welding is clarified based on the relationship between the achievement ratio of the weld single crystal growth and fraction of high-angle (>15<sup>o</sup>) solidification boundaries, which affect severe dendrite coalescence undercooling. The lower fraction of the high-angle weld solidification grain boundaries attributed to the superior achievement ratio of weld single crystal growth, which reduces the dendrite coalescence undercooling and BTR. Consequently, it enhances the solidification crack propagation resistance.

Microstructural study of microwave welded austenitic stainless steel

The joining of austenitic stainless steels through microwave hybrid heating has gained much attention due to its advantages, like uniform and volumetric heating. However, to establish a microwave joining process for promising applications, a microstructural study of the welded joint is still an area to explore. In the present study, microstructural analysis of austenitic stainless steel (SS304) joints is explained through optical microscopy, scanning electron microscopy (SEM), elemental mapping, X-ray diffraction (XRD) and Vickers' micro-hardness tester. Further, detailed microstructural analysis of interlayer and heat-affected zone (HAZ) is studied through electron back-scattered diffraction (EBSD). XRD results reveal the development of various Ni, Cr and the M23C6 type carbides at the joint interlayer region. SEM results point toward non-epitaxial grain evolution along the fusion zone. The mean microhardness of 350 ± 20 HV was determined at the joint interlayer region. A relatively fine grain structure was observed along the fusion boundary. However, moving away from the joint, two different forms of coarse grain heat-affected zone (CGHAZ) were observed. The presence of two different forms of CGHAZ is confirmed through inverse pole figure maps and grain size plots. The presence of high-angle boundaries (HABs) at 60° rotation describes the twin boundaries characterized along the <111> axis. The existence of improved fractions of HABs in the microwave-joined specimen of SS will favor the microstructure to retard bulk deformation. The joint specimen endures a maximum force of 6.98 kN, and the ultimate tensile strength of 613 MPa with 4.75% of elongation. It is observed that microhardness observations and the microstructural studies in the research help establish a solid microstructure-property correlation for the joined material.

Core shift controls grain boundary energy scaling in Cu and Al

Grain boundary energies in different elements are correlated. The proportional scaling constants relating the energies of crystallography-equivalent boundaries in any two f.c.c. elements are nearly constant, with the notable exception of aluminum where these constants are known to vary significantly. However, the origins of the exceptional behavior of aluminum are not understood. Previously, we reported that for fcc metals there is a preference for tilt boundaries to shift their tilt axis across the (11¯0) plane towards [112] and to ultimately form low energy [112] core shifted boundaries (CSBs). By comparing grain boundary energies in copper and aluminum with different tilt axes in (11¯0) plane, we now report the existence of a well-defined scaling behavior for the case of low angle boundaries. In contrast, the scaling constant for high angle boundaries is essentially fixed regardless of their tilt axis shift. This results in a gradual change in the scaling constants from low angle to high angle boundaries, which is responsible for the apparent exceptional scaling behavior found in aluminum. An analysis of structure evolution during core shifting points to the significance of boundary-core dissociation, a form of correlated relaxation of individual atoms at boundaries, in controlling the scaling of boundary energies.

Annealing behavior of AlSi10Mg alloy fabricated by laser powder bed fusion

The AlSi10Mg alloy fabricated by laser powder bed fusion was subjected to isochronal annealing for 2h at temperatures in the range of 100–520 °C. Recovery and grain growth of the annealed samples were investigated by combining EBSD and in situ TEM techniques. A bimodal grain size distribution with grain sizes of 0.9 μm and 2.3 μm was developed in the initial sample, where large grains were found in the interiors of melt pools and most of small grains were distributed along melt pool boundaries. In-situ TEM investigations revealed that nucleation that occurs at the melt pool boundaries is as a consequence of migration of high angle boundaries during annealing. A slight increase in grain sizes was discovered during high-temperature heat treatment, indicating the remarkable thermal stability of the alloy compared to conventional cast Al-Si alloys. This is explained by the fact that the stable eutectic Si structure, which was originally formed during rapid solidification, can effectively retard grain boundary migration during recrystallization.

Dynamic recrystallization behavior of selective laser melted 316L stainless steel

The present work deals with the hot deformation behavior of selective laser melted 316L stainless steel with an emphasize on the involved recrystallization mechanisms. Toward this end, a series of hot compression test have been conducted in the temperature range of 700–1000 °C under the various strain rates of 0.001, 0.01 and 0.1s−1. The flow pattern analysis through Jonas-Poliak method verifies the occurrence of dynamic recrystallization in whole investigated temperature range. The intensified substructure development and continuous recrystallization is identified as the main softening mechanism in the course of thermomechanical processing. This causes appearance of a broad peak and low softening fraction in corresponding flow curves. The cellular substructure in the as-printed state facilitates low-angle into high-angle boundaries conversion, which provide a proper condition for the occurrence of continuous mechanisms and increases the rate of recrystallization. However, detailed investigation of the sub-boundary maps indicates the contribution of discontinuous and geometric dynamic recrystallization mechanisms. Dynamic recrystallization causes substantial refinement; and the percentage of the grain holding size lower than 10 μm increases from 2 % in printed microstructure to 16.8 % and 24 % in the microstructure deformed at 700 and 1000 °C.

Microstructure of the Advanced Titanium Alloy VT8M-1 Subjected to Rotary Swaging.

In this study, the microstructural behavior of the advanced Ti-5.7Al-3.8Mo-1.2Zr-1.3Sn-0.15Si (VT8M-1) alloy during rotary swaging (RS) was investigated. VT8M-1 has increased heat resistance and is considered a replacement for the Ti-6Al-4V alloy. It was shown that, during RS, the evolution of the primary a phase is characterized by the formation of predominantly low-angle boundaries according to the mechanism of continuous dynamic recrystallization. The density of low-angle boundaries increases three times: from 0.38 µm-1 to 1.21 µm-1 after RS. The process of spheroidization of the lamellar (a + b) component is incomplete. The average size of globular a and b particles was 0.3 μm (TEM). It is shown that the microstructures after RS (ε = 1.56) and equal-channel angular pressing (ECAP) (ε = 1.4) are significantly different. The temperature-velocity regime and the predominance of shear deformations during ECAP contributed to a noticeable refinement of the primary a-phase and a more complete development of globularization of the lamellar (a+b) component. EBSD studies have shown that RS leads to the formation of a structure with a higher density of low- and high-angle boundaries compared to the structure after ECAP. The results are useful for predicting alloy microstructure in the production of long rods that are further used in forging operations.

Effect of thermal aging at 550 °C on microstructure and mechanical properties of a 9Cr martensitic steel with high silicon content

Effect of thermal aging at 550 °C on microstructure and mechanical properties of a 9Cr martensitic steel with high silicon content (named as HSi steel) was investigated by field emission scanning electron microscopy (FESEM), electron backscattered diffraction (EBSD), transmission electron microscopy (TEM) and tensile and impact test. During aging, the martensitic matrix (including lath size, effective grain size and high angle boundaries) basically keeps stable. The M23C6 in HSi steel gradually coarsens with increasing aging time and the morphology of M23C6 on grain boundary tends to be chain-shaped, and then gradually becomes discrete again. Meanwhile, the Laves phase will precipitate adjacent to M23C6 due to a Si-rich layer around M23C6 and coarsens quickly before aging for 3000 h. With further increasing aging time, Laves phase is stable with a size of about 136 nm. The mechanical properties test results show that yield strength and elongation have little change during aging. However, the impact energy of HSi steel drops obviously after precipitation of Laves phase and the impact fracture presents cleavage fracture morphology. When the aging time is over 3000 h, the coarsening rate of Laves phase is very low and the HSi steel exhibits a relatively stable impact energy about 113 J. After aging up to 10,000 h, HSi steel still has excellent comprehensive mechanical properties.

Heat treatment effect on microstructure evolution of two Si steels manufactured by laser powder bed fusion

Additive manufacturing technology represents a valid alternative for the production of ferromagnetic components in Si steels (FeSi) with high Si content. In this work the effect of heat treatments on microstructural evolution of two steels with standard (3.0 wt.% - FeSi3) and high (6.5 wt.% - FeSi6.5) Si content manufactured by Laser Powder Bed Fusion (L-PBF) was investigated. Four different temperatures ranging from 900 °C up to 1150 °C (soaking time of 1 h) were tested for each steel.In as-built condition, all the grains of FeSi3 are columnar with the [100] direction parallel to the building direction (BD) whereas in the case of FeSi6.5 there is a mix of columnar and equiaxed grains. The average grain size of FeSi6.5 (11.3 ± 0.6 μm) is about one order of magnitude lower than that of FeSi3 (103.1 ± 5.2 μm). EBSD analysis revealed that there is a prevalence of low-angle boundaries (LABs) in FeSi3 and of high-angle boundaries (HABs) in FeSi6.5. The different initial microstructure affects the response to heat treatments of the two alloys: grain size and shape of FeSi3 do not remarkably change, on the contrary significant grain growth takes place in FeSi6.5 without a weakening of the [100] texture. No brittle ordered phases were detected in as-built and heat treated samples of both materials. The results indicate that heat treatments at high temperature can be exploited to improve the magnetic characteristics of printed FeSi6.5 and reach similar properties of commercial steels.

Effect of filling materials on microstructure and properties of CMT-laser beam oscillation hybrid welding behavior of dissimilar Al–Mg–Si alloys

Laser oscillation-assisted cold metal transfer hybrid welding was conducted on 2.0 mm-thick 6082 and a new 6xxx aluminum alloys. The influence of wire composition on microstructure and mechanical properties of the 6082 and new 6xxx alloy joints is studied. Our results show that, due to the difference in surface tension, the cross section of a joint filled with ER4047 wire typically presents a wider U-shape profile, while the ER5183 filled joint shows a relatively narrower V-shape profile. The average grain size of the ER5183 joint is smaller than that of the ER4047 joint. The ER4047 joint exhibits better fluidity, resulting in a higher proportion of high-angle boundaries compared to the ER5183 joint, which we attribute to the higher Si content. Equiaxed grains are also observed near the fusion boundary on the new 6xxx side, while no equiaxed grain formation is observed on the 6082 side. EDS data demonstrates severe elemental segregation in the center of weld and the partially melted zone, suggesting that multiple secondary phases form unevenly within inter-dendritic regions and grain boundaries of α-Al during the welding process. The microhardness of the weld zone and the heat-affected zone on both sides in the ER5183 joint is higher than that of the ER4047 joint. The corrosion test evidences that the ER5183 joint has a slightly better corrosion resistance than the ER4047 joint. Our findings provide a new understanding on joining of dissimilar Al–Mg–Si alloys and guide the selection of appropriate filling materials and welding processes.

Hydrogen embrittlement in a metastable high Mn TWIP-assisted steel: Correlation between grain size and hydrogen-enhanced ε-martensite

The correlation between hydrogen-enhanced deformation-induced ε-martensite and hydrogen embrittlement at two different grain sizes has been investigated in a metastable high Mn steel. The variation in the fraction of ε-martensite was detected to be 34% for coarse grain (CG) and 10% in (FG) after hydrogen charging. The significant difference in ε-martensite fraction between CG and FG was attributed to a higher concentration of hydrogen per unit area (CHgb) of high-angle boundaries (θ > 15°) in CG compared with FG one, which caused a local decrease of stacking fault energy (SFE) at the vicinity of these boundaries in favor of martensitic transformation. Intergranular cracks were identified as the primary form of hydrogen damages, occurring more frequently than transgranular features in both alloys. The drastic hydrogen embrittlement in CG was attributed firstly to higher hydrogen concentration per grain boundary unit area and secondly to the more severe hydrogen cracking in CG than FG resulting from explosive hydrogen-enhanced martensite formation. In the FG , deformation twins and single-variant ε-martensite were the primary mechanisms triggering intergranular crack nucleation. However, they played a dual role in crack propagation, either arresting or facilitating crack growth. They were capable of arresting the crack if suitably aligned with the direction of crack propagation and the tensile axis. Conversely, in CG structure, the intense impingement of multiple-variant ε-martensite grains with each other and with grain boundaries can create severe microstructural stress concentration zones, promoting severe hydrogen-induced cracking.

Topological characteristics of grain boundary networks during severe plastic deformations of copper alloys

The grain boundary character distribution strongly affects the properties of polycrystalline materials. Grain boundaries of similar characters form networks, whose topological invariants can be considered as distribution descriptors. Understanding the evolution of such descriptors during severe plastic deformations (SPD) can elucidate the evolution of properties and underpin the design of processing routes for target behaviour. For topological analysis of grain boundary networks, polycrystalline materials are considered here as polyhedral complexes with grain boundaries classified into two types — low-angle and high-angle. Changes of grain boundary types are calculated using sub-grain rotations, which reflects the physical mechanism of microstructure evolution during SPD. A non-physical approach, by direct conversions of low-angle to high-angle boundaries, is also explored as a reference to demonstrate the impact of the physical constraint imposed by rotations. Reported is the discovery of topological phase transitions in the grain boundary networks which might take place during severe plastic deformations of different copper alloys. Depending on the evolution approach, the transitions correspond to zeros of the Euler characteristic, or of the logarithm of the inverse connectivity, of the grain boundary network. The relations between these transitions and the fraction of high-angle grain boundaries, and between the fraction of high-angle grain boundaries and plastic strain obtainable experimentally, provide new perspectives for grain boundary engineering and network design. Determining the dominant evolution mechanism and critical accumulated strain for a given material and processing route requires further experimental studies of triple junction evolution.

Effect of laser energy density on surface morphology, microstructure and mechanical behaviour of direct metal laser melted 17-4 PH stainless steel

Abstract The surface and microstructural characteristics of 3D printed parts play a significant role under mechanical loading. The authors have explored the effect of laser energy densities on the surface morphology, microstructure and mechanical behaviour of 17-4 precipitation hardened stainless steel fabricated under the direct metal laser melting technique. The considered processing parameters were laser energy density and its technical parameters: laser power, layer thickness, hatch spacing and scanning speed. The mechanical and metallurgical properties of the as-printed samples appeared better than the wrought counterpart due to the higher densification level (99.74 %) induced by the rotating scanning strategy. X‐ray diffraction revealed the presence of both the martensitic α phase and austenitic γ phase in the as-printed sample. There is no significant anisotropy in the mechanical behaviour as the build direction has a random texture with a fine columnar grain structure. The high laser energy density with low layer thickness results in an excellent surface finish. The tensile strength (1180 MPa) and the elongation for the as-printed sample (45.0 %) were considerably more significant than that for the wrought sample (1160 MPa and 26.0 %), which is attributed to the combination of low and high-angle boundaries, as confirmed by the electron backscatter diffraction results.

Microstructural Insights into the Evolution of Ophiolitic Chromite from Luobusha

The podiform chromitite found within the Luobusha ophiolite comprises characteristic nodules and massive chromitites. However, the exact origin of these formations remains a topic of ongoing debate. In this study, the microstructures of olivine and chromite are investigated to unravel their formation processes and shed light on the associated geodynamic mechanisms. EBSD analysis provides insights into chromitite and host peridotite deformation mechanisms. Olivine grains in the host dunite and nodular chromite exhibit crystallographic preferred orientations (CPOs) with D-type fabrics, which show a girdle distribution in the [010] and [001] axes, normal to the foliation plane of the sample. The massive and disseminated chromitite displays B-type and C-type olivine fabric, with a concentration of [001] axes parallel to the lineation of the sample. Crystal plastic deformation can be observed in the Luobusha chromite grains, highlighting intercrystalline deformation processes. Small grains lacking misorientation observed in the massive chromitite are likely attributed to heterogeneous nucleation. Chromite nodules are found to be a patchwork of subgrains with various orientations and high-angle boundary misorientation. The formation of Luobusha chromitite involves deep-seated crystallization, followed by amalgamation, and subsequent deformation within the mantle peridotite. These findings distinguish Luobusha chromitite from other ophiolitic chromite deposits, offering valuable insights into the deformation history and formation processes.

The Correlation of Kissing Bond, Microstructure and Mechanical Properties of the 2195 Al-Li Alloy Friction Stir Welding Joints at Different Welding Speeds

Reasonable welding speeds are a prerequisite for obtaining high-quality joints by friction stir welding (FSW). In this paper, 2195-T8 Al-Li alloy FSW joints were successfully fabricated at different welding speeds (100–600 mm/min) with a constant rotation speed. The effect of welding speed on the microstructure and mechanical properties was analyzed under different experimental methods. Microstructural characterization was conducted using optical microscopy (OM), scanning electron microscopy (SEM), electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM). Mechanical properties were measured by a hardness test and a tensile test. The results showed that the original T1 precipitates disappeared in the nugget zone (NZ), generating many dislocations. With welding speeds increasing, joints obtained at lower welding speeds developed coarser T1 precipitates in the heat-affected zone. Also, the equiaxed grains with a bigger size and a higher fraction of high angle boundaries (HABs) were detected in the NZ of these joints. As the welding speed increased, the area of hardness value changes gradually shrunk, which was consistent with the trend of the cross-section morphology. A kissing bond and macroscopic cracking were observed in the joints that were fabricated at the higher welding speeds. The appearance of those defects significantly reduced the ultimate tensile strength and elongation of the joints at high welding speeds. Fracture morphologies of the different joints were all characterized in quasi-cleavage fractures.

Twinning, dynamic recrystallization, and texture evolution in as-solution AZ80 Mg alloy during hot compression

The characteristics of twins, dynamic recrystallization (DRX), and texture evolution of as-solution AZ80 Mg alloy at different strains, strain rates and temperatures during hot deformation were investigated by EBSD analysis systematically. The activation of twin variants, their interactions, and their role in microstructure and mechanical response are emphasized. The results show that: a large number of {10–12} tension twins and a twin “cross” structure containing three types of twin-twin boundaries (TTBs) were formed at a strain of 0.10, a strain rate of 0.1 s−1, and a temperature of 300 °C. Meanwhile, the original grain is subdivided by high-angle boundaries (HAGBs) converted from twin boundaries (TBs), and the flow stress curve exhibits a feature of concave curvature at a higher strain rate and a lower temperature. As the strain increases, the dominant DRX mechanism is converted from continuous DRX (CDRX) to discontinuous DRX (DDRX) after peak stress, while particle stimulation nucleation (PSN) occurs throughout the process. The strong base texture is weakened by twinning and DRX, where twinning deviates from the parent grain by 86° and DDRX grains deviate from the parent grain by 5°–20°. The diagram of relationships between flow stress and microstructure evolution and texture is established.

Structural Evolution of 10% Cr–3% Co Steel Microalloyed with Re and Cu during Creep Au 923 К

Structural evolution of the tempered lath martensite of the 10% Cr--3% Co steel microalloyed with rhenium and copper with a low nitrogen content and a high boron content during creep at 923 K was investigated for the purpose of establishment of the rison of decrease in creep resistance of this steel under the low applied stress. The tempered martensite lath structure of 10%Cr-3%Co steel with an average lath size of 370 nm and a high dislocation number density of 2 ×1014 m–2 was observed after normalizing at 1323 K with the following tempering at 1043 K for 3 h. The structure was stabilized by M23C6 carbides, M6C carbides, and NbX carbonitrides. During long-term creep, the lath structure strongly experienced an evolution: the width of the martensitic laths increased significantly, dislocation density decreased, the Laves phase and Cu-enriched particles remarkably coarsen. Such structural evolution correlates with an appearance of creep strength breakdown on curves “Applied stress vs. Time to failure” and “Minimum creep rate vs. Applied stress”. Significant coarsening of the Laves phase particles and Cuenriched particles via formation of the large particles with sizes more than 250 nm along high-angle boundaries and full dissolution of the fine particles with sizes less than 50 nm along low-angle boundaries of martensite laths is considered to be the main cause of degradation of the creep resistance of the steel studied.

Role of alloy composition on micro-cracking mechanisms in additively manufactured Ni-based superalloys

Inherent micro-cracking mechanisms in two contrasting high-gamma-prime (high-γ’) Ni-based superalloys processed by laser powder bed fusion (LPBF) were investigated. RENÉ 65 (R65) has a 42% γ’ volume fraction including Al, Ti, and Nb, while RENÉ 108 (R108) has a higher γ’ volume fraction (63%) including Al, Ta, and lesser Ti. Quantitative analysis showed R108 exhibits 329% higher micro-cracking density than R65. All micro-cracks propagate along high angle boundaries (HABs) and exhibit interdendritic morphologies suggesting solidification cracking is the dominant micro-cracking mechanism. Moreover, secondary phases detrimental for solid-state cracking are not observed at the HABs. Atom probe tomography (APT) showed preferential segregation of Hf, Mo, C, B at the R108 HAB and Zr, Ti, Mo, C, B at the R65 HAB. The Kou solidification cracking criterion showed that Hf and Zr partitioning along the grain boundaries increases micro-cracking susceptibility. Gamma prime did not form during the LPBF process and titanium has a higher tendency than tantalum to partition in the last liquid to solidify.

An investigation into the reverse transformation of ferrite to austenite during friction stir processing of a duplex low-density steel

The reverse transformation of ferrite-to-austenite during friction stir processing of a duplex lightweight steel resulted in an abrupt decrease of the ferrite volume fraction within the stirred zone. This was explained in terms of diffusion-governed phenomena from the thermodynamic points of view. However, owing to the recorded thermal history of the process and the low exposure time at the maximum temperature, the probability of shear assisted diffusion was considered. The fraction of high angle and low angle boundaries within the stirred zone was extremely higher than those observed in thermomechanically affected zone or heat affected zone. These high/low angle boundaries acted as preferred diffusion paths and provided a proper condition to increase the lattice diffusion. The results of electron probe micro-analysis indicated that the alloying elements remained relatively un-partitioned in long range order. In addition, the grains were arbitrarily sheared towards the shear surface/ direction and volume fraction of shear texture component was substantially higher than that of recrystallization texture component. The above observations suggest that the abrupt drop in the ferrite content at the vicinity of the rotating tool in the shear layer may be also induced by shear assisted massive transformation.