Low Boundaries Research Articles

Evolution of the Lamellar Pearlite Structure of Rail Steel under Tension

In this paper, the properties of rail steel subjected to fracture under uniaxial tension deformation are studied. Mechanical properties of rail steel, defective substructure of lamellar pearlite, and fracture surface of rail steel are investigated using methods of modern physical materials science.
 Tensile strength is found to vary from 1247 to 1335 MPa, and the fracture strain of the samples changes from 0.22 to 0.26. The deformation of the rail steel is accompanied by the process of breaking the ferrite plates into fragments by low angle boundaries along with the significant increase in the scalar dislocation density to 7.9-1010 cm-2 (the scalar dislocation density of the original rail steel is 3.2 -1010 cm-2). Also, there is a formation of internal stress fields appearing in the form of bend extinction contours. The sources of stress fields are identified. It is revealed that cementite plates are fractured by cutting and dissolution with subsequent transfer of carbon to ferrite plates by moving dislocations and formation of round-shaped nanodimensional (8.3 nm) round shaped particles of tertiary cementite. It is shown that the dissolution of cementite plates is accompanied by fragmentation (into coherent scattering regions with an average size of 9.3 nm).

Electrochemical corrosion behavior in sulfuric acid solution and dry sliding friction and wear properties of laser-cladded CoCrFeNiNbx high entropy alloy coatings

316 stainless steel (SS) was coated with CoCrFeNiNbx (x: atomic ratio, x = 0, 0.1, 0.2, and 0.3) high entropy alloy coatings (HEACs) by laser cladding (LC). The impacts of Nb content on the phase composition, microstructure, microhardness, wear resistance and corrosion properties of CoCrFeNi based HEACs were examined. Different from the canonical dendritic (DR) - interdendritic (ID) microstructure presented by the other three HEACs, the ID of Nb0.3 HEACs also existed nanoscale Nb-rich Laves phase composed of NbNi and NbCo. The inclusion of Nb significantly reduced the average grain size of CoCrFeNi based HEACs from 59.43 μm to 11.38 μm. It also greatly diminished the formation of high angle grain boundaries (HAGBs), increasing the percentage of low angle grain boundaries (LAGBs) from 31 % to 61 %. The microhardness of Nb-containing HEACs was significantly promoted due to the solid solution strengthening and fine grain strengthening caused by large size Nb atoms. Affected by Laves phase with high coordination number and space filling ratio and higher proportion of LAGBs, the microhardness of Nb0.3 HEAC was even as high as 530 HV. The increase in Nb concentration also strengthened the HEACs' resistance to wear. Nb0, Nb0.1, Nb0.2, and Nb0.3 HEACs had corresponding specific wear rates of 0.36, 0.32, 0.23, and 0.20 mm3/Nm. Unlike 316 SS, Nb0 and Nb0.1 HEACs, which had both adhesive wear and abrasive wear, Nb0.2 and Nb0.3 HEACs were mainly wear by abrasive wear. The corrosion behavior of CoCrFeNiNbx HEACs in 0.5 M H2SO4 solution was similar to 316 SS, but the corrosion resistance was stronger than 316 SS. Nb0.3 HEAC with the Ecorr of −164 mV and Icorr of 6.40 × 10−7 A/cm2 showed the best acid corrosion resistance, which could be attributed to the stable and dense passivation film on its surface. Distinct from the intergranular corrosion of other coatings, the corrosion of Nb0.3 HEAC mainly occurred in DR, because the FCC phase acted as the anode site in it when FCC and Laves phase co-existed.

Overcoming the strength-ductility trade-off in additive manufactured AlSi10Mg plates by friction stir welding at a low rotational speed

The main problem in post-treatment of laser powder bed fusion (L-PBF) AlSi10Mg is the sharp reduction in strength when the ductility enhances, i.e. the problem of strength-ductility trade-off. Friction stir welding (FSW) at a low rotational speed of 200 rpm was employed to join AlSi10Mg plate produced by laser-powder bed fusion in conjunction with overcoming the strength-ductility trade-off. The microstructure, texture, and mechanical properties of the as-built and friction stir welded samples were examined. FSW resulted in the elimination of porosities, formation of ultrafine-grains with an average grain size of 0.7 μm, increment in dislocation density and Taylor factor, and fragmentation of Si particles. In addition, after FSW, B/B¯ and C shear texture components were mainly formed in the center of the stir zone, while at the shoulder-dominated zone the A1∗(111)[1¯1¯2] and A2∗(111)[112¯] components might be formed. The main grain structure formation mechanism was continuous dynamic recrystallization occurring by a gradual increase in misorientation of low angle grain boundaries and geometrically dynamic recrystallization. Consequently, tensile elongation was enhanced by ≈173%, while a negligible decrease in strength (≈9%) was observed. During tensile loading, the joint was fractured from the stir zone by the initiation of cracks from the surface of the sample, resulting in a bimodal dimple structure.

An experimental and numerical study on strain controlled fatigue response of TRIP steel under influence of hydrogen

The present work aims to study the influence of hydrogen charging on the transformation induced plasticity steel under strain-controlled low cycle fatigue (LCF). Hydrogen charging had detrimental effect on strain-controlled LCF lives. Cyclic hardening in un-charged specimen due to strain-induced martensitic transformation (SIMT) dominated over cyclic softening. In contrast, generation of large fraction of low angle grain boundaries in hydrogen charged specimen leads to cyclic softening and supresses cyclic hardening due to SIMT. Additionally, Cyclic plasticity modelling using Ohno-Wang kinematic hardening model is validated by simulating cyclic stress–strain hysteresis loop and cyclic softening curve of uncharged and hydrogen charged specimens.

Examination of the Effect of Cooling Rate after Extrusion to Formability of 6061 Automotive Profiles

As part of the present project, an inquiry is being conducted into the impact of the cooling rate subsequent to extrusion on both the mechanical characteristics and microstructure of 6061 alloy extruded profiles tailored for application in the automotive industry. Water quenching, air cooling, and step-cooling (combination of air cooling and water quenching) were performed after a solution heat treatment for simulating different cooling processes on the exit of the extrusion press. Microstructure examination was performed after artificial aging for every cooling method accompanied by three-point bending and tensile testing for investigation of differences in formability characteristics in each one of the three cases. Electron fractography, texture, and grain boundary misorientation analysis consisted the main analytical techniques, allowing the correlation between grains orientation resulting from the extrusion process with cracking initiation behavior in mechanical testing and for the determination of the regions which were more prone to cracking. From the examination, the positive role of rapid cooling for improved formability was highlighted. Through the grain boundary misorientation analysis and the formation of Taylor factor maps, it was shown that crack initiation preferably took place at subsurface regions even though “roughening” of the bent surface was obvious and expected to lead to crack initiation in the more ductile samples. Considerable amounts of LAGBs (Low Angle Grain Boundaries) (14.7%) and SGBs (Subgrain Boundaries) (4.5%) were detected in the sample which was subjected to step cooling accompanied by an outer and inner surface layers (surface zone) of 200–250 μm thicknesses exhibiting different orientations. The results of this project will be used for optimization of the automotive extruded profiles production process, ensuring improved mechanical performance and resistance to premature fracture.

Effect of multiple thermal cycling on the microstructure and microhardness of Inconel 625 by high-speed laser cladding

Establishing the relationship between thermal cycling and the induced evolution of microstructures and cladding layer properties is crucial to achieving microstructural and performance control, given the complexity involved in high-speed laser cladding process. We hereby report our new findings from laser cladding of Inconel 625 on Q245R steel substrate using a high-speed laser cladding technique. The microstructure evolution and microhardness variations in the first cladding track with different thermal cycles were examined by XRD, SEM, EBSD, TEM, and microhardness tests in detail. Numerical simulation was also conducted to characteristically illustrate the complex thermal cycles during the laser cladding process. Our work revealed that the first six thermal cycles resulted in significant variations of microstructure in the laser cladding layers. The dislocation density and low angle grain boundaries (LAGBs) of the first cladding track decreased after two thermal cycles due to the high peak temperature (above 1000 °C) of thermal cycles and subsequently increased when subjected to further thermal cycles with low peak temperature (below 500 °C). The microhardness value increased from 220 HV0.2 to 320 HV0.2, mainly caused by the formation of precipitated phase during the first four thermal cycles, and through by the enhancement of dislocations after six thermal cycles.

Elucidating the deformation behavior of as-cast B2 CoTi intermetallic using in-situ tensile testing

This study deals with microstructural aspects of the deformation behavior of as-cast homogenized CoTi intermetallic by using in-situ tensile testing coupled with EBSD analysis. CoTi exhibits excellent wear resistance and noticeable room temperature ductility. Therefore, it is commonly used as coating material or a reinforcing phase in superalloys. In-situ study of this intermetallic is pertinent as it will provide information about the microstructural evolution during deformation under tensile loading. During the initial stage of deformation, the plastic strain localization was observed to occur at softer grains due to the activation of {011}⟨100⟩ slip systems, which resulted in the accumulation of GNDs and increase in fLAGBs (fraction of low angle grain boundaries). However, fLAGBs did not increase significantly at higher strains. In addition, the contribution of texture towards deformation behavior was found to be weak. The intermetallic exhibited a mixed-mode failure, where both transgranular and intergranular features were observed on the fracture surface. A slightly higher strain to failure in the intermetallic has been attributed to both the increase in the GNDs density at the initial stage of deformation and increase in SSDs at the later stage of deformation, leading to strain hardening.

Atomic diffusion, segregation, and grain boundary migration in nickel-based alloys from molecular dynamics simulations

Grain boundary diffusion and metal mobility in alloys control material performance in many applications and yet remain poorly understood at a mechanistic level. With advances in accessible time and length scales for computational molecular simulations, and recent force field developments, we now possess tools to help unravel those mechanisms. Using large-scale molecular dynamics simulations, here we examined vacancy-mediated diffusion processes in Ni-5Cr alloy with low and high-energy grain boundaries. We show that atomic diffusion inside the grain boundary plane is about four times higher than bulk diffusion, at any temperature, and exhibits a typical Arrhenius behavior with a very small energy barrier (∼0.8 eV for Cr and 0.7 eV for Ni within 1300–1600 K). Additionally, the fastest diffusing species inverts; Cr diffusion was faster than Ni in the bulk but slower in the grain boundaries. This is attributed to the creation of high cohesive energy clusters of Cr at the grain boundary. Grain boundary migration was also observed to be temperature dependent and appears to be two times higher in the 5% Cr alloy than in pure Ni, highlighting the important role of the alloying element on grain boundary motion.

Analysis of the Tensile Deformation Behaviors and Microstructure Characterization under Various Temperatures of MarBN Steel by EBSD

The uniaxial tensile behavior of MarBN steel with a constant strain rate of 5 × 10-5 s-1 under various temperatures ranging from room temperature to 630 °C was analyzed. This study aimed to identify the effect of the temperature on the tensile behavior and to understand the microstructure deformation by electron backscatter diffraction. The tensile results showed that the yield and ultimate tensile strength decreased with increasing temperature. Serrated flow was observed from 430 °C to 630 °C. The electron backscatter diffraction analysis showed that the low-angle grain boundaries decreased at the medium deformation and increased at the maximum deformation. In contrast, they decreased with increasing temperatures. In addition, the number of voids increased with the increasing plastic strain. As the strain increased, the voids joined together, and the tiny cracks became larger and failed. Three mechanisms were responsible for the tensile deformation failure at various temperatures: grain rotation, the formation and rearrangement of low angle grain boundaries, and void nucleation and propagation. Finally, the formation of the low-angle grain boundaries and voids under different degrees of deformation is discussed.

Effects of bimodal basal texture on bending stress and microstructure evolution of Mg−2.6Nd−0.55Zn−0.5Zr alloys

AbstractThe extrusion direction (ED) and transverse direction (TD) samples were cut from Mg−2.6Nd− 0.55Zn−0.5Zr alloy sheet, and the effects of bimodal basal texture on the bending stress and microstructure evolution were investigated. Results showed that the ED sample with weak basal texture possessed much lower bending stress than the TD sample. This anisotropy is attributed to the role of different textures on the evolution of deformation mechanisms and microstructures. For the ED sample with weak basal texture, the dominant deformation mechanisms were basal slip in the majority with minor supplement of tension twin in the inner region while basal slip and prismatic slip in the outer region. In comparison, the TD sample with strong basal texture activated more tension twin in the inner region and more prismatic slip in the outer region. The activation of tension twin promoted the development of 〈0002〉//compression direction type texture with the confined ±30° angle range in the inner region and reduced the emergence propensity of high dislocation density and low angle grain boundaries. The massive activation of tension twin allowed the TD sample to maintain a high strain-hardening capacity, which was the fundamental reason for the high bending stress in this sample.

Sc/Zr ratio-dependent mechanisms of strength evolution and microstructural thermal stability of multi-scale hetero-structured Al–Mg–Sc–Zr alloys

• A tensile strength of ∼605 MPa and ∼10% elongation were obtained in Al–7Mg–0.3Sc–0.1Zr. • The strength-ductility synergy owes to a multi-scale heterogeneous microstructure. • While Al–7Mg–0.1Sc–0.3Zr had higher thermal stability at ≥ 400 °C than high-Sc/Zr ratio alloys. • High-content Zr atoms are helpful in enhancing precipitation and coarsening resistance of Al 3 (Sc,Zr). Microstructure and its thermal stability are critical in the development of high-performance Al–Mg alloys. Here, we attempt to tailor Al 3 (Sc,Zr) precipitates and thus microstructure characteristics to manipulate mechanical properties and microstructural stability of Al–7Mg alloys fabricated by hot extrusion combined with two-pass hard-plate rolling via changing Sc/Zr ratio. Increasing Sc/Zr ratio leads to improved strength without any loss of ductility. A strength-ductility synergy, i.e. yield strength of ∼548 MPa and ultimate tensile strength of ∼605 MPa with an impressive ductility of ∼10% elongation was achieved in the Al–7Mg–0.3Sc–0.1Zr alloy. The good strength-ductility synergy is ascribed to the multi-scale heterogeneous microstructure promoted by the high Sc/Zr ratio, i.e. a bimodal grain structure, profuse low angle grain boundaries, dispersed nano-sized Al 3 (Sc,Zr) precipitates coexisting with intragranular Mg-Zr co-clusters segregated at dislocations. Upon thermal exposure, the Al–7Mg–0.3Sc–0.1Zr alloy maintained higher hardness at below 250 °C, whereas Al–7Mg–0.2Sc–0.2Zr and Al–7Mg–0.1Sc–0.3Zr alloys exhibited higher hardness in moderate- and high-temperature range of 250-350 °C and ≥ 400 °C, respectively. Atom-probe tomography analysis illustrates that slow-diffusing Zr atoms enhance Al 3 (Sc,Zr) coarsening resistance through forming a higher-content Zr-enriched protective shell around a Sc-enriched core in Al–7Mg–0.1Sc–0.3Zr. Meanwhile, the high Zr content promotes concurrent Al 3 (Sc,Zr) precipitation during thermal exposure at high temperatures. The improved microstructural thermal stability in Al–7Mg–0.1Sc–0.3Zr alloy is further discussed in terms of the recrystallization resistance and grain growth behavior. The present study reveals the feasibility for designing high-strength and thermally stable hetero-structured Al–Mg–Sc–Zr alloys via tailoring Sc/Zr ratios for different application temperature ranges.

Mechanism of continuous dynamic recrystallization of Ti−6Al−4V alloy during superplastic forming with sub-grain rotation

AbstractThe continuous dynamic recrystallization (CDRX) mechanism of Ti−6Al−4V alloys during the superplastic forming (SPF) was investigated, aiming to clarify the dominant mechanism for various misorientation evolution processes. The tensile tests were designed at 890, 920, and 950 °C with 10−4−10−2 s−1, the electron back-scattered diffraction (EBSD) observations and constitutive model were applied to describing the microstructure evolution. The geometrically necessary dislocations (GNDs) are responsible for the low angle grain boundaries (LAGBs) evolution. Furthermore, the mechanism of the transformation from LAGBs to high angle grain boundaries (HAGBs) includes: (1) the migration from HAGBs to LAGBs; (2) the remaining dislocations after dynamic recovery will move to the LAGBs; (3) the sub-grain rotation also promotes the grain nucleation. Based on the physical constitutive model, the sub-grain rotation is verified just before the transformation from LAGBs to HAGBs. Particularly, the dislocation motion is dominant before the misorientation reaches the saturated value during the grain boundary misorientation evolution, otherwise, the sub-grain rotation will be activated.

Effect of welding speed on the microstructure and texture development in the individual weld zone of friction stir welded DP780 steel

In this study, microstructural and texture development in the individual weld zone during friction stir welding (FSW) of DP780 steel at similar tool rotation speed (200 rpm) different welding speeds (200 mm/min and 400 mm/min) was investigated. Phase transformations and competition between recovery and recrystallization mechanisms in the distinct weld zones were explored by electron back scattered diffraction (EBSD). More pronounced grain refinement was observed in stir zone (SZ) of sample welded at higher speed (0.62 μm) as compared to lower welding speed (0.51 μm) which is attributed to the enhanced Zener pinning effect at higher welding speeds. Continuous dynamic recrystallization (cDRX) was found to be the dominating recrystallization mechanism in SZ of both welded samples. Orientation distribution function (ODF) based texture investigations of SZ revealed the development of D1 {1¯1¯2}<111> and D2 {112¯}<111> shear components along with the cube {001}<100> texture. The lower texture intensities witnessed in SZ of higher welding speed sample was due to the accelerated recrystallization kinetics at higher welding speeds. The microstructural investigations of thermo-mechanically affected zone (TMAZ) of both welded samples revealed that cDRX was predominant recrystallization mechanism while microshear bands, grain boundary bulging and geometric DRX (gDRX) also assisted the grain refinement. Texture analysis showed the development of D2 {112¯}<111>, J{110}<11¯2> and J¯{1¯1¯0}<1¯12¯> shear components in the TMAZ lower welding speed sample and D1 {1¯1¯2}<111>, D2 {112¯}<111>, J{110}<11¯2> and J¯{1¯1¯0}<1¯12¯> shear components in the TMAZ of higher welding speed sample. Moreover, heat affected zone (HAZ) of both samples showed higher evolution of low angle grain boundaries (LAGBs) indicating that the grains are in the dynamic recovery stage due to lower temperatures and strain.

Significance of Melt Pool Structure on the Hydrogen Embrittlement Behavior of a Selective Laser-Melted 316L Austenitic Stainless Steel.

The hydrogen embrittlement (HE) behavior of a selective laser-melted (SLM) 316L austenitic stainless steel has been investigated by hydrogen charging experiments and slow strain rate tensile tests (SSRTs) at room temperature. The results revealed that compared to the samples without H, the ultimate tensile strength (UTS) and elongation (EL) of specimens were decreased from 572 MPa to 552 MPa and from 60% to 36%, respectively, after 4 h of electrochemical hydrogenation with a current density of 100 mA/cm2. The negative effects of hydrogen charging were more pronounced on the samples' ductility than on their strength. A quasi in situ EBSD observation proved that there was little phase transformation in the samples but an increased density of low angle grain boundaries, after 4 h H charging. After strain was applied, the surface of the H-sample displayed many hydrogen-induced cracks along the melt pool boundaries (MPBs) showing that these MPBs were the preferred areas for the gathering and transferring of hydrogen.

Accurate segmentation of lung nodule with low contrast boundaries by least weight navigation

Accurate segmentation of lung nodule with low contrast boundaries by least weight navigation

Compressive properties of a severely deformed fine-grained Mg−6Gd−3Y alloy

An extruded Mg−6Gd−3Y alloy was processed by 6 passes of simple shear extrusion (SSE) at 553 K to refine the microstructure. The microstructural studies were performed through electron back-scattered diffraction (EBSD) and also transmission electron microscopy (TEM) to investigate the microstructural features which emerged after SSE. It was revealed the SSE-processed alloy had a duplex microstructure containing both fine dynamically recrystallized (DRX) grains and some large deformed grains. A relatively high fraction of low angle grain boundaries (LAGBs), induced within the un-DRXed deformed grains, was identified by EBSD results. TEM studies revealed the formation of round Mg5Gd-type nano-particles through dynamic precipitation only in the DRXed regions of microstructure in the SSE-processed alloy. Compressive mechanical properties and strain hardening behavior of the alloys were assessed by compression testing of both alloys at room temperature. SSE processing resulted in enhanced yield stress and reduced ductility. Both the extruded and SSE-processed alloys exhibited a three-stage strain hardening behavior with a much more pronounced stage II of hardening in the latter one. According to the mechanical properties results, there was a noticeable difference in the strain hardening rate and also dislocation storage rate of the extruded and SSE-processed alloys. This discernible strain hardening behavior of the two alloys was attributed to the formation of fine DRXed grains, dynamic precipitation of nano-sized Mg5Gd particles, and induction of LAGBs, which emerged after SSE.

Grain boundary and triple junction characteristics analytics of additive manufactured Inconel 625 superalloy using selective laser melting

Selective laser melting (SLM) is a rapidly developing technology with many advantages over traditional manufacturing methods. SLM is gaining popularity in many industries due to its potential to revolutionise designs and efficiency. To facilitate more widespread adoption of the technology, it is vital to understand the microstructure characteristics of materials produced by it. In this study, IN625 superalloy were produced by SLM are analysed by microstructural characteristics including grain boundaries and triple junctions and mechanical properties by hardness. Electron backscatter diffraction (EBSD) and data analysis were employed to investigate the effect of SLM on grain structures, crystal orientation distribution, grain boundary characteristic distribution (GBCD) and triple junction characteristic distribution (TJCD) in IN625. Investigation of microstructure characteristics revealed that there is increase in the volume of grain boundaries and dislocation barriers perpendicular to the build direction. The grain boundaries in the SLM manufactured IN625 superalloy were found to be mostly low angle (<15°) and further investigation on triple junctions revealed that the majority of triple junction's present being the preferred type of triple junctions where two or more low angle or coincidence site lattice (CSL) grain boundaries meet. High volume of low angle boundaries, anisotropic microstructure and preferred type of triple junctions found in SLM manufactured IN625 leads to increase in the failure resistances when compared to conventional manufactured IN625. These microstructures were anisotropic in nature which leads to anisotropic mechanical properties.

Effect of molybdenum partitioning on the evolution of microstructure, grain boundary constitution and corrosion behaviour of electrodeposited zinc-molybdenum coatings

Effect of compositional partitioning on microstructural evolution and corrosion behaviour of Zn-Mo coatings (Mo: 2.8 – 6.5 wt%) electrodeposited on mild steel substrate has been explored. Corrosion resistance of Zn coating increased with Mo addition until an optimum addition (Zn-4.2 wt% Mo) beyond which it decreased for higher Mo content (Zn-6.5 wt% Mo). The corrosion current density (icorr) values for pure Zn, Zn-2.8 wt% Mo, Zn-4.2 wt% Mo, Zn-6.5 wt% Mo coating was 17 μA/cm2, 12.25 μA/cm2, 7.5 μA/cm2, 14.2 μA/cm2 respectively. Incorporation of Mo resulted in the formation of Zn-Mo solid solution and Zn3Mo2 compound phases along with segregation of Mo at Zn grain boundaries. High corrosion resistance of Zn-4.2 wt% Mo coating was due segregation of Mo at Zn grain boundaries, higher fraction of low angle grain boundaries and coincidence site lattices when compared to the pristine Zn coating. Lower corrosion resistance of Zn-6.5 wt% Mo coating was primarily due to higher coating strain because of the solid solution of Mo in Zn and formation of Zn3Mo2 particles in the coating matrix.

The effect of build orientations on mechanical and thermal properties on CuCrZr alloys fabricated by laser powder bed fusion

Laser powder bed fusion (LPBF) is a kind of popular additive manufacturing process to fabricate sophisticated components in recent years. However, it is still challenging to fabricate copper alloys due to excellent thermal properties and high reflective to laser beams during LPBF process. As such, green laser beam has been employed in present work to fabricate copper-chromium-zirconium (CuCrZr) alloys in present work. To enhance performance, post-heat-treatment was employed on the alloys. The mechanical properties in present work are superior to most of LPBF-built CuCrZr alloys using conventional infrared laser. So far, most of studies about additive manufactured (AM-ed) CuCrZr alloys only focus properties of alloys perpendicular to build directions. Hence, the mechanical properties and thermal properties of LPBF-built alloys along different build orientations were investigated. The slender columnar grains were observed. Nevertheless, heat-treated samples along transverse directions own better mechanical properties (yield strength (YS): 505 MPa, ultimate tensile strength (UTS): 585 MPa, Elongation (EL): 14.4%) and better thermal properties (307 W/mK). While the heat-treated samples along build directions own lower strength (YS: 472 MPa, UTS: 495 MPa) with better ductility (EL: 17.8%) and poor thermal conductivity (255 W/mK). The mechanical and thermal anisotropy attributes the stronger {110} grain orientation and lower fraction of low angle grain boundaries (LAGBs) along build directions due to the columnar structure with fine equiaxed grains.

Bimodal grain structure formation and strengthening mechanisms in Mg-Mn-Al-Ca extrusion alloys

The effects of small additions of calcium (0.1% and 0.5%11All compositions in weight percentage.) on the dynamic recrystallization behavior and mechanical properties of as-extruded Mg-1Mn-0.5Al alloys were investigated. Calcium microalloying led to the formation of Al2Ca in as-cast Mg-1Mn-0.5Al-0.1Ca alloy and both Mg2Ca and Al2Ca phases in Mg-1Mn-0.5Al-0.5Ca alloy. The formed Al2Ca particles were fractured during extrusion process and distributed at grain boundary along extrusion direction (ED). The Mg2Ca phase was dynamically precipitated during extrusion process, hindering dislocation movement and reducing dislocation accumulation in low angle grain boundaries (LAGBs) and hindering the transformation of high density of LAGBs into high angle grain boundaries (HAGBs). Therefore, a bimodal structure composed of fine dynamically recrystallized (DRXed) grains and coarse unDRXed regions was formed in Ca-microalloyed Mg-1Mn-0.5Al alloys. The bimodal structure resulted in effective hetero-deformation-induced (HDI) strengthening. Additionally, the fine grains in DRXed regions and the coarse grains in unDRXed regions and the dynamically precipitated Mg2Ca phase significantly enhanced the tensile yield strength from 224 MPa in Mg-1Mn-0.5Al to 335 MPa and 352 MPa in Mg-1Mn-0.5Al-0.1Ca and Mg-1Mn-0.5Al-0.5Ca, respectively. Finally, a yield point phenomenon was observed in as-extruded Mg-1Mn-0.5Al-xCa alloys, more profound with 0.5% Ca addition, which was due to the formation of (101¯2)extension twins in unDRXed regions.