Fraction Of Twin Boundaries Research Articles

Enhancing the Pitting Corrosion Resistance of Fe‐36Ni Invar Alloy via Introducing Mg

The primary objective of this study is to investigate the corrosion resistance of Fe‐36Ni Invar alloys with varying Mg contents in a 3.5 wt% sodium chloride solution. The electrochemical results reveal that the incorporation of Mg amplified the corrosion behavior of Fe‐36Ni Invar alloy. The inclusion compositions undergo a transformation with the increase of Mg content, evolving from MnO–MnS in 0 Mg alloy to MnO–MnS–MgO in 0.0015 Mg alloy, and ultimately to MnS–MgO–MgS in 0.0030 Mg alloy. During the corrosion process, the small‐sized MnS–MgO–MgS inclusions exhibit greater stability compared to the MnO–MnS inclusions, rendering them less susceptible to attack and dissolution. Adding Mg diminishes the size and number density of inclusions, which effectively decreases the susceptibility to pitting initiation. The introduction of Mg refines the microstructure and elevates the fraction of twin boundaries, which also is responsible for the enhancement of corrosion resistance.

Fatigue mechanisms at 450°C of a highly twined (>70%) and HIP-densified IN718 superalloy additively manufactured by laser beam powder bed fusion

Fatigue resistance at elevated temperatures is crucial for certifying aerospace structures additively manufactured by the laser beam powder bed fusion (PBF-LB) method with IN718 superalloy. This study employed a multi-step, supersolvus heat treatment process with hot isostatic pressing (HIP), called RHSA, to minimize pores and brittle phases. Stress intensity factor (△K) calculations using data from X-ray computed tomography and shape factors referencing finite element analysis (FEA) studies confirmed the suppression of △K below the threshold of conventional IN718 (∼5 MPa√m), shifting fatigue behavior to grain-structure-dominated. Despite a very high twin boundary (TB) fraction (>70%), fatigue tests at 450°C and R = 0.1 demonstrated low scatter. Slip trace analysis and high-resolution electron backscatter diffraction (EBSD) revealed that TB-induced strain concentration became prominent only at high △K, causing cracking at 45⁰ to the loading direction. The randomly oriented TBs with higher angles (60⁰) compared to high-angle grain boundaries (HAGBs) (30–40⁰) likely enhanced slip resistance and provided a net strengthening effect, which can explain the lower-than-average TB% along fracture paths. These insights suggest that a high TB fraction is not detrimental if fatigue stress is not excessive, alleviating concerns about annealing twins during defect minimization in AM IN718, allowing novel processes to improve fatigue resistance in PBF-LB IN718.

Unveiling the Correlation among Microstructure, Texture, Slip System Activity, and Tensile Property in a Hot Rolled Ni Containing Medium‐Mn Steel

The microstructure–texture–tensile property relationships in a Ni‐containing medium‐Mn steel (Ni‐MMS), subjected to limited thermomechanical processing steps (i.e., hot forging and hot rolling), have been established in this study. The microstructure of the specimens hot rolled (HR) at 1173 (HR‐1173K) and 1373 K (HR‐1373K) exhibits ferrite and austenite phases. The austenite phase fraction in the HR‐1173K specimen is found to be higher than the HR‐1373K condition. The volume fraction of austenite to martensite transformation during tensile testing is also observed to be higher in the HR‐1173K specimen (≈13%) than the HR‐1373K condition (≈2%). This suggests the occurrence of more efficient transformation‐induced plasticity effect in the HR‐1173K specimen in comparison to the HR‐1373K condition, resulting in improved strength–ductility synergy in the former specimen. The smaller grain size of both the phases and higher fraction of twin boundaries as well the evolution of higher intensity γ‐fiber <111>//ND in the HR‐1173K specimen leads to better tensile properties. In addition, the overall activation of the primary slip systems (combination of face‐centered cubic and body‐centered cubic phases slip system), estimated through visco‐plastic self‐consistent simulation, is higher in HR‐1173K specimens, resulting in improved strain hardening response as compared to HR‐1373K one.

Effect of hold-type on cyclic life and microstructural evolution of an austenitic stainless steel

The present work investigates the effect of different type of hold on the change in microstructure and cyclic life. i.e., number of cycles to failure of the 304LN grade austenitic stainless steel at an elevated temperature. The strain-controlled low cycle fatigue and creep-fatigue interaction tests were carried out in air at 540 °C for constant total strain amplitude levels of ± 0.5 % and ± 0.7 %. The duration of hold-time was maintained at a constant level of 600 s at peak tensile, compressive and both peak tensile-compressive strain during different creep-fatigue interaction tests. Due to incorporation of creep damage, the cyclic life of the creep-fatigue interaction-tested samples has been found to be lower than that of the low cycle fatigue-tested samples. The tensile-hold appears to have maximum impact on reduction in cyclic life during creep-fatigue interaction tests followed by tension-compression hold and compressive hold. The scanning electron microscopy and electron back scattered diffraction analyses of creep-fatigue interaction-tested samples have revealed that the grain size coarsening, reduction in twin boundary fraction and increase in average Kernel Average Misorientation are the key factors in reduction of cyclic life.

Synergistic interplay between residual stress, hardness, and microstructural evolution in laser peened stainless steel 347

This study investigates the correlation between the properties of stainless steel 347 samples processed using Laser Shock Peening without Coating (LSPwC). While optimizing the process under different surface confinement conditions, the primary focus is on exploring the synergistic interplay between residual stress, hardness, and microstructural evolution for the selected condition. LSPwC with a water confinement layer at 6 GW cm−2 power density, showed significant Compressive Residual Stress (CRS) of −447.50 ± 48.71 MPa at 60 μm, reaching approximately 76 % of the yield stress and the dislocation density at 60 μm depth is 9.830 × 1014 m−2. Additionally, hardness increased to 256.50 ± 14.15 HV0.1 (∼+53 %) at 50 μm depth. Moreover, there was an increase in the grain boundary fraction to 0.3766 (∼+81 %), along with a rise in the twin boundary fraction to 0.1480. At 6 GW cm−2, correlation analysis revealed a strong relationship between compressive residual stress and hardness (correlation coefficient of 0.972), and between hardness and dislocation density (correlation coefficient of 0.968). Consequently, the 6 GW cm−2 condition emerges as the most practical and effective choice, offering substantial compressive stresses, improved hardness, and efficient grain refinement, and thus favours the 6 GW cm−2 power density condition as the optimal choice for LSPwC of stainless steel 347.

Driving force for enhancing the cold silver sinter joining using time domain-dependent Ag–Au epitaxy and interfacial stress

We reveal the mechanism of the cold silver sinter joining process using time-dependent characteristics of Ag–Au interactions grown in the highly (111) orientation. In the initial (∼15 min) state at 175 °C, the Ag–Au inter-diffusion layers developed strongly in the (001) direction, and with increasing time (∼60 min), they switched epitaxial growth in the (111) orientation. The die-bonding strength was significantly improved from 14.9 MPa to 37.8 MPa when the sintering time increased from 15 min to 60 min. The gradual Ag–Au epitaxial growth behavior with a higher stacking-fault energy (SFE) at 175 °C was the dominant factor linearly increasing the cold silver sinter joining strength and ultimately maximizing energy absorption leading to fracture. The surface finish of Au, which can contribute to the time-dependent growth of Ag–Au inter-diffusion layers, underwent a dramatic transformation from a Cube texture to a γ-fiber texture over time. This change was attributed to the micro/nanoscale shear deformation, and reduction of the twin boundary fraction. This study unveiled the in-depth fundamental mechanism for strengthening and pressureless cold sinter joining of Ag–Au epitaxial layers by comprehensively considering this texture behavior, changes in X-ray residual stress, and mechanical properties.

Grain refinement and strain delocalization in TRIP high-entropy alloys during hot deformation

Transformation-induced plasticity (TRIP) high-entropy alloy (HEAs) have shown promising mechanical properties, making them potential candidates for various applications. The preparation of structures with fine grains and heterogeneous orientation is important to improve the strength and ductility of TRIP-HEAs. However, the evolution and mechanism of grain refinement with heterogeneously oriented structures of TRIP-HEAs during thermal deformation and its effect on strain delocalization are not clear. In this paper, the microstructure evolution and strain delocalization of a TRIP-HEA, Co35Cr25Ni15Mn15Fe10 (atomic percent, at %) was examined through hot compression tests conducted at temperatures ranging from 900 ℃ to 1000 ℃, and strain rates ranging from 0.001 s−1 to 0.1 s−1. The results revealed that the average grain size of the alloy decreases with increasing deformation temperature and reducing strain rate due to the appearance of dynamic recrystallization (DRX) and annealed twins. After deformation at 1000 °C-0.001 s−1, the average grain size of the alloy decreases from 169.85 μm (initial sample) to 21.5 μm, and the volume fraction of DRX and twin boundaries (TBs) increases to 81.9 % and 37.6 %, respectively. The DRX mechanism of the TRIP-HEA transformed from discontinuous dynamic recrystallization (DDRX) at 900 °C-0.1 s−1/0.01 s−1 to geometric dynamic recrystallization (GDRX) at 900 °C-0.001 s−1 and then to continuous dynamic recrystallization (CDRX) at 950 °C/1000 °C-0.001 s−1. Furthermore, the occurrence of DRX promotes the formation of annealed twins, resulting high volume fraction of high angle boundaries (HAGBs) and twin-matrix pairs, and contributing to grain refinement. Encouragingly, these HAGBs and twin-matrix pairs not only contributes to grain refinement, but also facilitates strain delocalization via dispersing strain and forming strain gradient, thus leading to coordinated deformation. This result is of great significance for understanding and improving the mechanical properties of TRIP-HEAs, and it can help in the design of high-performance TRIP-HEAs for various applications.

Anomalous twin boundary formation in magnesium alloys by rapid solidification

The aim of this work is to investigate the formation of an anomalously high fraction of twin boundaries (TBs) during laser directed energy deposition (DED) of a Mg-3Al-1 Zn (wt.%) alloy. With that goal, single tracks are deposited using different combinations of scan speed and powder feed rate. The melt pool dimensions are related to the DED parameters. Irrespective of the processing conditions, the resulting microstructures are always formed by fine grains with a very weak texture. However, some DED parameter combinations give rise to a fraction of TBs that is significantly higher than that corresponding to a random texture. These TBs are characterized by an irregular morphology and by the presence of segregated atoms, solute clusters, and nanoparticles. Additionally, the lattice in the vicinity of the TBs is highly distorted and contains a high population of stacking faults. All these characteristics set them apart from conventional tensile TBs, which exhibit a large degree of coherency and limited segregation. Rapid solidification is thus presented here as a novel avenue to design TBs with unique properties in Mg alloys. This work paves the way for future studies on grain boundary design via additive manufacturing methods, a field that is still in its infancy.

Achieving a Columnar-to-Equiaxed Transition Through Dendrite Twinning in High Deposition Rate Additively Manufactured Titanium Alloys

AbstractThe coarse β-grain structures typically found in titanium alloys like Ti–6Al–4V (wt pct, Ti64) and Ti–6Al–2Sn–4Zr–2Mo–0.1Si (Ti6242), produced by high deposition rate additive manufacturing (AM) processes, are detrimental to mechanical performance. Certain modified processing conditions have been shown to lead to a more refined grain structure, which has generally been attributed to a change in the solidification conditions with respect to the experimental Hunt diagram proposed by Semiatin and Kobryn. It is shown that with Wire Arc AM (WAAM) increasing the wire feed speed (WFS) is effective in promoting a columnar-equiaxed transition (CET). Conversely, estimates of the dendrite-tip undercooling using the KGT model suggest that this will be too small for free nucleation without the addition of artificial nucleants, due to the very low solute partitioning in Ti alloys. It is also shown that it is difficult to promote a CET with plasma transferred arc WAAM as computational fluid dynamics (CFD) melt-pool simulations indicate that the solidification parameters remain within the columnar region on the Semiatin-Kobryn Hunt map, within the constraints of a stable process. However, a high fraction of twin boundaries was observed in the refined β-grain structures seen at high WFS. This has been attributed to departure of $$\left\langle {001} \right\rangle _{\beta }$$ 001 β alignment from the direction of maximum thermal gradient, caused by the curvature of the fusion boundary, stimulating dendrite twinning during solidification. In addition, it is shown that increasing the WFS leads to a change in melt-pool geometry and a reduction of remelt depth, which promoted dendrite twinning and grain refinement.

Microstructure and compressive deformation behavior of W-Ni3Al-Co model alloys by two-step sintering

The regulation of microstructure can improve the mechanical properties of tungsten alloys. To regulate the microstructure, the W-Ni3Al-Co alloys with different Ni3Al/Co ratios were prepared using a two-step sintering method, and the microstructure and compressive deformation behavior were investigated. The results showed that the particle sizes of the W-rich phase in the network structure were refined with the increase of the Ni3Al/Co ratio, and a large fraction of twin boundaries were detected in the adhesive phase. The compressive deformation behavior of a W-7Ni3Al-3Co alloy with excellent compressive properties was investigated by the EBSD technique. The results demonstrated that twins in the FCC binder phase hindered the stress concentration in the binder phase and at the interface of the W/binder phases through twin-dislocation interactions. The W network structure underwent the uniform global deformation that restricted grain boundary sliding during deformation and prevented crack extension and consolidation. Twins in the binder phase and the W network structure were both important factors in improving the compressive properties of W-Ni3Al-Co alloys.

Suppression of anti-phase boundary defects in Mn-Al-Ti permanent magnets

Permanent magnets (PMs) based on manganese show significant potential for applications in electric motors and devices as an alternative to Rare-earth PMs (REPMs). The metastable ferromagnetic τ phase of the Mn-Al system has magnetic performance between the REPM Nd-Fe-B magnets and the lower performance ferrite magnets. However, the maximum performance in Mn-Al PMs has not reached its theoretical limit, in part due to challenges in controlling crystalline defects such as anti-phase boundaries (APBs). APBs act as nucleation sites for domain reversal in Mn-Al and negatively affect magnetization by interrupting the L10 ordering, placing Mn atoms into AFM-coupled Mn-Mn configurations. In this study, Ab-initio modeling was used to screen ternary elements based on their affinity to segregate to the APB and on their preference for FM configuration. The ternary element addition of 1 at.% Ti lowered the density of APBs as observed in a transmission electron microscope. The 1 at.% Ti addition was observed to improve (BH)max over the base alloy, by preserving Hci and improving Mr. It also significantly increased the fraction of twin boundaries in the τ phase. AFM behavior that was observed in the base alloy during TC heating experiments (Néel behavior) and high-field VSM measurements (spin-flop behavior) was effectively suppressed with the addition of Ti. Additionally, the Ti addition thermally stabilized Hci at 550 °C, improving τ phase processability. Other candidate elements, Cr, V, and Zr, were modeled to have similar potential for minimizing APBs when added to Mn-Al.

EFFECT OF SOLUTION TREATMENT ON THE MICROSTRUCTURE AND MECHANICAL BEHAVIOR OF THE NICKEL-BASED ALLOY N06625

We have studied the effects of different solution temperatures and holding times on the microstructure and room-temperature mechanical properties of the alloy using methods such as electron backscatter diffraction, hardness testing and tensile testing. The results showed that the average grain size increased significantly as the solution temperature increased, and many annealing twins appeared in the grains. The fraction of twin boundaries reached its maximum value at 1080 °C. The hardness, yield strength and tensile strength of the alloy decreased with an increase in the solution temperature, but abnormal yielding occurred at 1160 °C. The elongation after fracture and the area reduction increased with an increase in the solution temperature. The fracture morphology of the alloy was irregular, conical, accompanied by a large number of ductile dimples, and the material exhibited typical ductile fracture. At the same time, as the solution temperature increased, the number of dimples on the fracture surface continued to increase, the depth of the dimples gradually increased, and the distribution became more uniform.

Multiple cycles of combined cold rolling and short-time annealing on the microstructure and properties evolution of NiAl bronze

In this study, three cycles of combined cold rolling and annealing (CRA) were applied to study the mechanical behavior of NiAl bronze (NAB) alloy. The underlying mechanisms for microstructure and mechanical properties were elucidated. Microstructure characterization and quantitative analysis demonstrated significant morphology changes by CRA. The results showed that large α-Cu phase is compressed, lamellar κIII phase is broken and sphericalized, and κII phase accumulates and forms a hard phase band skeleton. Finally, the microstructure with spheroidized κIII phase embedded in the skeleton of the κII phase is obtained. The optimal ultimate tensile strength of 903 MPa and yield strength of 596 MPa are obtained after one cycle of CRA, which exhibits an improvement of about 40 % and 130 % compared to the as-cast samples, respectively. The best ductility of 18.0 % is obtained after three cycles of CRA, with an improvement of about 75 % over the as-cast samples. The high strength is mainly attributed to the grain refinement and high density of dislocations. Additionally, the higher density of low-angle grain boundaries can promote the formation and growth of twins. The saturation fraction of twin boundaries in experimental alloys of about 66 % is achieved after two cycles of CRA.

Achievement of grain boundary engineering by transforming residual stress in selective laser-melted Inconel 718 superalloy

The complicated geometric parts produced by selective laser melting (SLM) are not amenable to grain boundary engineering (GBE), which is primarily implemented via thermomechanical processing. Herein, the high internal residual stress produced by SLM processing is utilized as stored activation energy for direct GBE in a SLM-fabricated Inconel 718 superalloy. The grain boundary distribution characteristics, grain size, texture, and corresponding mechanical properties of the as-built specimens subjected to different heat treatments were investigated. As the annealing temperature increased, the recrystallized fraction increased, accompanied by an initial increase and subsequent decrease in the number of special grain boundaries. The numerous annealing twin boundaries effectively divided the grains, despite grain coarsening induced by grain boundary migration at a higher temperature. Compared to the as-deposited sample, the yield strength and ultimate tensile strength increased from 707 to 1071 MPa and from 947 MPa to 1271 MPa, respectively, and the ductility increased from 17% to 21% in the sample annealed at 1200 °C for 1 h followed by double aging treatment because this sample had the highest fraction of twin boundaries (64.89%). The “Additive Manufacturing + GBE” strategy based on utilizing the residual stress offers a feasible pathway for broadening the application scope of additive manufactured alloys.

Enhancement of the mechanical properties of textured AZ31 Mg alloy sheets by bimodal-orientation microstructure: Basal orientation and RD-texture

In the present study, a bimodal-orientation microstructure including the basal orientation and RD-texture was prepared by twinning deformation of an AZ31 Mg alloy sheet. Experimental results show that 21%–24% of tension twin boundaries are produced and that the recrystallization content decreased by up to 30.5%–52.0% after hot deformation. The variant selections of island twin and parallel tension twin pairs conform to the Schmid law, while the connected twin pairs and the twin transmitting are controlled by the strain coordination. Twin-free grains are observed because of the accommodation strain controlled by the hard deformation mode. After final post-annealing, the bimodal-orientation microstructure is balanced with a tension twin boundary fraction and recrystallization content of 16.6%–22.2% and 45.0%–74.3%, respectively. Finally, the anisotropy weakening, and improvement of formability are obtained due to the orientation adjustment and work hardening elimination, with the average Lankford values (r‾) decreasing from 2.84 to 0.43 and the IE value increasing from 2.25 to 4.63.

The strain rate history effect in a selective-laser-melt 316L stainless steel

The strain rate history effect in a selective laser melt 316L (SLM-316L) alloy was investigated through quasi-static (10−3 s−1) and high strain rate (1600-3200 s−1) interrupted and reloading compression tests. The specimens pre-tested until about prescribed strains at quasi-static and high strain rates were reloaded dynamically and quasi-statically, respectively. The results revealed that the flow stress depended on strain and strain rate as well as strain-rate history. Quasi-static reloading the dynamically pre-tested specimens until about prescribed strains induced a higher flow stress than the specimens tested quasi-statically. The strengthening was ∼70 MPa at 0.11 pre-strain and decreased as the dynamic test pre-strain was increased due to adiabatic heating. On the other side, reloading the quasi-statically pre-tested specimens dynamically at 0.11 pre-strain resulted in ∼60 MPa lower flow stress than the specimens tested dynamically. The grains of the quasi-statically tested specimens until 0.11 strain were shown to have a lower Taylor factor for twinning and geometrically necessary dislocation density, indicating more potential for twinning than dynamically tested specimen. Although, quasi-statically and dynamically tested specimens were deformed predominantly by the twinning induced plasticity, a higher fraction of twin boundaries was shown microscopically in the dynamically pre-tested specimens until 0.11 pre-strain. This phenomenon of boundary strengthening could be used as a tool of strengthening of SLM-316L alloy at low strains.

Synthesis mechanism of Ni–Co alloy uniting ultrahigh strength with tensile ductility via abrasive-assisted electroforming

Persistent efforts are being made to combine strength and ductility in metallic nanomaterials. We successfully prepared a homogeneous nanocrystalline Ni−27%Co alloy exhibiting ultrahigh strength and sufficient ductility using abrasive-assisted electroforming (AAE) with free ceramic beads. The AAE process permits superior control of the microstructure of nanocrystalline Ni–Co alloy. Our results demonstrated that the structure of the Ni−27%Co alloy (AAE) consists of ultrafine equiaxed nanocrystals with an average grain size of 88 nm and a twin boundary fraction of 24%. The alloy structure results from the combined effect of the addition of Co as an alloying element and the electrochemical and physical action of the moving ceramic beads on the nucleation and growth. The Ni−27%Co alloy (AAE) exhibited superior yield and tensile strength of 1103 ± 47 and 1639 ± 19 MPa, respectively, while having excellent ductility with tensile elongation to failure of 8.5 ± 0.5%. The yield strength of the Ni−27%Co alloy (AAE) was approximately 1.6 times that of the Ni−26%Co alloy (Direct current electroforming, DCE). This result can be attributed to the contribution of strengthening mechanisms that involve frictional stress, dislocation, grain boundary, solid solution, and twin boundary strengthening. Among them, the grain boundary (Hall-Petch) strengthening, arising from grain boundary-dislocation interactions, and twin boundary strengthening, arising from the intersection of the twin-dislocation, account for approximately 70% of the total yield strength. The tensile ductility of the Ni−27%Co alloy (AAE) at ultrahigh flow stress can be preserved by simultaneous strain and strain-rate hardening using the AAE method. This study provides a novel strategy for synthesizing ductile ultrahigh strength materials for applications in microelectromechanical systems, surface coatings, and X-ray telescopes.

Temperature-dependent dynamic compressive properties and failure mechanisms of the additively manufactured CoCrFeMnNi high entropy alloy

CoCrFeMnNi high entropy alloy (HEA) parts were fabricated by laser powder bed fusion (LPBF), and their dynamic compressive properties at different temperatures as well as the resulting microstructures were analyzed. The HEAs showed an unprecedented strength-ductility combination, especially at a cryogenic temperature of 77 K and a high strain rate of 3000 s−1. Under this testing condition, the yield strength (YS) of the HEAs amounted to 665 MPa. Regardless of the testing temperature, the deformation mechanism of all investigated HEAs was dominated by a synergistic effect consisting of deformation twinning and dislocation pile-up around twins. The fraction of twin boundaries and dislocation density within the deformed microstructure of the HEA correlated with the test temperature. At 77 K, the formation of nanotwins together with dislocation slip prevailed and contributed to pronounced twin-twin and twin-dislocation interactions which effectively restricted the dislocation movement and, hence, contributed to a higher YS as well as strain hardening rate in comparison to that of the HEAs at room temperature of 298 K. The LPBF-fabricated HEAs showed unpronounced thermal softening even at a high testing temperature of 1073 K. Continuous dynamic recrystallization was restricted in the HEA because of its inherent sluggish dislocation kinetics and low stacking fault energy.

Effect of grain size on fatigue limit in CrMnFeCoNi high-entropy alloy fabricated by spark plasma sintering under four-point bending

In order to investigate the effect of grain size on the fatigue limit of single-phase equiatomic high-entropy alloys (HEAs); CrMnFeCoNi, fabricated by spark plasma sintering, four-point bending fatigue tests were conducted at stress ratio of 0.1 under ambient condition. The surfaces of fatigued specimen were observed by in-situ optical microscopy, and fatigue crack initiation and propagation behaviors were analyzed by electron backscatter diffraction to elucidate the fatigue fracture mechanism in CrMnFeCoNi alloy with different grain sizes. The grain size of CrMnFeCoNi sintered compacts decreased with decreasing particle size of CrMnFeCoNi powders, and their fatigue limits tended to increase with decreasing grain size due to the Hall-Petch effect. Furthermore, fatigue cracks in the coarse-grained CrMnFeCoNi sintered compact were initiated at the boundary of annealing twins, whereas annealing twin boundaries were not observed near the crack initiation site of the fine-grained one. The fraction of twin boundaries in CrMnFeCoNi alloy tended to decrease with decreasing grain size; therefore, the grain refinement is an effective approach for increasing the fatigue limit of CrMnFeCoNi alloy because of both the Hall-Petch effect and the increase in the fatigue crack initiation resistance.

Twin Boundary Induced Grain Coarsening in Friction Stir Welding of Fine- and Ultra-Fine-Grained Commercially Pure Titanium Base Metals

The mechanical properties of commercially pure titanium can further be improved through the grain refinement processes; however, welding fine-grained materials is challenging due to the grain coarsening in the weld area and hence the weakening of the mechanical properties locally. Meanwhile, friction stir welding is a promising process in which the metallurgical bonding is established through the solid-state mechanical mixing of materials to be welded; no studies have reported friction stir welding of the ultra-fine-grained commercial purity titanium to date. In this research, friction stir welding of fine-grained and ultra-fine-grained commercially pure titanium (1.58 and 0.66 μm, respectively) was conducted. The effect of the microstructural feature of base metals on the microstructural evolution of the stir zone and the feasibility of the friction stir welding process for those materials were discussed. It was found that the fraction of twin boundaries in ultra-fine-grained material was higher than in fine-grained material. It accelerated dynamic recrystallization and recovery in the stir zone, hence inducing the grain coarsening and the loss of ultra-fine-grained structure and character after welding.