Mechanism Of Grain Refinement Research Articles

Unravelling the mechanism of columnar-to-equiaxed transition and grain refinement in ultrasonic vibration assisted laser welding of Ti6Al4V titanium alloy

In this study, the microstructural evolution and mechanical properties of Ti6Al4V titanium alloy welded joints subjected to ultrasonic assisted laser welding were scrutinized, while numerical simulations were employed to explicate the grain refinement mechanism. The simulations indicate that the ultrasonic vibration significantly improves the overall fluidity and temperature of the molten pool. Under the identical condition of laser power and welding speed (1500 W, 1.3 m/min), the presence of 0.2A ultrasonic current yields a more uniform refinement of columnar grains, along with a denser arrangement of acicular martensite. The refinement mechanism can be attributed to the small temperature gradient, cavitation effects, and stress induced by ultrasonic vibration. Notably, the welded joint attains a peak tensile strength of 945.2 MPa under the aforementioned 0.2A condition, distinctly demonstrating the characteristics of ductile fracture. This research further reveals the underlying mechanism of grain refinement in Ti6Al4V alloy laser-welded joints induced by ultrasonic vibration, providing valuable references for optimizing process parameters and improving the quality of such welded joints.

Enhanced heterogeneous nucleation in Al alloys by melt ultrasonic treatment

The mechanisms of grain refinement in the A1050 alloy through melt ultrasonic treatment (UT) and isothermal holding were investigated. The results showed that UT resulted in considerable grain refinement, with refining efficiency tending to stabilize after 15min of holdings. Meanwhile, it was found that UT breaks down natural oxide films into fine γ-Al2O3 particles with {111} or {110} exposed surfaces. The well-defined cube-on-cube orientation relationships between γ-Al2O3 and α-Al verify that γ-Al2O3 dispersoids can act as potent substrates for the nucleation of α-Al grains. Based on experimental observations and theoretic calculations, enhanced heterogeneous nucleation in A1050 alloys can be attributed to the high potency of γ-Al2O3 particles with atomic misfits of ~2% on all exposed surfaces and their increased number density induced by the synergistic effects of cavitation and acoustic streaming.

Dramatically Enhanced Mechanical Properties of Nano-TiN-Dispersed n-Type Bismuth Telluride by Multi-Effect Modulation.

Bismuth telluride (Bi2Te3)-based alloys have been extensively employed in energy harvesting and refrigeration applications for decades. However, commercially produced Bi2Te3-based alloys using the zone-melting (ZM) technique often encounter challenges such as insufficient mechanical properties and susceptibility to cracking, particularly in n-type Bi2Te3-based alloys, which severely limit the application scenarios for bismuth telluride devices. In this work, we seek to enhance the mechanical properties of n-type Bi2Te2.7Se0.3 alloys while preserving their thermoelectrical performance by a mixed mechanism of grain refinement and the TiN composite phase-introduced pinning effect. These nanoscale processes, coupled with the addition of TiN, result in a reduction in grain size. The pinning effects of nano-TiN contribute to increased resistance to crack propagation. Finally, the TiN-dispersed Bi2Te2.7Se0.3 samples demonstrate increased hardness, bending strength and compressive strength, reaching 0.98 GPa, 36.3 MPa and 74 MPa. When compared to the ZM ingots, those represent increments of 181%, 60% and 67%, respectively. Moreover, the thermoelectric performance of the TiN-dispersed Bi2Te2.7Se0.3 samples is identical to the ZM ingots. The samples exhibit a peak dimensionless figure of merit (ZT) value of 0.957 at 375 K, with an average ZT value of 0.89 within the 325-450 K temperature range. This work has significantly enhanced mechanical properties, increasing the adaptability and reliability of bismuth telluride devices for various applications, and the multi-effect modulation of mechanical properties demonstrated in this study can be applied to other thermoelectric material systems.

Mechanism of Grain Refinement Induced by Mn Element in Wire Arc Addition Manufacturing Al-Mg Alloy

Mechanism of Grain Refinement Induced by Mn Element in Wire Arc Addition Manufacturing Al-Mg Alloy

Study on the grain refinement mechanism of the machined surface of Inconel 718

In machining processes, the high temperature and stress can lead to plastic deformations of the workpiece material. Thermomechanical loading has a significant impact on the microstructure of the machined surface, including grain deformation, recrystallization, and refinement. Among these factors, grain refinement plays a crucial role in determining the mechanical properties of the materials. To investigate the mechanism of grain refinement on the machined surface of Inconel 718 at different cutting speeds, the experiments and finite element simulations were conducted. The microstructural characteristics were quantitative analyzed, including grain size, grain boundary distribution characteristics, average orientation (KAM), and geometrically necessary dislocation (GND) density distribution. The mapping relationship between the multi-physical fields (equivalent plastic strain, temperature) during cutting and the microstructural evolution of Inconel 718 was established. The research indicated that the increasing the cutting speed leads to a higher degree of grain refinement. At a cutting speed of 450 m/min, the maximum grain refinement is achieved with an approximately grain size of 2.48 μm. The contribution from plastic deformation (dynamic recuperation) gradually increases while high temperature remains dominant as factors influencing grain refinement. Conversely, the equivalent plastic strain becomes the primary factor contributing to grain refinement, with a gradual increase in the proportion due to dynamic recrystallization at cutting speeds of 250 m/min and 350 m/min.

Controllable reinforcement phase distribution at the grain scale via a simple precipitation process

Due to the inability of Zr to refine Al-containing Mg alloys, achieving pronounced grain refinement in cast AZ31 alloy has been a significant challenge for researchers. Surprisingly, we found that the reinforcement phases demonstrated a regular distribution at the grain scale after adding 0.9 wt% Nd and 0.3 wt% Y: Al2RE inside the grains and Al11RE3 agglomerated near the grain boundaries. Additionally, the grain size of AZ31 alloy was significantly reduced from 258 µm to 65 µm. The mechanisms of reinforcement phase distribution and grain refinement were explored through the combination of transmission electron microscope (TEM) and thermal analysis during the solidification process. Al2RE phase precipitates prior to α-Mg, and exhibits a specific orientation relationship with the matrix: 011̅0Mg∥011Al2RE，0002Mg∥22̅2Al2RE. This suggests that Al2RE serves as potentially effective heterogeneous nucleation sites for α-Mg. Moreover, Al11RE3 phase can restrict grain growth in the later stage of solidification. The tensile results show that a regular distribution of the reinforcement phase at the grain scale can not only effectively alleviate stress concentration, but suppress the formation of coarse twins by inducing grain refinement during alloy solidification process, significantly improving the strength and ductility of the alloy.

Analysis of Microstructure, Texture Evolution, and the Mechanism of Grain Refinement of Commercially Pure Titanium During Cold Rolling: Experiment and Simulation

A comprehensive understanding of the microstructures and texture evolutions during the rolling process is of great significance for the preparations of high‐quality commercially pure titanium (CP‐Ti) sheets. Therefore, in the present work, the effects of rolling reductions on the microstructures and textures of CP‐Ti are investigated by optical microscope, electron backscatter diffraction, and X‐ray diffraction. A crystal plasticity finite element model (CPFEM) is developed to accurately predict the rolling deformation behaviors of CP‐Ti sheets. The results show that the slip and twinning including , compression twins and extension twins are prone to plastic deformations during the whole rolling process. The c‐axes of grains gradually tilt toward the transverse direction and the grains gradually deviate from the original //rolling direction (RD) to //RD. As a result, the mixed textures of and are developed in the specimen. The grain refinements are closely linked with the developments of and twins. In addition, the twin fractions and texture evolutions are well predicted by the CPFEM model. These results documented in this work should not only help to elucidate the microstructures and texture evolutions of CP‐Ti, but also shed important light on improving its properties.

Effect of ultrasonic power on degassing and microstructure of large-scale 7085 aluminum alloy ingots

In this work, five sets of large-scale 7085 Al alloy ingots (diameter 830 mm × length 3600 mm) were manufactured by introducing different ultrasonic powers (0–750 W) in the Direct-Chill (DC) casting process. The difference among the ultrasonic degassing, the α-Al grains, elemental segregation and eutectic phase with ultrasonic treatment were investigated. The results indicated that with the increase of ultrasonic power, the efficiency of ultrasonic degassing was gradually improved, which was decreased from 0.085 cm3/100 g to 0.056 cm3/100 g in ingot center part. The grain size was also dramatically refined, in which the grain refinement rate reached 38.2%, 42.8% with 550 W, 750 W power, respectively. The segregation of alloying elements (Zn, Mg, Cu) in the ingot was significantly suppressed, which the maximum positive deviations ∆C of Zn, Mg, and Cu were reduced by 59.5%, 55.9%, and 60.8%, respectively. In addition, the new Al14Cu2Fe3 (α−Fe) phase was the first to be discovered in the low power state and the coarse eutectic phase was more refined in the high power. The mechanisms of degassing, grain refinement and suppression of macroscopic segregation during ultrasonic casting were further revealed by calculating the acoustic energy density (I).

Strengthening of CoCrNi medium entropy alloy with Ti additions

A series of (CoCrNi)100-xTix (x = 0, 1.0, 2.0, 3.0 and 4.0) medium entropy alloys (MEAs) was manufactured by high-vacuum arc melting in the present study to examine the effects of Ti additions on the microstructures and mechanical properties of the MEAs. For x = 0, 1.0 and 2.0, the alloys remained a single-phase FCC solid solution. The Ti(Ni, Co)3 precipitates of HCP phase formed for x = 3.0 and 4.0. The presence of Ti, which had the larger atomic radius than the other constituent elements, in the solid solution enhanced the lattice distortion and resulted in grain refinement. The addition of Ti led to the enhanced mechanical properties with the ultimate tensile strength and Vickers hardness increased from 866 MPa and 212 HV at x = 0 to 1490 MPa and 409 HV, respectively, at x = 4.0. The strengthening mechanisms of solid solution strengthening, grain refinement and precipitation strengthening were discussed, and the predicted strengthening was compared with measurements.

Mechanisms of macrozone elimination and grain refinement of near α Ti alloy via the spheroidization of the Widmannstätten structure

Regions with sharp local textures, i.e., macrozones, are common microstructural inhomogeneities in titanium (Ti) alloys that significantly reduce the fatigue resistance of Ti alloy parts. In this study, a two-stage thermomechanical operation was developed to eliminate macrozones and optimize microstructure in a near α Ti alloy. Multiscale microstructure characterization and full-field crystal plasticity (CP) modeling were conducted to uncover the underlying mechanisms. Spheroidizing through dynamic recrystallization (DRX) and superplasticity are indispensable ingredients. The alloy exhibited multiple DRX phenomena, which are strongly associated with grain morphologies. The discontinuous DRX and the main spheroidizing mechanism of the equiaxed primary α phase failed to eliminate macrozones but exacerbated microstructure heterogeneities due to the strong anisotropy of the hexagonal close-packed α phase. The Widmannstätten structure (WS), fabricated through the β heat treatment that generated abundant nano-defects and serrated grain boundaries, significantly promoted the occurrence of geometric dynamic recrystallization (GDRX). A complete GDRX of lamellar α grains can generate a sufficiently fine-grain structure to achieve superplastic deformation in the subsequent hot compression at a moderate deformation. The full-field CP simulations considering slip only versus slip + superplasticity confirmed that the dislocation slip dominated deformation aggravated deformation heterogeneity and the formation of macrozones. In contrast, the superplastic deformation suppressed the heterogeneous deformation and randomized the grain rotation, which in turn homogenized the microstructure and eliminated macrozones. Based on these observations, the β heat treatment followed by α hot compression is an effective method to achieve grain refinement and macrozone elimination of Ti alloys.

Subgrain-assisted spontaneous grain refinement in rapid solidification of undercooled melts

The grain refinement mechanism for rapid solidification of undercooled melts is still an open problem even after 60 years of on-going studies. In this work, rapid solidification of undercooled Ni and equi-atomic FeCoNiPd melts was studied and spontaneous grain refinement was found at both low and high undercooling. After a detailed electron backscattered diffraction analysis, subgrain-induced grain orientation scattering and splitting were found to occur along with the transition from coarse dendrites to fine equiaxed grains at low and high undercooling, respectively, indicating that subgrains play an important role during the formation of fine equiaxed grains. On this basis, a compromise mechanism of subgrain-assisted spontaneous grain refinement was proposed. Because the dendrite re-melting induced thermo-mechanical process and fluid flow induced dendrite deformation occur simultaneously during the post-recalescence stage, stress accumulation would be maximum at both low and high undercooling, thus inducing dynamic recrystallization, during which the formation and rotation of subgrains make the grain orientations scattering and even splitting. Furthermore, the grain/subgrain size of undercooled FeCoNiPd ascribing to its unique co-segregation behavior keeps almost invariable from low to high undercooling, indicating that the co-segregation strategy would be effective to inhibit grain growth after rapid solidification and would be useful in practice.

Mechanical properties and microstructure revolution of vibration assisted wire arc additive manufacturing 2319 aluminum alloy

In this study, a hybrid wire arc additive manufacturing (HWAAM) process that combines low-frequency vibration with cold metal transfer (CMT) arc is proposed to address the problems of porosity, coarse grains, and low mechanical performance encountered in WAAM aluminum alloys. The effects of different vibration frequencies on the microstructure and mechanical properties were investigated. The results indicated that as the vibration frequency increases, the grain size refinement reaches a maximum of 19.1% compared with the non-vibrated specimen. Vibration can enhance the fluidity and the cooling rate of the weld pool. The fatigue fracture of dendrite arms is the main mechanism of microstructure grain refinement caused by vibration. The dynamic bending stress induced by vibration on the dendrite arms and the root remelting phenomenon are the dominant factors for the dendrite fragmentation. Increasing the vibration frequency did not result in an obvious improvement in the tensile strength of the specimens, but did significantly increase the elongation. With the increase in vibration frequency, the maximum transverse and longitudinal elongation reached 22.2% and 23.5% respectively, a raise of 15.6% and 38.2% compared with non-vibrated specimen. Vibration reduced the size and distribution density of pores on the fracture surface, and increased the dimpled microstructure, improving the mechanical performance of the aluminum alloy.

Grain-refining mechanism in hypereutectic Al-20Si alloy with minor Sr - Sc - La and ultrasonic vibration treatment

The grain-refining mechanism with minor Sr - Sc - La and ultrasonic vibration treatment (UVT) in the hypereutectic Al-20Si alloy were studied. The results demonstrated that the microstructure of the hypereutectic Al-20Si alloy could be refined significantly, further improve its mechanical properties. The desirable refinement of the microstructure was achieved using 0.2% Sr, 0.15% Sc, and 0.3% La under UVT, achieving the highest grain circularity coefficient, hardness, elongation, and area reduction. The tensile strength was the largest with the addition of 0.2% La. The findings of this study provide theoretical and experimental guidelines for the fabrication of structural materials for application in automotive, aerospace, and deep-sea equipment.

Study on Microstructure Evolution Mechanism of Gradient Structure Surface of AA7075 Aluminum Alloy by Ultrasonic Surface Rolling Treatment.

The materials with grain size gradient variation on the surface, which were prepared with mechanical-induced severe plastic deformation, always show high resistance to high and low cycle fatigue and frictional wear because of their good strength-ductility synergy. The ultrasonic surface rolling treatment (USRT) has the advantages of high processing efficiency, good surface quality, and large residual compressive stress introduced to the surface after treatment. The USRT was used to prepare aluminum alloy (AA7075) samples with a surface gradient structure; meanwhile, the microstructural evolution mechanism of the deformation layers on the gradient structure was studied with XRD, SEM, and TEM. The microstructure with gradient distribution of grain size and dislocation density formed on the surface of AA7075 aluminum alloy after USRT. The surface layer consists of nanocrystals with random orientation distribution, and high-density dislocation cells and subgrains formed in some grains in the subsurface layer, while the center of the material is an undeformed coarse-grained matrix. The results show that the dislocation slip dominates the grain refinement process, following the continuous cutting and refinement of dislocation cells, subgrains, and fragmentation of the second precipitates. This study systematically clarified the mechanism of grain refinement and nanocrystallization on the surface of high-strength aluminum alloys and laid a theoretical foundation for further research on mechanical behavior and surface friction and wear properties of high-strength non-ferrous materials with gradient structure.

Texture tailoring and microstructure refinement induced by {11−21} and {10−12} twinning in an extruded Mg-Gd alloy

An extruded Mg-17Gd (wt%) alloy with < 0001 > fiber texture was compressed along different directions at room temperature. Electron backscatter diffraction characterization was used to investigate the texture tailoring, twinning and dynamic recrystallization (DRX) behaviors. The results indicate that {10−12} and {11−21} twinning activated. The [0001] pole tilted to compression direction with the strain increasing and twinning dominated the texture transition. Activation of {10−12} twinning can be governed by Schmid law, while it is more appropriate to explain the activation of {11−21} twinning by the basal slip transfer condition in the twinned grains according to the slip trace analysis. In addition, operation of {11−21} twinning can facilitate the nucleation of {10−12} twin in the same grain by changing Schmid factor (SF) in the vicinities of {11−21} twin. Grain refinement occurred in the specimens with large strain and the average grain size decrease from ∼27 µm to ∼12 µm. The mechanism of grain refinement can be summarized as follows: the mobility of twin boundaries will be restricted because of the twin-twin and dislocation-twin interaction and converted to random high angle grain boundaries (HAGBs). The random HAGBs offer the nucleation sites and the strain accumulation within twins offer the driving force which facilitate the formation of DRXed grains at random HAGBs. Additionally, the SF difference within parent grain may result in the formation of HAGBs, and these HAGBs can connected the random HAGBs that result from the dislocation-twin interaction which result in the formation of new grains.

Deformation mechanisms and mechanical properties of the high-strength and ductile Al-Zn-Mg-Cu alloys processed by repetitive continuous extrusion forming process with different heat treatments

Continuous severe plastic deformation (SPD) processes, which overcome the limitations of economic efficiency and sample size for classical SPD approaches, are promising industry technologies for manufacturing aluminum alloy products with refined microstructures and enhanced properties. In the present work, the repetitive continuous extrusion forming (R-Conform) process combined with auxiliary thermal treatments was implemented in Al-Zn-Mg-Cu alloys, which had poor workability. By combining microstructural observation, mechanical testing and finite element simulation, the influence of auxiliary heat treatments on the mechanisms of grain refinement and strengthening for Al-Zn-Mg-Cu alloys processed by R-Conform were systematically investigated. The results indicated that the grains of Al-Zn-Mg-Cu alloys were significantly refined by a continuous dynamic recrystallization (CDRX) during the R-Conform process with special thermomechanical conditions. The increased supersaturated solid solution concentration and constituent precipitate fraction caused by prehomogenization were found to accelerate CDRX for grain refinement through the pinning effect on dislocation motion. Due to the simultaneous effect of shear deformation and online solution, the R-Conformed alloys contained massive lattice distortions and high concentrations of solute atoms. As a result, the processed Al-Zn-Mg-Cu alloys combined with prehomogenization and direct aging treatments possibly possess a good combination of strength and ductility due to the integrated functions of fine grains, massive lattice distortion, and evenly distributed nanosized precipitates.

Evaluation of microstructure and tensile properties of Mg–5Zn-xZr-yCa alloys

In this work, an attempt was made to study the synergetic effects of changes in alloying elements including Zr and Ca, as well as the hot extrusion process on the microstructure, texture, and mechanical properties of Mg–Zn-xZr-yCa (x= 0.2 and 0.8 wt %; and 0<y<2) alloy. To do so, a variety of tests (OM, SEM, EBSD, EDS, TEM, APT, hardness and tensile tests) were performed after sample preparation. It was shown that Ca/Zr ratio plays an essential role in modifying the microstructure and mechanical properties of the alloy. For Ca/Zr<1, the grains size was remarkably refined to 1.2 μm, the wrought texture was moderately weakened, and the mechanical properties were significantly increased to 429 MPa. However, for Ca/Zr>1 grain size was almost unchanged, the texture of hot-extruded alloys became more equiaxed after hot extrusion, and tensile strength decreased compared to with Ca/Zr=1. Microstructural analysis revealed that Zr-bearing phases (Zn22Zr and Zn39Zr5) and solute theory were the dominant mechanisms of grain refinement in the as-cast state. An atom probe tomography technique was utilized to assess the segregation of solute elements into the grain boundaries. It was shown that particle stimulated nucleation is not likely to be the main mechanism of texture weakening of extruded alloys; and the solute segregation into grain boundaries facilitates this phenomenon. Finally, it was seen that values of yield and ultimate tensile strengths of Mg–5Zn-0.8Zr-0.5Ca were 372 and 429 MPa, respectively.

Grain Refinement of Cast Aluminum by Heterogeneous Nucleation Site Particles with High Lattice Matching

A number of studies have been carried out to identify the mechanism of grain refinement in cast aluminum alloys by addition of grain refiners containing potent heterogeneous nucleation site particles. Although some mechanisms and theories have been proposed, in this review, the role of heterogeneous nucleation site particles on grain refining performance will be focused. The content of this article is as follows. First, the morphology of Al3Ti particles in the Al–Ti refiners is presented, since the grain refining performance of the Al–Ti refiners is affected by the morphology of the Al3Ti particles. Evaluation methods of lattice matching between heterogeneous nucleation site and crystallized phase are, then, discussed. Since we conclude that M value is potentially applicable to predict the effective heterogeneous nucleation site, M value at elevated temperatures is described. Next, fragmentation behavior of Al3Ti particles caused by deformation of the refiners is presented, because the number density of the Al3Ti heterogeneous nucleation site particles is increased by the fragmentation of Al3Ti particles. At the same time, the morphology of the Al3Ti particles, which affects the grain refining performance, can be changed. The concept of grain refiners with high-symmetry L12 modified (Al1−xMex)3Ti intermetallic compound particles, fabrication method and some results on grain refining performance are also given. Finally, application of heterogeneous nucleation for other fields are described.

Modeling energy consumption inside cutting deformation zone to predict the stress distributions, temperature and microstructure by micro irreversible entropic thermodynamics

It is important to analyze the process of chip formation by means of the energy consumption during metal cutting, which is related to the cutting power (or cutting force), the cutting temperature and the machined surface integrity. In this study, the consumed energy field inside the cutting deformation zone is established by considering four micro irreversible processes including the dislocation generation, annihilation, movement, and continuous dynamic recrystallization. A new shear-angle solution is proposed on the basis of the steady conditions of irreversible processes (i.e., the entropy production rate reaches the minimum), whose adequacy is verified by the experimental results of the shear angle and friction coefficient along the tool-chip interface. Based on the predicted consumed energy field inside the cutting deformation zone, the deformation mechanism, microstructure, stress and temperature are predicted and analyzed. The results show that three stages including the loading stage, unloading stage and steady stage are divided during chip formation according to the consumed energy field. The consumed energy and statistically stored dislocation density are both of gradient distribution in the primary shear zone, and continuous dynamic recrystallization is the main mechanism of grain refinement during chip formation, which is dominated in the secondary deformation zone. The stress and temperature inside the cutting deformation zone as well as the hardness of the machined surface are predicted according to the energy balance relationship and consumed energy field, indicating the feasibility of using energy consumption to comprehensively describe the internal changes of workpiece material inside the cutting deformation zone.

Demonstration Experiments on Grain Refinement Caused by Vibration Mold Using Ammonium Chloride Solution

For the purpose of understanding the mechanism of grain refinement in cast metal by mechanical vibration, demonstration tests were conducted using an ammonium chloride solution. Ammonium chloride solution and molds with various shape were prepared for the water model experiment. In the case of an open mold, ammonium chloride crystals crystallized on the mold wall and grew toward the center of the mold without vibration. However, when vibration was applied, countless crystals generated at the upper side of the solution immediately after pouring. In the case of a full plugged mold, no wave and convection occurred in the solution even when vibration was added, and the behavior seen was similar to the case without vibration. When only convection occurred in the full plugged mold, crystals were found to move from the mold wall into the solution. When a weir was set in the riser, it helped prevent the generation of crystals on the mold wall of the riser from migrating to the specimen. When a mold with a weir was used for Al–2%Cu alloy, crystals in the specimen did not become refined as seen in the water model experiments.