Boundary Migration Research Articles

Role of thermal stress-driven dislocation and low-angle grain boundary migration in surface plastic deformation and grain orientation evolution of tungsten under thermal shock

Role of thermal stress-driven dislocation and low-angle grain boundary migration in surface plastic deformation and grain orientation evolution of tungsten under thermal shock

Achieving synergy between strength and ductility in metastable β-titanium alloy through a novel multi-morphological microstructure

In order to overcome the trade-off of strength and ductility in traditional homogeneous microstructure, a novel multi-morphological microstructure in metastable β-titanium alloy Ti55531 was prepared by thermomechanical processing and post heat treatment. The multi-morphological microstructure consists of two shapes of equiaxed and lamellar, three scales of micro, submicron and nanometer α phase, and nanometer residual β phase. Micrometer-sized lamellar α phase and equiaxed α phase are obtained through the (α+β) phase region rolling, and nanometer-sized lamellar α phase is derived from aging treatment, with two step aging treatment leading to submicron-sized and nanometer-sized lamellar α phase. Meanwhile, the micrometer-sized α phase effectively prevents grain boundary migration, resulting in small β grains (less than 10 μm). This new multi-morphological microstructure results in a simultaneous enhancement in both strength (1175MPa) and ductility (20%) of the Ti55531 alloy. The synergistic improvement mechanism of strength and ductility was qualitatively analyzed in this research. The relationship between the yield strength and microstructure of the alloy was calculated, and the predicted strength was in agreement with the experimental values. It is concluded that precipitation strengthening is the primary strengthening method for multi-morphological Ti55531 alloy, and the strengthening of grain boundary increases sevenfold when the β grain size is smaller than 10 μm.

Real-time scan-free non-line-of-sight imaging

Real-time non-line-of-sight imaging is crucial for practical applications. Among existing methods, transient methods present the best visual reconstruction ability. However, most transient methods require a long acquisition time, thus failing to deal with real-time imaging tasks. Here, we provide a dual optical coupling model to describe the spatiotemporal propagation of photons in free space, then propose an efficient non-confocal transformation algorithm and establish a non-confocal time-to-space boundary migration model. Based on these, a scan-free boundary migration method is proposed. The data acquisition speed of the method can reach 151 fps, which is ∼7 times faster than the current fastest data acquisition method, while the overall imaging speed can also reach 19 fps. The background stability brought by fast scan-free acquisition makes the method suitable for dynamic scenes. In addition, the high robustness of the model to noise makes the method have the capability of non-line-of-sight imaging in outdoor environments during the daytime. To further enhance the practicality of this method in real-world scenarios, we exploit the statistical prior and propose a plug-in-and-play super-resolution method to extract higher spatial resolution signals, reducing the detector array requirement from 32 × 32 to 8 × 8 without compromising imaging quality, thus reducing the device expense of detectors.

Grain refinement for indium zinc oxide ceramic targets by praseodymium doped induced blocked boundary migration

How to refine the grain has been a difficult problem in the preparation of high-performance ceramic targets. In this work, the doping-induced grain refinement strategy was proposed, indium zinc oxide doped with different concentrations of Pr (Pr-doped IZO, PrIZO) targets were obtained by optimizing the sintering time and holding temperature. Effects of Pr doping content on the density, phase microevolution, grain size and resistivity during the densification process as well as the kinetics of the grain growth and the mechanism of grain refinement of PrIZO targets were investigated in detail. The results demonstrated that PrIZO targets with the atomic ratios of Pr:In:Zn=0.01-0.03:1:1 all exhibited the excellent performance with high densification (>99.10%) and mere average grain size (<3.0 μm) at low sintering temperature of 1350 °C. Additionally, XRD and EDS analysis indicated that PrIZO targets were composed of In2O3 and Zn3In2O6 with slight PrInO3, which formed by the limited solid solubility of Pr element. Combined with theoretical calculations, it inferred that the mechanism of grain refinement was attributed to the solute transformation and fine PrInO3 distributed at the grain boundaries of In2O3 phases, which produced the drag effect of grain boundary migration, then hindering the further growth of grains.

Evolution of microstructure and mechanical properties in TiBw/Ti65 composites during vacuum solid-phase diffusion bonding

This paper examines the solid-phase diffusion bonding (DB) of TiBw/Ti65 composites, focusing on the effects of processing parameters such as temperature, time, pressure, and interlayer thickness. The findings show that lower processing conditions resulted in slower atomic diffusion and more defects, whereas higher conditions reduced pores but led to grain growth. Optimal bonding was achieved at 950°C, 10MPa, and 60minutes, yielding a shear strength of 686MPa, which is 81.86% of the base material's strength. The introduction of pure titanium interlayer results in a concentration gradient, which serves as an additional driving force for atomic diffusion, thereby enhancing the quality of diffusion bonding. Shear strength varied with interlayer thickness, peaking at 668MPa for a 5 μm interlayer. Microplastic deformation, dynamic recrystallization (DRX), and grain boundary (GB) migration affected the microstructure and mechanical properties of the bonding interface. TiB whiskers (TiBw) near the bonding interface significantly influenced DRX and GB migration during the DB process.

Mechanical response of carbon ion implanted ferrite via atomic simulations

Ion implantation plays a nontrivial role in improving the mechanical properties of materials. Unfortunately, the atomic-scale understanding and awareness of the improvement mechanisms remain insufficiently clear and accurate. This paper investigates the nanostructural evolution of carbon ion implanted ferrite and the mechanical response under uniaxial tension leveraging molecular dynamics (MD) simulation, providing direct atomic-scale evidence of alloy strengthening. Regarding nanostructural evolution, grain boundary migration induced by carbon ion implantation becomes significant with increasing doses. However, point defects and amorphous structures caused by collision cascades tend to saturate gradually with increasing implantation doses. Uniaxial tensile test results indicate that the strength of all ion-implanted samples is appreciably enhanced compared to non-implanted samples, especially with an implantation dose of 6.23 × 1013 ions/cm2, where the strength increases by 39%. The underlying strengthening mechanism is that defects, amorphous structures, and lattice distortions induced by ion implantation collectively act as formidable barriers to dislocation motion during plastic deformation, strongly governing dislocation propagation and multiplication. More importantly, the interaction between carbon atoms from ion implantation and dislocations renders the formation of Cottrell atmospheres, which further enhances solid solution strengthening by pinning dislocations. These results advancing the fundamental understanding of nanostructural evolution and strengthening mechanism under ion implantation suggest a mechanistic strategy for augmenting alloy strength.

Effects of Postweld Heat Treatment on Interfacial Behavior and Mechanical Properties of Joints Welded with Cu/Ni-Cr Alloy.

Welded cable composed of nickel-chromium (Ni-Cr) alloy and copper is a crucial component in the resistance heating technology used for heavy oil production. Tungsten inert gas (TIG) welding was employed to join the copper and Ni-Cr alloy using copper filler wire, and the stability of the welded joint was analyzed under high-temperature service conditions. We examined the changes in the microstructure and properties of the welded joint after postweld heat treatment (PWHT) at 600 °C for 3, 6, and 12 days. The results showed that the welded joint was appropriately formed, with fractures occurring in the copper substrate. The average tensile strength of the welded joint was 240 MPa. The copper and nickel dissolved into each other, forming a Cu0.81Ni0.19 strengthening phase. A columnar crystal diffusion layer formed at the interface between the Ni-Cr alloy and the fusion zone after welding. Grain boundary migration promoted the continuous growth in the columnar crystals as the PWHT duration increased, eliminating the microdefects and inhomogeneities caused by welding. The microhardness progressively decreased from the Ni-Cr alloy side to the copper side. However, the nanoindentation results at the Ni-Cr fusion line initially decreased and then increased with increasing PWHT duration, which contrasted the overall hardness trend observed across the joint after PWHT.

A linear equivalent method for determining rotor stability under large disturbances

The stability of rotor systems under large disturbances is often a concern in the design process of rotating machinery. Under this condition, rotor systems show significantly different dynamic behaviors from linear systems. This paper proposes a method for determining rotor system stability under large disturbances. Equivalent stiffness and damping coefficients are defined and a linear system is constructed equivalent to the original system from an energy perspective, so as to indirectly determine the stability of the original system. The influence of large disturbances is described by the oil film boundary migration effect. The proposed method is verified by directly solving the nonlinear dynamic equations. The results show that the method proposed gives results consistent with the direct solving method, while avoids solving the divergent process, resulting in lower computational costs. Furthermore, this paper investigates the effects of several factors under large disturbances in rotor systems with the proposed method. This work may be helpful in qualitative analysis of rotor stability under large disturbances.

Tailoring nanotwinned Cu interlayers for localizing anisotropic plastic deformation during low energy input ultrasonic welding of robust Cu-Cu joints

Ultrasonic welding, known for its severe plastic deformation, faces the challenge of balancing sufficient deformation at the welding interface with minimizing damage to the substrate. This study utilizes the anisotropic deformation mechanisms and mechanical properties of nanotwinned Cu (nt-Cu). Specifically, Cu coatings featuring nanotwin layers aligned parallel to the ultrasonic vibration direction were employed as interlayers in ultrasonic welding of Cu-Cu joints. The effects of the nt-Cu interlayer on the welding quality and the deformation mechanisms under the various welding pressures are investigated. Experimental and molecular dynamics simulations demonstrate that at low welding pressures, the nt-Cu interlayer undergoes deformation and detwinning primarily through twin boundary migration. This mechanism effectively mitigates work hardening during the welding process, localizes deformation at the welding interface, and significantly enhances the strengths of the Cu-Cu joints. The maximum enhancement proportion occurs at a welding pressure of 8 psi, up to 26.75% compared to conventional coarse-grained copper. As the welding pressure increases, the strengthening effect gradually weakens. The deformation mechanism of nt-Cu transitions to dislocation transverse and threading. The interaction between dislocations and twin boundaries forms incoherent twin boundaries and 9R phases, resulting in work hardening of the interfacial regions and reduction of the strengthening effect.

Influence of grain boundary segregation on the deformation behavior of nanocrystalline Ni50Co50 solid solution alloys investigated by molecular dynamics simulations

Molecular dynamics simulations were carried out to investigate the tensile deformation behavior of Ni50Co50 solid solution nanocrystalline alloys with varying degrees of grain boundary segregation. A comprehensive analysis disclosed that the observed strengthening in the Ni-rich grain boundary model is primarily due to the formation of Lomer-Cottrell locks and dislocation networks which impede dislocation motion. Plastic deformation, on the other hand, is predominantly governed by enhanced grain boundary migration, grain rotation, and deformation twinning. In the Co-rich grain boundary model, the interactions among twins, dislocations, and grain boundaries, stemming from the formation of extensive stacking faults and twinning, dictate the plastic deformation and strengthening. Fluctuations in grain boundary composition promote stress concentration, thereby facilitating plastic deformation and dislocation accumulation within the grain, which in turn enhances strain hardening. The study reveals that the Ni50Co50 solid solution nanocrystalline alloys exhibit both high tensile strength and favorable plasticity for models with grain boundary segregation around ±10%. These insights provide valuable guidance for the development of Ni50Co50 nanocrystalline alloys with optimized strength and toughness.

Twinning and 9R Phase Transition Mediated Extraordinary Cold-drawn Deformability in NiCoCrFeMo High-Entropy Alloy.

The production and application of materials are evolving towards the low-dimensional micro-nano scale. Nevertheless, the fabrication of micron-scale alloy fibers remains a challenge. Herein, a novel Ni-Co-Cr-Fe-Mo high-entropy alloy (HEA) fiber with a cold-drawn reduction rate of 99.9995% and a strain (ɛ) of 12.19 is presented without requiring intermediate annealing. The exceptional deformation strain of 11.62 within the fiber leads to extraordinary tensile strengths of 2.8GPa at room temperature and 3.6GPa at 123K. The in-depth investigation of the microstructure of fibers has revealed the cold drawing deformation mechanisms mediated by the synergistic effects of plane defects. Specifically, various geometrically necessary dislocation interfaces, such as dislocation walls and microbands, along with deformation twins and long-period 9R structures, form in response to external stress when ɛ≤2.7. As the strain increases, the saturated layered structure emerges and progressively evolves into a 3D equiaxed crystal. Moreover, the formation and evolution of the 9R structure (i.e., the migration of incoherent twin boundaries), coupled with the interaction of partial dislocations and the role of deformation twins, are crucial factors determining the fiber's plastic response. This work provides a novel approach to discovering new high-strength metallic fibers with excellent deformability through plane defects engineering.

Nearly Barrierless Polarization Switching Mechanisms in ZrO2 Having Perpendicular In-Plane Domain Walls.

The polarization switching mechanism in ferroelectric ZrO2 involves the nucleation and subsequent migration of nonpolar domain boundaries; however, the fundamental mechanism driving this process remains inadequately understood. The present study introduces a mechanism for nearly barrierless polarization switching in 180° domain walls, facilitated by a the half-unit-cell nonpolar phase between oppositely polarized domains. Based on density functional theory (DFT) calculations, two types of 180° domain walls are explored, featuring head-to-head and tail-to-tail polarization boundaries, with nonpolar orthorhombic Pbcm and tetragonal P42/nmc phases as the respective domain walls. The calculations reveal exceptionally low energy barriers of 17.5 and 10.1 meV for migrating these Pbcm and P42/nmc boundaries by half a unit cell through the domains. An evaluation of the impact of defects on domain wall motion is achieved by introducing 2% oxygen vacancies or 3% Si doping, which induces a significant increase in the barrier motion activation energy, suggesting that defects serve as barriers to polarization switching.

Unraveling dynamic recrystallization mechanisms in Ni-based wrought superalloy GH4698 through comprehensive EBSD analysis

A comprehensive examination of dynamic recrystallization (DRX) mechanisms in wrought superalloy GH4698 has been conducted, leveraging Gleeble thermal simulation experiments and advanced Electron Backscatter Diffraction (EBSD) characterization techniques. Rigorous analysis elucidates four distinct recrystallization processes: Discontinuous Dynamic Recrystallization (DDRX), instigated by strain-induced boundary migration (SIBM); Continuous Dynamic Recrystallization (CDRX), characterized by progressive lattice rotation; Particle Stimulated Nucleation (PSN), driven by the Zener pinning effect of MC-type carbides; and Twin-induced Dynamic Recrystallization (TDRX), facilitated by the interaction between twin boundaries and existing dislocations.

The effect of Y2O3 particles on the microstructure and mechanical properties of tungsten fiber-reinforced tungsten composites

Tungsten fiber-reinforced tungsten (Wf/W) composites are a promising plasma-facing material, and the microstructure at the interface between W fibers and the W matrix greatly impacts performance. In this study, a new kind of Wf/W-Y2O3 composites was developed with the addition of Y2O3 particles to the W matrix in order to modify the interface microstructure between W fibers and the matrix. The microstructure and mechanical properties of these composites were investigated through sintering at temperatures ranging from 1500 °C to 1800 °C. The Y2O3 particles with a face-centered cubic crystal structure were formed at the interface between W fibers and the matrix, which is beneficial for weakening the bonding strength. The Wf/W-Y2O3 composites exhibited pseudo-plasticity at low sintering temperatures, which show the highest bending strength (261 MPa) and fracture energy (6.66 kJ/m2) at 1600 °C. The grains in W particles become the recrystallized nuclei on the surface of W fiber through the bonding of W powder and W fibers. Then, these recrystallized nuclei grow into W fibers by migration of bonding boundaries. However, the Y2O3 particles in the W powder can inhibit the migration of grain boundaries, thereby suppressing the abnormal grain growth behavior at the surface of W fiber.

Effect of friction stir processing on grain refinement and superplastic properties of binary Al-8 wt.% Si to Al-30 wt.% Si alloys

ABSTRACT As-cast binary Al–Si alloys containing ∼8, 12, 20, and 30 wt.% Si, representing the hypoeutectic, eutectic (12 wt.% Si), and hypereutectic compositions, were subjected to severe plastic deformation by friction stir processing (FSP) for grain refinement to 2.3–2.7 μm by dynamic recrystallization. Tensile tests conducted by differential strain rate test technique over the strain rates of 10−4 – 10−2 s−1 and test temperatures in the range of ∼840–530 K led to strain rate sensitivity index (m) varying from ∼0.04 to 0.40 depending on temperature, strain rate, and alloy composition. The constant initial strain rate tests conducted at 10−4 s−1 and 840 K exhibited a maximum elongation of 250% in the hypereutectic Al–20Si alloy, but the same increased linearly with m irrespective of alloy composition. Generally, with increasing Si content, the activation energy for deformation increased from 104.7 ± 14.4 kJ/mol for eutectic Al–12Si to 310.4 ± 32.3 kJ/mol for hypereutectic Al–30Si, which increased further to 572 ± 148 kJ/mol over the higher temperature range of 840–800 K. Analysis of observed deformation and microstructure behaviour supports the occurrence of superplasticity, whereby the accommodation of grain boundary sliding by grain boundary migration led to enhanced grain growth or else the local high-stress concentration at the particle-matrix interface led to cavity formation. There was no evidence of dynamic recrystallization during high-temperature tensile deformation but the flow softening observed is ascribed to the occurrence of concurrent cavitation.

A quasi-in-situ EBSD study on abnormal grain growth in 2219 aluminum alloy friction stir welded joints during post-weld heat treatment

Abnormal grain growth occurs in the friction stir welds of 2219 aluminum alloy during the solution treatment. To further investigate abnormal grain growth, the quasi-in-situ electron backscatter diffraction (EBSD) experiment, including the EBSD testing and heating by thermal simulation testing machine, was proposed to observe the grain growth process in the weld. The results show that the abnormal grain growth behavior in the stir zone and the thermo-mechanically affected zone is different, and different microstructures at different positions in the weld promote the growth of abnormal grains. In the stir zone, the modified Humphreys’ model is used to analyze the abnormal grain growth behavior. The grains with an advantage size and low strain are more likely to grow abnormally. The non-uniform pinning caused by the dissolution of second-phase particles further promotes abnormal grain growth. In the thermo-mechanically affected zone, the abnormal grains are formed by unstrained equiaxed grains near the stir zone or recrystallized sub-grains. The growth of abnormal grains in the thermo-mechanically affected zone is a strain-induced grain boundary migration process. The research is helpful to understand the abnormal grain growth in friction-stir welds during post-weld heat treatment.

Study on hot deformation behavior and dynamic recrystallization mechanism of recycled Al–Zn–Mg–Cu alloy

This study utilized Al–Zn–Mg–Cu alloy chips as raw materials to prepare recycled billets through a solid-state recycling process. Isothermal hot compression tests were conducted using a thermal simulation machine to establish an Arrhenius-type constitutive equation and the hot processing map based on the dynamic materials model. The microstructure of the recycled Al–Zn–Mg–Cu alloy after hot compression was characterized and analyzed using SEM, EBSD, and TEM, exploring the microstructural evolution mechanisms of the recycled alloy at different temperatures and strain rates. The results indicate that the optimal hot deformation parameters for the recycled Al–Zn–Mg–Cu alloy are 300–360 °C at a strain rate of 0.01–0.05 s⁻1 and 400–450 °C at a strain rate of 0.05–0.3 s⁻1. At the same strain rate, as the deformation temperature increases, low-angle grain boundaries gradually disappear, and the grain structure becomes more stable. Dynamic recrystallization replaces dynamic recovery as the dominant mechanism. At the same deformation temperature, low strain rates allow more time for the migration of low-angle grain boundaries, thereby promoting the growth of dynamically recrystallized grains and ultimately forming new, undistorted grains.

Effect of trace silver on the conductivity and softening temperature of high conductivity and high heat resistance pure copper

As a conductor material for high-power electronic devices, pure copper requires both conductivity and softening temperature. The effects of 0% (7N Cu), 0.015% (Ag150), 0.030% (Ag300), 0.045% (Ag450) and 0.059% (Ag590) Ag on the microstructure, conductivity and softening temperature of pure copper were studied. The results show that trace Ag can refine the microstructure of pure copper and improve the mechanical properties at room temperature and high temperature. The hardness values of 7N Cu, Ag150, Ag300, Ag450 and Ag590 were 112, 119.06, 122.14, 123.54 and 125.68HV, respectively, and the softening temperatures were 195, 310, 335, 344 and 345 °C, respectively. At the same time, the conductivity of the samples with different Ag content was more than 100.5 %IACS. The similar structure of Ag and Cu has little effect on the conductivity. On the one hand, the trace Ag element soluble in the matrix causes lattice distortion, inhibits recrystallization nucleation and grain boundary migration. On the other hand, the Ag element is polarized at the grain boundary, which reduces the grain boundary energy, decreases the atomic diffusion and increases the recrystallization temperature.

Low-temperature Cu-Cu direct bonding with ultra-large grains using highly (110)-oriented nanotwinned copper

With the application of Cu-Cu direct bonding technology in high-performance electronic devices, improving bonding reliability is essential for achieving high-density 3D IC integration. In this study, we innovatively applied highly (110)-oriented perpendicular nanotwinned Cu (p-ntCu) to achieve Cu-Cu direct bonding at low temperature and pressure. The anisotropic growth of p-ntCu during annealing at 200 °C resulted in the formation of ultra-large grains, which could improve the conductivity of the electrodeposited Cu film. Two Cu films were bonded at 200–250 °C with 2 MPa pressure for 1 h, and there was almost no void at the bonding interface. Anisotropic grain growth occurred within the bonding joint and grain boundary migration was observed at the bonding interface. The shear strength after bonding at 250 °C was measured as 58.3 MPa on average, indicating a relatively high quality of such Cu-Cu bonding. The p-ntCu can achieve Cu-Cu bonding and improve the conductivity by anisotropic grain growth at low temperature and pressure, which has great potential for Cu interconnect applications.

Hot deformation behavior and microstructural evolutions of a Mg-Gd-Y-Zn-Zr alloy prepared by hot extrusion

The hot deformation behavior of extruded Mg-9.5Gd-4Y-2Zn-0.5Zr alloy was studied through hot compression tests conducted at deformation temperatures of 350–500 °C and strain rates of 0.001–1 s−1. Under most deformation conditions, the flow stress first reached a maximum before decreasing to a stable value, demonstrating characteristics of dynamic recrystallization (DRX). The constitutive equation of the alloy was established as ε.=1.03*1011sinh0.01287*σp2.422exp(−171150/RT) through calculations and derivation. The influences of the deformation temperature and strain rate on the microstructure and DRX of the alloy during the hot compression process were investigated using electron backscatter diffraction. It was found that at temperatures between 350 °C and 450 °C, the alloy texture exhibited multiple {0001} peak rings distributed around the normal axis of the compression direction. When the strain rate was 0.001 s−1, the average equivalent grain diameter significantly increased from 3.3 μm to 22.4 μm with the rise in deformation temperature, whereas when the deformation temperature was 450 °C, the volume fraction of DRX grains noticeably decreased from 87.6 % to 13.2 % with the increasing strain rate. Further research revealed that due to the restriction of strain rate on grain boundary migration, discontinuous dynamic recrystallization and continuous dynamic recrystallization were the predominant DRX mechanisms at low and high strain rates, respectively.