Strong Pinning Effect Research Articles

Cu strengthened Al-Si-Cu semi-solid billet fabricated by liquid phase reaction sintering

A high-strength Al-6Si-6Cu semi-solid billet was prepared by liquid phase reaction sintering. Compared with the pure Al grains in the Al-6Si (wt%) semi-solid billet, Cu nanoparticles precipitated in Al grains of the Al-6Si-6Cu (wt%) semi-solid billet. The interface modified by geometrically necessary dislocations (GNDs) makes Cu nanoparticles show a strong pinning effect on dislocations. Under this pinning action, the Al-6Si-6Cu semi-solid billet possesses an outstanding strain-hardening rate, and its yield strength (YS) and ultimate strength (US) increased by 120% and 67.4%, reaching 220 MPa and 360 MPa respectively. The analysis of strengthening mechanism suggests that Orowan strengthening provided by Cu nanoparticles is the main strengthening mechanism of Al-6Si-6Cu semi-solid billet, which provides 83% of the total yield strength.

Anomalous precipitation of Al3Sc dispersoids on deformation behavior of a novel Al-Cu-Li alloy fabricated by direct energy deposition

In this work, precipitation and deformation behavior of a novel Al-Cu-Li-Mg-Ag-Sc alloy fabricated by direct energy deposition (DED) were investigated in detail. Dual-phase strengthening of Al3Sc and T1 (Al2CuLi) phases can be achieved with a single-stage T6 heat treatment compared to normal double-aging. The excellent compressive yield strengths achieved 512, 345 and 227 MPa at 20, 200 and 300 °C, respectively. Al3Sc and T1 phases can maintain strong pinning effects on dislocations, suggesting a good work hardening ability during deformation.

Improved mechanical performance of double-pass extruded Mg-Gd-Er-Zr alloys with various rare earth contents

The present work focuses on the effects of rare earth (RE = Gd and Er) contents on the microstructural evolution and mechanical performance of Mg-Gd-Er-Zr alloys. Three different alloys, i.e., Mg-12Gd-2Er-0.4Zr, Mg-14Gd-1Er-0.4Zr, and Mg-14Gd-2Er-0.4Zr, are fabricated and subjected to double-pass extrusion. Incomplete dynamic recrystallization (DRX) resulted in a non-homogeneous grain structure with coarse and refined equiaxed grains during single-pass extrusion. In contrast, the grain size decreased dramatically after double-pass extrusion, and a high number of fine Mg5RE particles precipitated in the Mg-14Gd-2Er-0.4Zr alloy than the other two extruded alloys. Moreover, the Mg5RE particles larger than 1 μm contributed to increase the DRX fraction, whereas the finer ones exerted a strong pinning effect on grain boundaries. The volume fraction of β′ precipitates increased with increasing RE content in the matrix. A large volume fraction of β′ precipitates in the alloy will primarily result in higher hardness and strength in the peak-aged condition. After peak-aging treatment, the double-pass extruded Mg-14Gd-2Er-0.4Zr alloy exhibited a good yield strength of 481 ± 3.7 MPa and an adequate elongation of 3.2 ± 0.6%. The influence mechanisms of grain refinement, fine particles, and the strengthening of β′ precipitates are discussed.

Achieving Superior High-Temperature Strength and Oxidation Resistance of TiAl Nanocomposite through In Situ Semicoherent MAX Phase Precipitation.

Increasing the service temperature of TiAl intermetallics is the main challenge for the development of next-generation aircraft. Dispersion-strengthening, an effective means to further improve the high-temperature performance of metals, fails to implement in TiAl intermetallics due to difficulties in interface optimization. Here, we successively fabricate a TiAl naocomposite with fully lamellar microstructures and homogeneously dispersed Ti2AlC nanoprecipitates via spark plasma sintering. The composite consisted of semicoherent interfaces among γ-TiAl/Ti2AlC precipitates/α2-Ti3Al, in addition to continuous polysynthetic nanotwins. Strong pinning effects as well as strain-induced nanoscale TiCr2 precipitation uplift the operation temperature of TiAl nanocomposites by more than 50 °C. Furthermore, we experimentally proved that semicoherent interfaces among in situ Ti2AlC precipitates and its surrounding matrix serve as oxygen diffusion barrier during isothermal oxidization and significantly drop down the mass gain of TiAl nanocomposites during operation, making the present nanocomposite a highly potential candidate for use as light-weight structural materials in automotive and aerospace industries.

Advanced tensile properties and strain rate sensitivity of titanium matrix composites reinforced with CaTiO3 particles

In the present work, a novel reinforcement, i.e., CaTiO3 particles, was adopted to produce high performance Ti matrix composites (TMCs), via a powder metallurgy process followed by hot extrusion. The results revealed that the CaTiO3 particles successfully improved the strength of Ti without sacrificing the elongation to failure. In contrast with most of ceramic particles that decompose or react with Ti matrix, CaTiO3 particles remain chemically unchanged in the matrix. Importantly, unlike titanium carbide (TiC) and other chemically stable particles that are reported with sharp corner and irregular shapes, the CaTiO3 particles display a round profile that may weaken the stress concentration in the matrix. It is found that the yield strength increases from 406 MPa for pure Ti to more than 800 MPa for TMCs with only 2.5 wt% addition of CaTiO3, while maintaining the elongation to failure over 25%. The attractive mechanical property is attributed to the hardened grain boundaries and increased geometrically necessary dislocations (GNDs) caused by the CaTiO3 particles. Besides, the experimental results show that, due to the particle-induced strong dislocation pinning and GNDs hardening effect, the strain rate sensitivity was also found increased for TMCs.

Insight into the recrystallization behavior and precipitation reaction of in-situ nano TiB2/Al Cu Mg composite during heat treatment

In this study, the quantitative results from both electron backscattered diffraction and transmission electron microscope analyses provided the fundamental insight into the recrystallized behavior and precipitation reaction of the in-situ nano TiB2/AlCuMg composite during heat treatment. The difference of coefficient thermal expansion between TiB2 particles and Al matrix enabled the formation of deformation zone around TiB2 clusters during water quenching which promoted the recrystallized behavior. Besides, the strong pinning effect induced by TiB2 distributed at recrystallized grain boundary was also observed. As a result, the fine recrystallized grains were generally distributed around particle clusters. During artificial aging process, the trace Mg remarkably enhanced the kinetics of precipitation reaction in the composite. The coexistence of θ″ and θ' precipitates confirmed the continuous transformation of θ″ into θ' via the atomic Cu rearrangement. Herein, the θ'/Al interface acted as a heterogeneous nucleation site of the Al3Zr precipitate was able to improve the nucleation and growth rate of the precipitate. Moreover, the misfit dislocations at TiB2/Al interface with larger lattice mismatch were confirmed. The synergistic effect of stress field caused by misfit dislocations and geometrically necessary dislocations facilitated the pipe diffusion was responsible for transporting solute atoms under depleted vacancy and solute atoms around the interface, benefited to the nucleation and growth of precipitates.

Internal friction and heat resistance of Al, Al–Ce, Al–Ce–Zr and Al–Ce–(Sc)–(Y) aluminum alloys with high strength and high electrical conductivity

To study the application performances of new aluminum alloy conductors, dynamic mechanical analysis and microstructure observation were used to characterize the heat resistance and internal friction. Severe recrystallization occurred in Al and Al–0.2Ce alloy above 320 °C. The hardness and electrical conductivity of Al decreased from 40.3 to 22.2 HV and 61.58 to 60.87 %IACS after temperature scanning from 50 to 500 °C, which was mainly attributed to the enhancement of electron scattering by recrystallized grain boundaries. The co-addition of Sc and Y element can obtain the best heat resistance. The diameter of Al3Sc precipitates enriched with Y atoms is lower than 40 nm under high temperature (400–500 °C) and the recrystallization behavior can be effectively inhibited by Al3Sc precipitates. The internal friction of Al, Al–0.2Ce and Al–0.2Ce–0.1Y alloys mainly originates from dislocation motion before PR peaks and the growth of recrystallization grains after PR peaks. The internal friction of Al–0.2Ce–0.2Sc and Al–0.2Ce–0.2Sc–0.1Y alloys increases evenly because of the strong pinning effect of Al3Sc precipitates on dislocations and boundaries. The internal friction is composed of weak dislocation motion and boundaries relaxation. Finally, the internal friction of Al–0.2Ce–0.12Zr alloy under high temperature is complicated and affected by the precipitation behavior of Al3Zr precipitates, dislocations motion, the interaction between Al3Zr precipitates and dislocations, recrystallization processes.

Effect of nitrogen on microstructure and mechanical properties of the CoCrFeMnNi high-entropy alloy after cold rolling and subsequent annealing

The effect of nitrogen on the structure and mechanical behavior of a CoCrFeMnNi high-entropy alloy after rolling and subsequent annealing was examined. The as-cast alloys doped with 0.5, 1.0, and 2.0 at% N were cold rolled to 80% thickness reduction. Further annealing in the temperature range of 700–1000 °C for 1 h has resulted in (i) development of recrystallization in the fcc matrix and (ii) precipitation of the M2N type nitrides in the alloys with 1.0 and 2.0 at% N. The recrystallization onset temperature lowered with an increase in the N content. Besides, the fcc grain size in the alloy with 2.0 at% N was considerably smaller than that in the alloys with a lower nitrogen content due to a strong pinning effect of the nitride particles. The cold-rolled alloys had high strength, but very limited ductility. The recrystallized microstructure has resulted in a much better strength-ductility synergy when tested at 293 K. For example, the alloy with 2.0 at% N after annealing at 900 °C had the yield strength of 673 MPa, ultimate tensile strength of 1021 MPa, and ductility of 47%. A decrease in the testing temperature to 77 K has resulted in a considerable increase in strength. For example, the same alloy had the yield strength and ultimate tensile strength of 1097 and 1548 MPa, respectively. The quantitative analysis of the strengthening mechanisms has revealed that (i) grain boundary strengthening provides the highest contribution at 293 K (ii) the increase in strength at 77 K is mostly associated with the lattice friction and interstitial solid solution hardening.

Efficient Ohmic contacts and built-in atomic sublayer protection in MoSi2N4 and WSi2N4 monolayers

Metal contacts to two-dimensional (2D) semiconductors are often plagued by the strong Fermi level pinning (FLP) effect which reduces the tunability of the Schottky barrier height (SBH) and degrades the performance of 2D semiconductor devices. Here, we show that MoSi2N4 and WSi2N4 monolayers—an emerging 2D semiconductor family with exceptional physical properties—exhibit strongly suppressed FLP and wide-range tunable SBH. An exceptionally large SBH slope parameter of S ≈ 0.7 is obtained which outperforms the vast majority of other 2D semiconductors. Such intriguing behavior arises from the septuple-layered morphology of MoSi2N4 and WSi2N4 monolayers in which the semiconducting electronic states are protected by the outlying Si–N sublayer. We identify Ti, Sc, and Ni as highly efficient Ohmic contacts to MoSi2N4 and WSi2N4 with zero interface tunneling barrier. Our findings reveal the potential of MoSi2N4 and WSi2N4 as a practical platform for designing high-performance and energy-efficient 2D semiconductor electronic devices.

Effect of grain and phase boundaries on soft magnetic properties of FeCoNiAlSi high-entropy alloys

The impact of grain and phase boundaries on the magnetic domain structure and soft magnetic properties of the FeCoNi(AlSi)x (0.1 ≤ x ≤ 0.7) high-entropy alloys (HEAs) have been studied in this work. The x = 0.1 alloy with single FCC phase and the x = 0.5 alloy with single BCC phase show obvious grain and phase boundaries, while the x = 0.3 containing FCC plus BCC phase and x = 0.7 alloy containing single BCC phase only present phase boundaries. Besides, The magnetic domains can cross grain and phase boundaries when the exchange coupling interaction is stronger enough. However, the domains are blocked for x = 0.5 alloy due to the strong pinning effect of grain boundaries (GBs). Moreover, the x = 0.1 and x = 0.5 alloys display good statically soft magnetic properties with relatively low coervivity (Hc) and high maximum permeability (μmax), whereas the x = 0.3 and x = 0.7 alloys exhibit good dynamic magnetic parameters with low alternating current (AC) Hc and total loss (Ps).

Lowering the Contact Barriers of 2D Organic F16 CuPc Field-Effect Transistors by Introducing Van der Waals Contacts.

2D organic crystals exhibit efficient charge transport and field-effect characteristics, making them promising candidates for high-performance nanoelectronics. However, the strong Fermi level pinning (FLP) effect and large Schottky barrier between organic semiconductors and metals largely limit device performance. Herein, by carrying out temperature-dependent transport and Kelvin probe force microscopy measurements, it is demonstrated that the introducing of 2D metallic 1T-TaSe2 with matched band-alignment as electrodes for F16 CuPc nanoflake filed-effect transistors leads to enhanced field-effect characteristics, especially lowered Schottky barrier height and contact resistance at the contact and highly efficient charge transport within the channel, which are attributed to the significantly suppressed FLP effect and appropriate band alignment at the nonbonding van der Waals (vdW) hetero-interface. Moreover, by taking advantage of the improved contact behavior with 1T-TaSe2 contact, the optoelectronic performance of F16 CuPc nanoflake-based phototransistor is drastically improved, with a maximum photoresponsivity of 387 AW-1 and detectivity of 3.7×1014 Jones at quite a low Vds of 1V, which is more competitive than those of the reported organic photodetectors and phototransistors. The work provides an avenue to improve the electrical and optoelectronic properties of 2D organic devices by introducing 2D metals with appropriate work function for vdW contacts.

Designing a new Mg-Zn-Ca-Y wrought alloy with high strength and ductility synergy

Herein, a new Mg-Zn-Ca-Y wrought alloy designed by micro-alloying of Ca and Y and subjected to an appropriate processing technique that showed an excellent combination of high strength and ductility at room and elevated temperatures. Density functional theory calculation indicates that Y atoms can incorporate into Ca2Mg6Zn3 phases by replacing Ca atoms, which increases the number density of the Ca2Mg6Zn3 phase and refine their size. The pinning effect of precipitates hinders grain growth and leads to a fine average grain size (~2.5 μm), which also results in high tensile properties. Meanwhile, the solid-soluted Zn, Ca, and Y elements, numerous nano-precipitates and Lüders elongation promote the dislocation accumulation during further tensile deformation, which enhances both strain hardening and strain hardening rate, leading to a high elongation. Moreover, the annealed ZXW600 alloy exhibits high strength at high temperatures (≤ 120 °C), which is attributed to the strong pinning effect of fine particles. This work will be helpful for the development of new high-alloyed Mg-Zn based wrought alloys with high performance and extensive application in industries.

Spin-Torque Memristors Based on Perpendicular Magnetic Tunnel Junctions for Neuromorphic Computing.

Spin‐torque memristors are proposed in 2009, and can provide fast, low‐power, and infinite memristive behavior for neuromorphic computing and large‐density non‐volatile memory. However, the strict requirements of combining high magnetoresistance, stable domain wall pinning and current‐induced switching in a single device pose difficulties in physical implementation. Here, a nanoscale spin‐torque memristor based on a perpendicular‐anisotropy magnetic tunnel junction with a CoFeB/W/CoFeB composite free layer structure is experimentally demonstrated. Its tunneling magnetoresistance is higher than 200%, and memristive behavior can be realized by spin‐transfer torque switching. Memristive states are retained by strong domain wall pinning effects in the free layer. Experiments and simulations suggest that nanoscale vertical chiral spin textures can form around clusters of W atoms under the combined effect of opposite Dzyaloshinskii–Moriya interactions and the Ruderman–Kittel–Kasuya–Yosida interaction between the two CoFeB free layers. Energy fluctuation caused by these textures may be the main reason for the strong pinning effect. With the experimentally demonstrated memristive behavior and spike‐timing‐dependent plasticity, a spiking neural network to perform handwritten pattern recognition in an unsupervised manner is simulated. Due to advantages such as long endurance and high speed, the spin‐torque memristors are competitive in the future applications for neuromorphic computing.

Secondary recrystallization behavior in magnetostrictive Fe-Ga thin sheets induced by nano-sized composite precipitates

Strengthening η texture (⟨001⟩//rolling direction) through secondary recrystallization can significantly improve the magnetostriction coefficient of Fe-Ga thin sheets and expand the use of Fe-Ga alloys in high-frequency applications. However, the secondary recrystallization of Goss texture in Fe-Ga thin sheets deteriorates significantly with the thinning of the sheet thickness under the current inhibitors of micron-sized NbC particles or precipitates. In this paper, a new composite inhibitor system, composed of nano-sized MnS and Nb(C,N) precipitates, is proposed to induce the secondary recrystallization Goss texture in Fe-Ga thin sheets with thicknesses less than 0.3 mm. The density and size of MnS and Nb(C,N) is designed based on the effect of alloy composition and hot rolling parameters on the precipitation kinetics of MnS and Nb(C,N), including the relative nucleation rate-temperature (NrT) and precipitation-time-temperature (PTT) curves. It is found that the stored deformation energy increases the maximum nucleation rate and the fastest precipitation temperature. A large number of lamellar and rectangular precipitates (MnS and Nb(C,N)) with a size of 5∼20 nm were precipitated after hot-rolling at a temperature of 800∼1,050 °C and a total thickness reduction of 80%. These nano-sized inhibitors provide a strong pinning effect for the primary recrystallized grains, and induce abnormal grain growth of Goss grains at an annealing temperature of 950∼1,050 °C. Resultingly, centimeter-sized Goss grains with a small deviation angle and maximum magnetostriction coefficient of 264 ppm are successfully achieved in Fe-Ga alloy thin sheets with a thickness of 0.25 mm.

Static Recrystallization in Aluminum/Graphene Composites

The aim of this work was to analyze the recrystallization behavior of cold rolled Aluminum/graphene composites during annealing. The Aluminum/graphene composite was cold rolled firstly, and then annealed at different temperature (250°C, 300°C, 350°C, 400°C) and for various time (1 h, 2 h, 8 h, 32 h). Full recrystallization did not occur until the annealing temperature was above 300 °C. With annealing temperature increasing from 250 to 300°C, the hardness of the composites decreased from 49.6 to 27.6 HV. Grain growth were not observed at high annealing temperature and longer annealing time, which suggested that Graphene has strong pinning effect on the grain boundary of Aluminum.

Interdependent Linear Complexion Structure and Dislocation Mechanics in Fe-Ni

Using large-scale atomistic simulations, dislocation mechanics in the presence of linear complexions are investigated in an Fe-Ni alloy, where the complexions appear as nanoparticle arrays along edge dislocation lines. When mechanical shear stress is applied to drive dislocation motion, a strong pinning effect is observed where the defects are restricted by their own linear complexion structures. This pinning effect becomes weaker after the first dislocation break-away event, leading to a stress-strain curve with a profound initial yield point, similar to the static strain aging behavior observed experimentally for Fe-Mn alloys with the same type of linear complexions. The existence of such a response can be explained by local diffusion-less and lattice distortive transformations corresponding to L10-to-B2 phase transitions within the linear complexion nanoparticles. As such, an interdependence between a linear complexion structure and dislocation mechanics is found.

The evolution of dislocation loop and its interaction with pre-existing dislocation in He+-irradiated molybdenum: in-situ TEM observation and molecular dynamics simulation

In-situ TEM observation and molecular dynamics simulation were used to investigate the evolution of irradiation-induced dislocation loops and their interactions with the pre-existing dislocation lines in molybdenum irradiated by 30 keV He+ at 673 K. The nucleation, annihilation, merging, size change, and types of dislocation loops were investigated in detail, and the hardening effect from dislocation loops was analyzed. With the increase of helium fluence, the volume number density of dislocation loops increased rapidly and then decreased slowly to a saturation value, while the average size of dislocation loops increased continuously until it reached an upper limit. Both 1/2<111> and <100> loops were formed, with a proportion of 60.2% to 39.8% at 3.95 × 1015 He+.cm−2 (0.07 dpa). The pre-existing dislocation lines would suppress the nucleation and growth of the dislocation loops, while the irradiation-induced dislocation loops had a strong pinning effect on the dislocation lines and impeded their motion. Molecular dynamics simulations were used to clarify the atomistic mechanisms of some major features of the line-loop interactions observed in the in-situ experiment, which showed in a good agreement. The different interaction processes depended on the Burgers vector and plane of the dislocation loop, the motion and reaction kinetics of dislocation segments, and the external loading.

Three-Dimensional Atomic Structure of Grain Boundaries Resolved by Atomic-Resolution Electron Tomography

Summary Grain boundaries (GBs) determine the properties of polycrystals, and tailoring the GB structure offers a promising method for the discovery and engineering of new materials. However, GB structures are far from well understood because of their structural complexity and limitations of conventional projection imaging methods. Here, we decipher three-dimensional atomic structure and crystallography of GBs in nanometals using atomic-resolution electron tomography. Unlike conventional descriptions, whereby they are either straight or curved planar planes with one-dimensional translational symmetry, we show that the high-angle GBs completely lose translational symmetry due to undulated curvature related to configurations of structural units. Moreover, we directly visualize kinks and jogs at the single-atom scale in dislocation-type GBs and investigate their mobilities. Our findings bring new insights to the conventional wisdom of GBs and show the importance of developing methods to include the non-planar nature of GBs to statistically evaluate the behavior of GBs in modeling studies.

Effects of Mo-doping on the microstructure and mechanical properties of CoCrNi medium entropy alloy

Abstract

Development of texture and grain size during extrusion of ZA63 alloy containing stable quasicrystalline i-phase and its effect on tensile and compression strength

We have focused on dispersion of quasicrystalline i-phase in a “Rare Earth-free” magnesium alloy Mg–6Zn–3Al (ZA63). The alloy was chill-cast to produce orthorhombic φ phase interdendritically and quasicrystalline i-phase particles with strong pinning effect at grain boundaries, giving the boundaries a very curved shape. The cast alloy was subjected to a two-step direct extrusion process to study phase (particle) distribution, grain structure, microtexture and related parameters using a host of techniques associated with scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and the resulting mechanical properties. The first extrusion (at 563 K) with a small extrusion ratio of 6.25:1 produced a dispersion of φ phase along the extrusion direction and i-phase decorating the boundaries of grains of size of about 30 μm with a mild basal texture. The second extrusion (at 483 K) with an extrusion ratio of 25:1 reduced the average grain size to ∼2μm, with comparable yield strength 287.0 ± 4.1 MPa in tension and 299.0 MPa in compression (elongations ∼ 21%). The i-phase was found to be stable by differential scanning calorimetry (DSC) and the φ phase appeared to transform to the i-phase.