Model of critical strain for dynamic recrystallization in 10%TiC/Cu-Al_2O_3 composite

Using the Gleeble-1500 D simulator, the hot deformation behavior and dynamic recrystallization critical conditions of the 10%Ti C/Cu-Al2O3(volume fraction) composite were investigated by compression tests at the temperatures from 450 °C to 850 °C with the strain rates from 0.001 s-1 to 1 s-1. The results show that the softening mechanism of the dynamic recrystallization is a feature of high-temperature flow true stress-strain curves of the composite, and the peak stress increases with the decreasing deformation temperature or the increasing strain rate. The thermal deformation activation energy was calculated as 170.732 k J/mol and the constitutive equation was established. The inflection point in the lnθ-ε curve appears and the minimum value of-(lnθ)/ε-ε curve is presented when the critical state is attained for this composite. The critical strain increases with the increasing strain rate or the decreasing deformation temperature. There is linear relationship between critical strain and peak strain, i.e., εc=0.572εp. The predicting model of critical strain is described by the function of εc=1.062×10-2Z0.0826.

Grain refinement and weak-textured structures based on the dynamic recrystallization of Mg–9.80Gd–3.78Y–1.12Sm–0.48Zr alloy

We utilized electron backscatter diffraction to investigate the microstructure evolutions of a newly developed magnesium-rare earth alloy(Mg–9.80 Gd–3.78 Y–1.12 Sm–0.48 Zr)during instantaneous hot indirect extrusion.An equiaxed fine-grained(average grain size of 3.4±0.2μm)microstructure with a weak texture was obtained.The grain refinement was mainly attributed to the discontinuous dynamic recrystallization(DDRX)and continuous DRX(CDRX)processes during the hot indirect extrusion process.The twin boundaries formed during the initial deformation stage effectively increased the number of high angle grain boundaries(HAGBs),which provided sites for new grain nuclei,and hence,resulted in an improved DDRX process.Along with DDRX,CDRX processes characterized by low angle grain boundary(LAGB)networks were also observed in the grain interior due to effective dynamic recovery(DRV)at a relatively high temperature of 773 K and high strain rates.Thereafter,LAGB networks were transformed into HAGB networks by the progressive rotation of subgrains during the CDRX process.

Effects of laser pulse energy on surface microstructure and mechanical properties of high carbon steel

Surface microstructure and mechanical properties of pearlitic Fe–0.8%C(mass fraction) steel after laser shock processing(LSP) with different laser pulse energies were investigated by scanning electron microscopy(SEM),transmission electron microscopy(TEM),X-ray diffraction(XRD) and microhardness measurements.After LSP,the cementite lamellae were bent,kinked and broken into particles.Fragmentation and dissolution of the cementite lamellae were enhanced by increasing the laser pulse energy.Due to the dissolution of carbon atoms in the ferritic matrix,the lattice parameter of α-Fe increased.The grain size of the surface ferrite was refined,and the microstructure changed from lamellae to ultrafine micro-duplex structure(ferrite(α)+cementite(θ)) with higher laser pulse energy,accompanied by the residual stress and microhardness increase.

Design of phase interface and defect in niobium-nickel oxide for ultrafast Li-ion storage

Niobium pentoxide(Nb2O5)has attracted much attention in lithium batteries due to its advantages of high operating voltage,large theoretical capacity,environmental friendliness and cost-effectiveness.However,the intrinsic poor electrical conductivity,sluggish kinetics,and large volume changes hinder its electrochemical performance at high power density,making it away from the requirements for practical applications.In this research work,we regulate the electron transport of niobium-nickel oxide(NiNbO)anode material with enhanced structural stability at high power density by constructing the two-phase boundaries between niobium pentoxide(Nb2O5)and nickel niobate(NiNb2O6)through simple solid phase reaction.In addition,the presence of lattice defects in NiNbO-F further speeds up the transport of Li+and promotes the electrochemical reaction kinetics more effectively.The two-phase boundaries and defect modulated anode material displays high Li+diffusion coefficient of 1.63×10^(−10) cm^(2) s^(−1),pretty high initial discharge capacity of 222.8 mAh g^(−1) at 1 C,extraordinary high rate performance(66.7 mAh g^(−1))at an ultrahigh rate(100 C)and ultra-long cycling stability under high rate of 25 C(83.4 mAh g^(−1) after 2000 cycles)with only 0.016%attenuation per cycle.These results demonstrate an effective approach for developing electrode materials that greatly improve rate performance and durability.

Microstructure and mechanical properties of Mg-Gd-Y-Sm-Al alloy and analysis of grain refinement and strengthening mechanism

The effects of Sm on the microstructure and mechanical properties of Mg-11 Gd-2 Y-0.6 Al alloy were investigated by X-ray diffraction,optical microscopy,scanning electron microscopy,energy dispersive spectrometry and high resolution transmission electron microscopy.Based on the theory of edge—edge matching and electronegativity theory,the mechanism of grain refinement is discussed.The strengthening mechanism is expounded conveniently from fine grain strengthening,coherent strengthening,precipitation strengthening and grain boundary strengthening.The results show that the micro structure of Mg-11 Gd-2 Y-0.6 Al alloy is mainly composed of a-Mg matrix,Mg5 Gd and Mg24Y5 phases.The addition of Sm forms Mg41Sm5 phase in the alloy and refines the alloy.The addition of Sm significantly improves the mechanical properties of the alloy at room and high temperatures.When the addition of Sm is 3 wt%,the tensile strengths of the alloy at room temperature and high temperature(200℃)reach the maximum value 292 and 321 MPa,respectively.The fracture mode of the alloy at different temperatures is mainly brittle fracture and intercrystalline fracture.

Mechanical Properties and Fracture Behavior of Cu-Co-Be Alloy after Plastic Deformation and Heat Treatment

Mechanical properties and fracture behavior of Cu-0.84Co 0.23Be alloy after plastic deformation and heat treatment were comparatively investigated. Severe plastic deformation by hot extrusion and cold drawing was adopted to induce large plastic strain of Cu 0.84Co-0.23Be alloy. The tensile strength and elongation are up to 476.6 MPa and 18%, respectively. The fractured surface consists of deep dimples and micro voids. Due to the formation of su- persaturated solid solution on the Cu matrix by solution treatment at 950℃ for 1 h, the tensile strength decreased to 271.9 MPa, while the elongation increased to 42%. The fracture morphology is parabolic dimple. Furthermore, the tensile strength increased significantly to 580.2 MPa after aging at 480 ℃ for 4 h. During the aging process, a large number of precipitates formed and distributed on the Cu matrix. The fracture feature of aged specimens with low elongation （4.6%） exhibits an obvious brittle intergranular fracture. It is confirmed that the mechanical properties and fracture behavior are dominated by the microstrueture characteristics of Cu-0.84Co 0.23Be alloy after plastic de- formation and heat treatment. In addition, the fracture behavior at 450 ℃ of aged Cu-0.84Co 0.23Be alloy was also studied. The tensile strength and elongation are 383.6 MPa and 11.2%, respectively. The fractured morphologies are mainly candy-shaped with partial parabolic dimples and equiaxed dimples. The fracture mode is multi mixed mechanism that brittle intergranular fracture plays a dominant role and ductile fracture is secondary.

Graphene oxide effects on the properties of Al2O3-Cu/35W5Cr composite

Graphene oxide(GO)nanosheets were dispersed into premixed powders(Cu-0.4 wt%Al/35W5Cr)by wet grinding and vacuum freeze-drying process.The 0.3 wt%GO/Al2O3-Cu/35W5Cr and 0.5 wt%GO/Al2O3-Cu/35W5Cr composites,used for electrical contacts,were fabricated by vacuum hot-pressing sintering.The microstructure was analyzed by field emission scanning electron and transmission electron microscopy.In addition,the Raman spectroscopy and X-ray photoelectron spectroscopy were used to investigate the structural changes of GO before and after sintering.The arc erosion behavior was investigated by the JF04 C electrical contact testing apparatus.Consequently,the Al2O3 nanoparticles were evenly dispersed in the matrix,causing dislocation tangles.GO was converted to reduced graphene oxide after sintering.A group of carbon atoms combined with Cr forming Cr3C2 in situ during sintering,which enhanced the interface bonding.Compared with the Al2O3-Cu/35W5Cr composite,the tensile strength of the two contact materials containing 0.3 wt%GO and 0.5 wt%GO was increased by 45%and 34%,respectively.Finally,pips and craters were present on the anode and cathode surfaces,respectively.Tungsten has undergone re-sintering during arcing and formed needle-like structures.Compared with Al2O3-Cu/35W5Cr,the GO/Al2O3-Cu35W5Cr composites have better welding resistance.The final mass transfer direction of the two composites was from the cathode to anode.

Characterization of the Hot Deformation Behavior of Cu–Cr–Zr Alloy by Processing Maps

Hot deformation behavior of the Cu-Cr-Zr alloy was investigated using hot compressive tests in the tem- perature range of 650-850℃ and strain rate range of 0.001-10 s-1. The constitutive equation of the alloy based on the hyperbolic-sine equation was established to characterize the flow stress as a function of strain rate and deformation temperature. The critical conditions for the occurrence of dynamic recrystallization were determined based on the alloy strain hardening rate curves. Based on the dynamic material model, the processing maps at the strains of 0.3, 0.4 and 0.5 were obtained. When the true strain was 0.5, greater power dissipation efficiency was observed at 800-850 ℃ and under 0.001-0.1 s-1, with the peak efficiency of 47%. The evolution of DRX microstructure strongly depends on the deformation temperature and the strain rate. Based on the processing maps and microstructure evolution, the optimal hot working conditions for the Cu-Cr-Zr alloy are in the temperature range of 800-850 ℃ and the strain rate range of 0.001-0.1 s-1.

Role of slip and {10-12} twin on the crystal plasticity in Mg-RE alloy during deformation process at room temperature

The deformation mechanism of slips and twins has a considerable influence on the plasticity of magnesium alloys. However, the roles of slips and twins in the room-temperature deformation of Mg-rare earth(Mg-RE) alloys with high contents of rare earth elements is rarely investigated. Here, the microstructural evolution and deformation mechanism of an aged Mg-5 Y-2 Nd-3 Sm-0.5 Zr alloy during uniaxial compression at room temperature were systematically investigated using in-situ electron-backscattered diffraction and transmission electron microscopy. The results indicated that in the early stage of deformation, the Mg-RE alloy was mainly controlled by the slip of dislocations in the basal plane and the coordinated c-axis strain of the {10-12} twin. With an increase in the strain, the grain orientation became more suitable for the initiation of pyramidal Ⅱ dislocations in the later stage of deformation;these dominated the deformation mechanism. In the twin evolution of the Mg-RE alloy, there were three types of twin-twin interaction behaviors:(i) single twin variant 'parallel' structure,(ii) single twin variant 'cross' structure, and(iii) multi twin variant 'cross' structure. In addition, three types of twin-grain boundary interaction behaviors were summarized:(i) twin 'refracting through' grain boundary,(ii) twin'parallel through' grain boundary, and(iii) twin 'fusing through' grain boundary, which are expected to act as new means and solutions for the twin strengthening of magnesium alloys.

Formation and Relative Stabilities of Core-Shelled L12-Phase Nano-structures in Dilute Al–Sc–Er Alloys

First-principles thermodynamic calculations were carried out at the interface level for understanding the precipitation of coherent L12-phase nano-structures in dilute Al–Sc–Er alloys.All energetics,relevant to bulk substitution,interface formation,interfacial coherent strain and segregation,were calculated and used to evaluate the nucleation and relative stabilities of various possible L12 nano-structures.Only matrix-dissolved solute Er(or Sc)can substitute Sc(or Er)in L12-Al3Sc(or Al3Er).The inter-substitution between L12-Al3Sc and Al3Er is not energy feasible.Ternary L12-Al3(Er x Sc 1.x)precipitates tend to form the Al3Er-core and Al3Sc-shell structure with a sharp core/shell interface.Three possible formation mechanisms were proposed and examined.The eff ects of Er/Sc ratio and aging temperature on the relative stabilities of L12-phase nanostructures in Al were also discussed.

Wall Thinning Characteristics of Ti-3Al-2.5V Tube in Numerical Control Bending Process

A finite element(FE) model for the numerical control(NC) bending of Ti-3 Al-2.5 V titanium alloy seamless tube is established, considering the variation in the contractile strain ratio(CSR) and elastic modulus(E). The wall thinning characteristics of Ti-3 Al-2.5 V tube under different geometric and process conditions were investigated. The results showed that the CSR-E variation can change the wall thickness, but has no remarkable effect on the change characteristics. The reasonable parameter ranges are as follows: a bending-radius range not less than 1.5 times the outer diameter, a bend angle up to 180?, and a mandrel extension of 0-3 mm. The friction coefficient between the pressure die and the tube should be in the range of 0.20-0.35, and between the bending die and the tube should be in the range of 0.05-0.15. As long as the performance meets the requirements, the relative push-assistant speed should be as small as possible.

Effects of Annealing Temperature on Cu-Doped ZnO Nanosheets

The synthesis of Cu-doped ZnO nanosheets at room temperature was reported in our previous paper. The effects of annealing temperature on Cu-doped ZnO nanosheets were studied in this paper. Cu-doped ZnO nanosheets were annealed at 200-500℃ in air. The annealed specimens were characterized by scanning electron microscopy （SEM）, X-ray diffraction （XRD） and transmission electron microscopy （TEM）. The results show that Cu concentration in Cu-doped ZnO nanosheets reduced with increasing annealing temperature. When annealing temperature was lower than Zn melting point （410℃）, the morphologies of the Cu-doped ZnO nanosheets remained nearly the same as that before annealing. However, when the annealing temperature was over Zn melting point, Cu-doped ZnO nanosheets changed to nanowires, wormlike nanosheets or did not change. The change of Cu concentration in Cu-doped ZnO nanosheets is explained by oxidation thermodynamics. A physical model is suggested to explain the morphology changes of Cu-doped ZnO nanosheets, based on the existence of Cu-rich layer beneath Cu-doped ZnO nanosheets.

Multistage desulfurization mechanism to reduce sulfur content of high ferrotitanium prepared using thermite method

High ferrotitanium is used as a deoxidizer and alloying agent in steelmaking processes and is mainly produced using high-cost remelting processes.The thermite method is a simple and low-cost method for preparing low ferrotitanium.However,the high levels of S,Al,and O residues in the product severely restrict its applicability in the low-cost preparation of good-quality high ferrotitanium.In this study,a novel multistage deep reduction method for preparing high-quality high ferrotitanium is proposed,and the multistage desulfurization mechanism is systematically investigated.The results indicate that multistage desulfurization is an effective method for reducing the S content of high ferrotitanium prepared via the thermite method.During the strong desulfurization stage,Ti_(2)S reacts with CaO at the slag-metal interface and produces CaS.The S content decreases,while the O content increases,with the increase of CaO in the CaO-Al_(2)O_(3)-based slag.During the deep desulfurization,Ti_(2)S is deeply reduced by the Ca and produces CaS,thus further reducing the S content.The S content decreases with the incremental addition of Ca and can be reduced to 0.035 wt%after multistage desulfurization.

Formation mechanism and properties of NiCoFeLDH@ZIF-67composites

Metal–organic frameworks(MOFs) have a regular porous structure and high porosity,which make them ideal electrode materials for supercapacitors.However,their capacitance performance is greatly limited by their poor conductivity.In this study,a multi-component hierarchical structure was obtained by growing NiCoFeLDH on the surface of ZIF-67,which increased the electron transfer between the MOF particles and greatly improved the capacitance of ZIF-67.The formation mechanism of the multicomponent layered hollow structure indicated that the hydrolysis acidity of metal ions and the coordination ability with ligands were the key factors for forming nanosheets and hollow structures.By controlling the type and valence state of the doped metals and the reaction time,the morphology transformation of MOF composites can be effectively controlled.Electrochemical studies showed that the specific capacitance of hollow NiCoFeLDH@ZIF-67 composite is 1202.08 F/g(0.5 A/g).In addition,aqueous devices were assembled and carefully tested.This scheme is crucial for the design of MOF-based materials used in supercapacitor devices and serves as a guide for the design of MOF-based composites.

Arc Erosion Behavior of Cu–0.23Be–0.84Co Alloy after Heat Treatment: An Experimental Study

The arc erosion behavior of Cu-0.23Be-0.84Co alloy after heat treatment was investigated experimentally by a JF04C electric contact test system. The arc duration, arc energy, contact resistance and contact pressure of Cu-0.23Be- 0.84Co alloy after solution treatment and aging treatment were analyzed. The arc erosion morphologies were contrastively observed by a three-dimensional measuring system and scanning electron microscopy. For the Cu-0.23Be-0.84Co alloy in solution state and aging state, the maximum values of arc duration are 90 and 110 ms, and the arc energies are 15,000 and 18,000 mJ, respectively. The maximum value of the contact resistance of Cu-0.23Be-0.84Co alloy in different states is about 33 mΩ The contact pressure of Cu-0.23Be-0.84Co alloy in solution state generally changes between 50 and 60 cN during whole make-and-break contacts, while in aging state, it has a larger fluctuation range. Moreover, the quality of moving contact （anode） decreases, while static contact （cathode） increases. The materials transfer from anode to cathode during make-and-break contacts. The total mass losses of Cu-0.23Be-0.84Co alloy in solution state and aging state are 3 and 1.2 mg, respectively. In addition, a number of discrete corrosion pits, molten droplet, porosity and cavity distribute on the surface of moving contact and static contact. The arc erosion model of Cu-0.23Be-0.84Co alloy in make-and-break contact was built. The arc erosion resistance of Cu-0.23Be-0.84Co alloy after heat treatment is closely related to the microstructure and the properties of contact materials. This experimental study is important to evaluate the anode or cathode electrocorrosion fatigue life.

Impacts of multiple laser shock processing on microstructure and mechanical property of high-carbon steel

Multiple laser shock processing （LSP） impacts on microstructures and mechanical properties were investigated through morphological determinations and hardness testing. Microscopic results show that without equal channel angular pressing （ECAP）, the LSP-treated lamellar pearlite was transferred to irregular ferrite matrix and incompletely broken cementite particles. With ECAP, LSP leads to refinements of the equiaxed ferrite grain in ultrafine-grained microduplex structure from 400 to 150 nm, and the completely spheroidized cementite particles from 150 to 100 nm. Consequentially, enhancements of mechanical properties were found in strength, microhardness and elongations of samples consisting of lamellar pearlite and ultrafine-grained microduplex structure. After LSP, a mixture of quasi-cleavage and ductile fracture was formed, different from the typical quasi-cleavage fracture from the original lamellar pearlite and the ductile fracture of the microduplex structure.