Features of Electroplastic Effect in Alloys with Martensite Transformation

Influence of different electric current modes(pulse and direct) on occurrence of the electroplastic effect under uniaxial tension in the coarse-grained alloys with martensite transformations is investigated. The materials are shape memory Ti_(49.3) Ni_(50.7) alloy and metastable austenite transformation-induced plasticity (TRIP) steel. The paper contains experimental results of current impact on the ‘‘stress–strain" curves of the material. It has been taken an experimental measurement of the sample temperature during the test. It is shown that the shape of a stress–strain curves and type of the serrate plastic flow, connected with the martensitic transformation and electroplastic effect, depend on the current modes. Impact of pulse current and direct current suppresses shape memory and TRIP effect.     

Stress relaxation of Si/Si_（1-x）Ge_x/Si structure prepared by ion implantation and subsequent annealing process

The stress relaxation of 74Ge+-implanted (100) silicon wafers was investigated. The implantation energy as a function of Si/Si1-xGex/Si struc-ture fluence and two kinds of thermal annealing were reported. The stress and stress relaxation after thermal annealing were calculated on the basis of Raman analysis, and were compared with those obtained from the calculation of virgin Si.     

Structural transformation and rheological processes in bulk metallic glass Zr41Ti14Cu12.5 Ni10Be22.5

The temperature dependence of strain and strain rate of the Zr41Ti14Cu12.5Ni10Be22.5 (Vitl) bulk metallic glass (BMG) under constant heating condition was derived from the static extension method with a dynamic thermal mechanical analyzer (DMA). A few strain rate peaks, which corresponds to the glass transition and multistep crystallization in the differential scanning calorimeter (DSC) examination, were observed in the curves of the relation between strain rate and temperature. The onset of viscous flow and the end of glass transition are interrelated, the first and second strain rate peaks correspond with the first and second crystallization transition processes, respectively.The influence of stress on strain and strain rate was researched. It is found that the rheological behaviour of BMG Vitl changes from elasticity to anelasticity, finally to the Newtonian viscous flow along with increasing temperature.     

Structure evolution in copper resulting from the effect of powerful current pulses

Features of structure formation and changes in microhardness of pre-deformed copper 99.9% Cu resulted from the effect of current pulses of high density (∼10 kA/mm²) and short durability (∼10² μs) at the heating rate of 10⁶–10⁷ K/s have been studied. The changes occurring are interpreted as a result of a rapid rate process of thermally activated recrystallization. The characteristics of the process are compared with the ones taking place during thermal annealing. The mechanisms responsible for the differences observed are discussed.     

Hot Deformation and Dynamic Recrystallization Behavior of Austenite-Based Low-Density Fe–Mn–Al–C Steel

The hot deformation and dynamic recrystallization(DRX) behavior of austenite-based Fe–27Mn–11.5Al–0.95 C steel with a density of 6.55 g cm-3were investigated by compressive deformation at the temperature range of900–1150 °C and strain rate of 0.01–10 s-1. Typical DRX behavior was observed under chosen deformation conditions and yield-point-elongation-like effect caused by DRX of d-ferrite. The flow stress characteristics were determined by DRX of the d-ferrite at early stage and the austenite at later stage, respectively. On the basis of hyperbolic sine function and linear fitting, the calculated thermal activation energy for the experimental steel was 294.204 k J mol-1. The occurrence of DRX for both the austenite and the d-ferrite was estimated and plotted by related Zener–Hollomon equations. A DRX kinetic model of the steel was established by flow stress and peak strain without considering dynamic recovery and d-ferrite DRX. The effects of deformation temperature and strain rate on DRX volume fraction were discussed in detail. Increasing deformation temperature or strain rate contributes to DRX of both the austenite and the d-ferrite, whereas a lower strain rate leads to the austenite grains growth and the d-ferrite evolution, from banded to island-like structure.     

Substructure Evolution of Ti-6Al-2Zr-1Mo-1V Alloy Isothermally Hot Compressed in α＋β Two-Phase Region

Substructure evolution significantly influences the flow behavior of titanium alloys in isothermal hot compression.This paper presents a physical experiment(isothermal hot compression and electron backscatter difraction,EBSD)and a cellular automaton(CA)method to investigate the substructure evolution of a near-αtitanium alloy Ti-6Al-2Zr-1Mo-1V(TA15)isothermally compressed in theα+βtwo-phase region.In the℃A model,the subgrain growth,the transformation of low angle boundaries(LABs)to high angle boundaries(HABs)and the dislocation density evolution were considered.The dislocation density accumulating around the subgrain boundaries provided a driving force and made the transformation of the LABs to HABs.The℃A model was employed to predict the substructure evolution,dislocation density evolution and flow stress.In addition,the efects of strain,strain rate and temperature on the relative frequency of the HABs were analyzed and discussed.To verify the℃A model,the predicted results including the relative frequency of the HABs and the flow stress were compared with the experimental values.     

Hot Deformation Behavior and Processing Maps of a High Al-low Si Transformation-Induced Plasticity Steel: Microstructural Evolution and Flow Stress Behavior

Hot deformation behavior of a high Al-low Si transformation-induced plasticity(TRIP) steel was studied by an MMS-300 thermo-simulation machine at the temperature range of 1050–1200℃ and strain rate range of 0.01–10s~(-1). The constitutive equations of the TRIP steel were established at high temperature by fitting the strain factor with a sixth-order polynomial. The instability during hot rolling was discussed using processing maps. The results reveal that two types of flow stress curves(dynamic recrystallization and dynamic recovery) were observed during the hot compression of the high Al-low Si TRIP steel. Flow stress decreased with increasing deformation temperature and decreasing strain rate. The predicted flow stress of experimental TRIP steel is in agreement with the experimental values with an average absolute relative error of 4.49% and a coefficient of determination of 0.9952. According to the obtained processing maps, the TRIP steel exhibits a better workability at strain rate of 0.1s~(-1) and deformation temperature of 1200℃ as compared to other deformation conditions.     

Influence of Vanadium Content on Hot Deformation Behavior of Low-Carbon Boron Microalloyed Steel

Single-pass compression tests were performed to investigate the hot deformation behavior of low-carbon boron microalloyed steel containing three various vanadium contents at 900-1100℃ and strain rate of 0.01-10 s~(-1) using the MMS-300 thermal mechanical simulator.The flow stress curves of investigated steels were obtained under the different deformation conditions,and the effects of the deformation temperature and strain rate on the flow stress were discussed.The characteristic points of flow stress were obtained from the stress dependence of strain hardening rate;the activation energy of investigated steels was determined by the regression analysis;the flow stress constitutive equations were developed;the effect of vanadium content on the flow stress and dynamic recrystallization(DRX) was investigated.The result showed that the flow stress and activation energy(3-6.5 kJ mol~(-1)) of the steel containing 0.18 wt% V were significantly higher than those of the steels with0.042 wt% and 0.086 wt% V,which was related to the increase in solute drag and precipitation effects for higher vanadium content.DRX analysis showed that the addition of vanadium can delay the initiation and the rate of DRX.     

Effects of electric parameters on microstructure and properties of MAO coating fabricated on ZK60 Mg alloy in dual electrolyte

Micro-arc oxidation (MAO) process was carried out in a dual electrolyte system of NaAlO2 and Na3PO4 on ZK60 Mg alloys to explore the effect of electric parameters including current density, frequency, duty cycle and oxidation time on the evolution of coatings and other characteristics. The microstructural characteristics of coatings were observed by scanning electron microscopy (SEM) combined with the analysis of voltage-time responses during MAO process. Test of weight loss was conducted at a 3. 5％ NaCl solution to assess the resistance to corrosion. The results indicate that the current density and duty cycle play key roles on the coating quality. The peak voltage during MAO process increased with the increase in current density but the coating would be more easily detached when the current density was beyond a critical value. Voltage during MAO and microstructure of the coating were affected remarkably by duty cycle, and corrosion resistance was improved greatly when duty cycle was 40％. By means of single variable experiments, MAO process with optimized electric parameters was developed, which corresponds to the current density of 20 A·dm-2, frequency of 500 Hz, duty cycle of 40％ and oxidation time of 15 min.     

Effects of austempering temperature on microstructure and surface residual stress of carbidic austempered ductile iron (CADI) grinding balls

The effects of austempering temperature on microstructure and surface residual stress of carbidic austempered ductile iron(CADI) grinding balls were systematically investigated in this work. The microstructures were oberserved by optical metallography and analyized by X-ray diffraction. The surface residual stress measured by the cutting method is mainly composed of thermal stress and phase transformation stress. The thermal stress in grinding balls was determined by ANSYS simulation technique, and the surface phase transformation stress was obtained by subtracting the simulated surface thermal stress from the measured surface residual stress. Results show that all microstructures consist of ausferrite, white-bright zones(mixture of martensite and austenite), nodular graphite, and carbides. The distribution of ausferrite shows uniform. With the increase of austempering temperature, the volume fraction and carbon content of austenite increase, whereas the amount of white-bright zone decreases. In addition, the surface residual stress increases with the increase of austempering temperature. Only the tension exists at the austempering temperature of 200 ℃, and the pressure exists at the austempering temperature of 220-260 ℃. The thermal stress changes from the tension on the inside with the radius of 0-35 mm to the pressure on the outside with the radius of 35-62.5 mm, and the stress balance state presents at the radius of 35 mm. It is also found that the transformation stress is related to the content of carbon-rich austenite, and will reduce by 5.03 MPa accompanied with 1 vol.% increase of the austenite. The thermal compressive stress and the transformation tensile stress on the surface both decrease with the increase of the austempering temperature.     

Fatigue response,fracture characteristic and life modeling of a near-alpha titanium alloy under typical cyclic loadings in service

Experimental investigations on the fatigue behavior of a near-alpha titanium alloy under typical cyclic loadings were carried out to simulate the service loading states applied on the engine blades.The axial stress-controlled tension–tension low-cycle fatigue(LCF) tests were carried out over a range of maximum stresses and stress ratios.The rotary bending tests were conducted using a step-loading procedure to reveal the high-cycle fatigue(HCF) limit stresses.The cyclic softening effect is observed in this material,and the strain ratcheting occurs obviously at the maximum LCF loading of 900 MPa.The LCF resistance is found to be dependent on both the maximum loading and the stress ratio.The HCF limit stresses for 1 9 107 and 1 9 106 cycles are determined as405.7 and 457.6 MPa,respectively.The macroscopic fatigue fracture mode and the failure features on fracture surfaces were analyzed by scanning electron microscope(SEM).     

Thermal Conductivity and Tensile Properties of Carbon Nanofiber-Reinforced Aluminum-Matrix Composites Fabricated via Powder Metallurgy: Effects of Ball Milling and Extrusion Conditions on Microstructures and Resultant Composite Properties

Carbon nanofiber(CNF)-reinforced aluminum-matrix composites were fabricated via ball milling and spark plasma sintering(SPS), SPS followed by hot extrusion and powder extrusion. Two mixing conditions of CNF and aluminum powder were adopted: milling at 90 rpm and milling at 200 rpm. After milling at 90 rpm, the mixed powder was sintered using SPS at 560 °C. The composite was then extruded at 500 °C at an extrusion ratio of 9. Composites were also fabricated via powder extrusion of powder milled at 200 rpm and 550 °C with an extrusion ratio of 9(R9) or 16(R16). The thermal conductivity and tensile properties of the resultant composites were evaluated. Anisotropic thermal conductivity was observed even in the sintered products. The anisotropy could be controlled via hot extrusion. The thermal conductivity of composites fabricated via powder extrusion was higher than those fabricated using other methods. However, in the case of specimens with a CNF volume fraction of 4.0%, the thermal conductivity of the composite fabricated via SPS and hot extrusion was the highest. The highest thermal conductivity of 4.0% CNF-reinforced composite is attributable to networking and percolation of CNFs. The effect of the fabrication route on the tensile strength and ductility was also investigated. Tensile strengths of the R9 composites were the highest. By contrast, the R16 composites prepared under long heating duration exhibited high ductility at CNF volume fractions of 2.0% and 5.0%. The microstructures of composites and fracture surfaces were observed in detail, and fracture process was elucidated. The results revealed that controlling the heating and plastic deformation during extrusion will yield strong and ductile composites.     

Study on Stress-softening Effect in CuZnAl Shape Memory Alloys

The effect of mechanical cyclings on the stress-strain(S-S) curves was studied systematically using tensile tests over a widetemperature range.Two-stage yielding was observed in the temperature range between M_s and the ultimate temperature to in-duce martensite,denoted by M_d.The first stage was confirmed to be associated with stress-induced martensite(SIM) and the sec-ond stage was associated with the yielding of SIM.When the samples were mechanically cycled between M_s and M_d.astress-softening phenomenon was observed.The cyclings made the martensite amount increase,critical stress σ_c of SIM de-crease according to exponential law,and the entropy(also enthalpy) of the parent to martensite transformation also decrease.Thevariations of SIM relative amount were determined by recording the changes of electrical resistances in the course of tension.     

Microstructural Evolution and Biodegradation Response of Mg–2Zn–0.5Nd Alloy During Tensile and Compressive Deformation

The effect of deformation behavior on the in vitro corrosion rate of Mg–2Zn–0.5 Nd alloy was investigated experimentally after uniaxial tensile and compressive stress. The microstructure and texture were characterized using electron backscattered diffraction and X-ray diffraction, while potentiodynamic polarization and immersion tests were used to investigate the corrosion response after deformation. The result reveals that applied compressive stress has more dominant effect on the corrosion rate of Mg–2 Zn–0.5 Nd alloy as compared to tensile stress. Both tensile and compressive strains introduce dislocation slip and deformation twins in the alloy, thereby accelerating the corrosion rate due to the increased stress corrosion related to dislocation slips and deformation twins. The {10ī2} tension twinning and prismatic slip were the major contributors to tensile deformation while basal slip, and {10ī2} tension twin were obtainable during compressive deformation. The twinning activity after deformation increases with the plastic strain and this correlates with the degradation rate.     

Low cycle fatigue properties and cyclic deformation behavior of as-extruded AZ31 magnesium alloy

The low cycle fatigue(LCF)properties of as-extruded AZ31 Mg alloy were investigated under total strain amplitudes in the range of 0.4%-1.2%with strain rate of 1×10- 2s -1.Due to the twinning effect in compression during loading and the detwinning effect during unloading,the alloy showed an asymmetric hysteresis loop.The cyclic stress response exhibited cyclic hardening at high total strain amplitudes.The cyclic deformation behaviors were discussed using the Coffin-Manson plot,which divided the plastic strain amplitudes into the tension side and the compression side.Through the LCF tests that were started from either tension or compression under a total strain amplitude of 1.0%,the interaction between the twinning effect and dislocation was analyzed.The twinning effect during the LCF test and the variation of the dislocation density were investigated using optical microscopy and transmission electron microscopy,respectively.     

Bake-Hardening Response of High Martensite Dual-Phase Steel with Different Morphologies and Volume Fractions

Bake-hardening behaviour of carbon steel with different martensite morphologies and volume fraction was investigated.The specimens with fibrous and bulky martensite were prestrained in tension by 4%.After this,they were unloaded and bake hardened at 180 °C for 10–160 min.It was found that dual-phase steel samples which were bake hardened at 180 °C for 20 min showed an increase in the yield stress(YS) and ultimate tensile stress(UTS) but a decrease in ductility.Further increase in the bake-hardening time of 80 or 160 min has reduced the YS and UTS,but increased the ductility.Dr(increase in stress due to bake hardening),YS and UTS values are higher for the microstructure containing fibrous martensite compared to the microstructure-containing bulky martensite.It was also observed that at a given baking temperature Dr,YS and UTS increased by volume of martensite.     

Improvement of wear resistance of AZ31 and AZ91HP by high current pulsed electron beam treatment

The surface modification of magnesium alloys （AZ31 and AZ91 HP） was studied by a high current pulsed electron beam（HCPEB）. The results show that the cross-sectional microhardness of treated samples increases not only in the heat affected zone（ HAZ）, but also beyond HAZ, reaching over 250μm. This is due to the action of quasi-static thermal stress and the shock thermal stress wave with materials, which result in its fast deformation on the surface layer and so increases microhardness. For the AZ91HP alloy, a nearly complete dissolution of the intermetallic phase Mg17Al12 is observed, and a super-saturated solid solution forms on the re-melted surface, which is due to the solute trapping effect during the fast solidification process. Measurements on sliding wear show that wear resistance is improved by approximately 5.6 and 2.4 times for the AZ31 and AZ91HP respectively, as compared with as-received samples.     

Numerical modeling of in-flight characteristics of inconel 625 particles during high-velocity oxy-fuel thermal spraying

A computational fluid dynamics (CFD) model is developed to predict particle dynamic behavior in a high-velocity oxyfuel (HVOF) thermal spray gun in which premixed oxygen and propylene are burnt in a combustion chamber linked to a long, parallel-sided nozzle. The particle transport equations are solved in a Lagrangian manner and coupled with the two-dimensional, axisymmetric, steady state, chemically reacting, turbulent gas flow. Within the particle transport model, the total flow of the particle phase is modeled by tracking a small number of particles through the continuum gas flow, and each of these individual particles is tracked independently through the continuous phase. Three different combustion chamber designs were modeled, and the in-flight particle characteristics of Inconel were 625 studied. Results are presented to show the effect of process parameters, such as particle injection speed and location, total gas flow rate, fuel-to-oxygen gas ratio, and particle size on the particle dynamic behavior for a parallel-sided, 12 mm long combustion chamber. The results indicate that the momentum and heat transfer to particles are primarily influenced by total gas flow. The 12 mm long chamber can achieve an optimum performance for Inconel 625 powder particles ranging in diameter from 20 to 40 μm. At a particular spraying distance, an optimal size of particles is observed with respect to particle temperature. The effect of different combustion chamber dimensions on particle dynamics was also investigated. The results obtained for both a 22 mm long chamber and also one with a conical, converging design are compared with the baseline data for the 12 mm chamber.     

Facile synthesis,morphology and structure of Dy_2O_3 nanoparticles through electrochemical precipitation

The aim of this research was to introduce a new, facile and simple method for synthesis of Dy_2O_3 nanostructures at room temperature. For the first time,galvanostatic electrodeposition was used to synthesize Dy_2O_3 particles, and the influence of the current density on the structure and morphology of the product was studied.The samples were characterized by X-ray diffraction(XRD), transmission electron microscopy(TEM) and Brunauer–Emmett–Teller(BET). The results show that the current density has little effect on the chemical composition but great effect on the structure and morphology of the samples. The average size of the particles decreases as the applied current density increases. The grain size of asprepared samples decreases from 500 to 70 nm when the current density increases from 0.5 to 6.0 mA·cm~(-2). To obtain oxide product, the as-prepared samples were heattreated at 1,000 °C. The results show that the heat-treated samples have smaller particles. The XRD results show that the similar patterns are observed in the samples synthesized at different current densities, and the only difference from the JCPDS card is the ratio of peak intensities. With the increase in the current density, a decrease in the current efficiency is observed.     

SIMULATION OF STEEL COIL HEAT TRANSFER IN HPH FURNACE

The mathematical model has been estublished for the simulation of steel coil＇s heat transfer during annealing thermal process in HPH （high performance hydrogen） furnace. The equivalent radial thermal conductivity is adopted by statistical analysis regression approach through the combination of a large quantity of production data collected in practice and theoretical analyses. The effect of the number of coils on circulating flow gas is considered for calculating the convection heat transfer coefficient, The temperature within the coil is predicted with the developed model during the annealing cycle including heating process and cooling process. The good consistently between the predicted results and the experimental data has demonstrated that the mathematical model established and the parameters identified by this paper are scientifically feasible and the effective method of calculation for coil equivalent radial heat transfer coefficient and circulating gas flow has been identified successfully, which largely enhances the operability and feasibility of the mathematic- model. This model provides a theoretical basis and an effective means to conduct studies on the impact that foresaid factors may imposed on the steel coil＇s temperature field, to analyze the stress within coils, to realize online control and optimal production and to increase facilily output by increasing heating and cooling rates of coils without producing higher thermal stress.