EFFECT OF COPPER AND BORON CONTENT ON STRAIN-INDUCED Nb(C,N) PRECIPITATION IN ULCB STEELS AT HOT DEFORMATION TEMPERATURE

The stress relaxation curves of Ultra-Low Carbon Bainitic(ULCB) steels with different Cu and B contents were measured by using Gleeble-1500 dynamic thermal-mechanical simulator. The results show that Cu and B added can accelerate the strain-induced precipitation reaction, and the effect of Cu and B is even more obvious with Cu and B combined addition or Cu content increased. The TEM analysis of precipitate engendered at the temperature of 850℃ C indicate that Nb(C,N) precipitate nucleates dominantly on the dislocation line, and grows with holding time extended while the precipitate particle size increases from 5 nm to 17 nm.

MODELING OF STRAIN-INDUCED PRECIPITATION KINETICS IN Nb MICROALLOYED STEELS

On the basis of the thermodynamic calculation of precipitation and considering the effect of strain on the precipitation behavior and chemical composition （Si and Mn）, the kinetics of precipitation from austenite has been investigated for different temperatures and strains. Nucleation theory and the solubility product of niobium, carbon, and nitrogen in austenite have been used to derive equations for the start time of precipitation as a function of temperature and composition. The value of n in Avrami equation was determined using the available experimental data from the published reports, which indicated that n is a constant independent of temperature and the end time of precipitation is a function of n and the start time of precipitation. The values of the start time and end time of precipitation predicted by the new model are compared with the experimental values and a good agreement was obtained between both.

Effects of strain-induced melt activation treatment on the microstructure and properties of Zn sacrificial anodes

The microstructural properties and electrochemical performance of zinc(Zn)sacrificial anodes during strain-induced melt activation(SIMA)were investigated in this study.The samples were subjected to a compressive ratio of 20%-50% at various temperatures(425-435℃)and durations(5-30 min).Short-term electrochemical tests(anode tests)based on DNV-RP-B401 and potentiodynamic polarization tests were performed in 3.5wt%NaCl solution to evaluate the electrochemical efficiency and corrosion behavior of the samples,respectively.The electrochemical test results for the optimum sample confirmed that the corrosion current density declined by 90% and the anode efficiency slightly decreased relative to that of the raw sample.Energy-dispersive X-ray spectroscopy,scanning electron microscopy,metallographic images,and microhardness profiles showed the accumulation of alloying elements on the boundary and the conversion of uniform corrosion into localized corrosion,hence the decrease of the Zn sacrificial anode’s efficiency after the SIMA process.

Influence of Carbides and Strain-induced Martensite on Wear Resistance of Austenitic Medium Manganese Steels

The abrasive wear behaviour of austenitic medium manganese steels was studied under weak corrosion-abrasive wear simulating the liner plate in wet metallic ore bail mill under non-severe impact-loading working condition. Results show that the work-hardening mechanism and the wear resistance of high carbon austenitic medium manganese steels differ from those of medium carbon austenitic medium manganese steel. Under non-severe impact and weak corrosion-abrasive wear,the wear resistances of high carbon and medium carbon austenitic medium manganese steels are 50-90% and 20-40% higher than that of Hadfield steel respectively.

Research on Stress-Induced Nucleation ofε-Martensite in Fe-17Mn-10Cr-5Si-4Ni Polycrystalline Alloy

The relation between stacking fault overlap and martensitic nucleation in Fe-17Mn-10Cr-5Si-4Ni alloy was studied.The arrayed structure of dislocations and characteristics ofε-martensite formation under stress were in-situ observed by TEM.The results reveal thatε-martensite nucleates at the top of stacking faults overlapped band,where the ordered arrayed structure of Shockley partial dislocations exists.External stress promotesε-martensitic nucleation ability because of accelerated ordering of Shockley partial dislocations in stacking faults overlapped band.

KINETICS OF STRAIN-INDUCED PRECIPITATION OF (Nb,V) CARBONITRIDE IN A MICROALLOYED STEEL

The kinetics of the isothermal precipitation of(Nb,V)CN in Nb-V alloys has been investi- gated by using the Formastor-press simulator and the extraction replica method.Under four deformation amounts(0,10,30,50%)and three temperatures(1100,1000,850℃), four types of kinetic curves were found.

Characteristics of Strain-Induced Ferrite in Low Carbon Steel

The strain-induced ferrite formed under different conditions was observed with SEM and optical microscope.The nucleation sites of strain-induced ferrite include grain boundary,grain inside,deformed band and annealing twin boundary.The shapes of the ferrite accordingly are equiaxed irregular polygonal,strip-shaped and acicular.

Energy Spectrum, g Factors, Strain-Induced Level-Splittings and R-Line Thermal Shift of MgO:V^(2+)

Traditional ligand-field theory has to be improved by taking into account both pure electronic contribution and electron-phonon interaction one （including lattice-vibrational relaxation energy）. By means of improved ligand-field theory, the R line, t^322T1 and t^322T2 lines, t^22（^3T1）e^4T2, t^22（^3T1）e^4T1 and t2e^2（^4A2）4T1 bands, g factors of t^32 ^4A2 and t32E, four strain-induced level-splittings and R-line thermal shift of MgO：V^2＋ have been calculated. The results are in very good agreement with the experimental data. It is found that for MgO：V^2＋, the contributions due to electronphonon interaction （EPI） come from the first-order term; the contributions from the second-order and higher terms are insignificant. In thermal shift of R line of MgO：V^2＋, the temperature-dependent contribution due to EPI is dominant. The results obtained in this work may be used in theoretical calculations of other effects of EPI.

Strain-induced insulator–metal transition in ferroelectric BaTiO_3(001) surface:First-principles study

The electronic properties of TiO2-terminated BaTiO3（001） surface subjected to biaxial strain have been studied using first-principles calculations based on density functional theory. The Ti ions are always inward shifted either at compressive or tension strains, while the inward shift of the Ba ions occurs only for high compressive strain, implying an enhanced electric dipole moment in the case of high compressive strain. In particular, an insulator–metal transition is predicted at a compressive biaxial strain of 0.0475. These changes present a very interesting possibility for engineering the electronic properties of ferroelectric BaTiO3（001） surface.

Dynamic recrystallization behavior of 7085 aluminum alloy during hot deformation

The dynamic recrystallization behavior of 7085 aluminum alloy during hot compression at various temperatures （573？723 K） and strain rates （0.01-10 s^-1） was studied by electron back scattered diffraction （EBSD）, electro-probe microanalyzer （EPMA） and transmission electron microscopy （TEM）. It is shown that dynamic recovery is the dominant softening mechanism at high Zener？Hollomon （Z） values, and dynamic recrystallization tends to appear at low Z values. Hot compression with ln Z=24.01 （723 K, 0.01 s？1） gives rise to the highest fraction of recrystallization of 10.2%. EBSD results show that the recrystallized grains are present near the original grain boundaries and exhibit similar orientation to the deformed grain. Strain-induced boundary migration is likely the mechanism for dynamic recrystallization. The low density of Al3Zr dispersoids near grain boundaries can make contribution to strain-induced boundary migration.

X-ray Diffractions of Deformation Structurein Polycrystalline Fe-32Mn-5Si Alloy

The change of microstructure with strain was investigated in a Fe-32Mn-5Si austenitic alloy at room temperature by X-raydiffraction profile analysis.The experimental results show that the Fe-32Mn-5Si alloy is deformed by the strain—induced γ→εtransformation and the twinning except dislocation slip at room temperature.The amount of strain-induced ε-martensite,thestacking fault probability and the twinning probability all exhibit parabolic relationship with increasing strain.The stackingfault probability is higher than the twinning probability.

Modeling of Strain-Induced Precipitation Kinetics and Evolution of Austenite Grains in Nb Microalloyed Steels

Considering the effect of strain and chemical composition onprecipitation behavior, new models for the start and end time of Nb（C,N） precipitation in austenite under the conditions of different temperatures and strains have been investigated for Nb microalloyed steel. The value of n in the precipitation kinetic equation has been determined by using the available experimental data in literature, which indicated that n is a constant and independent of temperature. The values of the start and end time of the predicted precipitation are compared with the experimental values. Calculated results are in good agreement with the experimental results. Also, the evolution of austenite grains before ferrite transformation is simulated by taking the effect of precipitation into consideration. The measured austenite grain size is in good agreement with predicted one prior to ferrite transformation.

Study on Phase Transformation Behavior of Strain-induced PLLA Mesophase by Polarized Infrared Spectroscopy

The structural transformation of mesophase to crystalline phase of strain-induced poly(L-lactic acid) has been investigated by differential scanning calorimetry(DSC) and in situ temperature dependent polarized Fourier transform infrared(FTIR) spectroscopy. It is found that, as the drawing temperature increases, melting of strain-induced mesophase in the heating process can remarkably interfere the crystallization behavior subsequently. Coupling with in situ polarized FTIR, from 60 °C to 76 °C, the mesophase melts partially rather than completely melting, and changes immediately to three-dimensional ordered structure. Of particular note, through monitoring the subtle spectral change in the critical phase transformation temperature from 60 °C to 64 °C, it is clearly demonstrated that relaxation of oriented amorphous chains initially takes place prior to the melting of mesophase.

Microstructure of semi-solid ADC12 aluminum alloy adopting new SIMA method

A semi-solid microstructure of ADC12 aluminum alloy containing spherical solid particles was studied.A new strain-induced melt activation(SIMA) process was proposed.In the treatment,chips were cut from ADC12 ingot by lathe machining and the plastic deformation was produced through cutting.The chips were put into a metal mold and compressed into a billet at 473 K.The microstructures of chip and billet were studied.The effect of the isothermal treatment on the semi-solid microstructure evolution of ADC12 aluminum alloy prepared by the new method was analyzed.A series of heating treatments were carried out from 823 to 838 K.The effects of heating temperature on microstructures were studied.The experimental results show the ADC12 aluminum alloy prepared by the new method can reap homogeneous and spherical grains at 828 K.The average grain size is about 82 μm.Also,the grain microstructure obtained by the present process is better than the traditional one.These results prove that the ADC12 aluminum alloy can be applied to semi-solid processing with the new SIMA method.

Effects of semi-solid isothermal process parameters on microstructure of Mg-Gd alloy

The effects of semi-solid isothermal process parameters on the microstructure evolution of Mg-Gd rare earth alloy produced by strain-induced melt activation(SIMA)were investigated.The formation mechanism of the particles in the process of the isothermal treatment was also discussed.The results show that the microstructure of the as-cast alloy consists ofα-Mg solid solution, Mg5RE and Mg24RE5(Gd,Y,Nd)phase.After being extruded with an extrusion ratio of 14:1 at 380℃,the microstructure of Mg-Gd alloy changes from developed dendrites to near-equiaxed grains.The liquid volume fraction of the semisolid slurry gradually increases with elevating isothermal temperature or prolonging isothermal time during the partial remelting.To obtain an ideal semisolid slurry,the optimal process parameters for the Mg-Gd alloy should be 630℃for isothermal temperature and 30 min for the corresponding time,respectively,where the volume fraction of the liquid phase is 52%.

Austenite Recrystallization and Controlled Rolling of Low Carbon Steels

The dynamic recrystallization and static recrystallization in a low carbon steel were investigated through single-pass and double-pass experiments. The results indicate that as the deformation temperature increases and the strain rate decreases, the shape of the stress-strain curve is changed from dynamic recovery shape to dynamic recrystallization shape. The austenite could not recrystallize within a few seconds after deformation at temperature below 900 ℃. According to the change in microstructure during deformation, the controlled rolling of low carbon steel can be divided into four stages： dynamic recrystallization, dynamic recovery, strain-induced ferrite transformation, and rolling in two-phase region. According to the microstructure after deformation, the controlled rolling of low carbon steel can be divided into five regions： non-recrystallized austenite, partly-recrystallized austenite, fully-recrystallized austenite, austenite to ferrite transformation, and dual phase.

Inhomogeneous Natural Network Promoting Strain-induced Crystallization:A Mesoscale Model of Natural Rubber

Although synthetic rubbers show continuously improved mechanical properties,natural rubber (NR) remains irreplaceable in the rubber family due to its superior mechanical properties.A mainstream viewpoint regarding the superiority of NR is that NR possesses a natural network formed by linking the poly(cis-l,4-isoprene) chain terminals to protein and phospholipid aggregates;after vulcanization,the natural network additionally contributes to rubber mechanics by both increasing the network density and promoting the strain-induced crystallization (SIC) behavior.However,the reason why the natural network promotes SIC is still unclear;in particular,only using the increased network density cannot explain our finding that the NR shows smaller onset strain of SIC than Gel (the gel component of NR with higher network density) and even vulcanized NR.Herein,we point out that the inhomogeneous chain deformation is the alternative reason why SIC of NR takes place at smaller strain than that of Gel.More specifically,although the natural network is homogenous on the subchain length scale based on the proton double-quantum NMR results,it is essentially inhomogeneous on mesoscale (100 nm),as revealed by the small angle X-ray scattering analysis.This inhomogeneous network also leads to the mesoscale deformation inhomogeneity,as detected by the orientation of stearic acid (SA) probe,thus resulting in the smaller onset strain of SIC of NR.Based on the experimental results,a mesoscale model is proposed to qualitatively describe the crucial roles of inhomogeneous structure and deformation of natural network in NR?s mechanical properties,providing a clue from nature to guide the development of high-performance rubbers with controlled structures at mesoscale.

Strain-induced Precipitation in Ti Micro-alloyed Interstitial-free Steel

Stress relaxation method was carried out on a Ti micro-alloyed interstitial-free （IF） steel at the temperature ranging from 800 to 1000℃. The results show that the softening kinetics curves of deformed austenite can be divided into three stages. At the first stage, the stress has a sharp drop due to the onset of recrystallization. At the second stage, a plateau appears on the relaxation curves indicating the start and finish of strain-induced precipitation. At the third stage, the stress curves begin to descend again because of coarsening of precipitates. Precipitation-time temperature （PTT） diagram exhibited a ＂C＂ shape, and the nose point of the PTT diagram is located at 900 ℃ and the start precipitation time of 10 s. The theoretical calculation shows that the strain-induced precipitates were confirmed as almost pure TiC particles. The TiC precipitates were heterogeneously distributed in either a chain-like or cell-like manner observed by transmission electron microscopy （TEM）, which indicates the precipitates nucleated on dislocations or dislocation substructures. In addition, a thermodynamic analytical model was presented to describe the precipitation in Ti micro-alloyed IF steel, which shows a good agreement between the experimental observation and the predictions of the model.

Effect of Adding Nitrogen on Microstructure and Property of Vanadium Microalloyed Reinforcing Bar Steel

The relation between stacking fault overlap and martensitic nucleation in Fe-17Mn-10Cr-5Si-4Ni alloy was studied. The arrayed structure of dislocations and characteristics of ε-martensite formation under stress were in-situ observed by TEM. The results reveal that ε-martensite nucleates at the top of stacking faults overlapped band, where the ordered arrayed structure of Shockley partial dislocations exists. External stress promotes ε-martensitic nucleation ability because of accelerated ordering of Shockley partial dislocations in stacking faults overlapped band.

MODELING OF STATIC RECRYSTALLIZATION KINETICS OF Nb-BEARING STEELS

By using isothermal double hit compression tests and applying the 2% offset method, a new model was developed to predict the microstructural evolution of Nb-bearing steels at temperatures above and below the start temperature of strain-induced precipitation （Tp）. The Tp was developed as a function of true strain, initial austenite grain size and the Nb content. The activation energy of static recrystallization （Qrex） was expressed as a function of the content of different alloy elements. It was found that Nb played the most important role in increasing the value of Qrex, The microstructural observations and measurements confirmed the validity of the model developed in the present investigation.