Flow Characteristics Analysis of TC18 Titanium Alloy during Hot Deformation Based on Phase Transformation

An accurate flow stress model was established by considering the parameters of strain rate,strain and temperature as well asβ→a+βphase transformation in order to develop the plastic forming theory of TC18 titanium alloy.Firstly,the phase transition kinetics of TC18 titanium alloy during isothermal and continuous cooling at 1073 and 1273 K was studied by thermodynamic calculation,meanwhile,the relationship of volume fraction of phase transition with temperature and time was obtained.Constitutive models were calculated by investigating flow behaviors under hot compression tests with the strain rates of 0.001-1s^(-1) and temperatures of 973-1223 K in the singleβand a+βregions in TC18 titanium alloy,respectively.By combining the phase transformation dynamic kinetics with constitutive models,an accurate flow stress model was established,providing theoretical basis and data support for the hot forging of TC18 titanium alloy.

COUPLED NUMERICAL SIMULATION ON COLD ROLLER'S TEMPERATURE FIFLD-PHASE TRANSFORMATION - STRESS FIELD DURING ITS QUENCHING PSOCESS

Complicated changns occur inside the steel parts during quenching process. The abruptly changed boundary conditions make the temperature field,micro - structure and stress field change dramatically in very short time, and these variables take a contact interactions in the whole process. In this paper, a three dimensional non - linear mathematical model for queeching process has been founded and the numerical simulation on temperature field,microstructre and stress field has been realized.In the FEM analysis, the incremental iteration method is used to deal with such complicated nonlinear as boundary nonlinear, physical property nonlinear,transformation nonlinear etc.The effect of stress on transformation kinetics has been considered in the calculation of microstructure. In the stress field anal- ysis,a thermo- elasto - plastic model has been founded, which considers such factors as transforma- tion strain,transformation plastic strain, themal strain and the effect of temperature and transforma- tion on mechanical propertier etc. The transient temperature field, microstructure distribution and stress field of the roller on any time can be displayed vividly,and the cooling curve and the changes of stress on any position can also be given.

Cyclic stress induced phase transformation in super-bainitic microstructure

The effects of cyclic stress loading on the microstructual evolution and tensile properties of a medium-carbon super- bainitic steel were investigated. Experimental results show that the cyclic stress can induce the carbon gathering in austenite and phase transformation from film-like retained austenite to twin martensite, which will obviously enhance the tensile strength and the product of tensile strength and ductility. The higher the bainitic transformation temperature, the lower the transformation rate of the retained austenite. The amount and thickness of the film-like retained austenite play an important role during the cyclic stress induced phase transformation.

Tensile Stress Induced Phase Transformations in Zn-Al Alloy

Both furnace cooled and as-cast eutectoid Zn-Al alloys were investigated under external tensile stress at 100℃. It was observed that the external tensile stress caused decomposition of two metastable phases η'T and η'S which derived from both original state of the alloy, and a phase transformation, αf +ε→T' +η， in both furnace cooled and as-cast eutectoid Zn-Al alloys. Also spheroidized structure formed partially during tensile testing. Superplasticity of the alloy has been discussed correlating with the phase transformations and microstructural changes.

FEM Simulation of Distortion and Residual Stress Generated by High Energy Beam Welding with Considering Phase Transformation

A series of experiments was carried out so as to elucidate the effect of the phase transformation in the cooling process on welding distortion and residual stress generated by laser beam welding (LBW) and laser-arc hybrid welding (HYBW) on the high strength steel (HT780). Then, the experiments were simulated by 3D thermal elasticplastic analysis with FEM (Finite Element Method) which was performed with using the idealized mechanical properties considering the transformation superplasticity. From the results, the effects of the phase transformation on welding distortion and residual stress generated by LBW and HYBW were elucidated. Furthermore, the generality of the idealization of the mechanical properties was verified.

HYSTERESIS OF PHASE TRANSFORMATION TEMPERATURE IN NiTi SHAPE MEMORY ALLOY

The relationship between structure and hysteresis of phase transformation temperature in NiTi shape memory alloy has been investigated by means of TEM observation,positron an- nihilation and electrical resistivity measurement.The sequence of hysteresis for the alloy aged under different regimes was found to be:plate martensite>R-phase>tie-like martensite. The reversible displaeement of phase boundaries of these transformations is blocked by the co- herent stress field around Ti_(11)Ni_(14)phase particles.A linear relationship between S paramet- er of positron annihilation and maximum values of temperature hysteresis showed that the mismatch dislocation and elastic stress field established by Ti_(11)Ni_(14)phase precipitation are the main factor to determine the temperature hysteresis of phase transformation in NiTi shape memory alloy.

Analysis of phase transformation from austenite to martensite in NiTi alloy strips under uniaxial tension

Phase transformation from austenite to martensite in NiTi alloy strips under the uniaxial tension has been observed in experiments and numerically simulated as a localized deformation. This work presents an analysis using the theory of phase transformation. The jump of deformation gradient across the interface between two phases and the Maxwell relation are considered. Governing equations for the phase transformation are derived. The analysis is reduced to finding the minimum value of the loading at which the governing equations have a unique, real and physically acceptable solution. The equations are solved numerically and it is verified that the unique solution exists definitely. The Maxwell stress, the stresses and strains inside both austenite and martensite phases, and the transformation-front orientation angle are determined to be in reasonably good agreement with experimental observations.

Effect of post rolling stress on phase transformation behavior of microalloyed dual phase steel

The slow phase transformation of microalloyed dual phase steel makes the nonuniform stress and temperature fields during the post rolling cooling process have a significant impact on the phase transformation process.Given the relatively slow phase transformation of DP780 steel within the microalloyed dual phase steel series,the influence of stress on the phase transformation behavior of DP780 steel was investigated.To quantify the nonuniform thermal and stress conditions in the steel coil,a thermo-mechanical coupled finite element model of the hot-rolled strip cooling process was established.Based on the simulation data,DP780 steel was chosen as the research material,and Gleeble 3500 thermal simulation equipment was used for experimental validation.The thermal expansion curves were analyzed through regression to establish the dynamic model of DP780 steel phase transformation under stress.Subsequently,metallographic analysis was conducted to determine phase transformation type and grain size of DP780 steel.The results confirmed that the stress promotes the occurrence of semi-diffusion-type bainite transformation.Furthermore,an appropriate level of stress facilitates the growth of bainitic grains,while the increased stress inhibits the growth of ferritic grains.

Phase transformation behaviors and shape memory effects of TiNiFeAl shape memory alloys

Measurements of electrical resistivity, X-ray diffraction, and tensile test at room temperature and ？196°C were performed to investigate the effects of Al addition substituting Ni on the phase transformation behaviors, the mechanical properties, and the shape memory effects of Ti50Ni47Fe2Al1 and Ti50Ni46.5Fe2.5Al1 alloys. It is found that 1at% Al addition dramatically decreases the martensitic start transformation temperature and expands the transformation temperature range of R-phase for TiNiFeAl alloys. The results of tensile test indicate that 1at% Al improves the yield strength of Ti50Ni47Fe2Al1 and Ti50Ni46.5Fe2.5Al1 alloys by 40% and 64%, but de- creases the plasticity to 11% and 12% from 26% and 27% respectively. Moreover, excellent shape memory effect of 6.6% and 7.5% were found in Ti50Ni47Fe2Al1 and Ti50Ni46.5Fe2.5Al1 alloys, which results from the stress-induced martensite transformation from the R-phase.

Prediction of the Residual Welding Stress in 2.25Cr-1Mo Steel by Taking into Account the Effect of the Solid-State Phase Transformations

A computational approach based on the thermal elastic plastic finite element method was developed for predicting welding residual stress in low carbon alloyed steel welds by taking into account the effect of the solid-state phase transformations. The kinetics of phase transformations was described by Johnson Mehl Avrami Kolmogrov （JMAK） equation for bainitic transition and by Koistinen-Marburger （K-M） relationship for martensitic transition. Moreover, an additive rule depending on volumetric phase fraction was adopted to represent the material property changes during heating and cooling. Consequently, the residual welding stresses in a 2.25Cr1Mo steel TIG welded plate were computed. Early calculation results suggest that the bainitic and martensitic transformations took place in the weld the heat-affected zone drastically reduce the residual longitudinal tensile stress in the region.

Residual stress model of pre-stressed dry grinding considering coupling of thermal, stress, and phase transformation

Pre-stressed dry grinding can result in a hardened layer on the part surface while the surface residual stress is controlled.Considering the factors of the thermal field,pre-stress,and microstructural transformation,a proximate model of surface residual stress for pre-stressed dry grinding is established using the ANSYS finite element simulation method and verified through experiment.The variation laws and mechanisms of the residual stress along with the grinding parameters are revealed.Under the comprehensive effect of pre-stress and phase transformation,the residual stress of pre-stressed dry grinding is revealed mainly as compressive stress.This increases as the pre・stress and grinding depth increase.Under the coupling effect,pre-stress has larger influence on the residual stress than the grinding depth.The model can analyze and predict the residual stress of pre-stressed dry grinding in general.

Effect of Ferroelectric Phase Transformation and Internal Stress on Mechanical Properties of PZT

1 Introduction The efficiency and characteristic stability of functional ceramics being used as devices are closely related to phase transformation and polarization, and the ageing process is affected by microcracking generated by internal stress. Relevant researches show that the mechanical behavior of ferro-（piezo-） electric materials depends on composition and microstructure, and is also controlled by phase transformation, and thus resulted internal stress, twinning and microcracking. Domain switching caused by polarization also has great effect on strength （σa） and fracture toughness (KIC). Clearly, variation in mechanical properties

RUDIMENTAL EQUATIONS FOR THERMO－ELASTO－PLASTIC STRESS ANALYSIS DURING CONTINUOUS CASTING WITH PHASE CHANGE

Coupled relations among temperature, phase transformation and stress have beendiscussed here in the present paper. Thermo-elasto-plastic constitutive equationsincluding creep and iterative finite element formulation during continuous casting withphase change have also been presented.

DEFORMATION-INDUCED α2→γ TRANSFORMATION IN A HOT DEFORMED Ti-45A1-10Nb ALLOY

High-resolution transmission electron microscope (HRTEM) was employed to investigate the deformation-induced α2→γ phase transformation phenomenon in a hot deformed Ti-45Al-10Nb alloy. Such a tronsformation can be nucleated either at α2/γ interfaces or at stacking faults on the basal planes of the α2 phase. The growth of deformation-induced γplate is accomplished by the motion of α/6<100> Shockley partials on alternate basal planes (0001)α2, and the α/6<100> Shockley partials move in coordination rather than sweep on (0001)α2 plane one by one. It appears that no atom transportation is involved in this stress-induced α2→γ transfromation.

固态变压器中隔离双向DC/DC变换器扩展移相下虚拟电流控制策略

该文以固态变压器后级隔离双向DC/DC变换器为重点研究对象,提出一种扩展移相下虚拟电流控制策略。通过分析后级变换器在扩展移相传输功率下相移量关系以获得最小电流应力最优解来提高变换器的传输效率,减少器件损耗。同时通过虚拟电流控制思想在线估算传输功率来提高系统动态响应性能,减少电流传感器使用。最后,对该文所提出的扩展移相下虚拟电流控制与扩展移相控制进行实验对比分析,验证了扩展移相下虚拟电流控制策略具有降低电流应力,显著提高变换器的动态响应的优点。

Effect of Spraying Condition and Material Properties on the Residual Stress in Plasma Spraying

The thermomechanical behavior and the distribution of residual stresses due to thermal spraying of NiCoCrAlY coating were studied by thermomechanical finite element analysis. The effects of phase transformation due to solidifying process of coating particles, thickness and material properties of coating on the residual stresses were discussed. Results showed that residual stress decreases little with the stress relaxation due to the phase transformation. For the substrates with the same thickness, the residual stress increases with the increase in coating thickness. The state of residual stresses relates to the material properties of coating and substrate closely. The stress-induced failure model of coating is also discussed.

Prediction of Welding Residual Stress in 2.25Cr-1Mo Steel Pipe

A numerical analysis method was proposed to predict the welding residual stress in 2.25Cr-1Mo steel pipe considering solid-state phase transformations. A thermal elastic plastic finite element (FE) model considering effects of martensite transformation was developed based on commercial ABAQUS software. Continuous cooling transformation (CCT) diagrams were employed to simulate the fraction of martensite in fusion zone, coarse-grained heat affected zone and fine-grained heat affected zone. The Koistinen-Marburger relationship was used to trace the formation of martensite. The effects of both volume change and yield strength change due to phase transformation on welding residual stress were considered using the proposed FE model. The result shows that the phase transformation has significant effects on the welding residual stress in multi-pass butt weld of pipe. The predicted simulation results by the proposed numerical method are generally in good agreement with experimental results.

Indenter Size Effect on Stress Relaxation Behaviors of Surface-modified Silicon:A Molecular Dynamics Study

Long-lasting constant loading commonly exists in silicon-based microelectronic contact and can lead to the appearance of plastic deformation.Stress relaxation behaviors of monocrystalline silicon coated with amorphous SiO_(2)film during nanoindentation are probed using molecular dynamics simulation by varying the indenter’s size.The results show that the indentation force(stress)declines sharply at the initial and decreases almost linearly toward the end of holding for tested samples.The amount of stress relaxation of SiO_(2)/Si samples indented with different indenters during holding increases with growing indenter size,and the corresponding plastic deformation characteristics are carefully analyzed.The deformation mechanism for confined amorphous SiO_(2)film is depicted based on the amorphous plasticity theories,revealing that the more activated shear transformation zones(STZs)and free volume within indented SiO_(2)film promote stress relaxation.The phase transformation takes place to monocrystalline silicon,the generated atoms of Si-II and bct-5 phases within monocrystalline silicon substrate during holding are much higher than those for smaller indenter.

Numerical investigation of welding residual stress in Q960 ultrahigh-strength steel multipass joints

This study investigates Q960 ultrahigh-strength steel as the research object.Based on software,a thermo-metallurgical-mechanical finite element model(FEM)is established to simulate the welding temperature field and residual stress distribution.At the same time,the hole-drilling(HD)method is used to measure the residual-welding stress distribution on the surface of the single-pass.Numerical simulation and experimental results show that the predicted value of numerical simulation agrees well with the experimentally measured value,which verifies the accuracy of the FEM.Based on the verification model,the surface and internal stress distribution characteristics of Q960 ultrahigh-strength steel during the multipass remelting of Q960 ultrahigh-strength steel considering solid-state phase transformation(SSPT)are analyzed.The results show that when SSPT is considered,after single-pass welding of Q960 ultrahigh-strength steel,the welded joint is dominated by tensile residual stress,and the peak stress is located in the heat-affected zone(HAZ).At the same time,the effect of SSPT can significantly reduce the size of the residual stress in the weld and affect the distribution of the lateral residual stress.Additionally,as the number of weld passes increased,the transverse residual stress at the center of the weld showed a“stepped”trend,and a local compressive stress peak appeared at the location of the HAZ.

Finite Element Method Analysis of Stress Field Considering Thermal Coupled Force During Laser Process

According to the prominent characteristic of rapid heating and cooling velocity in process of laser phase transformation hardening, it is important and necessary to consider the couple relation among temperature, phase transformation and stress. Based on the above, a mathematical model to analyze transformation and to reflect its characteristic is presented. Also temperature field control equation relative to stress and phase transformation is set up and thermal elastoplastic constitutive equation containing phase transformation was derived, In a given yield condition, temperature and stress field are numerically calculated by means of Finite Element Method. The result indicates the calculation value coincides with result of test Stress especially thermal stress varied violently in process of laser phase transformation hardening, which leaded to the residual stress on the surface and accorded to the fact. This process can improve the characteristic of material, so it is significant to study on thermal stress of laser quenching, to discuss the effect of phase transformation and stress on temperature, to optimize the laser quenching process and to improve the synthetic mechanic properties of quenching material.