Thermal deformation behavior and microstructure of nuclear austenitic stainless steel

Gleeble-1500D thermal simulation tester was employed in the hot-compression investigation of as-cast nuclear 304 austenitic stainless steel under conditions: deformation temperature 950―1200℃; deformations 30% and 50%; deformation rates 0.01 and 0.1 s?1. The results show that the flow stress decreases with temperature rise under the same strain rate and deformation, that the flow stress increases with deformation under the same temperature and strain rate, and that the flow stress increases with strain rate...     

Effect of the Cool System on Internal Stress of CaO-Al203-SiO2 Glass-ceramic System

The samples were attained through altering the cooling system of producing glass-ceramics. The X-ray diffraction was used to test the stress value of different samples. The relation of the cooling system and internal stress were also investigated. The experimental results show that the stress of glass-ceramic had a close relation with starting cool temperature. Above 800 ~C, glass-ceramic could be accelerated cooling and did not bring stress. Temperature between 500 ℃ and 800℃ was an important temperature range of the formation of stress in glass-ceramic, in which the glass-ceramic stress would change obviously. Cool system was the key on how to control and eliminate internal stress in order to reduce the destroy of materials crated by internal stress. In addition, glass particles size increase, glass-ceramic stress increase in consequent.     

Cooling mechanism of embankment with block stone interlayer in Qinghai-Tibet railway

In order to study the cooling mechanism of embankment with block stone interlayer under open and closed conditions,an experimental railway section was built and data within one freeze-thaw cycle were collected. The results explain well the cooling mechanism of embankment with block stone interlayer. Under the open condition in cold seasons,the enforced convection effect occurs within block stone interlayer when the wind speed is large;however,the weak air convection occurs within the block stone interlayer near the bottom of the embankment when the wind speed is slow. Under the open condition in warm seasons,heat conduc-tion occurs within block stone interlayer due to the change in wind speed and di-rection. Under the closed condition,however,the enforced convection within block stone interlayer is so weak that heat conduction is dominant in the whole year be-cause wind is blocked. Therefore,the cooling effect of embankment with a block stone interlayer to the soil beneath it is produced by enforced convection and weak free air convection;both its process and the cooling intensity are controlled by the local wind speed and direction. Because of the difference in the cooling effects,the soil temperature beneath the embankment has a temperature difference of 2℃―4℃ between the open and closed conditions.     

Relationship between rheological manufacturing process and optical performance of optical fiber coupler

Through theoretical analysis and experiments, the viscoelastic mechanical model of optical fiber coupler in the process of fused biconical taper was established, and the numerical analysis in non-uniform temperature field was made. The results show that the rheological parameters, such as drawing speed and fused temperature, have a tremendous influence on stress distribution and performance of optical fiber coupler, especially the influence of fused temperature. The change of fused temperature by 5 ℃ can lead to the change of the maximum stress by 30% and stress difference by 20% in the same cross section. The change of temperature gradient by 3% can result in the change of stress difference by 90%. In the present condition of rheological technology, rheological defects such as crystallizations and microcracks are easy to generate in the optical fiber coupler.     

Fast calorimetric scanning of micro-sized SnAgCu single droplet at a high cooling rate

The undercooling of the single micro-sized droplet of Sn-3.0Ag-0.5Cu(wt%)alloy has been studied via the newly developed fast calorimetric scanning technique,by which the fast heating and cooling treatment for a single droplet can be realized,with the maximum heating or cooling rate being 1×104K/s.Owing to the nearly spherical shape of the single droplet upon heating and cooling and the resul-tant geometric stability,the influence of the droplet size on the solidification process could be elimi-nated.As a re...     

Understanding of the influence of process parameters on the heat transfer behavior at the metal/die interface in high pressure die casting process

The current paper focuses on the influence of the process parameters on the peak values of the inter-facial heat transfer coefficient (IHTC) at metal/die interface during high pressure die casting (HPDC) process. A "step shape" casting and AM50 alloy were used during the experiment. The IHTC was de-termined by solving the inverse thermal problem based on the measured temperature inside the die. Results show that the initial die surface temperature (IDST, TDI) has a dominant influence while the casting pressure and fast shot velocity have a secondary influence on the IHTC peak values. By curve fitting, it was found that the IHTC peak value (hmax) changes as a function of the IDST in a manner of hmax =eαTγDI. Such relationship between the IHTC peak value and the IDST can also be found when the casting alloy is ADC12, indicating that this phenomenon is a common characteristic in the HPDC process.     

Statistical values of valence electron structure parameters applied to research on phase transition temperature and eutectoid reaction of titanium alloy

Based on the empirical electron theory of solids and molecules (EET), the statistical values of valence electron structure parameters Sn _A and SE _A which can characterize the properties of alloy phases are calculated, and influences of alloying elements (e.g., V, Nb, Mo, Hf, Zr, Fe, Mn, Co, Cr, Si, and so on) on the phase transition temperature and eutectoid reaction of titanium alloy are discussed with the statistical values of valence electron structure parameters. The research results agree well with real situations. Supported by the National Natural Science Foundation of China (Grant No. 50471022, 50741004) and National Key Basic Research Program of China (“973”) (Grant No. 2007CB613807)     

Behavior of high water-cement ratio concrete under biaxial compression after freeze-thaw cycles

The high water-cement ratio concrete specimens under biaxial compression that completed in a triaxial testing machine were experimentally studied. Strength and deformations of plain concrete specimens in two loading direction under biaxial compression with stress ratio of a=0, 0.25, 0.5, 0.75, 1.0 were obtained after 0, 25, 50 cycles of freeze-thaw. Influences of freeze-thaw cycles and stress ratio on the peak stress and deformation of this point were analyzed according to the experimental results, Based on the test data, the failure criterion expressed in terms of principal stress after different cycles of freeze-thaw, and the failure criterion with consideration of the influence of freeze-thaw cycle and stress ratio were proposed respectively.     

Microstructure and of mechanics microwave boriding

Microwave boriding layer microstructure of carbon steels and its diffusion mechanics were studied. The results show that the existence of microwave field in the boriding can＇t change the growth mechanics of boriding layer. Compared with conventional boriding, if the treatment temperature and time remain constantly, the descend rate of the boriding layer thickness with the increase of carbon content of steel is smaller. The diffusion activation energy ofT8 steel is 2.6× 10^5 J/mol between the temperature of 750 ℃ and 900 ℃ in microwave field, which is in the same order of conventional boriding.     

Modeling and optimal design of multilayer thermal cantilever microactuators

A model of curvature and tip deflection of multilayer thermal cantilever actuators is derived. The simplified expression received from the model avoids inverting complex matrices enhances understanding and makes it easier to optimize the structure parameters. Experiment is performed, the modeled and experimental results demonstrate the validity of the model, and it also indicates that Young’s module makes great contribution to the deflection; therefore, thin layers cannot be ignored arbitrarily. Supported by the National Natural Science Foundation of China (Grant No. 60576053) and Hi-Tech Research and Development Program of China (Grant No. 2007AA03Z333)     

Research and application of CO<Subscript>2</Subscript> refrigeration and heat pump cycle

The environmental problem caused by refrigerant has become the focus all over the world. As the most typical natural refrigerant, CO₂, of course, becomes the research focus. This paper introduces the development and application status of CO₂ refrigeration and heat pump technology. The researches on CO₂ refrigeration and heat pump, carried out by Thermal Energy Research Institute, Tianjin University, also are presented in this paper. Supported by the National Natural Science Foundation of China (Grant Nos. 50676064, 50506019) and the National Hi-Tech Research and Development Program of China (“863” Project) (Grant No. 2007AA05Z262)     

Effect of working condition on thermal stress of NiFe2O4-based cermet inert anode in aluminum electrolysis

Based on the FEA software ANSYS,a model was developed to simulate the thermal stress distribution of inert anode.In order to reduce its thermal stress,the effect of some parameters on thermal stress distribution was investigated,including the temperature of electrolyte,the current,the anode cathode distance,the anode immersion depth,the surrounding temperature and the convection coefficient between anode and circumstance.The results show that there exists a large axial tensile stress near the tangent interface between the anode and bath,which is the major cause of anode breaking.Increasing the temperature of electrolyte or the anode immersion depth will deteriorate the stress distribution of inert anode.When the bath temperature increases from 750 to 970 ℃,the maximal value and absolute minimal value of the 1st principal stress increase by 29.7% and 29.6%,respectively.When the anode immersion depth is changed from 1 to 10 cm,the maximal value and absolute minimal value of the 1st principal stress increase by 52.1% and 65.0%,respectively.The effects of other parameters on stress distribution are not significant.     

Examination of dynamic recrystallization during compression of AZ31 magnesium

This study aimed to investigate dynamic recrystallization (DRX) behavior during compression of magnesium alloy AZ31. Cylinder samples were cut from the extruded rod and hot rolled sheet AZ31 for compression test. The samples were compressed using a Gleeble 1500D at a temperature of 300℃ and a strain rate of 0.01 s-1. Grain orientations and misorientation angles across grain boundaries for the tested samples were obtained by using electron backscatter diffraction (EBSD) technique. The results showed that strong basal texture was observed after 50% compression (ε = 0.69) on both the extruded and hot rolled samples, which have different initial textures. It was observed that with increased strain, DRX grains gradually rotated to basal orientation, and grain boundaries with misorientation angle of near 30° was formed in the samples. At the strain of 0.69, a high fraction of high-angle (> 60°) bounda-ries was present in the extruded sample, whereas almost no high angle boundaries were observed in the hot rolled sheet sample.     

The oscillatory damped behavior of double wall carbon nanotube oscillators in gaseous environment

The mechanical oscillatory behaviors of multiwall carbon nanotube oscillators in gaseous environment are investigated using the molecular dynamics method. The effects of ambient gas and temperature on intertube frictional force and oscillation frequency are analyzed. It is found that the intertube frictional force increases with the ambient gas density and temperature. Higher gas density and higher temperature cause a more rapid decay in the oscillation amplitude and an increase of the oscillation frequency. Compared to the vacuum environmental condition, the collision between gas atoms and the nanotube walls is a main ingredient leading to the increase of the energy dissipation. Gas damping may be the main reason for the failure of carbon nanotube oscillators working in gas environment. The ambient temperature also has an important effect on oscillations and low temperature is advantageous to sustain oscillations. Supported by the National Basic Research Program of China (“973”) (Grant No. 2006CB300404), the National Natural Science Foundation of China (Grant Nos. 50676019, 50775017), the Jiangsu Province Natural Science Foundation (Grant Nos. BK2006510, BK2007113), and the Research Funding for the Doctor Program from Chinese Educational Ministry (Grant No. 20050286019)     

Molecular dynamics simulation of the test of single-walled carbon nanotubes under tensile loading

Molecular dynamics (MD) simulations were performed to do the test of sin-gle-walled carbon nanotubes (SWCNT) under tensile loading with the use of Bren-ner potential to describe the interactions of atoms in SWCNTs. The Young’s modulus and tensile strength for SWCNTs were calculated and the values found are 4.2 TPa and 1.40―1.77 TPa, respectively. During the simulation, it was found that if the SWCNTs are unloaded prior to the maximum stress, the stress-strain curve for unloading process overlaps with the loading one, showing that the SWCNT’s de-formation up to its fracture point is completely elastic. The MD simulation also demonstrates the fracture process for several types of SWCNT and the breaking mechanisms for SWCNTs were analyzed based on the energy and structure be-havior.     

Development of Flow Stress of AISI H13 Die Steel in Hard Machining

An approach was presented to characterize the stress response of workpiece in hard machining, accounting for the effect of the initial workpiece hardness in addition to temperature, strain and strain rate on flow stress in this paper. AISI H13 die steel was chosen to verify this methodology. The proposed flow stress model demonstrates a good agreement with experimental data. Therefore, the proposed model can be used to predict the corresponding flow stress-strain response of AISI H13 die steel with variation of the initial workpiece hardness in hard machining. Funded by the Natural Science Foundation of Jiangxi Province (No. 550067),the National Natural Science Foundation of China(No. 50465003) and Foundation of the State Key Laboratory of Plastic Forming Simulation and Die & Mould Technology (No. 06-3)     

Internal state variable models for microstructure in high temperature deformation of titanium alloys

There exists an interaction between microstructural evolution and deformation behavior in high temperature deformation of titanium alloys. And the microstructure of titanium alloys is very sensitive to the process parameters of plastic deformation process. In this paper, on the basis of plastic deformation mechanism of metals and alloys, a microstructural model including dislocation density rate equation and grain growth rate equation is established with the dislocation density rate being an internal state variable. Applying the model to the high temperature deformation process of Ti60 titanium alloy, the average relative errors of grain sizes between the experiments and the predictions are 9.47% for sampled data, and 13.01% for non-sampled data. Supported by the National Natural Science Foundation of China (Grant No. 50475144), the NPU Foundation for Research (Grant No. NPU-FFR-006), and the National Basic Research Program of China (“973”) (Grant No. G20000672)     

Effects of Phosphorus Additions on Microwave Sintering Properties of UltrafineWC-lOCo Cemented Carbides

Using the microwave sintering technology, the effects of phosphorus （P） additions on the microstructure and properties of the ultrafine WC-10Co alloys were investigated. The experimental results show that with only 0.3wt% P additions, full density WC-10Co cermets were obtained at temperature of 1250℃, which is 70 ℃ lower than that of the undoped counterparts. Lower sintering temperature can result in finer WC grain growth; therefore, the P-doped WC-10Co alloys exhibited higher hardness than the undoped ones. But at the same time, P doping could lead to sacrifice of fracture toughness ofWC-10Co cemented carbides.     

Preparation of Plate Fe60 Co8 Zr10 Mo5 W2 B15 Bulk Amorphous Alloy and Its Fracture Toughness

With processes of arc melting, inductive melting and copper mold suction casting, a plate Febased bulk amorphous alloy Fe₆₀Co₈Zr₁₀Mo₅W₂B₁₅ with a thickness of 1 mm was prepared. The surfaces and fractures of the cast bulk amorphous alloy were aglean and with typical metallic luster. The glass transition temperature (T_g), supercooled liquid region (δT_x) and reduced glass transition temperature (T_g) of prepared Fe-based amorphous alloy are 884 K, 63 K, and 0.611 respectively. The fracture toughness of the cast bulk amorphous alloy is at the level of 1.6 MPa·m^1/2. Funded by the Key Project of National Natural Science Foundation of China (No. 50431030) and the Natural Science Foundation of Jiangsu Province (No. BK2001053)     

Constitutive model for overconsolidated clays

Based on the relationships between the Hvorslev envelope, the current yield surface and the reference yield surface, a new constitutive model for overconsolidated clays is proposed. It adopts the unified hardening parameter, to which the potential failure stress ratio and the characteristic state stress ratio are introduced. The model can describe many characteristics of overconsolidated clays, including stress-strain relationships, strain hardening and softening, stress dilatancy, and stress path dependency. Compared with the Cam-clay model, the model only requires one additional soil parameter which is the slope of the Hvorslev envelope. Comparisons with data from triaxial drained compression tests for Fujinomori clay show that the proposed model can rationally describe overconsolidated properties. In addition, the model is also used to predict the stress-strain relationship in the isotropic consolidation condition and the stress paths in the undrained triaxial compression tests. Supported by the National Natural Science Foundation of China (Grant Nos. 50479001 and 10672010), the National Science and Technology Supporting Item (Grant No. 2006BAK12B12), and the National Basic Research Program of China (Grant No. 2007CB714203)