Uranium dioxide pellet is one of the crucial nuclear fuels. Conventionally, the manufacturing process of uranium dioxide pellet is high-temperature sintering (1750℃×10h) presently. The pellet sintered by this method has a large grain size and reliable capability, but this technology comsumes a large amount of energy and leads to a very high production cost. Therefore, the low-temperature sintering technology (1100℃×4h) of uranium dioxide pellet was studied at home and abroad. Compared with the high temperature singering process, low-temperature sintering process has lots of advantages such as much shorter sintering time, much lower energy consumption and production costing, easier to operate and manipulate, and besides it strengthens competitiveness of the nuclear power. Nevertheless, its grain size is so smaller which limits its use. In this paper, on purpose to explore the high temperature creep property of uranium dioxide pellet in the reactor, high-temperature creep experiment of uranium dioxide with different grain sizes was carried out to investigate whether the smaller grain size has effect on properties of uranium dioxide pellet.Comparative creep test is done for uranium dioxide with different grain sizes at high temperature.High-temperature creep property of uranium dioxide pellets with grain size of 23.8μm (high-temperature sintering) and grain size of 9.0μm (low-temperature sintering ) is tested under 20-40 MPa compression stress at 1400-1500℃. Creep curve is drawn when high temperature creep data were processed. The steady creep rate of each sample at 1400-1500℃is calculated, then the curve of compression ratio and time is fitted, similarly the logarithmic curves of steady creep rate and compression stress is also fitted, and the functional relationship bewteen stead creep rate and compression stress is deduced. The microstructure of uranium dioxide pellet after creep testing is observed by optical microscope and SEM (scanning electronic microscope), and the creep mechanism of urannium dioxide pellet is aslo discussed.Conclusions can be drawn from the test results:1. The lgε.–lgσrelationship between steady creep rate and stress of uranium dioxide pellet with different grain size are described as follows:The sample with large grain size at 1500℃: lgε1= 2.41808lgσ- 6.87601The sample with small grain size at 1500℃: lgε2=1.69804lgσ- 5.64667The sample with large grain size at 1400℃: lgε3= 2.45457lgσ- 6.99475 The sample with small grain size at 1400℃: lgε4= 1.45100lgσ- 5.482922. The steady creep rates of uranium dioxide pellet at 1400-1500℃under 10MPa compression stress were calculated. The rates of uranium dioxide pellet with grain size of 23.8μm、9.0μm are 0.3484×10-4mm /h and 1.1256×10-4 mm/h, at 1500℃, respectively. The rates of uranium dioxide pellet with grain size of 23.8μm、9.0μm are 0.2883×10-4 mm/h and 0.9294×10-4mm /h at 1400℃,respectively.There is almost no order difference of magnitude between the uranium dioxide pellets with different sizes, and this result can be referenced in the engineering design and application fields.3. Uranium dioxide pellet is re-pressure-sintered during creep process with crack formation, and the interface between two pateicles can not be seen.The amount of pores decreased and some of the pores merged into a larger crack. Nitrogen-oxygen-uranium compounds were discovered in every sample. Chemical reaction took place between nitrogen and sintered uranium dioxide pellets, and the reaction product exists in form of white substances. White these substances discovered in the sample exist in form of particle at 1400℃, but exist in form of bamboo leaf figure around the crack at 1500℃. With the pressure increasing, the white substances reduced. The amount of white substances in the sample with grain size of 9.0μm was less than that of the sample with grain size of 23.8μm.4. The results of the high-temperature experiments were compared with the theory value of Nabarro–Herring creep mode、Coble creep mode and Hamper-Dorn creep mode, and it indicates that the creep process of uranium dioxide pellet with grain size of 9.0μm is controlled by both Nabarro-Herring creep mechanism and Hamper-Dorn creep mechanism, and Hamper-Dorn creep is dominant mechanism.The creep process of uranium dioxide pellet with grain size of 23.8μm is only controlled by Hamper-Dorn creep mechanism.
Post about "high temperature creep"
The creep behavior of a fine-grained cast TiAl alloy with nominal chemical composition of Ti-44Al-5Nb-0.85W-0.85B（at.%） was studied.The material was HIPped（hot isostatic pressed） at the temperatures of 1260℃and 1340℃respectively to produce different microstructures:the B2+ωphase was segregated at lamellar colony boundaries after 1260℃HIPping,and it was completely removed after 1340℃HIPping.Tensile creep tests were performed at the temperature of 700℃under a constant stress ranging from 150MPa to 300MPa.The influence of B2+ωprecipitates on creep under different stresses was studied and discussed.The results showed that the two conditions of the cast alloy subjected to creep for up to 1000 hours,even at the temperature of 700℃and at the stress of 300MPa,were still in the secondary creep regime,no fracture occurred.This reveals a good creep resistance for the W-containing fully lamellar TiAl alloy.Also found is that the fully lamellar microstructure with B2+ωsegregation shows a better creep resistance than that without B2+ωsegregation.Creep mechanism has been found to change between 200MPa and 300MPa.The microstructure evaluation was performed by scanning electron microscopy （SEM） and transmission electron microscopy（TEM）.The results showed that coarseγlaths contained a relatively high dislocation density.Dislocation pile-ups were observed among lamellar tips,antiphase domain boundaries andα2/γinterfaces.Theα2+γlamellae exhibit relatively high stability.Parallel decomposition of coarseα2 laths was rarely found,the deformation twins were found only inγlaths,and no any lamellae were observed to break-up and globularise.The research has found that that blocky B2+ωordered phase can be removed completely after 1340℃HIPping.However,the ordered phase was precipitated again around the colony boundaries after creep.It is deduced that the refractory elements W,Nb still distribute nearby after the HIPping due to sluggish diffusion.They would contribute to the re-precipitation of B2+ωduring long-time creep under the high temperature and high stress.It is therefore concluded that the removing of B2+ωby HIPping is not an effective way to strengthen theα2 andγlamellae in TiAl alloys.
Study on Creep Behaviors of Candidate Material Used in Cladding of Super Critical Water Cooled Reactor
Super Critical Water Cooled Reactor (SCWR) is one of the candidate forms in the fourth generation reactor (GIV) units.In this paper, high temperature tensile test and creep test were carried out for Ni-based alloy C276, in order to study the high-temperature mechanical properties of the material in GIV notional aggregate at the condition of SCWR. Tensile test was carried out at 500℃、550℃、575℃、600℃and 650℃. And creep test was carried out at 600℃、650℃and 700℃with different stresses. After that, the tensile and creep properties of C276 were assessed with the micrograph of material being broken both by tensile and creep tests. And the data of the creep test were analysised with Norton’s creep damage formula and Kachanov’s creep damage formula. All of the results prove that the Ni-based alloy C276 has the good high-temperature mechanical property.
As an alternative material of WC-Co, TiC-Ni cermets is mainly used in machining aero. However the britleness of this material limits its scope of application. So far, there has been only a few study on high temperature mechanical properties especially high-temperature creep properties on TiC-Ni cermets. In this article, the author carried out a large number of high-temperature tensile/creep tests on TiC-Ni materials prepared by SHS-PHIP technology by using the CSS–3905 creep testing machine. Creep models were used to simulate experimental data. Then the minimum creep rates, stress exponents, and activation energy were also calculated. To investigate the relationship between damage and hardness, micro-Vickers hardness tester was used to measure the hardness of TiC-Ni before and after the tensile/creep experiments. Experimental results showed that the changes of hardness during the experiments had no defining characteristics.
With the development of modern industry, a lot of materials are working at high temperature. The property of materials and the distribution of stress change with the influence of temperature and time. Therefore, creep of materials must be considered.Automotive glass manufactured from flat glass is widely used in the automobile industry. First, the flat glass is heated to a molten state in hot bending stove. Then, the glass is shaped by forming machine in according to requirement. Molten glass will creep during the forming process, which can affect the quality of automotive glass directly. However, it’s significant to research the creep phenomenon of automotive glass.The experiments of uniaxial creep for car glasses at high temperatures and different stress levels are conducted, revealing the characteristics of nonlinear deformation. Under different temperature and loadings,the creep curves have similar shape. It’s observed that, at 550 o C~590 oC , the primary creep is not obvious, while the steady creep and the accelerating creep processes are dominant. And, the fracture data follow the Monkman-Grant relationship, not influenced significantly by temperature and stress. On the experiments, structure factor is considered into the strain rate, then, acceleration stage is built by fracture time and structure factor, at last, the paper presents a nonlinear uniaxial constitutive equation to describe the creep of glasses. By theoretical computation, it’s found that the nonlinear uniaxial constitutive equation proposed in this paper can describe the creep process of car glasses at high temperatures.
Effect of Nd and Reciprocating Extrusion on Microstructure and High Temperature Creep Resistance of Al2O3f/AZ91D Composite
Al2O3f/AZ91D composites with different Nd contents (0.0%,0.2%,0.5%and 0.8%) were prepared by pressureless infiltration method, and the volume fraction was 20%. Reciprocating extrusion was carried on the samples with 0.2%,0.5%and 0.8%Nd. The effect of Nd and reciprocating extrusion on the microstructure, phase composition and fracture mechanism were analyzed by OM, SEM, XRD and EDS. Effect of Nd on the hardness and high temperature creep resistance were studied, and the creep resistance of Al2O3f/AZ91D was studied by the high temperature compression tests. Creep process and creep mechanism were revealed by creep curves and creep stress exponent of composites, and the evolution of microstructure and dynamic recrystallization mechanism during the creep process were also discussed.The granular and rod shaped Al-Nd phase were formed and matrix microstructure was refined by adding Nd. The addition of Nd made Mg2Si phase become fine and dispersive. The hardness and the creep resistance of composites added with Nd were improved significantly, and the optimum content of the additives was 0.8%. The microhardness of Mg2Si phase increased first and decreased afterwards by adding different contents of Nd, and reached maximum at 0.5%.The matrix microstructure of Al2O3f/AZ91D composite was refined, the segregation of fiber was eliminated and the homogeneity of fiber distribution was improved after 2 passes of reciprocating extrusion. The creep resistance of composites after reciprocating extrusion was better than the composites as cast at temperature 250℃under pressure 90MPa. The hardness of composite was improved although the microhardness of Mg2Si phase was reduced by reciprocating extrusion.After high temperature compression creep test, the distribution of fiber became even, matrix microstructure was refined and deformation twins and cross sliding lines could be observed, and recrystallized grains were formed in fiber segregation area. According to the lnε-lnσcurve at temperature 250℃, stress exponent n is calculated to be 6.6, which indicates that the controlled high temperature creep mechanism is dislocation climb controlled by grain boundary diffusion and a constant substructure model. According to n theε1/n-σcan be plotted and the threshold stress is calculated to be 1.32MPa at temperature 250℃.
Research on High Temperature Mechanical Behavior of Copper Alloy Materials with Aerospace Power Systems
In this paper, high temperature tensile, high temperature fatigue and high temperature creep behavior of Cu-0.2%Zr and Cu-3%Ag-0.5%Zr alloy have been studied. Test by test, obtained different factors on two alloys’mechanical properties of high-temperature law. Methods of metallographic and SEM have been used,analysis the microstructure and failure fracture of CuZr and CuAgZr alloy systematically,discussion the fracture mechanism of two alloys under different microstructures at high temperature。CuZr and CuAgZr alloy high temperature tensile test results at the same temperature and stretch rate show that, the yield strength, tensile strength of smaller grain size material are batter than the bigger grain size. Results consistent with Hall-Petch relationship. The fracture morphology of SEM results show that, two alloys tensile fracture mechanism of high-temperature ductile fracture along the grain, the fracture of plastic materials, the better the smaller, share of the fracture zone and a larger proportion of fiber.Two alloys in the same stress ratio of high temperature and high cycle fatigue test results show that, under the same stress level, with increasing temperature, fatigue life of the two alloys decreased, smaller grain size materials high temperature and high cycle fatigue life are longer, obtained the fatigue limit at different temperatures of two alloys. At the stage of high-cycle fatigue, comparison with the test stress, temperature impact greater on the fatigue life of two alloys. Intergranular fracture of two alloys are ductile fracture, but fracture characteristics are no significant than temperature tensile fracture.CuZr and CuAgZr alloy at the same strain under high temperature low cycle fatigue test results show that, with increasing temperature, the fatigue performance of the two alloys is improved. CuAgZr alloy in fatigue process showed cyclic softening feature. Fracture surface of the two alloys are more holes, the fracture is mainly intergranular ductile fracture, fracture characteristics of the porous aggregate. Universal slope method using data obtained from the calculated equation are more practical and reliable, the method can be used to predict low cycle fatigue properties of two alloys as an effective criterion. Analysis shows that under the general slope, high temperature low cycle fatigue of CuZr alloy show better performance resistance.High temperature creep test results of two alloys show that, the microstructure of CuZr and CuAgZr alloy have significant changes after creep, and the larger grain size materials have better high temperature creep resistance. In intermediate temperature creep zone, viscous dislocation slip is CuZr and CuAgZr creep process control mechanisms. In high temperature creep zone, the creep process of CuZr and CuAgZr alloy are mainly controlled by grain boundary diffusion mechanism. High temperature creep fracture of CuZr and CuAgZr alloy showed ductile intergranular fracture, and exists secondary cracks. Creep rupture data of two alloys are found Monkman-Grant relationships.
With the development of science and technology, now the working environment of engineering structure and mechanical equipment have changed a lot,which is not only worked in alternating load, but also need to work under high temperature conditions. In many cases, when the engineering structure and mechanical equipment experiene high temperature creep and fatigue, Crack propagation is influeneed by material damage, causing leaking out or explosion.All thesestatus will lead to eeonomie losses and people casualty. So we study the material high-temperature creep reliability, it has a important significance when it is used to evaluate the safety of the structure.This paper described research that high temperature alloy steel GH36 has been tested on high temperature enduring strength and fracture experiments, and draw three factories GH36 steel high temperature enduring strength and fracture time data. And then these randomly distributed material properties of the measured datas are regard as random variables, mathematical statistical methods are used to deal with it. This paper are mainly researched on items listed below:First of all, there is a brief narration about the development of high-temperature creep experiments, and then creep test enginery characteristics and working principle are discussed, and according to the requirements of appropriate testing machine and test pieces.Secondly, the distribution characteristics, parameters estimation and hypothesis testing of the commonly used statistical distribution functions, which includes the normal distribution, lognormal distribution, the two-parameter and three-parameter Weibull are program by MATLAB, and the corresponding linear regression are charted.Thirdly, To handle the time data of GH36 steel high temperature enduring strength and fracture with the distribution characteristics, parameters estimation and hypothesis testing of the commonly used statistical distribution functions, which includes the normal distribution, lognormal distribution, the two-parameter and three-parameter Weibull are program by MATLAB, and draw the best distribution types of each data.Finally, in order to get to meet the credibility of the results of statistical analysis based on the best distribution types of each data, and draw the three factories confidence interval of high temperature enduring strength and fracture, and through the F inspection and t-test contrast analysis we can known that, the dispersion of GH36 steel high-temperature creep data of QiGang production is big than others, and there is no significant difference between erchong and 43chang about theirvariance, but erchong’s mean larger.
In high-temperature corrosive environment, composite plate are more and more widely applied in the petroleum chemical industry because it can be designed, have corrosion resistance and cost advantage. However, in the process of metallurgy, manufacturing and useing, inevitably, there were various defects in the structure of the composite steel pressure vessel. Especially, in the composite pressure vessel steel’s weld, due to the special welding process and weld structure, and there is a big difference of materials, thermal physical properties, chemical composition and organization between the two kinds of combination of the composite steel plate. After welding, welding seam often appear dislocation, inclusions and voids defects. Under high temperature in a long time, these defects is easy to produce creep crack, eventually leading creep rupture failure to the container, cause a great loss and dangerous to society, so it is need of stuty the composite steel pressure vessel’s weld creep.As the composite plate weld joint creep tensile specimen preparation difficult, creep test takes longer, expensive, experimental study of composite steel pressure vessel weld creep is difficult. But the finite element (FEA) method which is a comprehensive, relatively short time, inexpensive, widely used in high temperature performance of welded components.In terms of composite steel pressure vessel weld creep mechanism, composite steel pressure vessel weld different parts and different structures creep damage under high temperature, and composite steel pressure vessel safety assessment of high temperature creep crack and others, this paper completed the study about the composite steel pressure vessel weld creep.1. Useing metal viscoelastic material mechanics theory and damage mechanics theory analysis methods, studied composite steel pressure vessel weld creep mechanism, analyzed material characteristics about every layer composite steel plate,weld structure and characteristics of high temperature creep damage. Establish material differential constitutive of Generalized Kelvin model, and thus derived composite steel pressure vessel cylinder visco elastic constitutive equation.2. Adoption of international general finite element analysis software ANSYS10.0, completed numerical analysis about stainless steel composite plate (304+16MnR) pressure vessel weld high temperature creep,it is uniform with the results of similar experimental and theoretical studies. The numerical results show that: composite steel pressure vessel weld creep serious is higher than the base metal, the complex layer more serious than the primary, weld heat affected zone creep is the most serious in weld complex layer, followed by the weld interface transition layer. In addition,when t=0, the entire weld (complex, primary, intermediate layer) creep stress is common with membrane stress,enter into the first stage of creep, the complex layer creep stress is gradually increased, primary creep stress is gradually reduced and finally become stable, the maximum creep stress creep present in complex layer weld heat affected zone.3. this paper adopted the assessment of technology of the British Standard BS7910-1999 “metal structure defect evaluation method of acceptance of guidelines”, combined with composite steel pressure vessel itself structural characteristics, presented a set of composite steel plate pressure vessel high-temperature integrity assessment method which assessed separately the different functions structure, that testing and evaluation methods of foot-layer is in accordance with the general pressure vessels’methods, but the main testing and assessment of complex layer is cracks whether affected its corrosion function.This subject adopted finite element numerical analysis method, studied various composite steel pressure vessel weld interfaces creep strain field and stress field distribution, intuitively reflect various weld interfaces creep damage, provided a reliable theoretical basis for optimal design, nondestructive testing and security assessment of the composite steel pressure vessel.
Research on the Fabrication and High Temperature Mechanical Properties of TiC/Ni Cermet Composite Material
TiC/Ni cermet composite material has been synthesized by melt in-situ reaction from mixed Ti,C,Mo and Ni powders.Microstructure and phase composition of the cermet were analyzed by scanning electron microscopy and X-ray diffraction respectively.Relative density and hardness were measured.Because there have been a few study on high temperature mechanical properties especially high temperature creep properties on TiC/Ni cermet so far,in this article,a large number of high temperature tensile and creep tests were carried out on TiC/Ni cermet prepared by in-situ reaction on the CSS-3905 electronic creep test machine.at last,the creep damage of the creep specimen was characterized by the method of electric resistance,the method of Rockwell hardness and the method of elastic modulus.Results show that TiC/Ni cermet composite material is mainly consist of metal phase Ni and ceramic TiC,the addition of Molybdenum has effect on refining the size of the particle of ceramic phase;the relative density of TiC/Ni cermet composite material is becoming larger with the increasing of the content of TiC, when the content of TiC is constant,the relative density has a positive proportional to the content of Ni and has nothing to do with the content of the content of Mo.and Rockwell hardness is determinated by the phase compositions and the relative density;the results of the tensile test show that when the content of TiC is low,the mechanism of the fracture is ductile fracture,when the content of TiC is high,the mechanism of the fracture is brittle fracture.When the temperature is increasing,the strength of the material decreases,the strain increases.TiC/Ni with 20% volume fraction of TiC and 5% volume fraction of Mo has the best creep resistance.and the mechanism of creep fracture is intergranular fracture;the feasibility study on the method of electric resistance is verified,the method of Rockwell hardness has something difficult to survey.the method of elastic modulus is well to characterize the creep damage, variations of the curve of damage fator are concordant with variations of the curve of creep at different periods.