Improving creep strength of the fine-grained heat-affected zone of novel 9Cr martensitic heat-resistant steel via modified thermo-mechanical treatment

The infamous type Ⅳ failure within the fine-grained heat-affected zone (FGHAZ) in G115 steel weldments seriously threatens the safe operation of ultra-supercritical (USC) power plants.In this work,the traditional thermo-mechanical treatment was modified via the replacement of hot-rolling with cold rolling,i.e.,normalizing,cold rolling,and tempering (NCT),which was developed to improve the creep strength of the FGHAZ in G115 steel weldments.The NCT treatment effectively promoted the dissolution of preformed M_(23)C_(6)particles and relieved the boundary segregation of C and Cr during welding thermal cycling,which accelerated the dispersed reprecipitation of M_(23)C_(6) particles within the fresh reaustenitized grains during post-weld heat treatment.In addition,the precipitation of Cu-rich phases and MX particles was promoted evidently due to the deformation-induced dislocations.As a result,the interacting actions between precipitates,dislocations,and boundaries during creep were reinforced considerably.Following this strategy,the creep rupture life of the FGHAZ in G115 steel weldments can be prolonged by 18.6%,which can further push the application of G115 steel in USC power plants.

Creep Rupture of Mismatched Welded Joints of Steels with Dissimil ar Creep Strengths

The finite element analysis of mismatched welded jo ints with a 30°groove angle was performed to study the mechanical behavior of D MWJs (dissimilar metal welded joints). It is concluded that the distribution of stress triaxiality in the DMWJs is uneven, especially near the fusion lines. The degree of creep strength mismatch has remarkable effect on the distribution. Th e higher the level of mismatch is, the more uneven the distribution is and the e asier for premature failure to occur in the joint.

On the microstructural origin of premature failure of creep strength enhanced martensitic steels

The creep strength enhanced martensitic steels are key material for the main power generating units in ultra-supercritical plants.Studies on the evaluation of their creep rupture life show there is an overestimation of rupture life after long-term creep,which is known as premature failure.However,the microstructural origin of the premature failure remains unclear.Here in this study,we have carefully investigated the microstructural transformations and their influences on creep rupture behavior,showing that the evolution of martensite and M_(23)C_(6) carbides as well as Laves phase are responsible for the premature failure.By using multi-step TTP-LMP method,we confirmed a three-stage creep rupture behavior under different stress regions.Further quantitative analysis showed that the coarsening of M_(23)C_(6) carbides and recovery of martensite exert equal and dominant effects on the premature failure in the medium stress region,while precipitation and coarsening of Laves phase are responsible for the premature failure in the low stress region.

The effects of DS blade's geometry features on material's creep strength

Because of the better creep performance,the directional solidification(DS)Nickel-based turbine blades have been widely used in advanced aero-engines.However,the DS turbine blade's different abrupt geometrical changes at different regions cause a variation of temperature field at those regions.Subsequently,the variable temperature field is very likely to lead to a different grain structures at those different region,and those different grain structure finally give rise to a variation in material's creep performance at different region in DS turbine blade.To study the variation in creep strength among different regions of a DS turbine blade,this article designed and manufactured three types of DS specimens to simulate the geometry features of platform,shroud and body part of a typical DS turbine blade.Creep tests on these specimens were conducted under the stress level of 608 MPa and temperature of 850。C,the creep rupture life of platform-like and shroud-like specimens are 93%and 73%of body-like specimens'respectively,which support the assumption that there exists a certain variation in material's creep strength among different regions in DS turbine blades.The fracture positions of these specimens also support above conclusion.It is suggested that the material's creep strength variation among different locations of DS turbine blades should be considered in future turbine blade life design and prediction.

TENSILE STRENGTH AND CREEP RESISTANCE OF Mg-9Al-1Zn BASED ALLOYS WITH CALCIUM ADDITION

Small amount of calcium addition to the Mg-9Al-0.8Zn-0.2Mn (AZ91) alloy resulted in obvious influence on mechanical properties. The yield strength of the alloys increased with the increase of Ca addition and the maximum strength was obtained from the alloy containing 0.15% of Ca. The creep resistance at the temperatures between 150-220°C was also significantly increased with Ca addition. The creep rate (at 200°C, 50 MPa) of the alloy with 0.15% Ca addition was one order of magnitude lower than that of the base alloy (AZ91). Microstructural observations revealed that the addition of calcium refined the microstructure and enhanced the thermal stability of the β precipitates, which accounted for the improvement of creep resistance at high temperatures.

Development of a Nickel-base Cast Superalloy with High Strength and Superior Creep Properties

Derived from Russian alloy CHS88U, six experimental Ni-base alloys named as A to F in the Ni-Cr-Co-W-Ti-Al-Hf system are designed, evaluated and processed. One of these alloys, F, shows excellent high temperature tensile strength and ductility with superior creep rupture properties. As predicted by using modeling tools such as PHACOM and NEW PHACOMP, there is hardly the tendency for formation of topologically close-packed phase (TCP) phase in alloy F. Furthermore, through microstructural observation, it is also found that no TCP phase is formed in alloy F after long-time exposure at high temperature. So alloy F has well balance of phase stability and mechanical properties in view of application for gas turbines. It is proved that d-electron approach can be applied for design and development of nickel-base superalloys for gas turbine application.

The Effect of Cold Working on Creep Rupture Strength and Microstructure of Ni-23Cr-7W Alloy

In order to clarify the reason why the creep rupture time of pre-strained Ni-23Cr-7W Alloy (HR6W) is longer than that of the non-pre-strained HR6W, microstructures of HR6W after a series of creep tests were investigated. The creep tests were conducted at 750°C, 90 and 100 MPa. In the pre-strained samples, the grain boundary shielding ratio by precipitates was larger than that of the non-pre-strained sample. In addition, in the pre-strained samples the size of the M23C6 carbide in the grains was finer than in the non-pre-strained sample. The W content in the M23C6 carbide in the pre-strained samples tended to be larger than in the non-pre-strained sample. Therefore, the Ostwald ripening of the carbide was delayed and the size of M23C6 carbide was thought to be fine for a long time. These observations show that creep strength in the pre-strained samples is higher than that of the non-pre-strained sample because of both precipitation strengthening inside of the grains and grain boundaries.

MECHANICAL PROPERTIES OF FINE PITCH DEVICES SOLDERED JOINTS BASED ON CREEP MODEL

Nonlinear analyses of quad flat package （QFP） on printed circuit board （PCB） assemblies subjected to thermal cycling conditions are presented. Two different solders are considered, namely, Sn37Pb and Sn3.5Ag. The stress and strain response of fine pitch devices soldered joints was investigated by using finite element method based on Garofalo-Arrheninus model. The simulated results indicate creep distribution of soldered joints is not uniform, the heel and toe of soldered joints, the area between soldered joints and leads are the creep concentrated sites. The similar phenomena of stress curves simulated based on Garofalo-Arrheninus model and Anand equations is confirmed, and the creep strain value of Sn3.5Ag soldered joints is lower than that of Sn37Pb soldered joints. Thermal cycling results show that Sn3.5Ag strongly outperforms Sn37Pb for QFP devices under the studied test condition. This is well matched with the experimental outcome analyzed. In addition, the soldered devices were tested by micro-joints tester, the tensile strength of Sn3.5Ag soldered joints is found to be higher than that of Sn37Pb soldered joints. By analyzing the fracture microstructure of soldered joints, it is found that fracture mechanism of Sn3.5Ag soldered joints is toughness fracture, while fracture mechanism of Sn37Pb soldered joints includes brittle fracture and toughness fracture. The results of this study provide an important basis of understanding the mechanical properties of fine pitch devices with traditional Sn37Pb and Sn3.5Ag lead-free soldered joints.

Elevated temperature endurance and creep properties of extruded 2D70 Al alloy rods

The elevated temperature performances of 2D70 Al alloy hot extrusion rods after two-stage homogenization and intensive deformation were studied by measuring the elevated temperature enduring strength and the creep ultimate strength. The fracture morphology of some selected samples after testing at different elevated temperatures was observed by scanning electron microscopy （SEM）. The results indicate that, as the test temperature increases, the elevated temperature enduring strength of 2D70 Al alloy decreases gradually. In a comparison between 150 C and 240 C, the notch enduring strength drops from 375 to 185 MPa and the smooth enduring strength drops from 337 to 130 MPa. Enduring strength is not sensitive to the notch. The notch sensitivity ratio （NSR） coefficient is in the range of 1.119 to 1.423 from 150 C to 240 C. The creep test results show that, as the test temperature increases from 150 C to 240 C, the creep ultimate strength of 2D70 Al alloy rods drops gradually from 312 to 117 MPa.

Research on the creep damage and interfacial failure of dissimilar metal welded joint between 10Cr9Mo1VNbN and 12Cr1MoV steel

The mechanical properties, creep damage, creep rupture strength and features of interfacial failures of welded joints between martensite (SA213T91) and pearlite steel (12Cr1MoV) have been investigated by means of argon tungsten pulsed arc welding, high temperature accelerated simulation, creep rupture, mechanical property tests and scanning electronic microscope (SEM). The research results indicate that the mechanical properties of overmatched and medium matched joint deteriorate obviously, and they are susceptible to creep damage and failure after accelerated simulation operation 500 h, in the condition of preheat 250℃, and post welding heat treatment 750℃×1 h. However, the mechanical properties of undermatched joint are the best, the interfacial failure tendency of undermatched welded joint is less than those of medium and overmatched welded joint. Therefore, it is reasonable that low alloy material TR31 is used as the filler metal of weld between SA213T91and 12Cr1MoV steel.

Laboratory creep tests for time-dependent properties of a marble in Jinping Ⅱ hydropower station

In order to investigate the time-dependent behaviors of deep hard rocks in the diversion tunnel of Jinping II hydropower station, uniaxial creep tests were carried out by using the triaxial testing machine RC-2000. The axial compressive load was applied step by step and each creep stage was kept for over several days. Test results show that： （1） The lateral deformation of rock specimens is 2-3 times the axial compressive deformation and accelerates drastically before damage, which may be employed as an indicator to predict the excavation-induced instability of rocks. （2） The resultant deformation changes from compression to expansion when the Poisson＇s ratio is larger than 0.5, indicating the starting point of damage. （3） In the step-loading stages, the Poisson＇s ratio approximately remains constant; under constantly imposed load, the Poisson＇s ratio changes with elapsed time, growing continuously before the specimen is damaged. （4） When the applied load reaches a certain threshold value, the rock deteriorates with time, and the strength of rocks approximately has a negative exponent relation with time. （5） The failure modes of the deep marble are different in long- and short-term loading conditions. Under the condition of short-term loading, the specimen presents a mode of tensile failure; while under the condition of long-term loading, the specimen presents a mode of shear failure, followed by tensile failure.

Triaxial creep damage characteristics of sandstone under high crustal stress and its constitutive model for engineering application

The creep characteristics of rock under high crustal stress are of important influence on the long‐term stability of deep rock engineering.To study the creep characteristics and engineering application of sandstone under high crustal stress,this study constructed nonlinear creep damage(NCD)constitutive mode based on the triaxial graded loading‒unloading creep test of sandstone in the Yuezhishan Tunnel.A numerical NCD constitutive model and a breakable lining(BL)model were developed based on FLAC3D and then applied to the stability analysis of the Yuezhishan Tunnel.Based on the creep test results of sandstone,a power function of creep rate and stress level was constructed,by which the long‐term strength was solved.The results show that the long‐term strength of the red sandstone based on the related function of the steady‐state creep rate and stress level is close to the measured stress value in engineering.The NCD model considering damage factors reflects the instantaneous and viscoelastic plasticity deformation characteristics of the red sandstone.The numerical NCD constitutive model and the BL model can reflect surrounding rock deformation characteristics and lining failure characteristics in practical engineering.The research results provide theoretical references for long‐term stability analysis of rock engineering and the deformation control of surrounding rock under high crustal stress.

Magnesium nanocomposites:An overview on time-dependent plastic(creep) deformation

Magnesium(Mg) nanocomposites are created when nano-size particles are embedded into the Mg(or Mg alloy) matrix. The Mg nanocomposites, cited as high-strength energy-saving materials of future, are a group of emerging materials with excellent combination of strength and ductility and superior specific strength property(strength-to-weight ratio). Having said this, Mg nanocomposites are considered as promising replacement for other structural alloys(i.e. aluminum and titanium) wherever low density and high strength are required, i.e. transportation, aerospace, defense, etc. To be able to apply this group of materials for real components, different failure mechanisms at ambient and elevated temperatures under static and dynamic loading condition must be well documented. Compared with other metals and alloys,rate-dependent plastic deformation(creep), at ambient and elevated temperatures, of these novel materials is not yet well studied which seems a tangible lack of knowledge. This is required since the materials in service are often exposed to medium and elevated temperatures and/or static loads for long duration of time and this encourages creep failure on them. To this end, the information and the controlling mechanisms on time/temperature-dependent response of the material need to be developed to be able to predict the response of the Mg nanocomposites where the materials are under creep conditions. This paper aims at providing an overview on(i) creep-resistant Mg alloys(as matrix) and their chemical compositions, and(ii) responses of the Mg nanocomposites at different creep conditions(time and temperature). The controlling mechanisms contributing to the strength and ductility of the Mg nanocomposites due to the presence of the nanoparticles have been reviewed briefly in the present article. In this paper both traditional(uniaxial) and depth-sensing indentation creep of Mg nanocomposites are reviewed. Also, some fundamental questions and possible explanations have been raised on the creep characteristics of Mg nanocomposites and the contribution of micro structural features(i.e.grain boundaries, twins, precipitates, nanoparticles). This overview article provides a comprehensive summary to understand one of the failure modes(creep) at ambient and elevated temperature in the energy saving Mg nanocomposites that would be of interest for those in academia who explore novel nanocomposites.

Aging Coefficient, Creep Coefficient and Extrapolating Aging Coefficient from Short Term Test for Sealed Concrete

Aging coefficient and creep coefficient are important parameters for creep analysis of any structure. With the aim to obtain those parameters, an experimental investigation is carried out on sealed concrete. Altogether ten specimens were tested, out of which four were for creep, two for shrinkage and remaining four for relaxation test. A year of relaxation test and two years of creep test results were analyzed to compute aging coefficient and creep coefficient. From regression analysis of observed and calculated aging coefficient using formula of Bazant and Kim, modification for aging coefficient is performed and extrapolation equations are generated. Instead of ACI model, B3 model has been used to obtain compliance function as the parameters of B3 model seems more relevant to creep mechanism of concrete compared to that of ACI model.

Study on Strength and Life of Anisotropic Single Crystal Blade - Part I: Crystallographic Constitutive Models and Applications

The single crystal blade is one of the key technologies for improving the performance, durability and reliability of aero-engines and ground gas-turbine engines. However, the anisotropic mechanical properties of the single crystal material makes a great deal of difficulties on the development and the application of the single crystal blade, which is a challenge for the engineering application of the single crystal superalloy and the theoretic bases of the application. Some researches on the strength analysis and the life prediction of the anisotropic single crystal blade were carried out by the authors' research team. They are as follows. The crystallographic constitutive models for the plastic and the creep behaviors and the method of the rupture life prediction were established and verified. The tensile or the creep experiments for DD3 single crystal alloy with different orientations under different temperatures and different tensile rates or under different temperatures and different stress levels were carried out. The experimental data and the anisotropic properties at intermediate and high temperatures revealed by the experiments are significant for the application of the single crystal alloy. In addition, the experimental research for a kind of single crystal blade was also made. As the application of the researches the strength analysis and the life prediction were carried out for the single crystal blade of a certain aero-engine. In this part, the constitutive models and their applications are described, and the experimental research work will be described in part II.

Effects of Manufactured-sand on Dry Shrinkage and Crccp of High-strength Concrete

The influences of natural sand, manufactured-sand （MS） and stone-dust （SD） in the manufactured-sand on workability, compressive strength, elastic modulus, drying shrinkage and creep properties of high-strength concrete （HSC） were tested and compared. The results show that the reasonable content （7%-10.5%） of SD in MS will not deteriorate the workability of MS-HSC. It could even improve the workability. Moreover, the compressive strength increases gradually with the increasing SD content,and the MS- HSC with low SD content （smaller than 7%） has the elastic modulus which approaches that of the natural sand HSC, but the elastic modulus reduces when the SD content is high. The influence of the SD content on drying shrinkage performance of MS-HSC is closely related to the hydration age. The shrinkage rate of MS-HSC in the former 7 d age is higher than that of the natural sand HSC, but the difference of the shrinkage rate in the late age is not marked. Meanwhile the shrinkage rate reduces as the fly ash is added; the specific creep and creep coefficient of MS-HSC with 7% SD are close to those of the natural sand HSC.

RHEOLOGICAL BEHAVIOUR FOR AN ANHYDRITE SPECIMEN AND DETERMINATION OF ITS LONG TIME STRENGTH

Based on the method of torsional creep, the creep laws of ananhydrite specimen are studied in this paper. When a shearing stressapplied to the specimen is less than a value, only the primary stagetakes place. How- ever, when the shearing stress is more than anothervalue, all the three stages of a creep curve, i. e. primary, steady-state and accelerated are exhibited.

High strength and high creep resistant ZrB2/Al nanocomposites fabricated by ultrasonic-chemical in-situ reaction

In this study, the ZrB2/Al nanocomposites were fabricated via in-situ reaction of the Al-K2ZrF6-KBF4 system, assisted with ultrasonic vibration and spiral electromagnetic stirring. Microstructure, tensile property and creep behavior of the fabricated nanocomposites were further investigated. Microstructure observation showed that the ultrasonic vibration could prevent the fast growth as well as break the clusters of in-situ synthesized nanoparticles in melt, resulted in smaller size (10-50 nm) and relatively more uniform distribution of the in-situ nanoparticles located on the boundary of and/or inside the aluminum matrix grains in the final composites. The fabricated nanocomposites exhibited an enhancement in both strength and ductility, due to the elevated work hardening ability, i.e., improved dislocation propagating ability and decreased dynamic recovery of the existing dislocations induced by the in-situ nanoparticles. Meanwhile, the nanocomposites exhibited excellent creep resistance ability, which was about 2-18 times higher than those of the corresponding aluminum matrix. The stress exponent of 5 was identified for the fabricated nanocomposites, which suggested that their creep behavior was related to dislocation climb mechanism. The enhanced creep resistance of the nanocomposites was attributed to the Orowan strengthening and grain boundary strengthening induced by the ZrB2 nanoparticles. Thus, the ultrasonicchemical in-situ reaction promises a low cost but effective way to fabricate aluminum nanocomposites with high strength and high creep resistance.

Improvements of Elevated Temperature Strength and Creep Resistance of Mg-12Zn-4Al Based Alloy with Calcium Addition

A series of Mg-12Zn-4Al-0.3Mn(ZA124) based alloys with additions of calcium have been prepared and their microstructure and properties have been investigated.The results show that Mg-12Zn-4Al-0.3Mn alloy consists of the α-Mg matrix and block quasicrystal and exhibits excellent creep resistance compared to commonly used AZ91 alloy.A small amount of calcium addition to the ZA124 based alloys increased the yield strenghth at both ambient and elevated temperatures as well as creep resistance,although the ductility decreased slightly.Microanalysis indicated that the addition of calcium decreased the quasicrystalline phase and caused the formation of some lamellar precipitates of Al2Mg5Zn2 with orthorhombic crystal structure.This Ca-containing Al2Mg5Zn2 phase with high stability straddled the grain boundaries and strengthened grain boundaries by inhibiting crack propagation during creep tests at elevated temperature.

天然和饱水红砂岩三轴压缩蠕变变形试验研究

蠕变特性和长期强度是红砂岩地区岩体工程安全评价的关键要素,饱水软化效应对红砂岩蠕变特性和长期强度具有重要影响.采用全自动岩石三轴压缩伺服流变仪,开展天然与饱水红砂岩的三轴压缩试验和三轴压缩蠕变试验.试验结果表明:峰值强度、弹性模量和蠕变破坏应力因饱水软化效应而降低,围压对其降幅有削弱作用,蠕变破坏应力与峰值强度的比值在0.75~0.98之间;饱水软化效应能增强蠕变瞬时变形,减弱蠕变变形及蠕变应变速率;环向应变比轴向应变更早进入稳态蠕变阶段和加速蠕变阶段;稳态蠕变速率与偏压的变化关系符合指数函数关系;长期强度因饱水软化作用而降低,却因围压增加而增大,长期强度与峰值强度的比值在0.62~0.92之间;裂纹损伤应力与长期强度相差较大,裂纹损伤应力视为长期强度有待进一步考究.