Creep mechanism of as-cast Mg-6Al-6Nd alloy

The creep mechanism of as-cast Mg-6Al-6Nd alloy was studied. The stress exponent for creep is 5.8 under the applied stresses of 50–70 MPa at 175°C. The activation energy for creep is 189 kJ·mol⁻¹ under the applied stress of 70 MPa in the range of 150–200°C. The true stress exponent and threshold stress for creep are calculated as 4.96 and 10.2 MPa, respectively. The true stress exponent indicates that its creep mechanism belongs to the dislocation climb-controlled creep, which is in agreement with the microstructure changes before and after creep. The high value for stress exponent is attributed to the interaction of Al₁₁Nd₃ phase with dislocations. The activation energy is more than the self-diffusion activation energy of Mg, which is attributed to the load transfer taking place from the matrix to Al₁₁Nd₃ phase during creep.     

Mg-5.5Zn-2Gd-0.6Zr铸造镁合金的蠕变机制

在ZM-1(Mg-5Zn-0.6Zr)合金的基础上,适量增加Zn的含量并加入重稀土元素Gd,设计了Mg-5.5Zn-2Gd-0.6Zr实验合金。采用砂型铸造工艺制备实验合金试样,在不同温度和应力条件下对该实验合金和ZM-1合金的蠕变曲线进行了测试。结果表明:在相同条件下,Mg-5.5Zn-2Gd-0.6Zr实验合金的稳态蠕变速率较ZM-1合金的降低了一个数量级;当施加应力为40 MPa时,实验合金的蠕变激活能Q200-250℃=142.0 kJ/mol,接近镁的自扩散激活能,蠕变受位错攀移控制,而ZM-1合金在相同应力下蠕变激活能Q200-250℃=88.5 kJ/mol,接近镁的晶界扩散激活能,蠕变受晶界滑移控制。合金在200℃条件下的应力指数n=4.21,而ZM-1合金的应力指数n=2.21。因此,认为加入重稀土元素Gd后实验合金的蠕变机制发生改变,200℃时的蠕变机制为位错攀移机制。     

工业纯钛TA2的低温蠕变行为(英文)

对工业纯钛TA2在150℃（精对苯二甲酸装置钛冷凝器的工作温度）下的蠕变行为进行了研究。单轴拉伸蠕变试验结果表明,150℃下TA2材料蠕变第1阶段服从指数规律 ε = βt°。第1阶段蠕变衰减速度（时间指数口值）分别在160-200MPa和220-240MPa的试验应力范围内呈分阶段恒定的趋势。在较低的应力条件下（低于200MPa）,蠕变第1阶段的衰减速度较大,第2阶段存在蠕变饱和现象,且随着应力的降低,达到饱和所需的时间缩短。由此可以推论,第2阶段蠕变饱和的出现与第1阶段蠕变衰减速度密切相关,且在低温和较低的应力条件下,蠕变行为主要以第1阶段为主。     

CREEP BEHAVIOR MODELING OF UDIMET 720Li SUPERALLOY

Constant load creep tests on isothermally forged Udimet 720Li, an advanced superalloy for gas turbine disc application, were run in the stress/temperature field 900-450MPa/650-700℃ produeing rupture times in the 20-5000h range. The creep curves have shown a predominant accelerating creep stage, that has been described by the following equation:     

Effect of Carbon Content on the Creep Rupture Properties and Microstructure of 316H Weld Metals

Two types of 316 butt welds with carbon contents of 0.016% and 0.062% have been produced using the gas tungsten arc welding process.The δ-ferrite content decreased from 7.2 to 2.8% in volume as the carbon content increased.The creeprupture strength and creep ductility of the two types of weld metals have been measured at 550℃ over the stress range of 290-316 MPa and at 600℃ over 230-265 MPa.The microstructure change and precipitation behavior of the weld metals were observed and related to the creep rupture properties.The creep rupture strength of the C2(0.062% C) weld metal was higher than that of the Cl(0.016% C) weld metal at both 550 ℃ and 600 ℃.At 550℃,as the decrease in the applied stress,the difference of the creep-rupture life between the two weld metals diminished due to the higher depletion rate of carbon by precipitation of M_(23)C_6 in the C2 weld metal,while at 600℃,the difference enlarged due to the massive precipitation ofσ phase and extensive crack formation and propagation along σ/austenite boundaries in the C1 weld metal.For both the C1 and C2 weld metal,the decrease in ductility was adverse with the transformation percentage and related to products of theδ-ferrite transformation.     

STEADY-STATE CREEP OF UNIDIRECTIONAL C/C COMPOSITE WITH PYROLYTIC CARBON MATRIX

1.IntroductionWhenstructuralcomponentsworkatelevatedtemperature,thenatureofthematerialvarieswiththechangeoftemperature,andthestressmayberedistributedduetotheeffectofthetemperature-timehistory.Engineeringdesignofstructuralcomponentsforhightemperatureapplicationscanbeseverelyrestrictedbythecreeppropertiesofthematerialsinuse.Theapplicationofcarbon/carbon(C/C)compositesintheaircraftgasturbineforlongtimeserviceathighoperatingtemperatureisamainresearchtrendrecentlylllZI.Besidesthesolutiontotheprob…     

Effects of microstructure on creep behavior of Mg-5%Zn-2%Al（-2%Y） alloy

Creep tests of Mg-5%Zn-2%Al（-2%Y） （mass fraction） alloy were carded out at 175 ℃ under the load of 50 MPa, on the cast and solution condition, respectively. Relationship between the creep behavior and microstructure was studied by the optical microscopy, SEM and TEM. The result shows that the primary creep strain is relative to the thermo-stability and the quantity of intermetallic compounds in grain boundary area. During the secondary creep period, the minimum creep rate of solution-treated specimens is substantially decreased as a result of dynamic MgZn2 precipitation.     

Creep properties and controlled creep mechanism of as-cast Mg-5Zn-2.5Er alloy

The creep behaviors of as-cast Mg-5Zn-2.5Er alloy(mass fraction,%) ,under various applied stresses(50-70 MPa) and creep temperatures(150-200℃) for 100 h,were investigated.The stress exponent n is in the range of 1.5-5.8,and the activation energy Qc is in the range of 28.3-77.1 kJ/mol.With respect to the calculated n and Qc as well as the microstructures after creep,it is suggested that there is a transition region between grain boundary sliding(GBS) dominated creep to dislocation creep mechanism(from n3 to n3) ,arising in the steady-stage creep rate value of 2.89×10-9 s-1.     

TiAl基铸造合金的蠕变行为研究

研究了名义成分为Ti-46Al-2Cr-2Nb-0.15B合金在700～850℃,140～300MPa的蠕变条件下的蠕变性能和蠕变机制。研究表明,在蠕变过程中随着温度和载荷的增加,合金的最小蠕变速率随之增大;TiAl基合金在700～850℃,140～300MPa下的最小蠕变速率可用蠕变方程εmin=A(σ)4.7exp(-280/RT)来描述;在该蠕变条件下的蠕变行为主要受位错攀移控制。     

RELATIONSHIP BETWEEN CREEP PROPERTIES AND STRUCTURE OF STEEL 20Cr11MoVNbNB

Studies were made of the creep properties and structure change of steet 20Cr11MoVNbNB at550-650℃.It is found that at 550℃ when creep stress>180 MPa there exists linear rela-tion with two different slopes β_1 and β-2 between transitional creep strain ε_β and creeptime t~(1/3),and β_1(in initiol period)>β_2(in later period).The Creep rate of stable stage at550℃ can be expressed by =Aσ exp(-Q_ /RT),where,n=4.7.A exp(-Q_ /RT)=5.37×10~(-16).Apparent creep activation energy Q_ =430 kJ/mol at 550-650℃,which ismuch higher than the self-diffusion activation energy of atoms for matrix Fe.In creepprocess,as the creep stress and time increase,the total weight of precipitated phases and rela-tive weight of Cr,Mo and V increase,but those of Fe and Nb decrease in precipitated phases,     

C和Nb含量对TP347HFG钢在650℃析出相参量和持久寿命的影响

研究了C和Nb含量对TP347HFG钢在650℃析出相参量（成分、体积分数和尺寸）和持久寿命的影响.对ASME成分范围内的2种不同C和Nb含量的TP347HFG钢在650℃,230和150 MPa条件下进行持久实验,持久寿命分别为199,420 h和2426,8837 h,其中C含量较低Nb含量较高的样品持久寿命较长.对持久管样的EPMA-EDS+MPSM和TEM-EDS分析表明,较低的C含量和较高的Nb含量对应较少的M₂₃C₆和较多的MX,并阻碍了M₂₃C₆的聚集粗化,同时基体中可保留较多的Cr,有利于延长持久寿命.此外,运用热力学软件Thermo-Calc分析了在500-1300℃范围内C和Nb组合含量的变化对各相成分和体积分数的影响,与实验结果相吻合.     

Creep resistance of Mg-4 %Al-2 %RE-2 ? alloy

The creep resistance of the alloy Mg-4Al-2RE-2Ca（AEC422） and the base alloy AE42 was studied. The results reveal that the precipitated phases of AEC422 consist of Al2La and Al2Ca by contrast with the precipitated phase Al11La3 in AE42, which is instable and decomposes to A1l2La and Al at high temperature. Creep resistance of AEC422 is significantly improved compared with that of AE42. The microstructure of AEC422 has no obvious changes after creep test at 175℃ and 70MPa, as compared to that before creep test, indicating that Al2La and Al2Ca have high thermal stability. Especially Al2Ca phase largely increases the strength of the grain boundaries in AEC422, which accounts for the creep resistance improvement.     

镍基单晶高温合金在不同条件下的蠕变性能和组织演化(英文)

研究[001]取向的镍基单晶高温合金在不同测试条件下的蠕变性能,采用扫描电镜和透射电镜研究合金蠕变断裂后的γ′相、TCP相和位错组织演化特征。结果表明:合金具有良好的蠕变性能,蠕变曲线显示出两种不同的蠕变变形特征。在(760°C,600 MPa)、(850°C,550 MPa)条件下,蠕变第一阶段较长;在(980°C,250 MPa)、(1070°C,140 MPa)和(1100°C,120 MPa)条件下,蠕变第一阶段很短。蠕变断裂后,在(760°C,600 MPa)条件下γ′相形态变化不大;在(850°C,550 MPa)条件下γ′相已经合并长大;在(980°C,250 MPa)条件下基体γ被γ′相包围;在(1070°C,140 MPa)条件下基体γ不再连续;在(1100°C,120 MPa)条件下基体γ厚度进一步增加。在(760°C,600MPa)、(850°C,550 MPa)和(980°C,250 MPa)条件下合金无TCP相析出,而在(1070°C,140 MPa)和(1100°C,120MPa)条件下有针状TCP相析出。在低温高应力下,变形特征为位错包括层错的剪切机制;在高温低应力下为位错绕过机制,并在γ/γ′相界面形成位错网。     

Primary creep deformation behaviors related with lamellar interface in TiAl alloy

Constant tensile stress creep tests under the condition of 760~816°C/172~276 MPa in an air environment are conducted, and the microstructural evolution during primary creep deformation at the creep condition of 816°C/172 MPa was observed by transmission electron microscopy (TEM) for the lamellar structured Ti-45. 5Al-2Cr-2.6Nb-0.17W-0.lB-0.2C-0.15Si (at.%) alloy. The amount of creep strain deformed during primary creep stage is considered to be the summation of the strains occurred by gliding of initial dislocations and of newly generated dislocations. Creep rate controlling process within the primary stage seems to be shifting from the initial dislocation climb controlled to the generation of the new dislocations by the phase transformation of 2 to as creep strain increases.     

C和Nb含量对TP347HFG钢在650℃析出相参量和持久寿命的影响

研究了C和Nb含量对TP347HFG钢在650℃析出相参量（成分、体积分数和尺寸）和持久寿命的影响.对ASME成分范围内的2种不同C和Nb含量的TP347HFG钢在650℃,230和150 MPa条件下进行持久实验,持久寿命分别为199,420 h和2426,8837 h,其中C含量较低Nb含量较高的样品持久寿命较长.对持久管样的EPMA-EDS+MPSM和TEM-EDS分析表明,较低的C含量和较高的Nb含量对应较少的M₂₃C6和较多的MX,并阻碍了M₂₃C₆的聚集粗化,同时基体中可保留较多的Cr,有利于延长持久寿命.此外,运用热力学软件Thermo-Calc分析了在500—1300℃范围内C和Nb组合含量的变化对各相成分和体积分数的影响,与实验结果相吻合.     

工艺参数对蠕变时效2124铝合金力学性能和微观组织的影响

研究2124铝合金在蠕变时效过程中工艺参数对力学性能和微观组织的影响。结果表明，蠕变量和蠕变速率随着时效时间、温度、应力的增大而增大。硬度随着时间和应力的增加呈类似于先增加后减小的趋势。在实验温度185～195℃范围内，温度对硬度的影响不大。当蠕变条件为200MPa、185℃、8h时，试样得到最佳的力学性能，此时试样基体内同时存在强化相S＂相和S＇相。透射电镜观察表明外加应力能促进析出相的析出和长大，基体中没有发现明显的应力位向效应。     

P91钢的瞬态蠕变行为分析

系统研究了P91钢在600和620 ℃下不同应力水平时的瞬态蠕变变形行为。通过将蠕变过程中材料内部位错与析出相间相互作用耦合于内应力,结合Orowan方程,应用非线性拟合的方法确定了瞬态蠕变模型参数,对P91钢在瞬态蠕变阶段的变形曲线进行了数值模拟。结果显示,数值模拟结果与试验数据较为吻合,表明应用该模型对瞬态蠕变状态进行研究具有一定的可行性。同时,结合瞬态蠕变时间和归一化瞬态应变对温度和应力的依赖关系,进一步证实了P91钢在600和620 ℃下,135~175 MPa的应力范围内的瞬态蠕变主要是受位错攀移过程控制。     

Microstructure Evolution of a Single Crystal Nickel-Base Superalloy During Heat Treatment and Creep

Microstructure evolution of a single crystal nickel-base superalloy during heat treatment and tensile creep at 1010℃ and 248 MPa for 30h was observed and analyzed. Internal stresses because of lattice mismatch between γ and γ‘ phase provided the driving force for γ‘ shape evolution during heat treatment. More than 65 vol. % distorted cubic γ‘ phase keeping coherency with the γ matrix precipitated after solution at 1295℃ for 32h. The shape of γ‘ phase was perfectly cubic with increasing precipitate size during the two-step aging treatment. Due to the applied stress and intemal stress field the continuous γ-γ‘ lamellar structure perpendicutar to the apptied stress was fonmed after 30h tensile creep.     

MECHANICAL PROPERTIES OF TiaSiC2 AT HIGH TEMPERATURE

Creep and stress relaxation behavior, the elastic modulus and fracture toughness of machinable Ti3SiC2 at various temperatures from 20 to 1250℃ were investigated by means of three-point bending tests. The experiments were performed respectively at: (i) fixed stress and changed temperatures, and (ii) fixed temperature and changed stresses. A creep resistance parameter that represents the probability of creep deformation in a given condition was defined as a function of both applied stress and the threshold stress, varying in a range from 0 to 1. Elastic modulus at high temperatures was measured through comparing relative slopes of loading curves in cyclic loading curve. The fracture toughness measured by SENB method showed a stable value in the range of 25-1000℃, but over 1000℃, it declined abruptly from ～6.7MPa.m^1/2 to～2.0MPa.m^1/2 at 1200℃.     

C-HRA-3耐热合金850 ℃持久性能及组织特征

研究了C-HRA-3耐热合金在850 ℃持久试验过程的碳化物特征。结果表明:当持久应力为120 MPa时,持久寿命为37.8 h,主要析出相包括M₂₃C₆相和Ti(C, N)相,高应力状态下,M₂₃C₆相沿变形带析出。当持久应力为80 MPa时,持久寿命延长至491 h,晶界处M₂₃C₆相部分回溶,导致晶界特征不明显,同时细小M₂₃C₆相重新析出,晶内析出的Ti(C, N)相尺寸增加。不同应力下试样的断裂方式均为沿晶断裂,裂纹源优先在尺寸较大的析出相(M₂₃C₆和Ti(C, N))周围萌生,导致伸长率随持久寿命延长而减小。