TA15钛合金的高温蠕变行为

研究TA15钛合金在500~525℃下的高温蠕变行为,实验应力为250~350 MPa。计算合金在不同应力、不同温度下的稳态蠕变速率和应力指数以及蠕变激活能,并通过引入临界应力的概念对稳态蠕变的Arrhenius方程式进行修正,得出不同温度下的临界应力以及合金的真实蠕变应力指数,在此基础上研究其蠕变变形机制。研究结果表明,蠕变应力为350 MPa时,合金的蠕变激活能appQ=403.1 kJ/mol;500℃和600℃下,TA15合金的蠕变临界应力0?值分别为82.15 MPa和34.79 MPa;500℃,TA15合金的真实蠕变应力指数P值为1.7~4.3,600℃时,合金的P值为4.0~6.0;在实验温度和应力范围内,位错的攀移和滑移在TA15合金蠕变变形过程中的作用很大,其中以位错攀移为主,位错滑移为辅。     

退火处理工艺下Ti40合金蠕变组织研究

对Ti40合金进行600℃×4h／AC退火处理,并测试合金在500～600℃温度范同内250MPa应力下的蠕变性能、,实验结果表明,蠕变应力为250MPa的条件下,当蠕变温度不超过520℃时,合金蠕变性能较好,当蠕变温度升高到535℃时,合金蠕变性能急剧恶化,不能满足使用要求。Ti40合金蠕变稳态阶段是位错滑移塞积和攀移释放应力的动态平衡。当蠕变进入第三阶段,出现主位错的分解与合并以及位错之间的交割和缠结。在蠕变过程中,位错的缠结和塞积导致应力集中,最终在晶界处形成蠕变空洞。     

温度和应力对Ti-47.5Al-6(Cr,Nb,W,Si,B)%合金蠕变性能的影响

研究了Ti-47.5Al-6(Cr,Nb,W,Si,B)%合金在不同试验温度和试验应力下的蠕变性能,并分析了不同试验条件下的蠕变机制.试验结果表明,该合金在760℃,100～150MPa具有良好的蠕变性能,在200MPa,700～800℃温度范围内蠕变激活能为U≈299kJ·mol-1,蠕变机制受原子扩散过程控制.在760℃和100～200MPa应力范围内,蠕变应力指数n从2.1变到4.2,故蠕变变形由高密度界面滑移控制变为位错攀移控制的回复蠕变.     

Al-Fe-Sc合金压缩蠕变行为及形貌演变研究

通过压缩蠕变测试及Al-Fe-Sc合金形貌观察，研究了不同压缩蠕变参数对合金蠕变行为及形貌的影响。实验采取压缩蠕变参数：70和90 MPa压应力，90,120和150℃蠕变温度。发现Al-Fe-Sc合金在70 MPa/90,120和150℃条件下，稳态蠕变速率为1.57×10^-7,1.37×10^-5和3.35×10^-6 s^-1，硬度为HV 40.58,HV 35.16和HV 42.32；在90 MPa/90,120和150℃条件下，稳态蠕变速率为1.79×10^-6,1.36×10^-5和7.60×10^-6 s^-1，硬度为HV 45.54,HV 34.46和HV 50.34。样品形貌随温度的升高和压力的增大由各向异性趋向于多种取向，锯齿状晶粒晶界越明显。透射电镜(TEM)图像显示Al₃Sc沉淀相弥散分布在亚晶粒中和亚晶界处，强烈地阻碍了位错和晶界的移动，起到强化力学性能和抑制压蠕变变形的作用。     

Mg-9Gd-3Y-0.3Zr合金的蠕变行为

研究Mg-9Gd-3Y-0.3Zr合金在不同温度(200~300℃)和应力(30~110MPa)条件下的蠕变行为,利用金相显微镜、透射电镜等分析蠕变过程中合金组织的演变。结果表明:温度较低时(200~250℃),蠕变曲线分为瞬时和稳态蠕变两部分,利用Arrhenius公式计算出合金的平均应力指数n=2,由此判断蠕变机制是晶界滑移机制,平均蠕变激活能Q=85.6kJ/mol;当温度为300℃时,合金经过短暂的瞬时蠕变和稳态蠕变阶段后,很快进入断裂阶段。n=4.2,蠕变机制为位错攀移机制,Q=145.5 kJ/mol。在温度较低时,稀土元素所形成的析出相β￠相阻碍位错的运动,从而提高合金的抗蠕变能力;随蠕变温度升高,析出相转变为β相,在晶界处聚集长大,使晶界处易产生应力集中,促使孔洞的形成,导致合金发生蠕变断裂。     

UO2烧结块高温蠕变性能的试验研究

对晶粒度分别为9.0μm 、23.8μm的UO2 烧结块进行了高温蠕变试验.结果表明:在相同温度、相同载荷下,UO2 烧结块晶粒尺寸越小,稳态蠕变速率越大;在相同温度下,稳态蠕变速率的对数值与应力的对数值之间存在线性关系,且随着载荷增大,UO2 烧结块的稳态蠕变速率增大;在相同载荷下,随着温度升高,UO2 烧结块的稳态蠕变速率增大.对于晶粒度分别为9.0μm 、23.8μm的UO2 烧结块,在载荷为10 MPa下的高温蠕变速率没有数量级的差别.     

Nb-Cr-Mo合金蠕变行为研究

通过Gleeble3500型热模拟机上的恒温恒应力压缩试验,研究了成分为Nb-22.5Cr-2.5Mo(%,原子分数)的Nb-Cr-Mo合金的高温蠕变行为。结果表明:Nb-Cr-Mo合金的稳态蠕变速率随应力的增加和变形温度的升高而加快, 1000℃和200 MPa条件下, Nb-Cr-Mo合金的稳态蠕变速率为5.3×10~(-5) s~(-1)。随着变形温度的升高, Nb基体中位错运动阻力减小,在温度和外力的作用下,有形成亚晶的趋势;随着变形温度的升高, Nb/NbCr_2两相颗粒间由于热膨胀系数不匹配和弹性模量之间的差异所导致的界面压应力进一步加大,从而促使Laves相颗粒中更多原子的相对运动,使得同步剪切机制更加明显,组织中的层错/孪晶结构密度明显增加,合金的蠕变抗力明显降低。蠕变变形过程中, Nb基体中位错的滑移、攀移,多边形化和Laves相NbCr_2中的同步剪切是Nb-Cr-Mo合金蠕变变形的主要方式。相对于未合金化的Nb-22.5 Cr合金,由于Mo对Nb基体的固溶强化,在基体中产生了柯氏气团钉扎位错,提高了合金的抗蠕变能力。     

新型马氏体耐热钢G115的蠕变性能研究

G115是钢铁研究总院与宝钢联合开发的应用于600~650℃的超超临界锅炉用管钢。对G115热挤压管在130MPa应力下,分别进行625、650和675℃的蠕变性能试验,通过不同温度下蠕变曲线对比,发现G115钢的持久蠕变性能对温度参数较为敏感,提高持久蠕变温度,导致蠕变试样的稳态蠕变速率大大增加;同时对在650和675℃下的持久蠕变断裂试样的金相组织进行分析,两个温度下断裂的两个持久蠕变试样的断裂机制主要是晶界蠕变孔洞的出现和晶内马氏体板条密度降低导致。     

一种单晶合金的高温蠕变行为及其变形特征

通过测定一种单晶镍基合金的高温拉伸蠕变曲线及位错运动的内摩擦应力σ0,建立了综合蠕变方程,计算出稳态蠕变期间的表观蠕变激活能及相关参数.结果表明：在蠕变期间,位错运动的内摩擦应力σ0,随外加应力的提高略有提高,随温度的升高而明显降低.蠕变后期,由于缩径使样品不同位置承受不同的有效的应力,导致筏状γ＇相具有不同的粗化特征,在近断口处,载荷的有效应力增大,使筏状γ＇相扭曲且粗化加剧.界面位错网对形变硬化和回复软化具有协调作用,并减缓位错切入γ＇相,因此有利于合金蠕变抗力的提高.     

Ti-5523钛合金热变形流变行为的研究

采用恒应变速率高温压缩模拟实验,对Ti-5523钛合金在应变速率为0.001～5.0 s-1,变形温度为600.900℃条件下的流变应力行为进行了研究,计算了变形激活能及相应的应力指数,建立了合金的应力.应变关系方程.结果表明:在恒温条件下,合金的流变应力随应变速率的增大而增大;在恒应变速率条件下.合金的流变应力随温度的升高而降低;变形激活能和应力指数分别为Q=317.811 kJ·mol-1和n=4.43;可用包含Arrhenius项的Zener-Hollomon参数描述Ti-5523钛合金高温塑性变形时的流变行为.     

Creep and rupture properties of an austenitic Fe-30Mn-9Al-1C alloy

The creep deformation behavior and rupture properties of as-quenched austenitic Fe-30Mn-9Al-1C alloy have been studied at 923, 948, and 973 K under applied stresses ranging from 50 to 350 MPa. The creep curves of the alloy exhibited an extended tertiary stage prior to failure. The stress and temperature dependencies of the minimum creep rate indicated two regimes of creep deformation as well as a transition from creep to power-law breakdown. These two regimes of creep deformation were identified as a low-stress creep regime having an activation energy of 140 kJ/mol and a stress exponent of about 1, and a power-law creep regime having an activation energy of 350 kJ/mol and a stress exponent of about 6. Transmission electron microscope (TEM) observations of the deformed specimens revealed that a low density of dislocations, coarse dislocation networks, and profuse slip bands were developed in the low stress, power law, and power-law breakdown regimes, respectively. Optical microscope and scanning electron microscope (SEM) observations of the ruptured specimens showed that creep cavitation shifted from round-type in the low-stress creep regime to wedge-type in the power-law breakdown regime. The observed creep and rupture characteristics of the alloy are interpreted in terms of creep mechanisms, which involve the Coble creep and dislocation climb creep.     

Steady state creep in two Ni-Al alloys

Creep of two Ni-AI alloys containing 4.8 and 7.0 wt pct Al was studied in the temperature range 873 to 1073 K and stress range 30 to 400 MPa. The former alloy represents the solid solution of aluminum in nickel, the latter a solid solution strengthened by NI3AI particles. As to its creep behavior the solid solution alloy belongs to the Class n of solid solu-tions,i.e. the creep controlling mechanism is the same as in pure nickel. From the analy-sis of an effective stress dependence of steady state creep rate it follows that the mo-tion of jogged screw dislocations can be considered as the most probable creep control-ling mechanism. The apparent activation energy of creep in the two phase alloy increases with tempera-ture. This effect is caused by changes in the volume fraction of second phase particles and by the onset of climb around particles at high temperatures. At lower temperatures particles are cut by dislocation pairs.     

Creep and low-cycle fatigue behavior of ferritic Fe-24Cr-4Al alloy in the dynamic strain aging regime: Effect of aluminum addition

Creep and low-cycle fatigue behavior of ferritic Fe-24Cr-4Al alloy was studied in the temperature range of 673 to 873 K, where dynamic strain aging (DSA) occurrence was found. The DSA of the alloy manifested in the form of serrated flow, negative strain rate sensitivity, and the peak or plateau in the variations of yield strength (YS) and ultimate tensile strength (UTS) with temperature. The characteristic creep behavior of the alloy was experimentally verified as that for a class I solid solution. However, this ferritic alloy showed an anomalous high stress exponent (n=5.7) and high activation energy (Q _c =285 kJ/mol) of the secondary creep, which were commonly exhibited by class II solid solutions. During cyclic deformation, the alloy displayed serration in the stress-strain hysteresis loops, increased cyclic hardening, and enhanced planarity of dislocations. On the basis of the observed experimental results and proper analysis, it was proposed that there was strong elastic interaction between solute aluminum atoms and dislocations in the DSA temperature domain. The anomalous creep and fatigue features were interpreted in terms of the interaction of aluminum with the dislocations.     

Tension creep of wrought single phase γ TiAl

The minimum strain rate, tertiary creep and damage behavior of a single phase gamma (γ) TiAl alloy over the temperature range 760–900°C at initial applied stress levels ranging from 32 to 345 MPa are reported. Two regions of creep deformation are identified. These consist of a region having a stress exponent of 6 and an activation energy of 560 kJ/mol and a region having a stress exponent of 1 and an activation energy of 192 kJ/mol. These are postulated to represent dislocation and boundary diffusion dominated creep respectively. The activation energy for dislocation creep is suggested to represent the energy to generate an appreciable density of dislocations in the minimum strain rate region. In the diffusional regime the minimum strain rates at 760°C lie well below the predicted minimum strain rates when compared to the Coble creep equation. In addition, a natural transition from diffusional creep to glide dominated deformation occurs at 760°C with increasing stress level. Tertiary creep of this material is found to correlate well with a two state variable approach. The initial stage of tertiary creep is dominated by an increase in the mobile dislocation density with increasing creep strain. Tertiary creep is found to obey a power law relationship with a stress exponent of 3 and an activation energy of 304 kJ/mol and is explained by the coupling of an increasing mobile dislocation density in the early stage of tertiary with constrained cavity growth in the late stage which leads to specimen failure.     

Creep rupture of a silicon carbide reinforced aluminum composite

The microstructure, texture, and whisker orientations in 6061 Al-20 wt pct SiC whisker composites have been examined using transmission electron microscopy and X-ray diffraction. Tension creep tests of the composite material have also been conducted in the temperature range 505 to 644 K (450 to 700 F). The steady state creep rate of the composite depends strongly on the temperature and applied stress. The stress exponent for the steady state creep rate of the composite is approximately 20.5 and remains essentially constant within the range of test temperatures. The activation energy is calculated to be 390 kJ/mol, nearly three times as high as the activation energy for self-diffusion of aluminum. No threshold stress was observed. Fracture surface examination using scanning electron microscopy shows that the composite fails by coalescence of voids in the aluminum matrix which originate at the aluminum-SiC interface. It is demonstrated that SiC paniculate composites are less creep resistant than SiC whisker composites.     

Impression Creep Behavior of an Mg–Zn–RE Alloy at Elevated Temperatures

Mg–Zn–RE alloys are promising candidates for automotive and aerospace applications as, among magnesium alloys, they have better corrosion and creep resistance abilities at elevated temperatures. This study evaluates the high-temperature creep behavior of ZE41 magnesium alloy, belonging to the Mg–Zn–RE family, using impression test. Impression tests were performed under a constant temperature and stress with a flat-ended cylindrical punch. Power law and Eyring relationships were used to analyze the creep mechanism. By applying the power-law relationship, it was found that the creep exponent decreased from 7.5 to 4 in the temperature range of 493 K to 593 K. Activation energy increased from 78.5 to 107.1 kJ/mol in the applied stress range of 350 to 500 MPa (normalized stress: 0.024 ≤ σ_imp/G ≥ 0.034). Using the Eyring relationship, a single activation energy of 25 kJ/mol for the entire stress and temperature range was obtained. Based on the creep exponent and activation energy, it is proposed that pipe-diffusion-controlled dislocation climb is the dominant mechanism, but grain boundary sliding also contributes at higher stresses.

     

Stress and temperature dependence of creep in Alloy 600 in primary water

The stress and temperature dependence of creep of commercial nickel-base Alloy 600 was investigated through constant load, step-load, and step-temperature creep tests in deaerated primary water containing 40 to 60 cc/kg hydrogen. To analyze creep rates for Alloy 600 in the mill-annealed (MA) condition, effective stresses were estimated using applied stresses and instantaneous strains. The apparent activation area was determined to be 7b ² by the multiple regression analysis of creep rates. The apparent activation energy for creep has a weak stress dependence and was determined to lie between 188 and 281 kJ/mole for the effective stress range of 117 to 232 MPa. Creep rates were better correlated with effective stress than applied stress and the stress exponent of Alloy 600 MA was determined to be 2.2 at 337 °C and 5.1 at 360 °C. The magnitudes of the stress exponent, activation energy, and activation area can be interpreted to support a creep mechanism controlled by dislocation-climb and nonconservative motion of jogs in commercial Alloy 600 MA. The activation area agreed with that determined from carbon in solution, implying thermally activated dislocation glide as another possible creep mechanism.     

Creep and ductility in an Al-Cu solid-solution alloy

High-temperature creep was investigated in an Al-3 wt pct Cu alloy at temperatures in the range of 773 to 853 K and at a normalized shear stress range extending from 10^-5 to 7 × 10^-4. The results show the presence of three distinct regions. In region I (low stresses), the stress exponent is 4.5 and the activation energy is 155 kJ/mole. In region II (intermediate stresses), the stress exponent is 3.2 and the activation energy is 151 kJ/mole. In region III (high stresses), the stress exponent is 4.5 and the activation energy is 205 kJ/mole. Creep curves obtained in the three regions exhibit a normal primary stage, but the extent of the stage is less pronounced in region II than in regions I and III. The creep characteristics in regions I and II, along with the values of the transition stresses between the two regions, are in conformity with the prediction of the deformation criterion for solid-solution alloys. While the advent of region III (high stresses) correlates well with dislocation breakaway from a solute-atom atmosphere, the creep characteristics in this region are not entirely consistent with any of the existing high-stress creep mechanisms. The plot of elongation to fracturevs initial strain rate at 853 K exhibits two peaks at strain rates of 1 × 10^-4 and 6 × 10^-4 s^-1. The first peak (1 × 10^-4 s^-1) is attributed to the variation of the stress exponent for creep in the alloy with strain rate, and the second peak (6 × 10^-4 s^-1) appears to reflect the effect of solute drag on dislocation velocity.     

Threshold Stress Creep Behavior of Alloy 617 at Intermediate Temperatures

Creep of Alloy 617, a solid solution Ni-Cr-Mo alloy, was studied in the temperature range of 1023 K to 1273 K (750 °C to 1000 °C). Typical power-law creep behavior with a stress exponent of approximately 5 is observed at temperatures from 1073 K to 1273 K (800 °C to 1000 °C). Creep at 1023 K (750 °C), however, exhibits threshold stress behavior coinciding with the temperature at which a low volume fraction of ordered coherent γ′ precipitates forms. The threshold stress is determined experimentally to be around 70 MPa at 1023 K (750 °C) and is verified to be near zero at 1173 K (900 °C)—temperatures directly correlating to the formation and dissolution of γ′ precipitates, respectively. The γ′ precipitates provide an obstacle to continued dislocation motion and result in the presence of a threshold stress. TEM analysis of specimens crept at 1023 K (750 °C) to various strains, and modeling of stresses necessary for γ′ precipitate dislocation bypass, suggests that the climb of dislocations around the γ′ precipitates is the controlling factor for continued deformation at the end of primary creep and into the tertiary creep regime. As creep deformation proceeds at an applied stress of 121 MPa and the precipitates coarsen, the stress required for Orowan bowing is reached and this mechanism becomes active. At the minimum creep rate at an applied stress of 145 MPa, the finer precipitate size results in higher Orowan bowing stresses and the creep deformation is dominated by the climb of dislocations around the γ′ precipitates.