新型Cr-Co-Mo-Ni合金的高温蠕变损伤

摘要：对500℃/950MPa条件下经845-8 h蠕变断裂的一种新型Cr-Co-Mo-Ni合金的蠕变损伤进行了分析,并且对蠕变孔洞的形成进行了研究。结果表明,蠕变断裂后,基体中呈链状分布的M6C相显著粗化,平均等效直径达到3.0μm,体积分数达到3.85%;马氏体板条上析出大量弥散细小的Laves相,尺寸在10～25nm之间,面积比达20%;蠕变孔洞在密集分布的链状M6C型析出相与基体结合界面上产生,其形成与M6C相的链状聚集和显著粗化有关;并且与高密度Laves相的析出有关;因此,控制链状M6C相的析出、聚集和长大能够提高该新型合金的抗高温蠕变性能。     

Creep behavior of the heusler type structure alloy Ni2AlTi

A specific method for improving the high temperature creep strength of β-NiAl by a ternary addition giving rise to an additional degree of order is examined. The ternary alloy thus formed has theA ₂BC or Heusler type structure, and the present study is devoted to the creep behavior of polycrystalline Ni₂AlTi of stoichiometric composition. Possible slip modes are predicted on the basis of the hard sphere model, and quantitative transmission electron microscopy is used to verify these predictions. All intracellular dislocations, and network dislocations have a α₀〈110〉 type Burgers vector; α₀ is the lattice parameter of a bcc cell of which the large Ni₂AlTi unit cell is composed. The creep strength of this alloy is ∼ 3 times that of NiAl in its most creep resistant form, na mely [100] axis single crystals.     

The stress-dependence of high temperature creep of tungsten and a tungsten-2 Wt pct ThO2alloy

Constant-load creep tests were conducted with pure tungsten and a W-2 wt pct ThO₂ alloy at temperatures between 1600° and 2200°C and at strain rates of about 1 × 10^-8 to 4 × 10^-5 sec^-1. The results were evaluated by the empirical correlations of Robinson and Sherby and also Mukherjeeet al. which describe the stress dependence of the creep of metals and alloys. The agreement of the present experimental data with these correlations was found to be poor. However, when the following empirical relationship was used: •ε _c =A’(σ _c /σ _f ) ⁿ the present creep data for tungsten and the tungsten alloy at various temperatures were much better correlated. Here, •ε _c is the experimental creep rate, σ_c is the applied stress for creep, σf is the flow stress of the material at the same temperature in a constant strain rate tensile test, andA’ is function of temperature, structure, and strain rate.     

Design considerations for a high temperature creep resistant nickel base alloy

Life of all high temperature materials is decided by their creep properties. The understanding of factors that control their high temperature properties is important in designing creep resistant alloys. Since dislocation movement is primarily responsible for creep, all microstructural parameters that increase resistance to dislocation motion like, low stacking fault energy, association of jogs and vacancies with dislocation and distribution of second phase particles influence the creep resistance of alloys. In this paper, detailed investigations carried out on the evolution of two phase microstructural parameters in nickel base binary and some commercial high temperature alloys are presented. The role of some of the observed microstructures in enhancing creep resistance will be discussed.     

High temperature creep of Ni-20Cr-2ThO2 single crystals

The high temperature creep properties of single crystals of Ni-20Cr-2ThO₂ have been investigated in the temperature range 650–1300°C. Single crystal tensile specimens were cut from plates having a large elongated grain structure and constant stress vacuum creep tests were conducted at various stresses and temperatures.By including the temperature dependence of the elastic modulus in the analysis of the data, the creep activation energy was found to be very nearly equal to that for self diffusion of Ni-20Cr. The stress dependence of the creep rate was observed to be large and variable, with power law exponents ranging from 9 to 75 over the temperature range studied. The ductility of these crystals as measured by percentage reduction of area was very high, typically ranging from 40 to 60%. The percentage elongation varied inversely with the stress exponent and ranged from 1 to 28%.The creep properties of these dispersion strengthened crystals can be described accurately by considering the creep strength to be given by a simple sum of the creep strength of unthoriated polycrystalline Ni-20Cr and the Orowan strength, as calculated from the measured ThO₂ particle distribution. This simple model describes the measured creep rates to within a factor of three at all stresses and temperatures.     

High temperature creep damage in steels and superalloys

The nucleation and growth of cavities was examined in steel bicrystals (Fe-3%-Si, X 8 CrNiNb 16 13) and in the ODS superalloy Inconel MA 754 (Inconel MA 754 (78% Ni; 20% Cr; 0.5% Ti; 0.3% Al; 0.6% Y₂O₃). Cavity density distributions were measured on metallographic sections and on cleaved grain boundaries as a function of time, strain, temperature and stress. Nucleation and growth laws were obtained by evaluating the distributions with appropriate models. For the fcc and bcc bicrystals, it was found that cavities nucleated continuously at sulfide and carbide particles during creep. They grew by grain boundary diffusion. But the growth rate was delayed with increasing creep strain due to cavities which nucleated in the surroundings of existing cavities. For the ODS alloy, however, many round cavities preexisted on quasi-boundaries consisting of the aggregate of coarse oxide and carbide particles. They grew initially by diffusion, but with increasing creep time (cavity size), the growth mechanism switched from growth controlled by grain boundary diffusion to growth controlled by power law creep. Implications for life predictions are discussed.     

High temperature creep in Fe-3 pct Si

The kinetics and structure dependence of the high temperature-low stress creep of Fe-3 pct Si has been studied and compared with the predictions of current creep theories. It is shown that the steady state creep rate is diffusion controlled and exhibits a power law stress dependence in the temperature range 1393 to 1678 K and stress range 6.9×10⁴ to 1.03×10⁶ Pa. In this same range of experimental conditions the dislocation density present during steady state creep is essentially stress independent. It is shown that current creep theories are incompatible with the experimental results.     

High temperature deformation of a commercial aluminum alloy

A study of high temperature deformation of a commercial aluminum alloy has been undertaken through tensile tests at strain rates ranging from 5.6×10⁻⁵ s⁻¹ to 5.6×10⁻² s⁻¹ and load relaxation testing in the temperature range 473 to 873 K. Experiments have established that maximum ductility is reached at about 623 K and at maximum strain rates. Maximum fracture ductility corresponds to minimum uniform elongation. The deformation and fracture mechanisms operating in the temperature range 473 to 573 K seem to differ from those between 623 K and 823 K; different strain rate sensitivities are also observed. Dynamic recovery is the dominant softening mechanism in high temperature plastic deformation—that is, a thermally activated process whose kinetics can be suitably described by an empirical power relation.     

High temperature deformation of a commercial aluminum alloy

A study of high temperature deformation of a commercial aluminum alloy has been undertaken through tensile tests at strain rates ranging from 5.6 × 10_-5 s_-1 to 5.6 × 10_-2 s_-1 and load relaxation testing in the temperature range 473 to 873 K. Experiments have established that maximum ductility is reached at about 623 K and at maximum strain rates. Maximum fracture ductility corresponds to minimum uniform elongation. The deformation and fracture mechanisms operating in the temperature range 473 to 573 K seem to differ from those between 623 K and 823 K; different strain rate sensitivities are also observed. Dynamic recovery is the dominant softening mechanism in high temperature plastic deformation—that is, a thermally activated process whose kinetics can be suitably described by an empirical power relation.     

High temperature creep cavitation mechanisms in a continuously cast high purity copper

Continuously cast high purity copper was used to study intergranular high temperature creep fracture mechanisms. With the help of an internal marker system due to impurity segregation, grain boundary sliding, GBS, was found to have occurred to a similar extent on cavitated and uncavitated boundaries. To explain this phenomenon a void nucleation model involving small nonwetting shearable particles is suggested. Metallographic observations and the apparent activation energy derived from fracture time data indicate the operation of the vacancy condensation mechanism at the lower temperatures and higher stresses. At the higher temperatures and lower stresses void growth is enhanced by GBS. This cavitation mechanism obtains strong support from measurements of the distribution of voids on grain boundaries as a function of the boundary angle with respect to the tensile direction. Computer analysis of these distributions, in terms of a model which properly accounts for the distribution of potential nuclei, yields bimodal curves exhibiting peaks at grain boundaries oriented for high shear stress (peak I), and for high normal stress (peak II). A phenomenological equation is proposed for the dependence of peak I on test conditions. Peak II is thought to be caused by nucleation by local GBS and growth by vacancy condensation under locally enhanced normal stress. A. RUKWIED, formerly Physicist, Engineering Metallurgy Section, Metallurgy Division, National Bureau of Standards, U. S. Department of Commerce, Washington, D. C.     

High temperature creep of silicon carbide particulate reinforced aluminum

The effect of stress on the creep properties of 30 vol.% silicon carbide particulate reinforced 6061 aluminum (SiC_p-6061 Al), produced by powder metallurgy, has been studied in the temperature range of 618–678 K. The experimental data, which extend over seven orders of magnitude of strain rate, show that the creep curve exhibits a very short steady-state stage; that the stress exponent, n, is high (n > 7.4) and increases with decreasing the applied stress; and that the apparent activation energy for creep, Q_a, is much higher than the activation energy for self-diffusion in aluminum. The above creep characteristics of SiC_p-6061 Al are similar to those reported for dispersion strengthened (DS) alloys, where the high stress exponent for creep and its variation with stress are explained in terms of a threshold stress for creep that is introduced by the dispersoid particles. Analysis of the creep data of SiC_p-6061 Al using the various threshold stress models proposed for DS alloys indicates that the threshold stresses introduced by the SiC particulates are too small to account for the observed creep behavior of the composite. By considering an alternate approach for the source of the threshold stress in SiC_p-6061 Al, an explanation for the asymptotic behavior of the creep data of the composite is offered. The approach is based on the idea that the oxide particles present in the Al matrix, as a result of manufacturing the composite by powder metallurgy, serve as effective barriers to dislocation motion and give rise to the existence of a threshold stress for creep.     

含Laves相TiCr2钛铬合金的高温氧化特性

研究了含铬18%～35%(质量分数)及Laves相TiCr2钛铬合金在650,700和780℃温度下的抗氧化性能.试验结果表明,含铬量对过共析钛铬合金的抗氧化性能有显著影响.在相同条件下,含铬量低于21%的合金的抗氧化性能不如纯钛,而含铬量达到26%以上时,抗氧化性能比纯钛高2～3倍.含TiCr2钛铬合金在高温下既发生了外氧化又发生了内氧化.外氧化层没有抗氧化保护作用.铬合金化提高含TiCr2钛铬合金的抗氧化性能主要是由于形成了连续、致密的含铬内氧化层.钛铬合金的高温抗氧化性能对氧化温度敏感,氧化温度提高,则抗氧化性能明显降低.     

温度对GH4169合金蠕变行为及机制的影响

在研究了温度对镍基高温合金GH4169蠕变行为及机制的影响基础之上,分析了其断口形貌和蠕变断裂机理。实验结果表明,随着蠕变温度的升高,GH4169合金的稳态蠕变速率逐渐升高,蠕变寿命显著降低,即该合金有极强的温度敏感性。蠕变过程中,γ″相长大聚集,并向δ相转变,随着蠕变温度的升高,γ″相向δ相转变速度加快,晶内的γ″相数量减少,δ相所占体积增加,尺寸增大,次生裂纹数量减少,尺寸减小。当蠕变温度为650 ℃时,断口中存在较多亮白色撕裂棱,韧窝尺寸大小不一,有少量析出物和碳化物;当温度提高到670 ℃时,韧窝尺寸减小,以浅韧窝为主,且出现解理面;当温度提高到690 ℃时,只存在少量韧窝,且δ相的数量显著增多,出现解理台阶,断裂方式为解理断裂或准解理断裂。      

High temperature deformation and fracture mechanisms in a dendritic Ni3Al alloy

The mechanisms that control high temperature deformation and rupture were studied in a Ni₃Al alloy that was thermo-mechanically treated to produce a non-porous dendritic grain structure. Comparisons of data corresponding to the dendritic grain morphology with that for the equiaxed grain structures indicate that the dendritic morphology results in significantly lower creep rates as well as substantially greater times to rupture. Comparison of the data with numerical calculations suggests that this difference in creep strength is due to an inherent resistance to grain boundary sliding by the dendritic grain structure. A constrained cavity growth model was adapted based on microstructural observations to account for cavitation within the dendritic microstructure. The success of the model indicates that rupture time is primarily determined by constrained cavity growth on isolated dendrite boundary segments.     

ODS-310合金的高温氧化行为

采用静态氧化增重实验及扫描电镜、能谱分析和X射线衍射分析等手段,研究了ODS-310合金在高温环境下的氧化行为,分析了氧化层的形貌、成分和物相,并对其高温氧化动力学曲线进行拟合.实验发现各个温度下的氧化动力学曲线均基本符合抛物线规律.在700℃和900℃氧化100 h以后,ODS-310合金均表现出优异的抗氧化性能.但是,当氧化温度为1100℃时,氧化层厚度明显增厚,而且氧化层有疏松和不连续的现象,不利于氧化层对基体的保护.氧化程度随着氧化温度的提高和氧化时间的延长而加剧.通过能谱和X射线衍射综合分析可知氧化层的物相为Cr₂O₃.     

The growth of creep cavities in a low alloy steel

Experimental observations of cavity sizes and spacings, after creep of a low alloy steel, are used to assess the applicability of some theoretical models for cavity growth. It is suggested that, for this type of material, various factors may control the growth of cavities, dependent on their size and the test temperature. There also exists a need for accurate diffusion data under these testing conditions. C. D. HAMM, formerly at University of Manchester     

RA330合金的耐高温腐蚀性能

RA330合金是镍铁铬系耐热合金,用于制作煤气加热炉隔热墙。在850℃,连续运行2 160h后,通过分析其表面氧化模式,研究了其在高温下的腐蚀性能。结果表明,其与煤接触表面主要是煤灰分形成的熔盐腐蚀,氧化膜不完整,表面形成蚀坑;与煤气接触表面主要是高温氧化腐蚀,氧化膜较完整,硫腐蚀不明显。