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。     

The effect of temperature and microstructure on the creep damage found in low alloy ferritic steels

Constant strain rate tests at 10^-5 s^-1 have been carried out in the temperature range 723 to 973 K on two 1 1/2 pct Cr · 1/2 pct V ferritic steels, the first steel with a 20 pct bainite, 80 pct ferrite microstructure and the second with a fully ferritic structure. Measurements of the quantitative strain, ε_gb, due to grain boundary sliding (gbs), were made and in both steels the γ values (where γ = ε_gb/ε_T) increased with increasing temperature. In both structures, sliding was found to occur on all boundaries. A qualitative study of cavitation damage and final fracture mechanisms was also made. It is suggested that in the mixed structure, cavities are nucleated by gbs at carbides whereas in the fully ferritic structure, cavity nucleation is by the interaction of intragranular slip with a grain boundary. Optical observations showed that the large scale cavitation behavior was superficially very similar in both steels, but scanning electron microscope observations showed remarkable differences in the fine scale cavitation damage. The implications of these results are discussed in terms of the relationship between matrix deformation, grain boundary deformation and creep fracture. Formerly of the Department of Metallurgy, University of Manchester.     

Room temperature creep of high strength steels

The room-temperature creep behaviour of three high strength steels has been investigated. Several parameters such as creep stress, loading rate, stress history and heat treatment has been altered and their influence on the low temperature creep has been reported. The primary creep in all three alloys agreed well with the logarithmic creep law and the creep mechanism has been identified as pure dislocation creep. Higher stresses and high loading rates led to increased creep strains and strain rates. Reloading after a period of creep resulted in significantly decreased creep strains and no recovery of the time dependent deformation could be detected. The yield strength of the materials per se had no influence on the room temperature creep whereas the same material with decreased 0.2% offset strength showed significantly reduced time dependent deformation. The possible interaction between primary creep and stress corrosion cracking has been 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 ductility loss in carbon-manganese and niobium-treated steels

The causes of embrittlement in several plain carbon-manganese and niobium-treated steels between 800 and 1200 °C have been investigated. Tensile ductility was measured as a function of temperature and strain rate. Percent elongation and reduction in area were used to characterize the temperature dependence and severity of the ductility loss. The size, distribution, and composition of grain boundary precipitates were measured on extraction replicas. Grain boundary segregation was measured by AES on samples that were deformed at 900 °C before being fractured under ultra-high vacuum at room temperature. Segregation of impurity residual elements and grain boundary precipitation are the primary factors responsible for the observed ductility loss. The embrittlement results in a low ductility fracture which is largely intergranular through the austenite grain boundaries. Segregation of Cu, Sn, and Sb was found on the fracture surfaces of the embrittled samples. High temperature deformation was necessary to produce segregation as no segregation was detected in undeformed samples. Grain boundary precipitation, particularly AIN but also Nb (C,N), contributed to the embrittlement when there was a relatively fine distribution of precipitates along the austenite grain boundaries. The most severe ductility loss occurred when grain boundary precipitation combined with Cu, Sn, and Sb segregation. Formerly Graduate Student, Lehigh University     

Compressive creep properties of Ir-base refractory superalloys

Ir-base alloys with the fcc and L1₂-Ir₃X (X = Nb, Zr) two-phase structure have been developed as next-generation high-temperature materials. The compressive creep behavior of Ir-Nb and Ir-Zr alloys was investigated at 2073 K under 137 MPa. The effect of addition of the third element, Zr, on the creep behavior of an Ir-Nb alloy was also investigated at 2073 K for 137 MPa. The creep rate became two orders lower by addition of a small amount of Zr. The lattice misfit change between the fcc and L1₂ two phase by addition of Zr and the deformation structure in binary and ternary alloys after a creep test were also investigated. The creep behavior is discussed in terms of the lattice misfit, precipitate shape, and their distribution. This article is based on a presentation made in the symposium entitled “Beyond Nickel-Base Superalloys,” which took place March 14–18, 2004, at the TMS Spring meeting in Charlotte, NC, under the auspices of the SMD-Corrosion and Environmental Effects Committee, the SMD-High Temperature Alloys Committee, the SMD-Mechanical Behavior of Materials Committee, and the SMD-Refractory Metals Committee.     

High temperature corrosion of superalloys in an environment containing both oxygen and chlorine

The corrosion behavior of a series of commercial superalloys in flowing argon-20 pct oxygen-2 pct chlorine at 900 °C has been investigated using thermogravimetric analysis and examination of the condensed corrosion products using X-ray diffraction analysis and optical and scanning electron microscopy. Most of the alloys exhibited decreases in mass due to formation of volatile chloride or oxychloride corrosion products. An exception is the alloy 214 which showed little change in mass due to the formation of a protective alumina scale. Alloys which are high in refractory metals, notably alloys S and C-276, snowed the highest rates of attack. In addition to the metal wastage, several of the alloys were subject to internal attack in the region near the corroded surface. Formerly Graduate Student, Department of Civil Engineering, Mechanics, and Metallurgy, University of Illinois at Chicago     

Mechanisms of strain accumulation and damage development during creep of prestrained 316 stainless steels

The effects of prestraining at room temperature and at the creep temperature of 848 K, as well as the responses to stress reductions during creep, have been studied for 316 stainless steels varying in composition and initial microstructure. The results are analyzed by contrasting the strengthening effects achieved by introducing high dislocation densities prior to creep exposure with the deleterious effects, which can occur when prestraining causes premature void nucleation at grain boundaries. In addition, by recognizing the differing contributions made by the grain interiors and the grain boundary zones to the overall rates of creep strain accumulation, a consistent explanation is provided for the diverse creep behavior patterns reported for different metals and alloys after various prestraining treatments.     

On the influence of grain morphology on creep deformation and damage mechanisms in directionally solidified and oxide dispersion strengthened superalloys

Directionally solidified (DS) and oxide dispersion strengthened (ODS) superalloys like CM 247 LC and MA 760 exhibit elongated macrograins. In uniaxial creep tests, the creep strength of such alloys in the direction of the longitudinal grains is higher than that of an equiaxed grain structure, because significantly less grain boundary (GB) segments are perpendicular to the axis of the applied stress. The present study investigates how creep in the longitudinal direction of these alloys is influenced (1) by deviations of individual grain orientations from the optimum growth direction during casting (CM 247 LC) and (2) by the spatial distribution of the small transverse GB segments (MA 760) for a given grain aspect ratio. In the case of creep ductile CM 247 LC, it was shown that if there is a large fraction of grains that are oriented for single slip, this results in higher creep rates and lower rupture times than if there is only a small fraction of such grains. The study of the influence of grain morphology on creep damage accumulation in the creep-brittle and notch-sensitive ODS alloy MA 760 showed that large scatter in creep rupture lives is related to (1) the stochastic nature of creep damage accumulation on transverse GB segments and (2) the spatial distribution of transverse GB segments. It is the combination of these two factors that results in increased scatter in rupture lives as compared to equiaxed fine grain structures. This article is based on a presentation made at the “High Temperature Fracture Mechanisms in Advanced Materials” symposium as part of the 1994 Fall meeting of TMS, October 2-6, 1994, in Rosemont, Illinois, under the auspices of the ASM/SMD Flow and Fracture Committee.     

High temperature creep in a semi-coherent NiAl-Ni2AlTi alloy

Creep experiments have been made on a Ni-Ti-Al alloy, which has a microstructure consisting of a distribution of semi-coherent NiAl(β) precipitates with a Ni₂AlTi(β′) Heusler phase matrix. The creep strength of this bcc type structure alloy is at least comparable with that of the nickel-base superailoy MARM-200 for values ofT/T _m in the range 0.68 to 0.82. Quantitative electron microscope experiments show that both undissociated α₀〈110〉 dislocations, and paired α₀〈100〉 dislocations coupled by a sublattice A.P.B. exist within the β′ phase;α ₀ is the lattice parameter of a bcc cell of which the large Ni₂AlTi unit-cell is composed. The sublattice A.P.B. is a crystallographic fault created by wrong bonds between atoms on the Al-Ti sublattice. Theoretically the energy γ of a sublattice A.P.B. is shown to be minimum on {100}, and the experimental value for γ on {100} is ~40 mJ/m₂.     

Features and effect factors of creep of single-crystal nickel-base superalloys

The creep behavior of two single-crystal nickel-base superalloys with [001] orientation has been studied by measuring the creep curves, internal friction stress of dislocation motion, transmission electron microscopy (TEM) observation and energy-dispersive X-ray analysis (EDAX) composition analysis. The results show that over the stress and temperature range, there are different creep activation energies, time exponents, and effective stress exponents in two alloys at different creep stages. The size and volume fraction of the γ′ phase in the tantalum-free alloy is obviously decreased with the elevated temperature. This results in the decrease of the internal friction stress during steady-state creep. Higher levels of tungsten in the alloy result in a smaller strain value and lower directional-coarsening rate during primary creep. During steady-state creep, the primary reason for the better creep resistance of the other alloy is that it contains more Al and also Ta, which maintains a high volume fraction of γ′ phase. The dislocation climb over the γ′ rafts is the major deformation mechanisms during steady-state tensile creep. The fact that the strain rate is decreased with the increase of the size and volume fraction of the γ′ rafts may be described by a modified constitutive equation that is based on a model of the rate of dislocation motion.     

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.     

高碳钢连铸板坯高温力学性能

采用Gleeble-1500热模拟试验机测量了高碳钢连铸板坯的高温力学性能,得到了第Ⅰ、第Ⅲ脆性温度区的温度范围.结果表明:第Ⅰ脆性温度区脆化的主要原因是晶界部位的低熔点物质在高温下首先熔化,从而导致试样沿晶界开裂;第Ⅲ脆性温度区脆化的主要原因是在奥氏体部位析出的网状铁素体导致试样沿晶界开裂;在奥氏体单相区,由于氮化铝的析出导致钢种的塑性恶化.     

Effect of initial gamma prime size on the elevated temperature creep properties of single crystal nickel base superalloys

The influence of initial γ′ size and shape on the high temperature creep properties of two single crystal nickel-base superalloys was investigated. The two alloys were chosen to represent different magnitudes of γ-γ′ lattice mismatch. A range of initial microstructures was produced by various quenching and aging treatments. Creep-rupture testing at 1000 °C was performed under stresses where γ′ directionally coarsens to form γ-γ′ lamellae in the early portion of the creep life. Both alloys exhibited a peak in creep resistance as a function of initial γ′ size. The peak corresponded to an initial microstructure consisting of cuboidal precipitates aligned along [001] directions. These aligned cuboidal γ′ particles directionally coarsened into a relatively perfect lamellar γ-γ′ structure in the early stages of creep, whereas the more irregularly shaped and distributed γ′ particles in both under and overaged material formed more irregular lamellae with more imperfections. The alloy with a lower magnitude of mismatch was less sensitive to initial γ′size and shape.     

Tertiary creep in nickel-base superalloys: analysis of experimental data and theoretical synthesis

It has recently been established that the extended period of tertiary creep of nickelbase superalloys with good ductility is not primarily a consequence of the thermal instability of the particulate microstructure: rather it is believed to be caused by a strain-induced instability of the dislocation substructure. The instability appears to be intrinsic to this class of alloy and the resulting isotropicallyaccelerating creep rate can be represented by a scalar parameter in constitutive laws. Evidence is presented to suggest that a similar strain-induced instability operates in certain low alloy ferritic steels and precipitation-hardened aluminium alloys. The micromechanism responsible for this intrinsic instability is still a matter for speculation, but recently it was suggested that, for NiCr alloys hardened with aluminium and titanium, the acceleration in strain accumulation may be influenced by the volume fraction of γ′ particles and by the magnitude of their lattice-parameter mismatch with the matrix. In this paper, a more substantial range of data for alloys of this type is presented and it is concluded that ductility in creep is the most significant parameter correlating differences in rates of strain-accumulation in the tertiary stage: thus, the lower the ductility the greater the rate of strain-accumulation. The various shapes of uniaxial creep curve found in nickel-base superalloys have been synthesised quantitatively by developing a model with two internal state-variables: one represents damage caused by the intrinsic instability while the other represents damage caused by cavitation. The model accounts for the engineering classification of alloys into so-called “von-Mises type” and “maximum principal stress type” and also rationalises the previously unexplained fact that there is no unique correlation between this classification and uniaxial ductility in creep.     

High temperature phase chemistries and solidification mode prediction in nitrogen-strengthened austenitic stainless steels

Nitronic 50 and Nitronic 50W, two nitrogen-strengthened stainless steels, were heat treated over a wide range of temperatures, and the compositions of the ferrite and austenite at each temperature were measured with analytical electron microscopy techniques. The compositional data were used to generate the (γ + δ phase field on a 58 pct Fe vertical section. Volume fractions of ferrite and austenite were calculated from phase chemistries and compared with volume fractions determined from optical micrographs. Weld solidification modes were predicted by reference to the Cr and Ni contents of each alloy, and the results were compared with predictions based on the ratios of calculated Cr and Ni equivalents for the alloys. Nitronic 50, which contained ferrite and austenite at the solidus temperature of 1370 °C, solidified through the eutectic triangle, and the weld microstructure was similar to that of austenitic-ferritic solidification. Nitronic 50W was totally ferritic at 1340 °C and solidified as primary delta ferrite. During heat treatments, Nitronic 50 and Nitronic 50W precipitated secondary phases, notably Z-phase (NbCrN), sigma phase, and stringered phases rich in Mn and Cr.     

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 superplastic creep and internal friction of yttria doped zirconia polycrystals

Creep tests in compression and internal friction experiments have been performed in order to investigate the influences of microstructural parameters such as the grain size and the amount of intergranular glassy phase on the plastic deformation of yttria stabilized tetragonal zirconia polycrystals. The values of the apparent activation energies obtained from creep tests in the low stress regime and those obtained from the internal friction tests are in good agreement. The two techniques appear to be complementary approaches to the study of plastic deformation of fine grained zirconia under low stress. The mechanisms of plastic deformation are discussed in terms of grain boundary sliding models.     

High temperature behavior of superalloys exposed to sodium chloride: I. mechanical properties

An investigation of the corrosion behavior of superalloys in the presence of sodium chloride at elevated temperatures has been conducted and the effect of corrosion on mechanical properties investigated. Residual mechanical properties as determined by a postexposure tensile test were found to be seriously degraded, the amount of degradation being dependent upon the partial pressure of oxygen in the environment. Creep rates were increased by as much as three orders of magnitude in the presence of NaCl.