Stress intensity factors and crack propagation in a single crystal nickel-based superalloy

A three-dimensional finite element method was used to calculate the stress intensity factors for corner cracked specimens of a single crystal nickel-based superalloy. The anisotropic material properties and inclinations of the cracks were shown to have significant effects on the stress intensities. Then the two-dimensional resolved shear stress approach for predicting the crack planes and crack growth directions in single crystals was extended to the three-dimensional case. Using this approach, the fatigue crack growth behaviour in single crystal corner cracked specimens could be explained.     

Effects of carburization on recrystallization behavior of a single crystal superalloy

The samples of single crystal superalloy DD6 were grit blasted and heat treated in the solution and aging treatment at vacuum atmosphere, then the effects of carburization on recrystallization of single crystal superalloy DD6 were investigated. The results showed that carbon was introduced into the grit blasted samples during carburization, producing a homogenous distribution of MC-type carbides in the form of submicron dispersed precipitates within the grit blasted region. These MC-type carbides hold back the recrystallized grain boundaries. As temperature increases, MC-type carbides dissolve and secondary M₆C-type carbides precipitate. The interaction between these M₆C-type carbides and recrystallized grain boundaries further hinders recrystallization. Thus the average depth of the recrystallization region and number of recrystallized grains observed in carburized grit blasted samples are less than those in the samples which underwent grit blasting process alone. Carburization can significantly reduce recrystallization in single crystal superalloy DD6 during solution heat treatment.     

Crystallographic analysis on small fatigue crack propagation behaviour of a nickel-based single crystal superalloy

In situ observations by scanning electron microscopy show that small fatigue cracks in a nickel-based single crystal superalloy are inclined to the loading direction and propagate in dominant crystallographic manners. In order to evaluate the driving forces for inclined crack propagation, three-dimensional anisotropic linear elastic finite-element analysis is conducted. The plastic zone size on the dominant slip plane has been calculated and proposed to correlate the fatigue crack growth. It is shown that this parameter takes into account both material anisotropy and octahedral fracture modes, and it can effectively characterize small crack propagation behaviour.     

Effect of strain rate on the tensile behavior of a single crystal nickel-base superalloy

The effect of various strain rates on the tensile behavior of a single crystal nickel-base superalloy was studied. Single crystals with 0 0 1 crystal orientation were tested at 800 and 1000 °C under three kinds of strain rate of 10⁻³, 10⁻⁴ and 6 × 10⁻⁵ s⁻¹. The yield strength increased with the increase of strain rate, while the configuration of the stress–strain curves was independent of strain rate. Additionally, fracture surface was related to strain rate at two temperatures. At 800 °C the amount of cleavage surface was different at three strain rates, which resulted from the difference of activated slip systems. The elongation increased with the decrease of strain rate, which was influenced by the heterogeneous ductile deformation. At 1000 °C the difference of fracture surface was attributed to the microvoid at higher strain rate, while the γ/γ′ interfaces also played an important role at lower strain rate; elongation rate was independent of strain rate.     

Simulation of crack growth under low cycle fatigue at high temperature in a single crystal superalloy

Crack growth tests have been performed at 950 °C with Single Edge Notch specimens of the Ni-based single crystal superalloy PWA1483. In particular, several orientations and frequencies have been investigated, thus allowing the assessment of the influence of these parameters on the crack growth rate. In addition, oxidation experiments have been carried out to characterize the kinetics of the outer oxide scale growth at the same temperature.On the other side, crack growth has been simulated with the Finite Element program ABAQUS in real test conditions by the node release technique. The nodes are released according to the measured crack growth rate.The simulation results are compared with the test results on the basis of the computed Crack Tip Opening Displacement (CTOD). For this purpose, the crack is propagated until a stabilized value of the CTOD is obtained. This is usually the case when the crack has crossed the initial plastic zone. The procedure provides an evaluation of the effects of cycle frequency, crystal orientation, plasticity and oxide induced crack closure.     

Temperature dependence of impact deformation behavior of single crystal Ni based superalloy

The impact test was carried out to investigate the intermediate temperature brittleness of single crystal Ni based superalloy. The samples were impacted at the velocity of ～5?m?s^??1. The results showed that the impact toughness also exhibited intermediate temperature brittleness, which is similar to the situation in tensile. The samples showed the highest impact toughness at 600°C but exhibited the lowest impact toughness at 760°C. Results showed that the variety of impact toughness was due to the deformation mechanism. At 600°C, a/2? dislocation slips on the {111} slip system were attributed to the high impact toughness; however, a/2? dislocation slips from octahedral {111} planes to cubic {100} planes resulted in significant work hardening, leading to the decrease of impact toughness.     

Temperature and concentration dependence of the critical resolved shear stress of cadmium-zinc alloy single crystals

The increase of the critical resolved shear stress of cadmium single crystals by additions of zinc has been investigated in the temperature range 77 to 295 K. The temperature dependence of the critical resolved shear stress can be divided into two temperature regions. At all temperatures the critical resolved shear stress was found to increase withc ^2/3 wherec is the atomic concentration of zinc as solute. The concentration dependence of the plateau stress is explained according to the theory of Labusch [5].     

Nanoscale void nucleation and growth and crack tip stress evolution ahead of a growing crack in a single crystal

A constrained three-dimensional atomistic model of a cracked aluminum single crystal has been employed to investigate the growth behavior of a nanoscale crack in a single crystal using molecular dynamics simulations with the EAM potential. This study is focused on the stress field around the crack tip and its evolution during fast crack growth. Simulation results of the observed nanoscale fracture behavior are presented in terms of atomistic stresses. Major findings from the simulation results are the following: (a) crack growth is in the form of void nucleation, growth and coalescence ahead of the crack tip, thus resembling that of ductile fracture at the continuum scale; (b) void nucleation occurs at a certain distance ahead of the current crack tip or the forward edge of the leading void ahead of the crack tip; (c) just before void nucleation the mean atomic stress (or equivalently its ratio to the von Mises effective stress, which is called the stress constraint or triaxiality) has a high concentration at the site of void nucleation; and (d) the stress field ahead of the current crack tip or the forward edge of the leading void is more or less self-similar (so that the forward edge of the leading void can be viewed as the effective crack tip).     

Rotary bending high-cycle fatigue behavior of DD32 single crystal superalloy containing rhenium

Smooth and notched specimens of single crystal superalloy DD32 were subjected to rotary bending high-cycle fatigue (HCF) loading at different temperatures. The experimental results demonstrate that fatigue strengths of the smooth and notched specimens reach the maxima and the minimum notch sensitivity displays at 760 °C. DD32 alloy exhibits excellent HCF properties compared to SRR99 alloy under the same test condition. As for the smooth specimens, slip bands moving through γ and γ′ phases as well as dislocation bowing are the main deformation modes. As for the notched specimens, the deformations are carried out by dislocation loop bowing and shearing of PSBs mode at intermediate temperatures; at 900 °C, the minimum fatigue strength results from dislocation climbing deformation and the degradation of γ′ precipitates. The fine secondary γ′ precipitates advantage the recovery of dislocations and further deformation of the fatigue specimens.     

Fatigue variability of a single crystal superalloy at elevated temperature

In order to develop more accurate life prediction tools, an improved understanding of the variability within the fatigue behavior of a material is required. Recent work has shown multiple failure mechanisms that drive the variability in fatigue life of polycrystalline titanium and nickel materials. In addition, the bimodal behavior in the fatigue response is not readily apparent when only a very small number of specimens are tested at each loading condition, as is normal practice.The objective of this work was to investigate the fatigue variability of a single crystal nickel-base superalloy at elevated temperature. PWA1484, a second generation single crystal alloy developed for advanced turbine airfoil applications, was the material of choice for this investigation. A large number of fatigue tests were performed at one condition (stress level, stress ratio, frequency and temperature) to determine the variability and identify the sources of uncertainty in life. Scanning electron microscopy was used to investigate the relationship between failure mechanisms and variability. Crack growth analyses were used to predict lowest life estimates and were compared to experimental data. The results show large variability in fatigue life at fairly high stresses. Evaluation of the fracture surfaces indicated that microstructural features such as carbides and eutectics were responsible for the failures. In addition, the size of the feature responsible for fatigue failure could not be directly related to the fatigue life. The lowest expected life based on fatigue crack growth analyses did agree with the shortest life found experimentally. However, more testing and analysis is required.     

Estimating the orientation of Stage I crack paths through the direction of maximum variance of the resolved shear stress

Our formalisation of the Shear Stress-Maximum Variance Method takes as a starting point the hypothesis that, in ductile materials subjected to fatigue loading, the crack initiation planes, i.e. the so-called Stage I planes, are those containing the direction experiencing the maximum variance of the resolved shear stress. From a computational point of view, the most remarkable implication of the above assumption is that, as soon as the variance and covariance terms characterising the considered load history are known, the effective time needed to estimate the orientation of the critical plane does not depend on the length of the load history itself. Further, such a computational efficiency is seen to be associated with an high-level of accuracy in estimating fatigue lifetime of both plain and notched engineering components, this holding true under constant as well as under variable amplitude uniaxial/multiaxial fatigue loading. In this scenario, by assuming that the orientation of Stage I planes can directly be determined through the orientation of Stage II crack paths, the present paper investigates whether, independently from the degree of multiaxiality and non-proportionality of the applied loading history, the direction of maximum variance of the resolved shear stress is also capable of accurately estimating the orientation of Stage I crack paths.     

基于不同原始组织预设变形第四代单晶高温合金的再结晶行为

第四代单晶高温合金标准热处理试样和铸态试样压痕后分别在1100,1150,1200,1250,1300℃和1340℃退火处理,采用光学显微镜、扫描电镜、电子背散射仪研究不同条件的再结晶组织。结果表明:1100,1150,1200℃退火处理后,标准热处理试样和铸态试样都出现胞状再结晶。1250℃退火处理后,标准热处理试样和铸态试样都为混合再结晶。1300℃退火处理后,标准热处理试样再结晶组织全部为等轴再结晶,而铸态试样仍为混合再结晶。1340℃退火处理后,标准热处理试样和铸态试样都形成了等轴再结晶。随着退火温度升高,标准热处理试样和铸态试样的再结晶层深度明显增加,标准热处理试样再结晶深度明显大于铸态试样,相同条件下标准热处理试样的再结晶晶粒更容易长大。再结晶与基体的界面为小角度晶界、大角度晶界,而再结晶晶粒之间为小角度晶界、大角度晶界和孪晶界。孪晶在单晶高温合金再结晶的过程中发挥了重要作用。