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.     

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 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.     

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。     

DD421单晶高温合金在无SOx气氛下的低温热腐蚀硫化行为

采用涂盐法研究表面涂覆有90％(质量分数,下同)Na2 SO4+10％NaCl混合盐(750℃熔融态)和纯Na2 SO4盐(750℃固态)的第二代镍基单晶高温合金DD421在750℃大气环境下(无SOx气氛)的热腐蚀行为.结果表明:在熔融混合盐腐蚀介质中,硫化反应主要由液相熔融盐侵蚀所导致.热腐蚀100 h后合金腐蚀产物主要为典型的简单氧化物(Al2 O3,Cr2 O3,TiO2)以及Ni/Cr/Ti的硫化物.而在纯Na2 SO4固态盐热腐蚀实验中,热腐蚀100 h后合金腐蚀产物与混合盐实验中的产物基本相同,但其腐蚀层厚度相对更薄,硫化物尺寸相对更大.结合热力学和微观组织分析,本研究明确了在无SOx气氛的腐蚀环境下合金元素能够与固态Na2 SO4盐发生硫化反应.     

Simulation of deformation and lifetime behavior of a fcc single crystal superalloy at high temperature under low-cycle fatigue loading

This work deals with the formulation of a three-dimensional crystallographic time-incremental lifetime rule for face-centered cubic (fcc) single crystals used for gas-turbine blade applications. The damage contribution rate of each slip system to the total damage is governed by the current values of the resolved shear stress and the slip rate on the corresponding slip system. The damage rule is combined with a crystallographic viscoplastic deformation model. For the nickel-base single-crystal superalloy CMSX4 at 950 °C, various strain- and stress-controlled uniaxial cyclic tests with and without hold-times can be described for different crystal orientations by one set of material parameters. For verification, simulation results for a single-crystal specimen with a notch have been compared with corresponding experimental results. The predicted lifetime is within the factor of two of the measured one.     

On the thin-section size dependent creep strength of a single crystal nickel-base superalloy

The combined effects of thin-section size, D, and microcracks on the creep behaviour of the single crystal MAR-M002 were investigated at the creep conditions of 300 MPa and 900 °C. It was observed that the creep rupture life, t _R is controlled by the mean microcrack size to thin-section size, (d _c/D), (or the total number, (N _m), of the mean-sized microcrack particles across the diameter, assuming D/d _c=N_m); reducing N _m continuously improves t _R. The creep rupture strain (or ductility), _R, can be improved sharply by increasing the total number, N _T, of microcrack particles across the cross-section, N _T D ² N _A, where N _A is the number of microcrack particles (cavity density) per cross-section. The behaviour of the creep rupture ductility was interpreted in terms of the weakest link, or largest-flaw concept; the observation of the higher proportion of the less likely dangerous (smaller in size) microcracks with increasing N _T was the underlining reason for the improvement in ductility.     

The effects of stress ratio and grain size on near-threshold fatigue crack propagation in low-carbon steel

The effects of the stress ratio and the grain size on the fatigue crack growth near the threshold in a low carbon steel were analysed based on the crack-closure measurement and the microscopic observations of cracktip slip deformation and the fracture surface. The low-rate region A was divided into regions A1 and A2 in the relation of the rate against the effective stress intensity range. In regions A2 and B, the rate was expressed in a unique power function of the effective range without respect to the stress ratio and the grain size. In region A1 very close to the threshold, the rate was slower for larger grain sized material, and the effective threshold stress intensity factor increased linearly with the square root of the grain size. The slip-band zone in this region was rather independent of the stress intensity and was sized by the grain dimension. A model of the crack-tip slip bands blocked by the grain boundary was confirmed to be useful for analysing very slow growth as well as the threshold condition. The shear-mode fracture surface observed on the surface in region A1suggests the repeated nucleation mechanism for crack growth. The effects of the stress ratio and the grain size on the crack closure behavior near the threshold was quantified.     

On the local variation of the crack driving force in a double mismatched weld

A material inhomogeneity in the direction of crack extension causes a difference between the near-tip crack driving force, J_tip, and the nominally applied far-field crack driving force, J_far. This difference can be quantified by a material inhomogeneity term, C_inh, which is evaluated by a post-processing procedure to a conventional finite element stress analysis. The magnitude of the material inhomogeneity term is evaluated for cracks in an inhomogeneous welded joint made of a high-strength low-alloy steel. Both a crack proceeding from the under-matched (UM) to the over-matched (OM) and from the OM to the UM weld metal are treated. The effects of the inhomogeneity of the different material parameters (modulus of elasticity, yield strength, and strain hardening exponent) on C_inh and J_tip are systematically studied. The results demonstrate that the material inhomogeneity term is primarily influenced by the inhomogeneity of the yield strength. A crack growing towards an OM/UM interface experiences an accelerated crack growth rate or a pop-in, an UM/OM interface leads to a reduced crack growth rate or a crack arrest. The application of global assessment methods of the mismatch effect which are included in the Engineering Treatment Model (ETM) or in the Structural Integrity Assessment Procedures for European Industry (SINTAP) is discussed.     

Microstructure evolution and analysis of a single crystal nickel-based superalloy during compressive creep

During compressive creep, the cubical γ′ phase in [0 0 1] orientation single crystal nickel-based superalloy is transformed into the rafted structure along the direction parallel to the applied stress axis. By means of the elastic stress-strain finite element method (FEM), the von Mises stress distributions of the cubical γ′/γ phases are calculated for investigating the influence of the applied stress on the stress distribution and the directional coarsening regularity of γ′ phase. Results show that the stress distribution of the cubical γ/γ′ phases may be changed by the applied compressive stress, and the coarsening orientation of γ′ phase is related to the von Mises stress distribution of the γ matrix channel. Thereinto, under the action of applied compressive stress, the bigger von Mises stress produced on (0 0 1) plane of the cubical γ′ phase is thought to be a main reason of the microstructure evolution. The expression of the driving force for the elements diffusion and the directional growing of γ′ phase during compressive creep are also proposed.