Prismatic slip in zirconium single crystals at elevated temperatures

Single crystals of Zr oriented favorably for prismatic slip have been deformed in tension over a range of strain rates at temperatures between 473 and 1113 K. A temperature independent plateau is observed between 600 and 800 K and dynamic strain aging occurs in the vicinity of 723 K. The flow stress is temperature dependent both above and below this temperature interval. Plastic flow above 850°K is represented by an equation of the form:γ = AT ⁿ e-Q/rT where y is the shear strain rate,A is a constant whose value is 680 ± 20 (MN/m²)^•4.3. The stress exponentn = 4.3 ± 0.3 and the activation energy Q = 2.05 = 0.15 eV. It is proposed that the high temperature prismatic slip in Zr is controlled by a glide-climb process where the rate of plastic flow is determined by the rate of climb of dislocations.     

Prismatic slip in zirconium single crystals at elevated temperatures

Single crystals of Zr oriented favorably for prismatic slip have been deformed in tension over a range of strain rates at temperatures between 473 and 1113 K. A temperature independent plateau is observed between 600 and 800 K and dynamic strain aging occurs in the vicinity of 723 K. The flow stress is temperature dependent both above and below this temperature interval. Plastic flow above 850°K is represented by an equation of the form: {ie1217-05} where {ie1217-06} is the shear strain rate,A is a constant whose value is 680 ± 20 (MN/m²)^−4.3. The stress exponentn = 4.3 ± 0.3 and the activation energyQ = 2.05 ± 0.15 eV. It is proposed that the high temperature prismatic slip in Zr is controlled by a glide-climb process where the rate of plastic flow is determined by the rate of climb of dislocations.     

一种单晶高温合金不同温度的高周疲劳性能

研究了一种单晶高温合金700 ℃和800 ℃的高周疲劳性能，采用扫描电镜和透射电镜分析了断口和断裂机制.结果表明，随着温度升高，合金的疲劳强度系数降低，Basquin系数增加，高周疲劳极限降低.合金700 ℃与800 ℃具有相同的高周疲劳断口，都有几个{111}面平面组成，为类解理断裂机制.疲劳断口由裂纹源区、扩展区和瞬断区3部分组成.裂纹起源于试样的表面或亚表面，并沿{111}面扩展.扩展区可见河流状花样、滑移带、疲劳弧线和疲劳条带特征.瞬断区可见解理台阶和撕裂棱.断裂后γ′相仍保持立方形状，位错不均匀分布在γ基体通道中.      

The effects of environment on the elevated temperature fatigue behavior of nickel-base superalloy single crystals

The effects of air and vacuum on the fatigue behavior of a nickel-base superalloy, Mar-M200, in single crystal form were investigated. Between 800° and 1400°F fracture is entirely in the Stage I mode in air and vacuum, and fatigue life is unaffected by environment. At 1700° F in both environments, fracture is predominantly in the Stage II mode and fatigue life in air is greater than that in vacuum. At both temperatures, fatigue cracking in air is internally initiated, whereas in vacuum cracking is generally initiated at the specimen surface. Identical fatigue lives in air and in vacuum between 800° and 1400° F are attributed to the fact that internally initiated cracks in air are actually propagating in a high vacuum, surface cracking being inhibited by dynamic oxidation of emerging surface slip offsets. The subsurface portion of the Stage I fracture surface produced in air tests and all of the Stage I fracture produced in vacuum tests shows a dimpled structure, whereas the Stage I fracture surface produced while the crack propagation is in air shows a cleavage appearance. At 1700° F, bulk oxidation of surface initiated cracks interferes with the plastic blunting mechanism of Stage II crack growth normally observed at this temperature, internally initiated cracks causing ultimate failure. Shorter lives in vacuum are thought to result from enhanced Stage II surface crack propagation. Formerly with Materials Engineering and Research Laboratory, Pratt and Whitney Aircraft, Middletown, Conn.     

Behavior of nickel-base superalloy single crystals under thermal-mechanical fatigue

The thermal-mechanical fatigue behavior of AM1 nickel-base superalloy single crystals is studied using a cycle from 600 °C to 1100 °C. It is found to be strongly dependent on crystallo-graphic orientation, which leads to different shapes of the stress-strain hysteresis loops. The cyclic stress-strain response is influenced by variation in Young’s modulus, flow stress, and cyclic hardening with temperature for every crystallographic orientation. The thermalmechanical fatigue life is mainly spent in crack growth. Two main crack-initiation mechanisms occur, depending on the mechanical strain range. Oxidation-induced cracking is the dominant damage mechanism in the lifetime of interest for turbine blades.     

Creep resistance of CMSX-3 nickel base superalloy single crystals

Creep deformation in 〈001〉 oriented nickel base superalloy single crystals has been studied in an effort to assess the factors which contribute to the overall creep resistance of superalloys with high volume fractions of γ′ phase. Detailed observations of three dimensional dislocation arrangements produced by creep have been made with the use of stereo electron microscopy. In the temperature range of 800–900°C at stresses of 552 MPa or lower, the dislocation-free γ′ precipitates are resistant to shearing by dislocations. As a result, creep deformation occurs by forced bowing of dislocations through the narrow γ matrix channels on {111} planes. At moderate levels of temperature and stress there are incubation periods in virgin crystals prior to the onset of primary creep. The incubations arise because of the difficult process of filling the initially dislocation starved material with creep dislocations from widely spaced sources. When the newly generated dislocations percolate through the cross section, incubation comes to an end and primary creep begins. In primary creep neither work hardening nor any type of recovery plays an important role. The creep rate decelerates because the favorable initial thermal misfit stresses between γ and γ′ phases are relieved by creep flow. Continued creep leads to a build-up of a three-dimensional nodal network of dislocations. This three-dimensional network fills the γ matrix channels during steady state creep and achieves a quasi-stationary structure in time. In situ annealing experiments show that static recovery is ineffective at causing rearrangements in the three-dimensional network at temperatures of 850°C or lower. The kinematical dislocation replacement processes which maintain the quasi-stationary dislocation network structures during apparent steady state creep are not understood and require further study. Because of the impenetrability of the γ′ precipitates, dislocations move through the γ matrix by forced Orowan bowing, and this accounts for a major component of the creep resistance. In addition, the frictional constraint of the coherent or semi-coherent precipitates leads to the build-up of pressure gradients in the microstructure, and this provides load carrying capacity. There is also a smaller component of solid solution strengthening. Work hardening is comparatively unimportant. Finite element analysis shows that the non-deforming precipitates are increasingly stressed as creep deformation accumulates in the matrix. In the later stages of steady state creep and during tertiary creep the stresses in the precipitates rise to high enough levels to cause shearing of the γ′ particles by dislocations entering from the γ matrix. The recovery resistance of the material is in part due to a very low effective diffusion constant and in another part due to the fact that the three-dimensional dislocation networks formed in the γ matrix serve to neutralize the misfit between the γ and γ′ phases.     

High-cycle fatigue of nickel-based superalloy ME3 at ambient and elevated temperatures: Role of grain-boundary engineering

High-cycle fatigue (HCF), involving the premature initiation and/or rapid propagation of cracks to failure due to high-frequency cyclic loading, remains a principal cause of failures in gas-turbine propulsion systems. In this work, we explore the feasibility of using “grain-boundary engineering” as a means to enhance the microstructural resistance to HCF. Specifically, sequential thermomechanical processing, involving alternate cycles of strain and annealing, was used to increase the fraction of “special” grain boundaries and to break up the interconnected network of “random” boundaries, in a commercial polycrystalline Ni-based superalloy (ME3). The effect of such grain-boundary engineering on the fatigue-crack-propagation behavior of large (∼8 to 20 mm), through-thickness cracks at 25 °C, 700 °C, and 800 °C was examined. Although there was little influence of an increased special boundary fraction at ambient temperatures, the resistance to near-threshold crack growth was definitively improved at elevated temperatures, with fatigue threshold stress intensities some 10 to 20 pct higher than at 25 °C, concomitant with a lower proportion (∼20 pct) of intergranular cracking.     

Low-cycle fatigue behavior of INCONEL 718 superalloy with different concentrations of boron at room temperature

Symmetrical push-pull low-cycle fatigue (LCF) tests were performed on INCONEL 718 superalloy containing 12, 29, 60, and 100 ppm boron (B) at room temperature (RT). The results showed that all four of these alloys experienced a relatively short period of initial cyclic hardening, followed by a regime of softening to fracture at higher cyclic strain amplitudes (Δɛ _t /2≥0.8 pct). As the cyclic strain amplitude decreased to Δɛ _t /2≤0.6 pct, a continuous cyclic softening occurred without the initial cyclic hardening, and a nearly stable cyclic stress amplitude was observed at Δɛ _t /2=0.4 pct. At the same total cyclic strain amplitude, the cyclic saturation stress amplitude among the four alloys was highest in the alloy with 60 ppm B and lowest in the alloy with 29 ppm B. The fatigue lifetime of the alloy at RT was found to be enhanced by an increase in B concentration from 12 to 29 ppm. However, the improvement in fatigue lifetime was moderate when the B concentration exceeded 29 ppm B. A linear relationship between the fatigue life and cyclic total strain amplitude was observed, while a “two-slope” relationship between the fatigue life and cyclic plastic strain amplitude was observed with an inflection point at about Δɛ _p /2=0.40 pct. The fractographic analyses suggested that fatigue cracks initiated from specimen surfaces, and transgranular fracture, with well-developed fatigue striations, was the predominant fracture mode. The number of secondary cracks was higher in the alloys with 12 and 100 ppm B than in the alloys with 29 and 60 ppm B. Transmission electron microscopy (TEM) examination revealed that typical deformation microstructures consisted of a regularly spaced array of planar deformation bands on {111} slip planes in all four alloys. Plastic deformation was observed to be concentrated in localized regions in the fatigued alloy with 12 ppm B. In all of the alloys, γ″ precipitate particles were observed to be sheared, and continued cyclic deformation reduced their size. The observed cyclic deformation softening was associated with the reduction in the size of γ″ precipitate particles. The effect of B concentration on the cyclic deformation mechanism and fatigue lifetime of IN 718 was discussed.     

Orientation effects on the elevated temperature fatigue of copper single crystals

Effects of orientation on dislocation structures which evolve during the elevated temperature fatigue of copper single crystals have been studied using crystals oriented for double slip. The resulting dislocation reactions produce sessile jogs, Cottrell-Lomer locks, or cells formed by coplanar slip. The relative strengths of the reaction products vary markedly with increasing temperature. At room temperature coplanar slip crystals are strongest, crystals forming Cottrell-Lomer locks weakest. At 678 K (0.5 T_m) Cottrell-Lomer lock crystals are strongest, those forming sessile jogs weakest. The orientation dependence of the saturation stress is much greater at 678 K than at room temperature. A plateau in the saturation stress of about 14 MPa is observed in sessile jog and Lomer lock crystals cycled at 523 K. Persistent slip bands (PSBs) with the familiar ladder structure are observed by electron microscopy in these crystals. No saturation stress plateau or TEM evidence of PSB formation was found at 678 K. Only cells, usually equiaxed, were seen.     

The room temperature fatigue behavior of nickel-base superalloy crystals at ultrasonic frequency

Previous investigations have invariably observed strain rate related deformation effects as the fatigue frequency is raised to the ultrasonic range. Through room temperature tests on strain rate insensitive nickel-base superalloy single crystals of Mar-M200, we have shown that another effect of increasing the fatigue frequency to the ultrasonic range is in the suppression of the deleterious influence of environment. It was found that above a stress amplitude of 30,400 psi the fatigue lives of crystals ultrasonically fatiguedin air increase with decreasing stress in a manner which is functionally similar to, that of crystals conventionally fatiguedin vacuum. Similarly, the fracture surfaces of ultrasonically fatigued crystals have a dimpled appearance over most of their areas which is characteristic of locally ductile failure and identical to, the appearance of crystals failed at conventionally frequency in vacuum. These results, along with a kinetic analysis of gaseous adsorption, indicate that the major effect of increasing the fatigue frequency to the ultrasonic, range is in the suppression of the influence of oxygen in enhancing the rate of crack propagation. In addition, the short test times involved in running large numbers of cycles have allowed for the determination of the fatigue limit in a nickel-base superalloy. This is the first indication of no-fail behavior in this type of alloy.     

Shear deformation and compaction of nickel aluminide powders at elevated temperatures

Powder compacts of nickel aluminide were compressed under uniaxial load above 1373 K, using a method that allowed the material to move laterally. Lateral and axial displacements were measured by means of three LVDTs. The resulting data fully described the applied stress state and the strain state as a function of time. That allowed us to obtain simultaneous measurements of the time dependent, and density dependent shear and densification behavior of the powder compact. The shear rate was non-linear in stress suggesting a dislocation flow mechanism. A model for densification by power law creep was applied to the data. It greatly overestimated the measured densification rates. Interestingly it was found that it is difficult to densify the powder to a value more than about 0.80 (relative) using uniaxial compression. In further experiments the powder was hot-pressed in a constraint cavity, allowing large hydrostatic pressures to be applied to the specimen. Near theoretical densities were obtained, presumably because the hydrostatic pressure promoted the diffusional transport mechanism of densification. The hot-pressing data were combined with the sinter forging data to obtain the correlation between densification rate and applied pressure. The diffusional mechanism of densification gave a good quantitative explanation for the densification behavior. In a broader context, we think that powder consolidation techniques ought to be optimized with a view to both shear strain as well as hydrostatic pressure. The shear strain can promote microstructure refinement through dynamic recrystallization, while pressure provides a driving force for diffusional densification.     

cyclic deformation of Ni3(Al,Nb) single crystals at ambient and elevated temperatures

In order to study the cyclic deformation of an alloy crystal and the asymmetry in the flow stress of the γ′ phase, Ni₃(Al, Nb) crystals have been cycled in strain control at room temperature, 400 and 700°C. The orientation of the crystals has been a major variable studied. At all temperatures, cyclic hardening has been found considerable and at the two lower temperatures, the asymmetry has been found to follow the predictions of the model by Paidar et al. [Acta metall. 32, 435 (1984)] which is based on the increase of the dislocation friction stress by thermally activated cross slip onto [001] planes. Moreover, TEM observations of the dislocation structures are consistent with the model. At 700°C, a dominance of the tensile stress in the asymmetry was observed rather surprisingly. The model of Paidar et al. does not apply at this high temperature because dislocation climb and cube slip occurs, and no asymmetry is expected. Cracking and life behavior are also reported.     

Induced creep and creep/fatigue of a nickel-base superalloy at ambient temperatures

The stress controlled fatigue of Nimonic*115, a typical γ’-strengthened nickel-base superalloy, was studied at ambient temperature, using a trapezoidal wave form at 1 Hz, with stresses chosen to produce failure in the lO⁴ to lO⁴ cycle range. In tests with maximum stress greater than the proportional limit, most of the fatigue damage occurs within the first few test cycles. Much of this strain is accumulated under static load and is therefore identified as creep strain. Transmission electron microscopy shows that these creep strains occur in slip bands which disrupt the ordered γ’ precipitates. Strain is found to follow a logarithmic time dependence, which suggests a low activation energy mechanism.     

Low cycle fatigue behavior of Ni-Nr lamellar eutectic composites at elevated temperatures

Low cycle fatigue properties of unidirectionally solidified lamellar eutectic Ni-51 Cr alloy were determined and compared with those of the cast microstructure in the temperature range of 300° to 760°C. Both materials exhibited an initial cyclic strain hardening followed by saturation over most of the temperature range. The rate and the amount of cyclic work-hardening decreased with temperature above 600°C. Rapid softening due to macro-crack propagation occurred at later stages of the fatigue process, which occupied an increasing portion of the fatigue life in the lamellar material as the strain amplitude was decreased. At Δ∈_T = 0.0190, the lamellar material exhibited longer fatigue life over the entire temperature range which has been related to the ability of Cr-rich lamellae to deflect fatigue cracks. At 625°C, the fatigue life (Nf) of both materials was related to the plastic strain range ( Δ∈_P) through the relationship (Δ∈_P/2 =K(2N_f)^c wherec andK are -0.39 and 0.068 for the lamellar, and -0.45 and 0.074 for the cast structure, respectively. At this temperature with decreasing strain amplitude lamellar material became more resistant to fatigue than as-cast structure, which has been related to the more efficient deflection of fatigue cracks by Cr-rich lamellae at lower strain amplitudes . Formerly with the Dept. of Metallurgical and Materials Engineering, University of Pittsburgh, Pittsburgh, Pa. Formerly Visiting Scientist, Department of Metallurgical and Materials Engineering, University of Pittsburgh Formerly Professor and Chairman, Department of Metallurgical and Materials Engineering, University of Pittsburgh     

Creep of precipitation-hardened nickel-base alloy single crystals at high temperatures

Single crystals of a γ′ precipitation-hardened nickel-base super alloy, Mar-M200, were tested in constant load creep at 1575°F. It was found that shear of the γ′ precipitate by pairs of α/2 (110) dislocations controlled deformation in both primary and steady-state creep. This contrasts with 1400°F creep behavior where shear of γ′ is dominated by α/3 (112) dislocations in primary creep, but by pairs of α/2 (110) dislocations in steadystate creep. The orientation dependence of the steady-state creep rate at 1575°F is explained by the nature of dislocation junction reactions for the different orientations. Crystals along the [001]-[1•11] boundary have the greatest creep resistance because of the formation of stable dislocation networks at the matrix-particle (γ⊃ interfaces, whereas the lower creep resistance of crystals oriented along the [001]-[0•11] boundary is a consequence of the low probability for the formation of stable junction reactions. Finally, evidence, in the form of resolvable α/2 (110) dislocation pairs within the γ′ precipitate, is presented for a reduction in the local antiphase boundary energy of γ′ at high temperatures.