Crystallographic fatigue crack growth in incompatible Aluminum Bicrystals: Its Dependence on Secondary Slip

The secondary slip behavior ahead of crystallographic fatigue cracks and its effect on the crack growth near the grain boundaries (GBs) in\([12\bar 1]\) tilt nonsymmetrical aluminum bicrystals under constant cyclic stress amplitude have been systematically examined. The displacement field ahead of short crack tips near the interfaces in two specimens has been measured by using a microfiducial grid technique. It has been observed that the critical persistent slip band (PSB) ahead of a short crack tip near the GB in a middle misoriented bicrystal was able to develop as long as the primary one and resulted in a temporary stage II growth. As a longer crystal- lographic crack grew into the grain boundary affected zone (GBAZ), activation of the critical slip ahead of the crack front and crack branching along the critical PSB occurred in all groups of the aluminum bicrystals, which reveals a crucial role of the critical slip in increasing the crack opening and triggering the slip in the adjacent grain. On the other hand, cross slip became the dominant slip mode ahead of the crystallographic crack front near the GB in a bicrystal of larger misfit angles and drove the crack along the cross PSB, a steep path with a remarkably high growth rate, until it propagated into the GBAZ. The resultant stress on the secondary slip system ahead of a crack front near the interface contributed by the internal stress due to both intergranular and intragranular incompatible strain, as well as the enhanced crack tip stress, has been evaluated and rationalizes the activation of the secondary slip systems.     

Crystallographic fatigue crack growth in incompatible aluminum bicrystals: Its dependence on secondary slip

The secondary slip behavior ahead of crystallographic fatigue cracks and its effect on the crack growth near the grain boundaries (GBs) in tilt nonsymmetrical aluminum bicrystals under constant cyclic stress amplitude have been systematically examined. The displacement field ahead of short crack tips near the interfaces in two specimens has been measured by using a microfiducial grid technique. It has been observed that the critical persistent slip band (PSB) ahead of a short crack tip near the GB in a middle misoriented bicrystal was able to develop as long as the primary one and resulted in a temporary stage II growth. As a longer crystallographic crack grew into the grain boundary affected zone (GBAZ), activation of the critical slip ahead of the crack front and crack branching along the critical PSB occurred in all groups of the aluminum bicrystals, which reveals a crucial role of the critical slip in increasing the crack opening and triggering the slip in the adjacent grain. On the other hand, cross slip became the dominant slip mode ahead of the crystallographic crack front near the GB in a bicrystal of larger misfit angles and drove the crack along the cross PSB, a steep path with a remarkably high growth rate, until it propagated into the GBAZ. The resultant stress on the secondary slip system ahead of a crack front near the interface contributed by the internal stress due to both intergranular and intragranular incompatible strain, as well as the enhanced crack tip stress, has been evaluated and rationalizes the activation of the secondary slip systems. On leave from Taiyuan University of Technology, Taiyuan, Shanxi 030024, People’s Republic of China.     

Comparison of cyclic deformation behavior between copper bicrystals and their component crystals

In this article, the cyclic deformation behavior of a combined copper bicrystal (CB), a naturally grown copper bicrystal (RB), and their component crystals (G1 and G2) was investigated under total strain amplitude control at room temperature in air. The bicrystal CB without grain boundary (GB) was prepared by joining the two single slip-oriented component crystals G1 and G2 in the gage. The results showed that the cyclic hardening rates and the axial stresses increased, in order, in the component crystal G2, the bicrystal CB, the bicrystal RB, and the component crystal G1. The cyclic stress-strain curves (CSSCs) of the component crystals G1 and G2 and the bicrystal CB exhibited a plateau behavior under the selected strain range. However, the cyclic saturation stress of the bicrystal RB increased with strain amplitude and did not show a plateau in its CSSC. Surface observations of the copper bicrystals and the component crystals revealed that the cyclic plastic strains were carried by the primary slip (B4) within the component crystals in the bicrystal CB and its component crystal G1 and G2. However, an additional slip (A3) within the component crystal G2 was observed in the vicinity of the GB in the bicrystal RB. The width of the additional slip region increased with strain amplitude and varied in the range from 265 to 650 μm. In order to compare the CSSCs of the copper bicrystals and their component crystals, an “orientation factor” for the copper bicrystal was introduced and the effect of the GB on the bicrystal RB is quantified during cyclic deformation.     

Deformation texture of channel-die deformed aluminum bicrystals with S orientations

The evolution of crystal lattice orientations in two aluminum bicrystals with S orientations, during channel die compression has been studied in detail. The boundary planes of the bicrystals were parallel to the plane of compression. The textures developed in each component crystal and at the boundary between the two crystals were examined as a function of deformation. In one of the bicrystal assemblies, the initial orientations of the component crystals were found to be unstable. They rotated to a common orientation. Hence, the bicrystal boundary, initially high angle in nature, became a low angle boundary. In the other bicrystal, the initial orientations of the component crystals were found to be largely stable. Except for some minor orientation adjustments, they remained essentially in their initial orientations, and the bicrystal boundary retained its high angle misorientation. Results of theoretical analyses using a numerical model appear to explain the texture development and the behavior at the boundary of the bicrystals very satisfactorily.     

Fatigue Crack Growth Behaviour of Conventional and Modified IN 718 Superalloys at 650 °C

In the present investigation, the fatigue crack growth behaviour of modified IN 718 superalloy (Ni-0.02 %, C-19.04 %, Cr-19.31 %, Fe-3.04 %, Mo-4.73 %, Nb-1.01 %, Al-1.16 %, Ti-0.0033 %, B, all in wt%) has been compared with conventional IN 718 superalloy (Ni-0.02 %, C-19.0 %, Cr-19.35 %, Fe-3.0 %, Mo-5.10 %, Nb-0.50 %, Al-1.00 %, Ti-0.0033 %, B, all in wt%) at 650 °C. Modified IN 718 superalloy exhibits marginally lower crack growth rate as compared to conventional alloy and was attributed to roughness induced crack closure.     

Characterization of bicrystals using kikuchi patterns

Three parameters, the misorientation angle, misorientation axis, and boundary normal, have been used to describe a general bicrystal,e.g., two adjacent grains, subgrains, or twins. An analysis developed to determine these parameters is presented. The angle and axis of misorientation are calculated from a misorientation matrix obtained from two Kikuchi patterns, one taken from each of the two crystals. To obtain the boundary normal, a specimen tilt is made inside the electron microscope. A rotation matrix specifying the actual specimen tilt is formulated from two Kikuchi patterns taken from the same crystal before and after tilt. With this rotation matrix and the change of projected boundary images before and after tilt, the boundary normal can be calculated. It is demonstrated that 1) for high-angle bicrystals, the misorientation angle may be determined to within ±0.2 deg, and the misorientation axis to within 0.1 deg; 2) for low-angle bicrystals, the misfit angle can be obtained to within ±0.1 deg, and the misfit axis to within 4 deg; and 3) the boundary normals so determined are generally accurate to 2 deg if suitable correction is made for the magnification change resulting from the specimen tilt.     

Very High Cycle Fatigue of Ni-Based Single-Crystal Superalloys at High Temperature

Very high cycle fatigue (VHCF) properties at high temperature of Ni-based single-crystal (SX) superalloys and of a directionally solidified (DS) superalloy have been investigated at 20 kHz and a temperature of 1000 °C. Under fully reversed conditions (R = ? 1), no noticeable difference in VHCF lifetimes between all investigated alloys has been observed. Internal casting pores size is the main VHCF lifetime-controlling factor whatever the chemical composition of the alloys. Other types of microstructural defects (eutectics, carbides), if present, may act as stress concentration sites when the number of cycles exceed 10⁹ cycles or when porosity is absent by applying a prior hot isostatic pressing treatment. For longer tests (> 30 hours), oxidation also controls the main crack initiation sites leading to a mode I crack initiation from oxidized layer. Under such conditions, alloy’s resistance to oxidation has a prominent role in controlling the VHCF. When creep damage is present at high ratios (R ≥ 0.8), creep resistance of SX/DS alloys governs VHCF lifetime. Under such high mean stress conditions, SX alloys developed to retard the initiation and creep propagation of mode I micro-cracks from pores have better VHCF lifetimes.     

High-temperature low-cycle fatigue behavior of K40S cobalt-base superalloy

An investigation was made on the strain-controlled low-cycle fatigue (LCF) of K40S cobalt-base superalloy at 900 °C in ambient atmosphere. The results show that K40S alloy possesses high LCF resistance in comparison with X-40 alloy. Under the testing conditions in this study, K40S alloy exhibits a cyclic stress response of initial hardening followed by softening. The cyclic stress response behavior has been attributed to dislocation-dislocation interactions and dislocation-precipitate interactions. The high response stress can lead to a large stress concentration at locations where inelastic strains of high amplitude accumulate, which account for the decreasing fatigue life with increasing strain rate. The well-distributed carbide particles are the “secondary” crack initiation sites. The secondary crack initiation relaxes the stress concentration at the crack tip, reducing the driving force of crack propagation. High-temperature LCF failure of K40S alloy results from the interaction of the mechanical fatigue and environmental oxidation.     

Prediction of crack paths in WCCo alloys

A crack propagating through the WCCo microstructure has to choose between paths along the binder/carbide interface and paths across binder regions. The latter paths are selected when the crack enters a binder region at a large angle from the nearest carbide interface, while the interface paths are preferred by cracks entering at a small angle. A critical angle can be defined for the switch from one type of crack path to the other. Empirical data for the area fractions of the two crack paths in widely different WCCo alloys can be accounted for by a single critical angle, φ_c = 25°. Finite element analysis of the stress field in a region of binder enclosed between carbide grains shows that the preferred site for the growth of stress-induced microvoids will move from the carbide grain flanks to the interior of the binder region when the entry angle of the crack exceeds 24°. Thus the observation of a critical angle deciding the crack path is verified by the stress field analysis and given a physical explanation in terms of the most likely site for microvoid formation.     

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.     

Accelerated Stress Corrosion Crack Initiation of Alloys 600 and 690 in Hydrogenated Supercritical Water

The objective of this study is to determine whether stress corrosion crack initiation of Alloys 600 and 690 occurs by the same mechanism in subcritical and supercritical water. Tensile bars of Alloys 690 and 600 were strained in constant extension rate tensile experiments in hydrogenated subcritical and supercritical water from 593 K to 723 K (320 °C to 450 °C), and the crack initiation behavior was characterized by high-resolution electron microscopy. Intergranular cracking was observed across the entire temperature range, and the morphology, structure, composition, and temperature dependence of initiated cracks in Alloy 690 were consistent between hydrogenated subcritical and supercritical water. Crack initiation of Alloy 600 followed an Arrhenius relationship and did not exhibit a discontinuity or change in slope after crossing the critical temperature. The measured activation energy was 121 ± 13 kJ/mol. Stress corrosion crack initiation in Alloy 690 was fit with a single activation energy of 92 ± 12 kJ/mol across the entire temperature range. Cracks were observed to propagate along grain boundaries adjacent to chromium-depleted metal, with Cr₂O₃ observed ahead of crack tips. All measures of the SCC behavior indicate that the mechanism for stress corrosion crack initiation of Alloy 600 and Alloy 690 is consistent between hydrogenated subcritical and supercritical water.     

Fatigue crack growth and fracture behavior of bismuth-doped copper bicrystals

Crack growth rate measurements in Cu-Bi bicrystals have shown that for a given cyclic stress intensity factor ΔK, the crack growth rateda/dN increases with increasing Bi concentration. The increase inda/dN is due to the surface energy reduction that occurs because of the presence of bismuth in the copper. For a given bicrystal orientation, the Paris exponentm was found to increase with increasing bismuth content. Them value was found to be between about 0.5 and 3. This finding points to the need for fatigue crack growth theories that incorporate a variation inm in their crack growth laws. The grain boundary fracture surfaces of weak bicrystals showed steps whose formation is thought to arise from the need for bismuth to segregate to low energy surfaces. The results also indicate that the fracture surface energy of weak bicrystals can be lower than that of pure bismuth. This result might help explain the finding that when present, bismuth particles at the boundary tended to pull out of the matrix of one of the single crystals instead of cleaving flush with the boundary. Bismuth coverage at the grain boundary fracture surfaces of weak bicrystals was found to be between about 0.5 and 2 monolayers. Formerly Postdoctoral Fellow with the Department of Materials Science and Engineering, Northwestern University.     

晶界位向对硅钢立方/旋转立方取向双晶轧制取向变化的影响

晶界和{100}柱状晶在硅钢生产过程中对织构的遗传和演变有关键作用,因此本文利用晶体塑性有限元方法进行立方和旋转立方取向双晶在晶界不同位向时晶体取向演变的全场模拟。模拟显示,三种晶界位向下,晶界都具有诱发晶内产生S形状形变不均匀和缓解局部形变不均匀区取向转动的特点,立方和旋转立方取向双晶在带有剪切作用的轧制条件下都显示明显的取向稳定性。GB⊥RD(表示晶界垂直于轧向)晶界位向时,旋转立方取向晶粒优先在晶界中心位置发生取向转动,而立方取向则优先在远离晶界的端部发生取向转动。GB⊥TD(表示晶界垂直于横向)的晶界位向下,其晶界阻碍作用最小,双晶内产生的取向漫散度大,织构强度较低;除绕TD转动外,也具有复杂的绕RD、ND的取向转动。GB⊥ND(表示晶界垂直于法向)的晶界位向下,取向转动与GB⊥RD时相近,但有少量取向绕ND转动。     

Optimal Design Experiment of Ageing Time and Temperature in Inconel-713C Superalloy Based on Hardness Objective

This study consists of assessing the influence of ageing heat treatments on characteristics of the microstructure and the hardness in the Inconel-713C nickel-based superalloy. The ageing process including the time and the temperature was modified based on the material hardness, by the design of experiments (DOE). For this objective, ageing treatments consisted of heating at 850, 890 and 930 °C for 8 and 16 h, after a solutioning treatment. Experimental results showed that the mean hardness reached the highest value (as 43.2 RC) in the sample that was age-hardened at 890 °C for 8 h, without requiring usual double ageing heat treatments. Besides, the DOE also predicted the highest hardness, which would be related to the specimen, hardened at 896.1 °C for 8 h and also 895.3 °C for 16 h, based on vertical and parallel lines, respectively. The highest hardness was associated with the optimum volume fraction (as 64%) of approximately fine size (as 500 nm) of \(\gamma '\) precipitates. Besides, M₂₃C₆-type carbides plus NbC had a positive effect to enhance the hardness value. X-ray diffraction (XRD) results indicated various phases created in different ageing heat treatments that affected the hardness.     

Stress Corrosion Cracking Behavior at Inconel and Low Alloy Steel Weld Interfaces

Three-dimensional microstructure observations, macro- to micro-scopic residual stress measurements by three methods and creviced bent beam SCC tests were performed for Inconel/low alloy steel (LAS) weld samples. The possible reasons for the suppression of SCC crack propagation near the weld interface found at a nuclear power plant were estimated to include the crack branching at the grain boundary (GB) parallel to the interface, i.e., Type II GB, compressive residual stresses in the LAS region and crack tip oxidation in the LAS at the interface. The formation mechanism of Type II GB and stress gradient in individual grains in the Inconel are also discussed.     

Beta brass bicrystal and tricrystal stress strain behavior

Beta brass single crystal, bicrystal and tricrystal stress strain behavior were determined for a series of isoaxial specimens in which the bicrystals and tricrystals were incompatible in shear along the grain boundary. Volume fractions of grain boundary deformation were also determined to ascertain the effect of increased constraint on grain boundary strengthening. It was found that bicrystal flow stress was considerably higher than single crystal flow stress and that tricrystal strain hardening was only slightly larger than that of the bicrystal. For the tricrystals the grain boundary deformation zone is larger than that of corresponding bicrystals because of both configurational and constraint effects. This larger zone indicates that secondary stresses extend further from the boundary in tricrystals than in bicrystals. It was concluded that despite increased constraint, , the average grain boundary stress for the tricrystals, is smaller than that for corresponding bicrystals and that because of the increased extent of the secondary stress,σ _{b i}, the stress in the center component away from the grain boundary, is higher than that of the single crystal at corresponding strains. The relationship of these observations to corresponding stresses in polycrystalline material is examined. where Taek Dong Lee was formerly a Graduate Student. This paper is based on a thesis to be submitted by Taek Dong Lee in partial fulfillment of the requirements for the degree of Doctor of Philosophy at Polytechnic Institute of New York.     

Competing Modes for Crack Initiation from Non-metallic Inclusions and Intrinsic Microstructural Features During Fatigue in a Polycrystalline Nickel-Based Superalloy

Cyclic fatigue experiments in the high and very high cycle fatigue regimes have been performed on a René 88DT polycrystalline nickel-based superalloy. The microstructural configurations that favor early strain localization and fatigue crack initiation at high temperature from 400 °C to 650 °C have been investigated. Competing failure modes are observed in the high to the very high cycle fatigue regime. Fatigue cracks initiate from non-metallic inclusions and from intrinsic internal microstructural features. Interestingly, as stresses are reduced into the very high cycle regime, there is a transition to initiation only at crystallographic facets. At higher stress in the high cycle fatigue regime, a significant fraction of specimens initiate cracks at non-metallic inclusions. This transition is analyzed with regard to microstructural features that favor strain localization and accumulate damage early during cycling.     

Deformation in NiAl bicrystals

The effects of a concentrated local stress arising from elastic and plastic incompatibility at internal NiAl grain boundaries have been investigated in oriented NiAl bicrystals. Observations of surface slip markings and TEM characterization of the dislocations responsible for bicrystal deformation have been made. The glide of a〈110〉 dislocations on {110} planes is the primary mode of deformation in the vicinity of the oriented bicrystal interfaces investigated. The dominant mode of a〈110〉 dislocation generation is the nucleation of loops away from the bicrystal interface. The macroscopic stress required to activate these dislocations, in the presence of a stress concentration such as that produced by a dislocation pile-up at a grain boundary, is the same as that required to nucleate a〈100〉 dislocations in “soft” single crystals of NiAl. Although the statistical relevance of this phenomenon as it pertains to the deformation of NiAl polycrystals has not been established, it is speculated that the glide of non-a〈100〉 dislocations may play an important role in the ductility of polycrystalline NiAl at the ductile-to-brittle transition temperature. The movement of a〈100〉 dislocations provides the primary mode of deformation for several bicrystals investigated far (> 3 mm) from the bicrystal interface. It is suggested that the vacancies required for the extensive climb of a[100] vacancy loops is supplied by the climb of a[100] and a[010] dislocations, and that the gradient in the concentration of vacancies provides the driving force for the motion of the a[100] and a[010] dislocations.     

Dependence of Competitive Grain Growth on Secondary Dendrite Orientation During Directional Solidification of a Ni-based Superalloy

The competitive grain growth in bicrystal samples during unidirectional solidification of a Ni-based superalloy was found to depend on secondary dendrites perpendicular to the grain boundary of bicrystal samples, rather than primary dendrites parallel to the thermal gradient as generally recognized. The primary dendrite orientation, however, had significance for the dendrite blocking in overgrowth processes and the resultant overgrowth rate during competitive grain growth.