Structure/property/continuum synthesis of ductile fracture in inconel alloy 718

Two microscopic ductile fracture processes have been established in a fracture tough superalloy, Inconel 718, aged to five strength levels. At yield strengths less than 800 MPa, the mechanism is a slow tearing process within large pockets of inhomogeneous carbides and nitrides, giving rise to plane strain fracture toughness (K _IC)values greater than 120 MPa-m^1/2. At yield strengths greater than 900 MPa, the mechanism involves fracture initiation at carbides and nitrides followed by off crack plane void sheet growth nucleated at the Laves (σ) phases. Here, the fracture toughness drops to about 80 MPa-m^1/2. A Mode I normal strain growth model for low yield strength conditions and a shear strain void sheet model for high yield strength ones are shown to model K_IC data obtained from a J-integral evaluation of compact tension results.     

Hydrogen-assisted ductile fracture in porous iron

The present paper has studied the effect of porosity and hydrogen on the deformation and fracture properties in porous iron using double notched specimen tests and quantitative metallography analyses to characterize void growth. The presence of hydrogen produced the greatest hardening effect in the sample with a porosity of 0.037. It was found that while the effect of hydrogen on the reduction in area was greatest in the specimen with a porosity of 0.037, its effect on the fracture energy did not depend on the porosity. The ductile fracture surface morphology remained the same in all the specimens without and with hydrogen except for the sample with porosity of 0.003 where hydrogen induced some quasi-cleavage fracture. The quantitative metallography analysis of voids in the region near the fracture surface and below the unfractured notch has demonstrated that hydrogen causes the void growth to increase prior to plastic instability. The effect of hydrogen on the void growth was found to be slightly enhanced as the porosity was increased. The microscopic process of hardening induced by hydrogen is presented. The mechanism for hydrogen-assisted void growth is discussed in light of a recently developed model using a dislocation theory.     

Modeling of local strains in ductile fracture

The concept of dividing microvoid coalescence (MVC) ductile fracture into three constituent processes, nucleation, growth, and coalescence, is discussed, with emphasis on needs for additional analytical and experimental work. Statistical and stochastic aspects of the problem are presented. Recent work on modeling of local strains during ductile fracture, and particularly as components of fracture toughness, is summarized and discussed in light of current knowledge of ductile fracture. Such local strain modeling is especially attractive because it permits micromechanisms of fracture to be explicitly included in the fracture model. This paper is based on a presentation made at the symposium “Stochastic Aspects of Fracture” held at the 1986 annual AIME meeting in New Orleans, LA, on March 2-6, 1986, under the auspices of the ASM/MSD Flow and Fracture Committee.     

Hydrogen-assisted ductile fracture in spheroidized 1518 steel

Hydrogen was introduced into smooth and notched tensile specimens of spheroidized 1518 steel by electrocharging under controlled galvanostatic conditions. The role of hydrogen during void nucleation and growth was investigated by identifying two different void nucleation modes. Hydrogen promoted void nucleation at average-sized carbide particles by reducing the critical interfacial strength, σ_C, from 1200 to 1000 MPa (using a dislocation model). The stress-induced hydrogen segregation to the particle interfaces during deformation was estimated to explain the hydrogen-reduced σ_C. Void growth in both longitudinal and lateral directions was enhanced by internal pressurization of hydrogen. In order to better quantify such hydrogen-enhanced void growth, the internal hydrogen pressure inside a void was calculated on the basis of thermodynamics. The final void coalescence stage was analyzed by assuming that the void nucleation rate follows a normal distribution.     

Layer thickness effect on ductile tensile fracture

Laminated metal composites containing equal volume percentage of ultrahigh carbon steel (UHCS) and brass were prepared in three different layer thicknesses (750, 200, and 50 μm) by press- bonding and rolling at elevated temperature and were tensile tested at ambient temperature. A dramatic increase in tensile ductility (from 13 to 21 to 60 pct) and a decrease in delamination tendency at the UHCS-brass interfaces were observed as the layer thickness was decreased. The layer thickness effect on ductility is attributed to residual stress whose influence on delamination is decreased as the layer thickness is decreased. Suppression of delamination inhibits neck for- mation in the UHCS layers, allowing for extended uniform plasticity. For a given layer thick- ness, the tensile ductility decreases as the ratio of hardness of component layers is increased.     

Cavity formation from inclusions in ductile fracture

The previously proposed conditions for cavity formation from equiaxed inclusions in ductile fracture have been examined. Critical local elastic energy conditions are found to be necessary but not sufficient for cavity formation. The interfacial strength must also be reached on part of the boundary. For inclusions larger than about 100? the energy condition is always satisfied when the interfacial strength is reached and cavities form by a critical interfacial stress condition. For smaller cavities the stored elastic energy is insufficient to open up interfacial cavities spontaneously. Approximate continuum analyses for extreme idealizations of matrix behavior furnish relatively close limits for the interfacial stress concentration for strain hardening matrices flowing around rigid non-yielding equiaxed inclusions. Such analyses give that in pure shear loading the maximum interfacial stress is very nearly equal to the equivalent flow stress in tension for the given state of plastic strain. Previously proposed models based on a local dissipation of deformation incompatibilities by the punching of dislocation loops lead to rather similar results for interfacial stress concentration when local plastic relaxation is allowed inside the loops. At very small volume fractions of second phase the inclusions do not interact for very substantial amounts of plastic strain. In this regime the interfacial stress is independent of inclusion size. At larger volume fractions of second phase, inclusions begin to interact after moderate amounts of plastic strain, and the interfacial stress concentration becomes dependent on second phase volume fraction. Some of the many reported instances of inclusion size effect in cavity formation can thus be satisfactorily explained by variations of volume fraction of second phase from point to point. This work has been presented in part orally at the Third International Conference on Fracture in Munich, Germany April 1973.     

Brittle to ductile transition in cleavage fracture

In many cleavable solids, cleavage cracks can propagate at steady state by laying down a trail of dislocations emanating from the crack tip and lying on planes inclined to the crack front. These crack-tip initiated dislocations produce shielding at the crack tip that reduces both the crack tip tensile stresses and the shear stresses on the inclined planes. They also blunt the crack. Cleavage cracks, can nevertheless, still propagate under appropriately increased stress intensity conditions to keep the crack tip tensile stress constant. A condition is reached in the propagation of such slightly blunted cracks where a small increment in temperature or a decrement in the crack velocity permits the nucleation of a new set of dislocations that produce additional shielding and blunting which tip the balance against the crack-tip tensile stresses. This results in a transition from brittle cleavage to ductile behavior. The steady state specific plastic work that can just be tolerated by a propagating cleavage crack before it catastrophically blunts is calculated to be only of the order of 10% of the specific surface energy. Although most geometrical details of the dislocation emission process are adequately modeled, the calculated brittle to ductile transition temperatures are found to be more than an order of magnitude higher than those that have been experimentally measured. This discrepancy is a result of the present inadequate methods of modeling activation configurations by considering the dislocation loop radius as the only activation parameter, while proper modeling of such configurations must consider also the Burgers shear displacement of the loop as an activation parameter. Such two parameter analyses, however, require accurate information on interlayer atomic shear resistance profiles for specific crystals which are presently not available. The analysis furnishes ready explanations of the toughening effects of so-called “ductilization” treatments and embrittling effect of aging and dislocation locking, as well as the relatively large difference between the lowest levels of toughness between fracture in polycrystals and in single crystals.     

Cavity formation from inclusions in ductile fracture

The previously proposed conditions for cavity formation from equiaxed inclusions in ductile fracture have been examined. Critical local elastic energy conditions are found to be necessary but not sufficient for cavity formation. The interfacial strength must also be reached on part of the boundary. For inclusions larger than about 100Å the energy condition is always satisfied when the interfacial strength is reached and cavities form by a critical interfacial stress condition. For smaller cavities the stored elastic energy is insufficient to open up interfacial cavities spontaneously. Approximate continuum analyses for extreme idealizations of matrix behavior furnish relatively close limits for the interfacial stress concentration for strain hardening matrices flowing around rigid non-yielding equiaxed inclusions. Such analyses give that in pure shear loading the maximum interfacial stress is very nearly equal to the equivalent flow stress in tension for the given state of plastic strain. Previously proposed models based on a local dissipation of deformation incompatibilities by the punching of dislocation loops lead to rather similar results for interfacial stress concentration when local plastic relaxation is allowed inside the loops. At very small volume fractions of second phase the inclusions do not interact for very substantial amounts of plastic strain. In this regime the interfacial stress is independent of inclusion size. At larger volume fractions of second phase, inclusions begin to interact after moderate amounts of plastic strain, and the interfacial stress concentration becomes dependent on second phase volume fraction. Some of the many reported instances of inclusion size effect in cavity formation can thus be satisfactorily explained by variations of volume fraction of second phase from point to point.     

An acoustic emission investigation of microscopic ductile fracture

A high-strength 4340 steel fracture-toughness specimen was heat treated to give a ductile-rupture type of slow crack growth under rising load. For evaluation of the step-wise growth process, the specimen was instrumented with acoustic stress wave emission (SWE) detection equipment. The resulting crack area swept out by the advancing crack was correlated to the magnitude and number of the acoustic emission pulses. A crack growth model was developed which accounts for the direct relationship between crack area swept out and the sum of the individual SWE amplitudes, and for the experimentally observed bimodal distribution of the SWE amplitudes. The model postulates that slow crack growth takes place in a step-wise mechanism. This involves a repeated two-step process where the first step is the formation of a multitude of individual thumbnail cracks and the second step is the simultaneous interconnection of these thumbnail cracks to form a new continuous crack front. Formerly , Postdoctorate Associate, Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minn.     

Hydrogen assisted ductile fracture of spheroidized carbon steels

The effects of hydrogen on ductile fracture were studied in two spheroidized plain carbon steels, containing 0.16 and 0.79 pct C. A combination of fractography and quantitative metallography on sectioned, deformed specimens permitted separation of the effects of hydrogen on the initiation, growth, and link-up of voids. In both steels, hydrogen was found to have no significant effect on either the initiation of voids at carbides, or early growth of voids, prior to link-up. In the higher carbon steel the fracture surface dimple size increased after hydrogen exposure with no other evident change in the fracture surface appearance; it is therefore inferred that hydrogen primarily assists void growth during link-up in this steel. In the lower carbon steel the fracture appearance changed and a decrease in void size due to hydrogen was found fractographically; thus, both initiation and growth of voids are apparently enhanced during the link-up phase of fracture in this steel. It is hypothesized that these effects may be due largely to a void pressure mechanism if hydrogen is transported by mobile dislocations. Formaly Graduate Student, Department of Metallurgy and Materials Science, Carnegie-Mellon University     

An acoustic emission investigation of microscopic ductile fracture

A high-strength 4340 steel fracture-toughness specimen was heat treated to give a ductile-rupture type of slow crack growth under rising load. For evaluation of the step-wise growth process, the specimen was instrumented with acoustic stress wave emission (SWE) detection equipment. The resulting crack area swept out by the advancing crack was correlated to the magnitude and number of the acoustic emission pulses. A crack growth model was developed which accounts for the direct relationship between crack area swept out and the sum of the individual SWE amplitudes, and for the experimentally observed bimodal distribution of the SWE amplitudes. The model postulates that slow crack growth takes place in a step-wise mechanism. This involves a repeated two-step process where the first step is the formation of a multitude of individual thumbnail cracks and the second step is the simultaneous interconnection of these thumbnail cracks to form a new continuous crack front.     

Ferritic-bainitic structure and ductile fracture resistance of high-strength pipe steels

The formation of the products of bainitic transformation in pipe steels of strength classes K60 (X70) and K65 (X80) during thermomechanical treatment is considered. It is shown that the structural factors that affect the ductile-fracture resistance of steel are the bainite morphology and the carbon distribution over structural constituents.     

一个基于孔洞演化机制的韧性断裂预测模型

在韧性断裂中微观孔洞演化机制的基础上,提出了一个基于孔洞演化机制的非耦合型韧性断裂预测模型.模型充分考虑了两种典型的孔洞演化机制:孔洞的长大机制和孔洞的拉长扭转机制.该模型引入了三个具有不同物理意义的材料参数:材料对不同孔洞演化机制的敏感度、应力状态敏感度系数和材料的损伤阈值,并使用等效塑性应变增量表征其对韧性损伤累积过程的驱动作用.为了使模型可以更好地反映三维应力状态对材料韧性断裂性能的影响,将该模型从主应力空间转换到由应力三轴度、罗德参数和临界断裂应变构成的三维空间,得到了由模型确定的三维韧性断裂曲面,并研究了相关参数对三维韧性断裂曲面及平面应力二维韧性断裂曲线的影响.利用5083-O铝合金、TRIP690钢和Docol 600DL双相钢三个典型的轻质高强板材的韧性断裂数据验证了该模型对不同材料和不同应力状态的适用性和准确性.     

Damage leading to ductile fracture under high strain-rate conditions

Quantitative metallographic studies of damage evolution leading to ductile fracture under high strain-rate loading conditions are presented. A model material is considered, namely, leaded brass, which contains a dispersed globular lead phase that acts as void nucleation sites. Interrupted tensile split Hopkinson bar tests have been performed to capture the evolution of porosity and void aspect ratio with deformation at strain rates up to 3000 s⁻¹. Both uniaxial and notched specimen geometries were considered to allow the effects of remote stress triaxiality to be investigated. Plate impact testing has also been performed to investigate the evolution of damage under the intense tensile triaxiality and extremely high rates of deformation (10⁵ s⁻¹) occurring within a spall layer. Quantitative metallographic measurements of damage within deformed specimens are used to assess predictions from a Gurson-based constitutive model implemented within an explicit dynamic finite element code. A stress-controlled void nucleation treatment is shown to capture the effect of triaxiality on damage initiation for the range of experiments considered. This article is based on a presentation given in the symposium entitled “Dynamic Behavior of Materials—Part II,” held during the 1998 Fall TMS/ASM Meeting and Materials Week, October 11–15, 1998, in Rosemont, Illinois, under the auspices of the TMS Mechanical Metallurgy and the ASM Flow and Fracture Committees.     

Camage leading to ductile fracture under high strain-rate conditions

Quantitative metallographic studies of damage evolution leading to ductile fracture under high strainrate loading conditions are presented. A model material is considered, namely, leaded brass, which contains a dispersed globular lead phase that acts as void nucleation sites. Interrupted tensile split Hopkinson bar tests have been performed to capture the evolution of porosity and void aspect ratio with deformation at strain rates up to 3000 s⁻¹. Both uniaxial and notched specimen geometries were considered to allow the effects of remote stress triaxiality to be investigated. Plate impact testing has also been performed to investigate the evolution of damage under the intense tensile triaxiality and extremely high rates of deformation (10⁵ s⁻¹) occurring within a spall layer. Quantitative metallographic measurements of damage within deformed specimens are used to assess predictions from a Gursonbased constitutive model implemented within an explicit dynamic finite element code. A stresscontrolled void nucleation treatment is shown to capture the effect of triaxiality on damage initiation for the range of experiments considered. J.P. FOWLER, formerly Research Associate with the Mechanical and Aerospace Engineering Department, Carleton University This article is based on a presentation given in the symposium entiled “Dynamic Behavior of Materials—Part II,” held during the 1998 Fall TMS/ASM Meeting and Materials Week, October 11–15, 1998, in Rosemont, Illinois, under the auspices of the TMS Mechanical Metallurgy and the ASM Flow and Fracture Committees.     

Grain boundary ductile fracture in precipitation hardened aluminum alloys

Published microstructural studies of grain boundary (gb) fracture in precipitation hardened aluminum alloys are reviewed with respect to the three main ideas that have been developed to explain the gb fracture surfaces. The ideas are

• 1.(1) microvoid growth at large gb precipitates,
• 2.(2) strain localization in the soft, and sometimes solute-free, gb precipitate free zones (pfz) and
• 3.(3) the influence of matrix precipitate shear giving rise to inhomogeneous “planar” slip that may apply large stress concentrations to the gb at the end of slip bands. Although the last two processes have a supporting role in many cases, the published evidence strongly suggests that the first process is of overwhelming importance. This conclusion has been tested by reversion experiments in model Al-Li alloys in which microstructures with increasing area fractions, A_f, of large stable δ precipitates (Al-Li) were produced, but with equivalent matrix structures and yield strengths. The materials show marked falls of toughness and of fracture strain as A_f was increased. Studies of surface slip markings in the Al-Li alloys suggested that slip was initiated at the large gb δ precipitates. Only very limited evidence for a role of planar slip in the fracture of the Al-Li alloys was found in contrast to observations on high purity Al-Zn-Mg-Cu alloys where planar slip seemed to show more importance. Brief studies on a nickel based alloy, MAR-M200, suggested that even in the absence of a pfz, strong room temperature embrittlement by gb precipitates was produced. The results of this study suggest that the marked problems of gb fracture in Al-Li alloys are associated with large gb δ precipitates. Jensrud and Ryum [Mater. Sci. Engng64, 229 (1984)] have shown how gb precipitate growth is facilitated in this system as the gb δ phase is very much less soluble than the strengthening δ′ (Al₃Li) phase.

     

Evaluation of static and dynamic fracture toughness in ductile cast iron

Crack extension behavior and fracture toughness of ductile cast iron were examined by three-point bend tests, where various detection methods of crack initiation under static and dynamic loading conditions were adopted. Loading on the specimens was interrupted at various displacement points, and the final fracture surfaces of the specimen were observed via scanning electron microscopy (SEM). Crack-tip opening displacement (CTOD) obtained under the dynamic loading condition was smaller than that under the static loading condition in ferritic ductile cast iron, and CTOD additionally decreased with increasing pearlite content in the matrix. The relationship between J (ΔC) obtained by the compliance changing rate method and J(R) established by the intersection of the crack extension resistance curve and the theoretical blunting line varied with pearlite content. The average value of .J(ΔC) and J(R), that is J (mid), was proposed to define the fracture toughness of ductile cast iron; J (mid) was considered to be a reasonable measure for the fracture toughness of ductile cast iron, irrespective of loading condition and the pearlite content in the matrix.