The rate of CO bubble nucleation at oxide metal interfaces within liquid iron alloys

Baker, Warner, and Jenkins found that levitated droplets of Fe-0.8 pet C alloys exploded when decarburized at 1660°C, whereas during the present investigation, the drops remained intact during decarburization at temperatures above 1850°C. Therefore, the object of this work was to determine whether heterogeneous nucleation of CO bubbles at an iron-iron oxide interface could occur at 1900°K but could not occur at 2200°K. An equation was developed to calculate the nucleation rate of CO bubbles at an iron-iron oxide interface in iron at 1900°K containing 0.8 pct C and in iron at 2200°K containing 0.1 pct C. The results of the calculation showed that an iron-iron oxide interface could not serve as a site for CO bubble nucleation. Therefore, a new mechanism is postulated in which cavities swept into the levitated droplet from the surface serve as nuclei for CO bubble formation instead of nuclei formed at the iron-iron oxide interface.     

Low cycle fatigue in rene 88DT at 650 °C: Crack nucleation mechanisms and modeling

Intermediate-temperature low-cycle-fatigue experiments were conducted on three microstructural conditions of the nickel-base superalloy Rene 88DT over several strain ranges. The most significant difference between the microstructural conditions was the grain size; two of the conditions had an average grain diameter of approximately 20 μm, while the third condition had an average grain diameter of 6 μm. Two dominant crack nucleation mechanisms were observed on the specimen fracture surfaces: crack nucleation from surface slip band damage accumulation and crack nucleation around subsurface inclusion clusters. The experimental results indicate that both the grain size and the applied strain range are contributing factors in the prevailing crack nucleation mechanism. The Fatemi-Socie parameter, a multiaxial fatigue damage parameter, was used to examine the ease and probability of surface slip band crack nucleation for these microstructural conditions. The parameter was modified to explicitly include grain size, fatigue R-ratio, and applied strain range, thus giving it a more physically meaningful basis. This modified Fatemi-Socie parameter was found to be a suitable parameter for characterizing the ease of surface slip band cracking for materials with different grain sizes and is able to explain the observed disparity in fatigue lives based on microstructural design.     

The role of impurities during creep and superplasticity at very low stresses

It is well documented that impurities play an important role in the deformation and fracture of polycrystalline materials. For example, the results of a number of studies have demonstrated that the presence of a very small of amount of impurities in polycrystalline materials can explain many phenomena such as temper embrittlement in steels, creep embrittlement, and enhancement of ductility in the intermetallic compound Ni₃Al. This article reviews the details of two high-temperature deformation phenomena whose characteristics are, according to very recent experimental evidence, influenced or controlled by impurities. The first phenomenon, micrograin superplasticity, deals with the ability of fine-grained materials (d<10 μm, where d is the grain size) to exhibit extensive neck-free elongations during deformation at elevated temperatures above 0.5 T _m, where T _m is the melting point. The second phenomenon, Harper-Dom creep, refers to the anomalous creep behavior of large-grained materials at very low stresses and temperatures near the melting point. It is shown that while these two phenomena are different in terms of the conditions of occurrence and the characteristics of deformation, they share three common features: (1) stresses applied to produce deformation are very small; (2) impurities control the deformation characteristics such as the shape of the creep curve, the value of the stress exponent, and the details of the substructure; and (3) boundaries play a key role during deformation. This article is based on a presentation made in the workshop entitled “Mechanisms of Elevated Temperature Plasticity and Fracture,” which was held June 27–29, 2001, in San Diego, CA, concurrent with the 2001 Joint Applied Mechanics and Materials Summer Conference. The workshop was sponsored by Basic Energy Sciences of the United States Department of Energy.     

Density measurements as an assessment of creep damage and cavity growth

The change in density during creep of several polycrystalline metals may be correlated through the expression —Δρ/ρ =B(∈t/d)(σ/G)^q exp (—Q _gb /RT) where —Δρ is the density change, p is the original density, e is the strain,t is the time,d is the linear intercept grain size,σ is the applied stress,G is the shear modulus,Q _gb is the activation energy for grain boundary diffusion,R is the gas constant,T is the absolute temperature, andB andq are constants withq ≃2 to 3. This expression is consistent with the theory of unconstrained grain boundary diffusion growth of cavities provided there is also concomitant strain-dependent nucleation. The expression does not support the power-law growth of cavities, growth by surface diffusion, or constrained grain boundary diffusion growth. Formerly Research Associate, University of Southern California.     

Morphological changes of carbides during creep and their effects on the creep properties of inconel 617 at 1000 °C

Creep tests have been correlated with microstructural changes which occurred during creep of Inconel 617 at 1000 °C, 24.5 MPa. The following results were obtained: 1) Fine intragranular carbides which are precipitated during creep are effective in lowering the creep rate during the early stages of the creep regime (within 300 h). 2) Grain boundary carbides migrate from grain boundaries that are under compressive stress to grain boundaries that are under tensile stress. This is explained in terms of 1 the dissolution of relatively unstable carbides on the compressive boundaries, 2 the diffusion of the solute atoms to the tensile boundaries and 3 the reprecipitation of the carbides at the tensile boundaries. The rate of grain boundary carbide migration depends on grain size. 3) M₂₃C₆ type carbides, having high chromium content, and M₆C type carbides, having high molybdenum content, co-exist on the grain boundaries. M₂₃C₆ type carbides, however, are quantitatively predominant. Furthermore, M₆C occurs less frequently on the tensile boundaries than on the stress free grain boundaries. This is attributed to the difference of the diffusion coefficients of chromium and molybdenum. 4) The grain boundaries on which the carbides have dissolved start to migrate in the steady state creep region. The creep rate gradually increases with the occurrence of grain boundary migration. 5) The steady state creep rate depends not so much on the morphological changes of carbides as on the grain size of the matrix.     

Creep-fatigue life prediction in terms of nucleation and growth of fatigue crack and creep cavities

In low cycle fatigue at elevated temperature, the interaction between fatigue crack and creep damages is known to be responsible for the significant reduction of the fatigue life. In this investigation, a model for the life prediction for low cycle fatigue with hold time at tensile peak strain is suggested for the temperature range of 0.5T _m. This model is formulated on the basis of the assumptions that the creep cavities are formed due to the vacancies generated during fatigue, and are grown during the hold period. The fatigue crack nucleated at the surface due to fatigue loading is affected by the creep damages for its propagation. The model is checked by experimental results with various hold time periods. The predicted creep-fatigue lives are in good agreement with experimentally observed ones for 304 stainless steel and 13CrMo44 steel. Formerly Graduat3e student, Department of Materials Science and Engeneering ,KAIST, Seoul, Korea.     

An investigation of the nucleation of creep cavities by 1 MV electron microscopy

Under constant strain rate conditions (~10⁻³ hr⁻¹) at 873 K intergranular cavities were observed by optical microscopy to form in a copper base alloy of grain size 16 /gmm average diameter at a tensile strain of about 0.2. Transmission electron microscopy at 1 MV demonstrated that cavity nucleation was associated with grain boundary particles. In particular, it was observed that cavities nucleated on one side of the particles and that the cavities were polyhedral in the very early stages of growth. The relationship between the particle size and the size of the cavity nucleated at the particle is discussed as is the frequency of cavities as a function of particle dimensions and inter-particle spacing. Constant load tests were carried out on material of 530 μm. grain size to determine the relationship between cavity nucleation and grain boundary sliding. It was established that a critical sliding displacement existed for nucleation (54 ± 5 × 10⁻¹⁰° m). This nucleation criterion is discussed in terms of the build-up of a dislocation network, the effective length of the dislocation pile-up at a particle and the various models for cavity nucleation by grain boundary sliding.     

Ferrite nucleation and growth during continuous cooling

The austenite decomposition has been investigated in two hypoeutectoid plain carbon steels under continuous cooling conditions using a dilatometer on a Gleeble 1500 thermomechanical simulator. The experimental results were used to verify model calculations based on a fundamental approach for the dilute ternary system, Fe-C-Mn. The austenite-to-ferrite transformation start temperature can be predicted from a nucleation model for slow cooling rates and small austenite grain sizes, where ferrite nucleates at austenite grain corners. The nuclei are assumed to have an equilibrium composition and a pillbox shape in accordance with minimal interfacial energy. For higher cooling rates or larger austenite grain sizes, early growth has to be taken into account to describe the transformation start, and nucleation is also encouraged at the remaining sites of the austenite grain boundaries. In contrast to nucleation, growth of the ferrite is characterized by paraequilibrium;i.e., only carbon can redistribute, whereas the diffusion of Mn is too slow to allow full equilibrium in the ternary system. However, Mn segregation to the moving ferrite-austenite interface has to be considered. The latter, in turn, exerts a solute draglike effect on the boundary movement. Thus, growth kinetics are controlled by carbon diffusion in austenite modified by interfacial segregation of Mn. Employing a phenomenological segregation model, good agreement has been achieved with the measurements. This article is based on a presentation made during TMS/ASM Materials Week in the symposium entitled “Atomistic Mechanisms of Nucleation and Growth in Solids,” organized in honor of H.I. Aaronson’s 70th Anniversary and given October 3–5, 1994, in Rosemont, Illinois.     

Application of heterogeneous nucleation theory to precipitate nucleation at GP zones

The accelerated nucleation of precipitates at GP zones is explained using heterogeneous nucleation theory. Nucleation at zone : matrix boundaries is encouraged by several factors: 1) the chemical interfacial energy of zone : matrix boundaries can significantly decrease the interfacial energy barrier to nucleation; 2) destruction of quenched-in excess vacancies at incoherent portions of the nucleus surface may make the change in the volume free energy significantly more negative; 3) the crystal structures of the zone and matrix are identical and parallel which permits the nucleus to be faceted in both phases. Some additional assistance to nucleation at GP zones is provided by: 4) the accelerated diffusivity resulting from the presence of excess vacancies and 5) the large area of zone : matrix boundary per unit volume of matrix. These factors can more than compensate for the decreased solute supersaturation due to the formation of GP zones and provide an explanation for the enhanced nucleation of precipitates in the presence of GP zones.     

Application of heterogeneous nucleation theory to precipitate nucleation at GP zones

The accelerated nucleation of precipitates at GP zones is explained using heterogeneous nucleation theory. Nucleation at zone : matrix boundaries is encouraged by several factors: 1) the chemical interfacial energy of zone : matrix boundaries can significantly decrease the interfacial energy barrier to nucleation; 2) destruction of quenched-in excess vacancies at incoherent portions of the nucleus surface may make the change in the volume free energy significantly more negative; 3) the crystal structures of the zone and matrix are identical and parallel which permits the nucleus to be faceted in both phases. Some additional assistance to nucleation at GP zones is provided by: 4) the accelerated diffusivity resulting from the presence of excess vacancies and 5) the large area of zone : matrix boundary per unit volume of matrix. These factors can more than compensate for the decreased solute supersaturation due to the formation of GP zones and provide an explanation for the enhanced nucleation of precipitates in the presence of GP zones.     

Creep at low stresses: An evaluation of diffusion creep and Harper-Dorn creep as viable creep mechanisms

High-temperature creep experiments often reveal a transition at very low stresses to a region where the stress exponent is reduced to a value lying typically in the range of ∼1 to 2. This region is generally associated with the occurrence of a new creep mechanism, such as grain-boundary sliding, diffusion creep, and/or Harper-Dorn creep. Several recent reports have suggested that diffusion creep and Harper-Dorn creep may not be viable creep mechanisms. This article examines these two processes and demonstrates that there is good evidence supporting the occurrence of both creep mechanisms under at least some experimental conditions. This article is based on a presentation made in the workshop entitled “Mechanisms of Elevated Temperature Plasticity and Fracture,” which was held June 27–29, 2001, in San Diego, CA, concurrent with the 2001 Joint Applied Mechanics and Materials Summer Conference. The workshop was sponsored by Basic Energy Sciences of the United States Department of Energy.     

An analysis of the effect of cavity nucleation rate and cavity coalescence on the tensile behavior of superplastic materials

A model utilizing a simple force-equilibrium approach was developed to establish the effect of the cavity nucleation rate and cavity coalescence on the uniaxial tensile behavior of superplastic metals. All cavities were assumed to be spherical and uniformly distributed within the material, irrespective of the degree of deformation. Material input parameters for the model comprised the cavity nucleation rate (N), the strain-rate sensitivity of the flow stress (m), and the growth parameter for individual cavities (η), which was taken to be a function of m. The effect of cavity coalescence on average void size and volume fraction was treated using an empirical relation, which correlates an average void growth rate to the growth rate of individual, noninteracting cavities. Model predictions indicated that the macroscopic quantities often used to describe cavitation behavior, i.e., “initial cavity volume fraction” (C _{v 0}) and “apparent cavity growth rate” (η _APP) describe the combined influence of cavity nucleation, growth, and coalescence. With regard to the overall tensile behavior, simulation results revealed that increasing cavity nucleation rates reduce ductility in a manner analogous to the effect of decreases in the strain-rate sensitivity. In addition, the failure mode was established with regard to the relative magnitudes of the cavity nucleation rate and the strain-rate sensitivity. Model predictions of tensile elongation and cavity-size distributions were validated by comparison to measurements found in the literature for cavitating superplastic materials.     

Forward and backward creep during screw rolling

The common features of lengthwise rolling, cross rolling, and screw rolling (which is the most general case) are found. A mathematical model is used to obtain the values of velocity coefficients at any point in the deformation zone, to reveal the position of the neutral line and the conditions affecting the relation between forward and backward creep zones, and to find the cases where a forward creep zone is absent.     

Internal stresses and structures developed during creep

Specimens of 304 stainless steel subjected to different thermomechanical histories develop different internal stresses, σ _i , and different substructures. Creep rate is uniquely related not to the applied stress, σ _A , but to the effective stress, σ^*=(σ _A −σ _i ). Values of σ^* are determined from experimental results and σ _i calculated from σ _i =(σ _A −σ^*). Results show σ _i increases with the applied stress according to σ _i ∝σ _A ^1.7 . Transmission electron microscopic observations show that the density of dislocations within subgrains, ϱ _D , and the subgrain diameter,D, vary with applied stress according to: ϱ _D ∝σ _A ^K ,D ∝ σ _A ^−0.8 , whereK=1.4 to 2.0. Subgrain misorientation is independent of creep stress, strain, or temperature. The contributions of these structural variables to the internal stress are discussed.