Creep cavity growth from tritium-induced helium bubbles in nickel

The growth of grain boundary cavities in nickel under creep conditions was investigated. Growth could be studied unambiguously and without the complications of radiation damage due to a unique sample preparation technique making use of the tritium-to-helium’ decay reaction. Resultant helium bubbles served effectively as creep cavity nuclei, the growth of which led to premature intergranular fracture. Constant stress tension creep tests under argon were performed on helium embrittled samples, revealing information on the stress and temperature dependence of the creep-fracture process. The cavity spacing developed during primary creep by bubble migration and coalescence persisted to fracture. Bulk plastic deformation was strongly suppressed by a matrix bubble population which stabilized a finer subgrain network than is characteristic of virgin nickel. This enhanced creep resistance permitted observation of a stress region in which void growth was controlled by classical Hull-Rimmer grain boundary vacancy diffusion. At higher stresses a transition to plasticity control appeared to take place. These results are interpreted in terms of a coupled grain boundary diffusive/ matrix plasticity model. This paper is based on a presentation made in the symposium “Crack Propagation under Creep and Creep-Fatigue" presented at the TMS/AIME fall meeting in Orlando, FL, in October 1986, under the auspices of the ASM Flow and Fracture Committee.     

Helium 3 precipitation in AISI 316L stainless steel induced by radioactive decay of tritium: Microstructural study of helium bubble precipitation

This article deals with the study of the influence of thermomechanical heat treatments, aging conditions (temperature and time), and helium concentration on helium bubble precipitation in a 316L austenitic steel. Helium was generated by the radioactive decay of tritium (tritium trick). Helium bubbles impede the grain growth in 316L steel aged at 1373 K and also the recrystal-lization reaction at this temperature if cold working is performed prior to aging. Transmission electron microscopy (TEM) observations indicated a weak helium precipitation at 1073 and 1223 K, presumably due to the presence of trapping sites for tritium, and no bubble growth after aging up to 100 hours. Precipitation sites are mainly dislocations in the matrix at 1073 K and grain boundaries and individual dislocations in the matrix at 1223 K. The large bubble size (50 nm) observed at 1373 K, even for short aging times (0.083 hour), can partly be attributed to bubble dragging by dislocations toward the grain boundaries. Cold deformation prior to aging leads to a larger bubble size due to growth enhancement during recrystallization. Decreasing the helium content leads to a smaller helium bubble size and density. Tritium trapping at helium bubbles may favor helium 3 accumulation on defects such as grain boundaries, as observed by tritium autoradiography. Formerly with CNRS, is Postdoctor, Department PuA, CEA-DAM, Bruyères Le Chatel, France.     

Crack and cavity nucleation at interfaces during creep

Athermal nucleation of microcracks and thermal nucleation of cavities during creep deformation are reviewed with an emphasis on effects of solute segregation to grain boundaries and cavity surfaces. The magnitude and the duration of stress concentration at a triple grain junction or at a grain boundary inclusion are estimated for transient Coble creep and steady state power-law creep conditions. Stable configurations of wedge-type microcracks are predicted by a Griffith-like crack model. The rate for thermal nucleation of cavities is obtained by the Fokker-Planck equation for vacancy clusters. Cracks and cavities are interdependent, and cavity nucleation occurs continuously throughout the three creep stages. The local stress concentration enhances microcrack and cavity nucleation. The cavity nucleation rate is generally increased as a result of solute segregation to the surfaces and interfaces and/or gas precipitation into cavity volume. This enhanced nucleation is more effective in a system with mobile solutes than with immobile solutes. Immobile solute or trace elements may affect the nucleation rate also by changing the grain boundary diffusivity. Experimental techniques for quantitative analyses of cavity nucleation processes are discussed. This paper is based on a presentation made at the symposium “The Role of Trace Elements and Interfaces in Creep Failure” held at the annual meeting of The Metallurgical Society of AIME, Dallas, Texas, February 14-18, 1982, under the sponsorship of The Mechanical Metallurgy Committee of TMS-AIME.     

Radiation-induced strengthening and embrittlement in aluminum

Commercially pure aluminum (1100 grade) in the annealed condition was irradiated to fast fluences in the range 1 to 3 × 10²⁶ neutron/m² (E > 0.1 MeV) and to similar thermal fluences at temperatures of 318 to 328 K, and was then tensile tested at temperatures between 298 and 643 K. This irradiation doubled the ultimate tensile strength and more than tripled the flow stress at all test temperatures. The work hardening exponent was severely reduced and there was a large loss in ductility. Strengthening is shown to be due to a fine precipitate of transmutation-produced silicon and an associated dislocation structure. At test tempera-tures below about 423 K the fracture mode was transgranular. Above 473 K grain boundary cavities were observed, the fracture mode become predominantly intergranular, and ductil-ity was further reduced. Unirradiated specimens containing cyclotron-injected helium showed no change in strength but displayed a loss in ductility at elevated temperatures. Concurrently holes were formed on the grain boundaries. Embrittlement in the neutron-irradiated specimens arises from two sources. One is through the defect structure which reduces the work-hardening exponent. The other is an additional effect at elevated temper-atures involving grain boundary failure by cavity growth and coalescence. Helium encour-ages cavity nucleation, the degree of cavitatlon increasing with increasing tensile strength.     

Mechanisms of strain accumulation and damage development during creep of prestrained 316 stainless steels

The effects of prestraining at room temperature and at the creep temperature of 848 K, as well as the responses to stress reductions during creep, have been studied for 316 stainless steels varying in composition and initial microstructure. The results are analyzed by contrasting the strengthening effects achieved by introducing high dislocation densities prior to creep exposure with the deleterious effects, which can occur when prestraining causes premature void nucleation at grain boundaries. In addition, by recognizing the differing contributions made by the grain interiors and the grain boundary zones to the overall rates of creep strain accumulation, a consistent explanation is provided for the diverse creep behavior patterns reported for different metals and alloys after various prestraining treatments.     

Characterization of Creep-Damaged Grain Boundaries of Alloy 617

Intergranular cracking and void nucleation occur over extended periods of time in alloy 617 when subjected to stress at high temperatures. Damage occurs inhomogeneously with some boundaries suffering failure, while others are seemingly immune to creep. Crack propagation associated with grain size, and grain boundary character was investigated to determine which types of grain boundaries are susceptible to damage and which are more resistant. Electron backscatter diffraction and a stereological approach to obtain the five-parameter grain boundary distribution were used to measure the proportions of each type of boundary in the initial and damaged structures. The samples were crept at 1273.15 K (1000 °C) at 25 MPa until fracture. It was found that in addition to low-angle and coherent twin boundaries, other low index boundary plane grain boundaries with twist character are relatively resistant to creep.     

Constrained cavity growth models of longitudinal creep deformation of oxide dispersion strengthened alloys

Two models of constrained cavity growth are developed to describe the long-term longitudinal creep behavior of nickel based oxide dispersion strengthened (ODS) alloys. For both models the rupture time is taken as the time for a transverse grain boundary to cavitate fully. A diffusive cavity growth law is assumed to govern cavitation. The applicability of the respective models is determined by the particular grain morphology achieved by thermal-mechanical processing. The first model assumes that longitudinal grain boundaries are unable to slide; hence displacements due to cavitation must be matched by displacements due to dislocation creep in adjoining grains. This model predicts a low stress exponent at the transition from single crystal to cavitation creep behavior, and higher stress exponents at stresses below this transition. Good agreement is found between the model predictions and creep data for MA 754 at 1000 and 1093 °C. A second model considers a grain morphology wherein longitudinal grain boundaries are able to slide by means of deformation of pockets of fine grains. Cavitation of transverse grain boundaries is thus controlled by grain boundary sliding. This model predicts a stress exponent of 1 at low stresses, and serves as an upper bound for the creep rate when a duplex grain morphology is present. Model predictions are in good agreement with creep data for a heat of MA 754 with a duplex grain morphology. Formerly Graduate Research Assistant in the Department of Materials Science and Engineering at Stanford University     

On the improvement of creep strength and ductility of Ni-20 pct Cr by small zirconium additions

The creep and fracture properties of high-purity Ni-20 pct Cr and Ni-20 pct Cr-0.11 pct Zr alloys are compared at 1073 K in vacuum. The Ni-20 pct Cr alloy cavitates at the grain boundaries and fractures intergranularly after strains of typically 20 pct. The observed cavity growth rates are in keeping with those predicted. Alloying with zirconium substantially increases the creep strength and ductility. Creep rupture associated with dynamic recrystallization occurs, and voids are observed only in heavily necked parts of the samples. In addition to Ni₅Zr and ZrO₂ inclusions, a Zr₄C₂S₂ carbo-sulfide was identified. Thus, the sulfur-gettering effect of zirconium even at very low residual sulfur levels (20 wt ppm) was confirmed. The zirconium-induced increase in the creep strength is discussed, and the inhibition of creep cavitation by zirconium is examined within the framework of thermal cavity nucleation. Lowering of the grain boundary diffusivity and the grain boundary free energy as well as dynamic recrystallization are likely to reduce cavity nucleation and growth rates in Ni-Cr-Zr and will thus increase its ductility. Finally, the results are used to illustrate the critical importance of minor alloying additions in constructing and using fracture mechanism maps.     

An analysis of cavity nucleation in superplasticity

Superplastic alloys possess either a quasi-single phase or a microduplex microstructure: in quasi-single phase alloys, cavities are observed to nucleate predominantly at coarse grain boundary particles whereas in microduplex alloys, cavities tend to form at interphase boundaries and at triple point junctions. A general analysis is presented for cavity nucleation, in both microstructures, under the stress concentrations caused by bursts of grain boundary sliding during superplastic deformation. In quasi-single phase alloys, calculations indicate the cavities nucleate at coarse particles located at grain boundaries because local interphase diffusion creep cannot accommodate the stress concentrations sufficiently rapidly. The analysis demonstrates that it is possible for cavities to nucleate at grain boundary ledges under some limited experimental conditions. It is demonstrated also that the present analysis is in agreement with the available experimental data on a quasi-single phase Cu-based superplastic alloy and a microduplex superplastic Zn-22% Al eutectoid alloy. Calculations show that small pre-existing cavities, if present, are likely to be sintered rapidly prior to superplastic déformation at elevated temperatures.     

Cavity nucleation at grain boundary ledges

The kinetics of cavity nucleation at grain-boundary ledges is examined for ceramics subjected to compressive loads. By considering grain boundary sliding and diffusion as concurrent processes, the analysis shows that sliding of faceted grain boundaries can induce time-dependent stress concentrations of sufficient magnitude to cause cavity nucleation at the ledges. The transient stress concentration has been found to depend on the ledge height and spacing, and to decrease rapidly with time and with distance from the ledge. Whether or not cavity nucleation occurs at the ledge depends on the local stress, the shape of the critical nucleus, and the characteristic time for cavity formation. The theoretical results are discussed in conjunction with nucleation rate measurements obtained for a sintered alumina using small-angle neutron scattering.     

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.     

Grain boundary sliding and stress concentration during creep

Importance of grain boundary sliding to creep intergranular fracture is focussed. Previous metallographic and fractographic studies of creep intergranular fracture on metal bicrystals and polycrystals are briefly reviewed in order to show the close relationship between grain boundary sliding and fracture. Deformation ledge and migration irregularity are shown to be potential sites of stress concentration and crack nucleation on sliding grain boundaries without particles. The effect of grain boundary structure on creep intergranular fracture is discussed on the basis of the effect of grain boundary structure on sliding, the contribution of sliding to the overall creep deformation, and a sliding-fracture diagram. Recent observations of the effect of grain boundary structure on creep intergranular fracture on alpha iron-tin alloy polycrystals are shown. This paper is based on a presentation made at the symposium “The Role of Trace Elements and Interfaces in Creep Failure” held at the annual meeting of The Metallurgical Society of AIME, Dallas, Texas, February 14-18, 1982, under the sponsorship of The Mechanical Metallurgy Committee of TMS-AIME.     

The influence of applied stress and stress sense on grain boundary precipitate morphology in a nickel-base superalloy during creep

Creep induced instability of strengthening precipitates at grain boundaries is of general concern in the applications of many high temperature alloys. Having shown that the general validity of the existing mechanism for such an instability in nickel-base superalloys may be considered suspect, this paper reports and discusses the effects of both tensile and compressive creep on γ′ grain boundary precipitate morphology in an alloy consisting of γ′ (Ni₃Al) precipitates in a γ (nickel solid solution) matrix. We find that the uniform distribution of γ′ precipitates is altered by the application of uniaxial creep stress, with the stress-induced precipitate morphology depending strongly on stress sense. Tensile creep results in the dissolution of γ′ precipitates at grain boundaries aligned more or less transverse to the stress axis, with an accompanying increase in volume fraction of γ′ precipitates at grain boundaries oriented parallel to, or almost parallel to the stress axis. In contrast, the reverse change in morphology occurs during compressive creep. The observed morphology changes and their dependence on stress sense are shown to be consistent with the flow of chromium atoms from grain boundaries that are under normal compression towards grain boundaries that are under normal tension. The results conclusively demonstrate that Herring-Nabarro type diffusion in multiphase, polycrystalline alloys can cause chemical changes in grain boundary regions which, in the extreme, result in phase changes at grain boundaries. The results and proposed mechanism are discussed in terms of the findings of other investigations.     

Combined matrix/boundary precipitation strengthening in creep of Fe-15 Cr-25 Ni alloys

High temperature creep behavior of carbide precipitation strengthened Fe-15 Cr-25 Ni alloys with different carbon content have been investigated. Grain boundary carbides obstruct dislocation annihilation at the grain boundary and, therefore, increase the dislocation density near the grain boundary. This gives rise to formation of a hard grain boundary region and significantly increase creep resistance of the alloy. The grain boundary precipitation strengthening and combined matrix/boundary strengthening are modeled following the concept of hard-soft composite structure, and a unified creep equation is derived by taking account of back stress from intergranular carbide particles, “boundary obstacle stress”. The models and analysis show that grain boundary precipitation strengthening is predominant for soft matrix but decreases with the increase of matrix strength, indicating the existence of coupled matrix/boundary strengthening.     

The effect of helium on the strength and microstructure of niobium

The effect of helium, from tritium decay, on the mechanical properties of single crystal noibium were determined. The yield stress in compression of helium charged niobium increased at all test temperatures. Annealing decreased the yield stress. Transmission electron microscopy was used to examine the as charged samples and to analyze the effects of annealing on nucleation and growth of helium bubbles. The results support the following conclusions: 1) Helium exists in niobium as clusters at ambient temperatures, 2) these clusters punch out prismatic dislocation loops, 3) the increase in strength due to the presence of helium can be explained by a cluster and bubble shearing model.     

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 fracture due to tritium and helium in 21-6-9 stainless steel

The effects of internal tritium and helium on the room-temperature tensile properties of a nitrogen-strengthened stainless steel, forged 21Cr-6Ni-9Mn (NITRONIC 40), were investigated by thermally charging tritium into tensile specimens and aging for selected times. The precipitation of helium as bubbles on dislocations greatly increased the yield strength, and as a consequence of dislocation pinning, the deformation mode changed from long-range dislocation motion to deformation twinning. The tensile specimens exhibited a 90 pct decrease in tensile ductility at 1438 appm 3 He, accompanied by a severe change in fracture mode from ductile rupture to a dominantly intergranular fracture mode. Grain-boundary facets showed multiple striations where deformation twins had intersected the boundaries. Twinning began immediately upon yielding, and at small strains, the microstructure evolved into a fully hardened state, normally observed at 40 pct or greater strain in unexposed or hydrogen-charged 21-6-9. Fracture occurs at low strains in tritium-charged and aged 21-6-9, in part, because helium bubble precipitation causes the deformed microstructure to evolve to a heavily deformation-twinned state at a lower strain. Helium bubble precipitation on the grain boundaries may have caused a further loss in ductility. Fracture appeared to nucleate at the intersection of deformation twins with the grain boundary.     

Creep mechanisms in beryllium

Observations of beryllium samples which have been creep tested between 922 K and 1422 K indicate that creep behavior is controlled by the relative strengths of the grain boundaries and the matrix. Since creep deformation can occur predominantly by grain boundary sliding or entirely by deformation within the grains, the creep strength was found to be controlled by the weaker of the two features. Low melting phases containing aluminum and silicon which formed along the grain boundaries acted as stress concentrations which favored localized grain boundary deformation, and recrystallization. Creep resistance was found to drop markedly when the BeO content was reduced substantially below 1 pct.     

Creep mechanisms in beryllium

Observations of beryllium samples which have been creep tested between 922 K and 1422 K indicate that creep behavior is controlled by the relative strengths of the grain boundaries and the matrix. Since creep deformation can occur predominantly by grain boundary sliding or entirely by deformation within the grains, the creep strength was found to be controlled by the weaker of the two features. Low melting phases containing aluminum and silicon which formed along the grain boundaries acted as stress concentrations which favored localized grain boundary deformation, and recrystallization. Creep resistance was found to drop markedly when the BeO content was reduced substantially below 1 pct.     

Nucleation kinetics of proeutectoid ferrite at austenite grain boundaries in Fe-C-X alloys

The nucleation kinetics of proeutectoid ferrite allotriomorphs at austenite grain boundaries in Fe-0.5 at. Pct C-3 at. Pct X alloys, where X is successively Mn, Ni, Co, and Si and in an Fe-0.8 at. Pct C-2.5 at. Pct Mo alloy have been measured using previously developed experimental techniques. The results were analyzed in terms of the influence of substitutional alloying elements upon the volume free energy change and upon the energies of austenite grain boundaries and nucleus: matrix boundaries. Classical nucleation theory was employed in conjunction with the pillbox model of the critical nucleus applied during the predecessor study of ferrite nucleation kinetics at grain boundaries in Fe-C alloys. The free energy change associated with nucleation was evaluated from both the Hillert-Staffanson and the Central Atoms Models of interstitial-substitutional solid solutions. The grain boundary concentrations of X determined with a Scanning Auger Microprobe were utilized to calculate the reduction in the austenite grain boundary energy produced by the segregation of alloying elements. Analysis of these data in terms of nucleation theory indicates that much of the influence of X upon ferrite nucleation rate derives from effects upon the volume-free energy change,i.e., upon alterations in the path of theγ/(α + γ) phase boundary. Additional effects arise from reductions in austenite grain boundary energy, with austenite-forming alloying elements being more effective in this regard than ferrite-formers. By difference, the remaining influence of the alloy elements studied evidently results from their ability to diminish the energies of the austenite: ferrite boundaries enclosing the critical nucleus. The role of nucleation kinetics in the formation of a bay in the TTT diagram of Fe-C-Mo alloys is also considered. Formerly Graduate Student, Department of Metallurgical Engineering and Materials Science, Carnegie-Mellon University