Creep crack growth in the absence of grain boundary precipitates in UDIMET 520

The effects of grain size and environment on creep crack growth (CCG) in Ni-base superalloy, UDIMET 520, were studied through experiments at 540 °C. Specially designed solution and aging treatments were used to produce γ′ strengthened microstructures with different grain sizes but without any M₂₃C₆ grain boundary precipitates. Five grain sizes, which fall into three groups (i.e., small, medium, and large), were employed. The creep crack growth rates (CCGRs) in specimens with small grain sizes were approximately 2.5 times lower than those with medium and large grain sizes, as a result of crack branching and the presence of some undissolved primary MC carbides at the grain boundaries. Otherwise, the CCGRs were insensitive to the grain size. Fractographic observations on the fracture surfaces and metallographic examinations on the cross sections of the interrupted CCG specimen revealed intergranular microcracks and a faceted intergranular mode of fracture in both air and argon environments. The test results suggest that the formation and propagation of intergranular cracks by grain boundary sliding (GBS) is the main micromechanism responsible for CCG in both air and argon environments at the relatively low test temperature employed. Grain boundary oxidation attack in the air environment simply accelerates the crack growth process. The present results are in agreement with the theoretical predictions of the GBS-controlled CCG model previously developed by the authors.     

Stress corrosion crack velocity and grain boundary precipitates in an Al-Zn-Mg alloy

The fracture kinetics of Al-5.5 Zn-2.5 Mg alloys submersed in 3 pct NaCl-H₂O solutions were varied by heat treatment. The steady state velocity, on a plot of velocity vs stress intensity, was compared with microstructure and it was found to be inversely proportional to the volume of MgZn₂ in the grain boundary. This behavior suggests that grain boundary precipitates can act as sacrificial anodes to retard intergranular stress corrosion cracking. Formerly Graduate Student at the University of Connecticut     

Grain growth and carbide precipitation in superalloy, UDIMET 520

The results of an experimental study on the grain coarsening behavior, M₂₃C₆ carbide precipitation, and secondary MC carbide precipitation kinetics in UDIMET 520 are presented. Primary MC carbides and M (C, N) carbonitrides strongly influence the grain growth, with their dissolution near 1190 °C and 1250 °C, respectively, resulting in two distinct grain coarsening temperatures (GCTs). M₂₃C₆ carbides precipitate in the alloy over a wide range of temperatures varying between 600 °C and 1050 °C. A discrete M₂₃C₆ grain boundary carbide morphology is observed at aging temperatures below 850 °C. Secondary MC carbides formed at temperatures ranging between 1100 °C and 1177 °C, in specimens in which primary MC dissolution had been obtained at solution treatment temperatures of 1190 °C to 1250 °C. A schematic time-temperature-transformation (TTT) diagram for understanding the microstructure and precipitation inter-relationships in UDIMET 520 alloy is also presented.     

Grain boundary sliding in the presence of grain boundary precipitates during transient creep

A constitutive rate equation for grain boundary sliding (GBS), in the presence of grain boundary precipitates, is developed. Langdon’s GBS model is modified by incorporating physically de-fined back stresses opposing dislocation glide and climb and by modifying the grain size de-pendence of creep rate. The rate equation accurately predicts the stress dependence of minimum creep rate and change in activation energy occurring as a result of changing the grain boundary precipitate distribution in complex Ni-base superalloys. The rate equation, along with the math-ematical formulations for internal stresses, is used to derive a transient creep model, where the transient is regarded as the combination of primary and secondary stages of creep in constant load creep tests. The transient creep model predicts that the transient creep strain is dependent on stress and independent of test temperature. It is predicted that a true steady-state creep will only be observed after an infinitely long time. However, tertiary creep mechanisms are expected to intervene and lead to an acceleration in creep rate long before the onset of a true steady state. The model accurately predicts the strain vs time relationships for transient creep in IN738LC Ni-base superalloy, containing different grain boundary carbide distributions, over a range of temperatures.     

An intergranular creep crack growth model based on grain boundary sliding

An intergranular crack growth model is developed to describe the effect of microstructural features such as grain size, grain boundary precipitates, and serrated grain boundaries on creep crack growth under grain boundary sliding (GBS) conditions. The model considers quantitatively that several deformation mechanisms contribute to the stress redistribution ahead of the crack tip through a stress relaxation process. The crack tip region is divided into three zones: (a) the intragranular-deformation-controlled stress relaxation zone, (b) the GBS-controlled stress relaxation zone, and (c) the elastic region. Intergranular creep crack growth is considered to occur as a result of the GBS-controlled process in all cases. The derived creep crack growth model shows a complex dependence of the creep crack growth rate (CCGR) on fracture mechanics quantities, such as C(t) (the path-independent energy integral with its steady-state value as C*) and K (the stress intensity factor). For creep-brittle materials, the model predicts that the CCGR depends on K to the power of 2 and this is verified experimentally; however, when environmental effects contribute to the crack growth process, the power exponent will increase. A semiempirical factor is introduced to account for the effects of oxidation on CCGR.     

Creep crack growth in udimet 700

Crack growth behavior in Udimet 700 was studied at 850°C (1560°F) and the crack growth rates are correlated with various fracture mechanics parameters,viz the stress intensity factor,J andC* integrals as well as with nominal stress. While there is considerable spread in the data in terms of all of these parameters, the crack growth rates seems to correlate better with the stress intensity factor than with the other three parameters. The crack growth behavior in the Udimet alloy is shown to differ significantly from that in previously studied Alloy 718 and these differences are attributed to the possible difference in the mechanisms of crack growth in the two alloys.     

Creep crack growth in alloy 718

Subcritical crack growth behavior in Alloy 718 was studied under creep conditions at 538, 649, and 760°C (1000, 1200, 1400°F) and crack growth rates were correlated using both linear and nonlinear elastic fracture mechanics. The results show that for a given stress intensity value crack growth rate increases significantly with increase in temperature from 538 to 649°C but either decreases or increases slightly with further increase in temperature to 760°C. On the basis of these results it is concluded that creep crack growth results from a balance of two competing processes, diffusion of point defects which contributes to crack growth, and creep deformation process that causes retardation of crack growth and even its arrest. Significance of these concepts in relation to enhancing the resistance of a given material to creep crack growth is discussed in detail.     

Interactions of grain boundaries with coherent precipitates during grain growth

Nimonic PE 16 has been heat treated below the γ′ solvus to initiate grain growth and strong interactions between coherent γ′ and migrating grain boundaries noted. In a small number of cases the γ′ is cut-through by the migrating grain boundary so as to retain coherency with the matrix. More usually very strong pinning of the grain boundary occurred as part of an attempt at by pass. In these cases the γ′ in contact with the grain boundary was found to competitively grow/coarsen at the expense of the surrounding matrix γ′ so that the local volume fraction of the γ′ on the boundary increased more than did its size. Direct measurements from electron micrographs have been used to establish the microstructural parameters characterising the γ′ dispersion and also the nature of the boundary-particle interactions. These measurements have been used to assess the local driving forces for grain boundary migration and the very large pinning effects due to the γ′ particles. The data are interpreted in terms of models proposed by Zener and by Ashby and colleagues.     

Crystallography of grain boundary α precipitates in a β titanium alloy

The crystallography of α(hcp) precipitates formed on the β(bcc) matrix grain boundaries has been studied with transmission electron microscopy (TEM) in a Ti-15V-3Cr-3Sn-3Al alloy. The α precipitates have a near-Burgers orientation relationship with respect to at least one of the adjacent β grains. Among the possible 12 variants in this orientation relationship, the variant that [11•20]_α is parallel to the 〈111〉_β closest to the grain boundary plane tends to be preferred by the α precipitates. Additionally, further variant selections are made so as to minimize the deviation of orientation relationship with respect to the “opposite“ β grain from the Burgers one. Such rules in variant selection often result in the formation of precipitates with a single variant at a planar grain boundary. Prior small deformation of β matrix changes the variant of α precipitates at the deformed portion of grain boundary. It is considered that the stress field of dislocations in the slip bands intersecting with the boundary strongly affects the variants of α precipitates. Discussion of these results is based upon a classical nucleation theory. Formerly Graduate Student, Department of Materials Science and Engineering, Kyoto University Formerly Graduate Student, Department of Materials Science and Engineering, Kyoto University 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.     

Effect of clustering of precipitates on grain growth

The present work shows that clustering of particles promotes deviation in the classical mathematical expressions describing the grain growth control by second-phase particles. On the basis of experimental results and theoretical laws, a semiempirical expression to predict the limiting grain size is presented. The latter expression takes account of agglomeration phenomena and can be extended to large volume fractions of particles, conditions under which classical theories clearly fail. The equation remains valid as far as the nucleation of precipitates takes place at random. From a practical point of view, it is shown that volume fractions larger than 0.12 cannot significatively control the grain size because of the increased probability for clustering.     

Effect of clustering of precipitates on grain growth

The present work shows that clustering of particles promotes deviation in the classical mathematical expressions describing the grain growth control by second-phase particles. On the basis of experimental results and theoretical laws, a semiempirical expression to predict the limiting grain size is presented. The latter expression takes account of agglomeration phenomena and can be extended to large volume fractions of particles, conditions under which classical theories clearly fail. The equation remains valid as far as the nucleation of precipitates takes place at random. From a practical point of view, it is shown that volume fractions larger than 0.12 cannot significatively control the grain size because of the increased probability for clustering.     

The dendritic growth of γ′ precipitates and grain

The growth pattern of γ precipitates in the grains and at the grain boundaries has been investigated in a Ni-24Co-4Al-4Ti-5Cr-5Mo (weight percent) alloy of very small lattice misfit between the precipitate and the matrix phases under varying heat-treatment conditions. When aged at temperatures lower than the solvus temperature (T _s = 1150 °C) by more than 30 °C after direct cooling from the solution-treatment temperature, the nucleation density is high. In this condition, the supersaturation is quickly removed because of the overlapping diffusion fields and the precipitates undergo Ostwald ripening from the early stage. The precipitates then have an equilibrium shape of spheres in the grains and truncated spheres at nearly straight grain boundaries. The precipitates at the grain boundaries are coherent with one of the grains, and their number density is not much larger than that in the grains, apparently because of a large contact angle (about 150 deg) with the grain boundary. Quenching the alloy after the solution treatment and aging at any temperature also produce high precipitate number density and equilibrium shapes. When aged at temperatures just belowT _s (above 1140 °C), the nucleation density is low, the precipitates grow dendritically in the grains, and the grain boundaries become serrated. The observed dendritic growth characteristics do not quantitatively agree with the predictions of Mullins and Sekerka theory, but the discrepancy may be due to the uncertainties in both the observed and calculated quantities. By deeply etching the matrix, it is shown that the grain boundary serration is produced by the precipitates growing preferentially in the direction of the incoherent boundary because of the rapid solute diffusion along the grain boundary. The dendritic growth and grain boundary serration can be obtained also by slowly cooling through the temperature range just belowT _s.     

Creep crack growth behavior of several structural alloys

Creep crack growth behavior of several high temperature alloys, Inconel 600, Inconel 625, Inconel X-750, Hastelloy X, Nimonic PE-16, Incoloy 800, and Haynes 25 (HS-25) was examined at 540, 650, 760, and 870 °C. Crack growth rates were analyzed in terms of both linear elastic stress intensity factor and J*-integral parameter. Among the alloys Inconel 600 and Hastelloy X did not show any observable crack growth. Instead, they deformed at a rapid rate resulting in severe blunting of the crack tip. The other alloys, Inconel 625, Inconel X-750, Incoloy 800, HS-25, and PE-16 showed crack growth at one or two temperatures and deformed continuously at other temperatures. Crack growth rates of the above alloys in terms ofJ* parameter were compared with the growth rates of other alloys published in the literature. Alloys such as Inconel X-750, Alloy 718, and IN-100 show very high growth rates as a result of their sensitivity to an air environment. Based on detailed fracture surface analysis, it is proposed that creep crack growth occurs by the nucleation and growth of wedge-type cracks at triple point junctions due to grain boundary sliding or by the formation and growth of cavities at the boundaries. Crack growth in the above alloys occurs only in some critical range of strain rates or temperatures. Since the service conditions for these alloys usually fall within this critical range, knowledge and understanding of creep crack growth behavior of the structural alloys are important.     

Computer simulation of morphological changes of grain boundary precipitates growing by the ledge mechanism

A previously developed computer model was modified to simulate the growth of grain boundary precipitates which grow by the ledge mechanism. The ledges were assumed to be nucleated in the grain boundary region at constant, parabolically decreasing, and random rates and to grow under the control of volume diffusion of solute to or from the riser of ledges. At lower under coolings at which the motion of individual ledges is slow, late-nucleated ledges soon catch up with first-nucleated ones, and precipitates tend to extend along the grain boundary: the overall precipitate shape is essentially that of a grain boundary allotriomorph. At larger undercoolings, first-nucleated ledges move fast to form a protuberance similar to Widmanstätten sideplates, while late-nucleated ones stay near the grain boundary region. The transition of precipitate shape from one to the other occurs in a very narrow range of supersaturation. The results are compared with various characteristics of the growth of proeutectoid ferrite allotriomorphs and sideplates in Fe-C alloys documented in the literature.     

Creep crack growth simulation under transient stress fields

The validity of the C -integral for correlation of creep crack growth under transient and steady-state stress fields has been investigated, using FEM analysis. In the steady-state regime, crack growth rates can be correlated withC*, however only for limited amounts of crack extension. When a crack grows in a transient regime no correlation of crack growth is found with any of the conventional crack tip parameters. For both regimes the most relevant parameter is theC-integral values obtained from the near-field region ahead of the crack tip. 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.     

Creep failure by degradation of the microstructure and grain boundary cavitation in a tensile test

For tensile test specimens under constant load creep failure is analysed, based on constitutive relations that account for the nucleation and growth of grain boundary cavities in polycrystalline metals at high temperatures. The material model is formulated in the context of finite strains, so that failure due to loss of cross-sectional area, as well as the possibility of necking, are incorporated in the analysis. Furthermore, the present investigation accounts for the possibility of creep acceleration induced by a gradual degradation of the microstructure. The results include cases in which the three types of failure mechanisms interact, as well as cases where any one of the mechanisms dominate. Most of the investigation is based on a simple one-dimensional model analysis; but a full axisymmetric numerical analysis is used to investigate a few cases, in which necking plays a role.     

Effects of pre-existing grain boundary microvoid distributions on fracture toughness and fatigue crack growth in low alloy steel

The role of dispersions of pre-existing grain boundary microvoids is investigated in fracture toughness and fatigue crack propagation behavior in a low alloy steel. Microvoid damage is achieved by prior exposure of the steel to gaseous hydrogen atmospheres at high temperatures and pressures, where carbon within the steel reacts with ingressed hydrogen to nucleate methane bubbles along prior austenite grain boundaries (hydrogen attack). It is shown that, whereas the crack initiation and crack growth toughness (i.e. K_Ic and the tearing modulus) are severely degraded, even for comparatively mild degrees of microvoid damage, rates of sub-critical crack growth by fatigue remain relatively unaffected. Such results are interpreted in terms of a mutual competition between microstructural damage generated by the grain boundary microvoids, which promotes crack growth by lowering the intrinsic resistance of the microstructure, and the resulting tortuous crack paths, which extrinsically retard crack growth at low stress intensities by lowering the local crack tip “driving force” (crack tip shielding). As shielding effects are minimized at high stress intensities, the degradation in intrinsic toughness is related to changes in ductility by means of a stress-modified critical strain model for ductile fracture, where the presence of small microvoid clusters is shown to promote coalescence through the easier onset of plastic strain localization. Fatigue behavior, conversely, is dominated by extrinsic shielding mechanisms and is modeled in terms of two-dimensional models of crack deflection and roughness-induced crack closure.     

On the growth kinetics of grain boundary ferrite allotriomorphs

Previous work has shown that the thickening kinetics of proeutectoid ferrite allotriomorphs in an Fe-0.11 pct C alloy are often more rapid than the kinetics calculated for volume diffusion-control from the Dube-Zener equation for the migration of a planar boundary of infinite extent, assuming the diffusivity of carbon in austenite,D, to be constant at that of the carbon content of the Ae3. Recalculating the thickening kinetics, using a numerical analysis of the infinite planar boundary problem previously developed by Atkinson in which the variation ofD with composition is taken fully into account, was found to increase this discrepancy. Measurements were then made of the lengthening as well as the thickening kinetics of grain boundary allotriomorphs in the same alloy. Application to these data of Atkinson’s numerical analysis of the growth kinetics of an oblate ellipsoid, in which the composition-dependence ofD is similarly considered, produced an acceptable accounting for nearly all of the data. It was concluded that the growth of ferrite allotriomorphs is primarily controlled by the volume diffusion of carbon in austenite; the presence of a small proportion of dislocation facets along one of the broad faces of the allotriomorphs, however, usually results in growth kinetics which are somewhat slower. An alternate treatment of the lengthening and thickening data upon the basis of the theory of interfacial diffusion-aided growth of allotriomorphs indicated that, in the temperature range investigated (735° to 810°C),the diffusivities of carbon along γ:γ and γ:α boundaries required for this mechanism to make a significant contribution to growth are too high to be physically plausible. Formerly with Scientific Research Staff Formerly with Scientific Research Staff, Ford Motor Company     

Creep crack growth of HK40 steel: Microstructural effects

Creep crack growth tests at 871 °C have been performed on compact tension specimens of HK40 steel having different microstructures. The skeleton-shaped carbides on the grain boundaries have a higher resistance to crack growth than the blocky-shaped carbides. The secondary carbide size and distribution explicitly affect crack growth behavior. There exists a critical size of the secondary carbides. With an increase in the secondary carbide size, the resistance to crack growth increases up to the critical size and decreases beyond the critical size.