Ashok Saxena

A critical assessment of fatigue crack nucleation and growth models for Ni- and Ni,Fe-based superalloys

Abstract Ni- and Ni,Fe-based superalloys are used extensively in the hot sections of gas turbines in the aerospace and power generation industries. One way to improve the performance of turbines is through increased operating temperatures and stresses. Therefore, an understanding of the factors that influence resistance of these materials to creep and fatigue is necessary to build models that can predict the lives of components in these harsh operating conditions. Predicting crack nucleation (or formation) and the subsequent rate of crack propagation is a complex problem because of the interactions between microstructure, cyclic deformation, and the high temperature effects of creep and environment; an additional influence is variable amplitude loading during the service life. This paper will discuss the pertinent research over the past three decades that has considered microstructural, temperature, environmental, frequency and loading effects on fatigue crack growth in these important intermediate temperature alloys and is divided into sections devoted to crack nucleation, short crack growth, and long crack growth.

Nanoindentation of single crystal and polycrystalline copper nanowires

Metal nanowires are attracting considerable interest because of their potential importance to the technology of miniaturization of electronic devices in need of metallic contacts. One of the important attributes of nanowires is their potentially high mechanical strength. Nanoindentation is the most realistic tool at the present time to determine the mechanical properties of nanowires. The load-displacement behavior during nanoindentation of electrodeposited single crystal and 500 nm diameter polycrystalline copper nanowires was performed and the results are reported in this paper. The behavior has also been compared with that of bulk nanocrystalline and annealed copper. The hardness values for 50 nm grain size polycrystalline nanowires and those of extruded bulk 50 nm grain size copper were comparable, 2.1 GPa, and that of a 50 nm single crystal copper nanowire was 1.8 GPa.

Plastic deformation of nanocrystalline copper-antimony alloys

Molecular dynamics simulations are used to evaluate the influence of Sb dopant atoms at the grain boundaries on plastic deformation of nanocrystalline Cu. Deformation is conducted under uniaxial tensile loading, and Sb atoms are incorporated as substitutional defects at the grain boundaries. The presence of randomly dispersed Sb atoms at the grain boundaries does not appreciably influence the mechanisms associated with dislocation nucleation in nanocrystalline Cu; grain boundary ledges and triple junctions still dominate as partial dislocation sources. However, the magnitude of the tensile stress associated with the partial dislocation nucleation event does increase with increasing Sb concentration and also with increasing grain size. The flow stress of nanocrystalline Cu increases with increasing Sb concentration up to 1.0 at.% Sb, with a maximum observed at a grain size of 15 nm for all Sb concentrations (0.0–2.0 at.% Sb).

Heterogeneous dislocation nucleation in single crystal copper–antimony solid-solution alloys

Molecular dynamics (MD) simulations are employed to study the partial dislocation nucleation process in single crystal copper with varying concentrations of antimony (0.0–2.0 at%Sb) under uniaxial tension. A well-established embedded-atom method potential is used to represent the Cu–Cu interactions and a recently developed Lennard-Jones potential is used for the Cu–Sb and Sb–Sb interactions. Antimony atoms are randomly distributed as substitutional defects in the Cu single crystal. MD simulations indicate that the tensile stress required for partial dislocation nucleation in the crystal decreases with increasing concentration of Sb. The strain field around Sb dopant atoms in the Cu lattice reduces the unstable stacking fault energy, which promotes heterogeneous nucleation of partial dislocations and reduces the tensile stresses required for plastic deformation. In addition, the role of Sb on the reduction in the stress required for dislocation nucleation is found to be orientation-dependent. Finally, both temperature and Sb distribution play a role in the statistical variation of the stress required for heterogeneous partial dislocation nucleation; this variation is maximum at moderate levels of Sb concentration (0.20–0.50 at%Sb).

Microstructure stability of nanocrystalline materials using dopants

By definition, nanocrystalline materials have grain sizes, d, less than 100 nm. Due to their reduced grain size, nanocrystalline materials have superior mechanical properties compared to their microcrystalline counterparts. Loss of these unique properties due to grain growth under the effect of high temperature and stress is a limitation to their use in many applications. Recently it has been proposed to use dopants (alloying elements) to reduce the driving force for grain boundary motion, leading to improved microstructural stability and resistance to deformation. Inclusion of dopants has been shown to alter properties of nanocrystalline materials, although their precise effect on mechanical and electrical properties is still unclear. In this brief review article, work done in the domain of stability of polycrystalline materials using dopants and their application in nanocrystalline materials is discussed. The importance of both experiment and molecular dynamics simulations is presented.

Characterization of creep-fatigue crack growth behavior in 2.25 Cr-1 Mo steel using (Ct)avg

Time-dependent creep-fatigue crack growth (CFCG) is an important consideration in the design and remaining life estimation of high temperature components. CFCG tests were carried out on compact type (CT) specimens of 2.25 Cr-1.0 Mo steel and its behavior, for hold times ranging from 10 seconds to 50 seconds, at 594°C (1100°F) was characterized using the average value of the Ct-parameter, (Ct)avg. The trends in the creep-crack growth (CCG) data for this material are also compared with the CFCG data. The analytically estimated values of (Ct)avg are compared with the experimental values of (Ct)avg obtained from the measured values of load-line deflection rates. It is also shown that even in the absence of accurate creep deformation constants, accurate estimates of the measured values of (Ct)avg can be obtained in CT specimens

Creep and creep–fatigue crack growth

Creep and creep–fatigue considerations are important in predicting the remaining life and safe inspection intervals as part of maintenance programs for components operating in harsh, high temperature environments. Time-dependent deformation associated with creep alters the crack tip stress fields established as part of initial loading which must be addressed in any viable theory to account for creep in the vicinity of crack tips. This paper presents a critical assessment of the current state-of-the-art of time-dependent fracture mechanics (TDFM) concepts, test techniques, and applications and describes these important developments that have occurred over the past three decades. It is concluded that while big advances have been made in TDFM, the capabilities to address some significant problems still remain unresolved. These include (a) elevated temperature crack growth in creep-brittle materials used in gas turbines but now also finding increasing use in advanced power-plant components (b) in predicting crack growth in weldments that inherently have cracks or crack-like defects in regions with microstructural gradients (c) in development of a better fundamental understanding of creep–fatigue–environment interactions, and (d) in prognostics of high temperature component reliability. It is also argued that while these problems were considered intractable a few years ago, the advances in technology do make it possible to systematically address them now and advance TDFM to its next level in addressing the more difficult but real engineering problems.

Elevated Temperature Fatigue Crack Growth rate model for NI-BASE Superalloys

Several time-dependent mechanisms are operational in the crack growth process of Ni-base superalloys at elevated temperature. Creep deformation during periods of sustained loading, oxygen diffusion at the crack tip, and oxidation reactions at and in front of the crack tip all contribute to the kinetics of crack growth. A crack growth rate model has been derived that attempts to capture the physics of these various rate processes. The proposed model assumes small-scale creep at the crack tip and incorporates the Hutchinson-Rice-Rosengren stress field equations to satisfy this condition. The model also includes stress-assisted diffusion of an environmental species at the crack tip. A process reaction rate is related to the time-rate of crack growth providing a model that accounts for these time-dependent processes. An evaluation of the form of the model is provided by comparison of the model with experimental crack growth data.

Fracture toughness behavior of weldments with mis-matched properties at elevated temperature

AbstractHigh temperature