M.E. Kassner

Creep cavitation in metals

Abstract This review concisely describes the state-of-the-art of the understanding of cavity, or r-type void, formation during stages I and II (primary and secondary) creep in polycrystalline metals and alloys, particularly at elevated temperatures. These cavities can directly lead to Stage III, or tertiary, creep and the eventual failure of metals. There have been, in the past, a variety of creep fracture reviews that omitted important developments relevant to creep cavitation or are less than balanced in their discussions of conflicting ideas or theories regarding various aspects of cavity nucleation and growth. This concise, comprehensive, review discusses all of the important developments over the past several decades relating to both the nucleation and growth of cavities. The nucleation section discusses the details and limitations of the approaches based on “classic” nucleation theory, slip-induced nucleation as well as grain boundary sliding effects. Growth is discussed starting from the Hull–Rimmer diffusion controlled cavity growth (DCCG) model. This will be followed by refinements to DCCG by others. Next, there will be a discussion of plastic cavity growth and diffusion-plasticity coupling theories. This will be followed by the particularly important development of constrained cavity growth, initially proposed by Dyson, and probably under-appreciated. Other growth effects by grain boundary sliding will also be discussed. All of these mechanisms will be compared with their predictions in terms of creep fracture phenomenology such as the Monkman–Grant relationship. Finally, there will be a discussion of creep crack propagation by cavitation ahead of the crack tip.

The dislocation microstructure of aluminum

Aluminum of 99.999 pct purity was deformed in torsion at 644 K and an equivalent uniaxial strain rate of 5.04 × 10−4 s−1 to various steady-state strains up to 16.33. The subgrain size and density of dislocations not associated with subgrain boundaries remained fixed throughout the wide steady-state strain range. The subgrain boundaries, however, underwent two important changes. At the onset of steady state (ε ~0.2) all of the subgrain boundaries had relatively small misorientation angles averaging about 0.5 deg. With increased strain, however, an increasing fraction of the subgrain facets were high-angle boundaries. At strains greater than about four nearly a third of the boundaries were high-angle. In specimens with both types of boundaries, the high-angle boundaries have misorientation angles (θ) greater than 10 deg, while θ for low-angle boundaries is nearly always less than 3 deg. Only rarely do subgrain boundaries have misorientation angles between 3 deg and 10 deg. In aluminum, the increased high-angle boundary area at larger strains originates from the extension of the initial boundaries through the mechanism, recently introduced by others, of “geometric dynamic recrystallization” in aluminum. The average misorientation across low-angle boundaries initially increases during steady state but eventually reaches a maximum value of about 1.2 deg at ε ≃ 1.2. Since the flow stress stays nearly constant, the dramatic changes in the character of the subgrain boundaries that are observed during steady state suggest that the details of the boundaries arenot an important consideration in the rate-controlling process for creep.

Dynamic restoration mechanisms in Al-5.8 At. Pct Mg deformed to large strains in the solute drag regime

An Al-5.8 at. pct Mg (5.2 wt pct Mg) alloy was deformed in torsion within the solute drag regime to various strains, up to the failure strain of 10.8. Optical microscopy (OM) and transmission electron microscopy (TEM) were used to analyze the evolution of the microstructure and to determine the dynamic restoration mechanism. Transmission electron microscopy revealed that subgrain formation is sluggish but that subgrains eventually (ε ≈ 1) fill the grains. The “steady-state” subgrain size (λ ≈ 6 μm) and misorientation angle (θ ≈ 1.6 deg) are reached by ε ≈ 2. These observations confirm that subgrains eventually form during deformation in the solute drag regime, though they do not appear to significantly influence the strength. At low strains, nearly all of the boundaries form by dislocation reaction and are low angle (θ < 10 deg). At a strain of 10.8, however, the boundary misorientation histogram is bimodal, with nearly 25 pct of the boundaries having high angles due to their ancestry in the original grain boundaries. This is consistent with OM observations of the elongation and thinning of the original grains as they spiral around the torsion axis. No evidence was found fordiscontinuous dynamic recrystallization, a repeating process in which strain-free grains nucleate, grow, deform, and give rise to new nuclei. It is concluded that dynamic recovery in the solute drag regime gives rise togeometric dynamic recrystallization in a manner very similar to that already established for pure aluminum, suggesting that geometric dynamic recrystallization may occur generally in materials with a high stacking-fault energy (SFE) deformed to large strains.

Silicide layer growth rates in Mo/Si multilayers

The thermal stability of sputter-deposited Mo/Si multilayers was investigated by annealing studies at relatively low temperatures (˜ 250-350 °C) for various times (0.5-3000 h). Two distinct stages of thermally activated Mo/Si interlayer growth were found: a primary surge, followed by a (slower) secondary steady-state growth in which the interdiffusion coefficient is constant. The interdiffusion coefficients for the interlayer formed by deposition of Mo-on-Si are higher than those of the interlayer formed by deposition of Si-on-Mo. Assuming that the activation energy is constant, an extrapolation of our results to ambient temperature finds that interlayer growth is negligible, suggesting long-term thermal stability in soft-x-ray projection lithography applications.

Large-strain torsional deformation in aluminum at elevated temperatures

Abstract A substantial amount of work has been produced recently in the area of large-strain deformation of aluminum at elevated temperatures using torsion tests. These studies have generated much interest because the equivalent uniaxial strain to failure of the aluminum can exceed 100, depending on the purity, strain rate and temperature. The research in this area has been performed principally by four groups and reported largely in less-circulated journals, proceedings and reports. The following review of the subject was written by three representatives from these groups. Emphasis was on establishing areas of substantial agreement as well as delineating aspects where further work will be helpful. The review is divided into two principal sections: macroscopic phenomenology and microstructural observations. The first includes discussions of ductility, stress-strain relationships and constitutive equations. The second includes discussions of grain elongation and thinning and grain boundary serration, subgrain sizes and morphologies, dislocation density, and subgrain boundary misorientation angles, as well as texture analysis. These investigations provide insight into the large-strain mechanical and microstructural phenomenology of materials in which elevated temperature softening proceeds by dynamic recovery.

Creep of Zirconium and Zirconium Alloys

Cumulative zirconium and zirconium alloy creep data over a broad range of stresses (0.1 to 115 MPa) and temperatures (300 °C to 850 °C) were analyzed based on an extensive literature review and experiments. Zirconium obeys traditional power-law creep with a stress exponent of approximately 6.4 over stain rates and temperatures usually associated with the conventional “five-power-law” regime. The measured activation energies for creep correlated with the activation energies for zirconium self-diffusion. Thus, dislocation climb, rather than the often assumed glide mechanism, appears to be rate controlling. The common zirconium alloys (i. e., Zircaloys) have higher creep strength than zirconium. The stress exponents of the creep data in the five-power-law regime were determined to be 4.8 and 5.0 for Zircaloy-2 and Zircaloy-4, respectively. The creep strength of irradiated Zircaloy appears to increase relative to unirradiated material. It was found that the creep behavior of zirconium was not sensitive to oxygen in the environment over the temperature range examined.

Energy dissipation efficiency in aluminum dependent on monotonic flow curves and dynamic recovery

In the hot working of Al, the flow curves are usually monotonic, reaching saturation at a lower strain εs and stress σs as temperature rises and strain rate declines. Microstructural examination confirms that the dislocation density rises to a steady-state level through formation of an equiaxed subgrain substructure with constant dimension that is larger for lower stress. The energy dis-sipation efficiency estimated by dynamic materials modeling for flow curves of the above type is the result of dynamic recovery, not of dynamic recrystallization, which is characterized by flow curves with a peak and marked softening to a steady-state regime.

Current issues in recrystallization: A review

Summary The authors have agreed that recrystallization is theformation of a new grain structure in a deformed material by the formation and migration of high angle grain boundaries driven by the stored energy of deformation. High angle grain boundaries are those with greater than a 10 to 15° misorientation. Recovery can be defined as all annealing processes occurring in deformed materials that occur without the migration of a high angle grain boundary. Grain coarsening can in turn, be defined as processes involving the migration of grain boundaries when the driving force for migration is solely the reduction of the grain boundary area itself. These definitions are consistent with some earlier definitions.

Evaluation and thermodynamic analysis of phase equilibria in the U-Al system

Abstract A reevaluation of phase equilibria in the binary U-Al system has been performed as part of the ASM-NBS Binary Phase Diagram Evaluation Program. The resulting diagram includes several modifications to the recently assessed diagram by Chiotti et al. The current assessment also emphasizes the uranium-rich portion of the diagram. The consistency between available thermodynamic data for the system (Gibbs energies of formation of UAl 2 , UAl 3 , and UAl 4 , and activity data for dilute solutions of uranium in liquid Al) and key points of the assessed diagram has been tested using the POTCOMP and FITBIN subroutines of the thermochemical computer code FACT. With the assumption of a regular solution model for Al-rich liquid, an ideal solution model for U-rich liquid, and Henrian models for U(β) and U(γ) solid solutions, there is reasonably good consistency between the thermo data and the assessed phase diagram.

The separate roles of subgrains and forest dislocations in the isotropic hardening of type 304 stainless steel

Tests on 304 stainless steel were conducted involving first warm working in torsion, then cold working in torsion, and finally measurement of the elevated-temperature yield strength in compression. These tests permitted separation of the effects of subgrain size and forest dislocation density on the isotropic part of the flow stress. Forest dislocation strengthening appears to dominate in this material. The results are best fitted by a root-mean-square summation of strength terms representing the contributions of solutes, forest dislocations, and subgrain boundaries. The same equation successfully predicts the flow stress during elevated-temperature transient deformation (under both constant strain rate and variable strain rate) from the transient dislocation substructure.