Kip O. Findley

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.

Structure and properties of lanthanide halides.

Lanthanum and cerium bromides and chlorides form isomorphous alloy systems with the UCl3 type structure. These scintillating alloys exhibit high luminosity and proportional response, making them the first scintillators comparable to room temperature semiconductors for gamma spectroscopy; Ce(III) activated lanthanum bromide has recently enabled scintillating gamma ray spectrometers with < 3% FWHM energy resolutions at 662 keV. However brittle fracture of these materials impedes development of large volume crystals. Low fracture stress and perfect cleavage along prismatic planes cause material cracking during and after crystal growth. These and other properties pose challenges for material production and post processing; therefore, understanding mechanical behavior is key to fabricating large single crystals, and engineering of robust detectors and systems. Recent progress on basic structure and properties of the lanthanide halides is reported here, including thermomechanical and thermogravimetric analyses, hygroscopicity, yield strength, and fracture toughness. Observations including reversible hydrate formation under atmospheric pressure, loss of stoichiometry at high temperature, anisotropic thermal expansion, reactivity towards common crucible materials, and crack initiation and propagation under applied loads are reported. The fundamental physical and chemical properties of this system introduce challenges for material processing, scale-up, and detector fabrication. Analysis of the symmetry and crystal structure of this system suggests possible mechanisms for deformation and crack initiation under stress. The low c/a ratio and low symmetry relative to traditional scintillators indicate limited and highly anisotropic plasticity cause redistribution of residual process stress to cleavage planes, initiating fracture. This proposed failure mechanism and its implications for scale up to large diameter crystal growth are also discussed.

Effects of alloy 718 microstructure on hydrogen embrittlement susceptibility for oil and gas environments

The effects of alloy 718 microstructure on hydrogen embrittlement susceptibility and tensile fracture mode were assessed through slow strain rate tensile testing and fracture surface analysis. Alloy 718 was annealed and aged to produce microstructures with variations in grain size and amount of grain boundary precipitates. Furthermore, the different ageing conditions likely resulted in differences in volume fractions and sizes of γ′ and γ′′ precipitates. The extent of grain boundary precipitation had the strongest effect on hydrogen embrittlement susceptibility, while grain size did not have any significant effect. Hydrogen embrittlement susceptibility was also correlated with differences in strength level, which was primarily controlled by the γ′ and γ′′ precipitate populations.

Strengthening and Toughening Mechanisms in Martensitic Steel

Low carbon martensitic steels are often produced by reaustenitizing and quenching (RA/Q). Direct quenching (DQ) has gained interest in the past few decades and requires quenching immediately after working above or below the austenite recrystallization temperature to form martensitic microstructures. In the current study, microalloyed ASTM A514 steel is used to produce martensite from either equiaxed or pancaked prior austenite grain (PAG) microstructures. The equiaxed PAG conditions simulate microstructures produced by RA/Q and the pancaked PAG conditions simulate microstructures produced by controlled rolling (CR) before DQ. Controlled rolling followed by DQ was simulated with double hit compression in a Gleeble® 3500. The prior austenite grain size (PAGS) was varied between 9 and 75 μm prior to controlled rolling. The strengthening and toughening mechanisms are being investigated in the as-quenched (AsQ), low temperature tempered (LTT: 200 °C), and high temperature tempered (HTT: 600 °C) conditions. The equiaxed PAG condition has a Hall-Petch (H-P) relationship between yield strength (or microhardness) and PAGS in the AsQ condition. There is not a H-P relationship between PAGS and microhardness in the CR-DQ conditions. The CR-DQ conditions generally exhibit higher microhardness than the RA/Q conditions with similar PAGS, with the most significant differences in the larger PAGS conditions. Toughness was only measured in the equiaxed PAG conditions. The smallest PAGS has the lowest ductile-to-brittle transition temperature (DBTT) with the highest strength in the AsQ and LTT conditions. The smallest PAGS has the lowest DBTT and the lowest strength in the HTT condition.

Effect of Microstructural Banding on the Fatigue Behavior of Induction- Hardened 4140 Steel

The fatigue behavior of induction-hardened calcium-treated 4140 steel with three different case depths was evaluated using rotating bending fatigue tests. The as-received microstructure of the steel was banded and the orientation of microstructural banding with respect to the fatigue specimen was varied. Due to the inclusion shape control resulting from the calcium additions, inclusions in the steel were not elongated in the direction of the banding. It was found that microstructure banding does not have a significant influence on the fatigue properties of the steel tested. Furthermore, the fatigue limit increase with case depth is primarily related to the bending stress near the location of crack nucleation.

Effects of Microalloy Additions and Thermomechanical Processing on Austenite Grain Size Control in Induction-Hardenable Medium Carbon Steel Bar Rolling

Three AISI 1045 steels: a base steel, one modified with vanadium (V), and one modified with V and niobium (Nb) were studied to evaluate microstructural conditioning prior to induction hardening. Simulated bar rolling histories were evaluated using fixed-end hot torsion tests with a Gleeble® 3500. The effects of chemical composition and thermomechanical treatment on final microstructures were examined through analysis of laboratory simulations of steel bar rolling and induction hardening processes in order to provide additional insights into the morphological evolution of austenite of microalloyed steels. Analysis of prior austenite grain size (PAGS) is complemented with analysis of austenite recrystallization and pancaking during rolling. The potential for utilizing TMP, in conjunction with microalloy additions, to enhance bar steel microstructures and subsequent performance is assessed by evaluating the induction hardening response of each steel systematically processed with different preconditioning treatments.

Application of Chords for Quantitative Characterization of Multi-Constituent Microstructures

A modified quantitative microstructure characterization technique has been developed based on constituent chord-length distribution (CLD). The modified technique, called the categorical CLD (CCLD), is capable of quantifying both size and volume fraction of the constituent of interest in a multi-constituent microstructure as a function of adjacent microstructure. The capability of the CCLD is demonstrated by its application to the investigation of the effect of microstructure on the stability of retained austenite (RA) during mechanical deformation in transformation-induced plasticity steels. By quantifying microstructures deformed to different levels of strains, it is found that RA is stabilized by fine grain size and adjacent bainitic ferrite.

A Dilatometric Study of Tempering Complemented by Mössbauer Spectroscopy and other Characterization Techniques

A new approach for non-isothermal tempering analysis utilizing dilatometry is proposed and was carried out on a medium carbon steel with high silicon and additions of Mo and V for secondary hardening. The method includes a second non-isothermal step performed with the same heating rate (2 °C/min) used for the first step in order to create a baseline for analysis. The results were correlated with several other characterization techniques. Mössbauer spectroscopy confirmed the formation of transition carbides by auto-tempering as well as the presence of retained austenite decomposition (stage II) and cementite precipitation (stage III), which demonstrated significant overlap. Electrical resistivity measurements were correlated with dislocation densities obtained through X-ray diffraction analysis. Transmission electron microscopy dark field images confirmed the secondary hardening assessment from dilatometry.

Analysis of Microstructure in Hot Torsion Simulation

Hot torsion is frequently employed to simulate multipass thermomechanical rolling. While flow behavior, observed through shear stress versus shear strain, is typically used to characterize hot deformation and softening behaviors, the resulting microstructures can also provide significant insight into microstructural evolution and strain accumulation during the hot deformation process. A preferred approach for the analysis of microstructural features resulting from hot torsion is presented. Torsional strain paths are reviewed and compared with traditional hot rolling deformations. A tangential sectioning technique, combined with supporting fundamentals, is also presented. Microstructural observation of steels thermomechanically deformed in hot torsion verified the ability to reasonably quantify strain from microstructural analysis. This approach offers a new method for assessing shear strain accumulation within local regions of a body plastically deformed in torsion, and should provide a useful complement to the assessment of mechanical responses in hot deformation studies.

The Effects of Pre Straining Conditions on Austenite Stability During Fatigue of Multiphase Trip Steels

A multiphase TRIP780 steel was subjected to tensile pre-strains of 0, 5, and 15% at room temperature and at −20 °C. The pre-strained specimens were subjected to strain-ontrolled, fully-reversed, axial fatigue testing at strain amplitudes ranging from 0.2 to 0.6%. The low-cycle fatigue life was largely independent of the pre-strain history. Pre-strained specimens had lower plastic strain amplitudes and higher stress ranges in the initial fatigue cycles before converging towards the same values as the specimens with zero pre-strain, due to cyclic strain softening. This suggests that the effects of pre-strain are eventually overcome by accumulated plastic deformation during fatigue cycling.