Crystallographic Mode Research Articles

Fracture Behavior of Single-Crystal Alloys Under Thermocyclic Loading

Experimental study results for thermal fatigue fracture of sand-glass specimens from a ZhS32 single-crystal alloy with different crystallographic orientations are presented over a wide range of maximum and minimum cycle temperature variations. Crystallographic, fractographic, and finite element data were used to identify crystallographic and noncrystallographic fracture modes of a single-crystal alloy. The noncrystallographic mode is realized at high maximum cycle temperatures and comparatively narrow temperature ranges. It is characterized by mode I crack growth. The crystallographic mode is realized at lower maximum temperatures and a wide range of cycle temperature variations. It is characterized by combined I–II mode crack growth in crystallographic plane {111}. The chart of fracture mechanisms in the maximum temperature-temperature range coordinates is proposed. The boundary between the regions permits of approximation, corresponding to the Arrhenius equation.

The Thermal Conductivity of Periodic Particulate Composites as Obtained from a Crystallographic Mode of Filler Packing

In this paper, an icosahedral non-body-centered model is presented to simulate the periodic structure of a general class of homogeneous particulate composites, by predicting the particle arrangement. This model yielded three different variations, which correspond to three different deterministic particle configurations. In addition, the concept of a boundary interphase between matrix and inclusions was taken into account. In this framework, the influence of particle vicinity on the thermomechanical properties of the overall material was examined in parallel with the concept of boundary interphase. The simultaneous consideration of these two basic influential factors constitutes the novelty of this work. Next, by the use of this advanced model, the authors derived a closed-form expression to estimate the thermal conductivity of this type of composite. To test the validity of the model, the theoretical predictions arising from the proposed formula were compared with experimental data found in the literature, together with theoretical results obtained from several accurate formulae derived from other workers, and an adequate accordance was observed.

First-principles study of crystallographic slip modes in \u03c9-Zr

We use first-principles density functional theory to study the preferred modes of slip in the high-pressure ω phase of Zr. The generalized stacking fault energy surfaces associated with shearing on nine distinct crystallographic slip modes in the hexagonal ω-Zr crystal are calculated, from which characteristics such as ideal shear stress, the dislocation Burgers vector, and possible accompanying atomic shuffles, are extracted. Comparison of energy barriers and ideal shear stresses suggests that the favorable modes are prismatic 〈c〉, prismatic-II langle 10bar{1}0rangle and pyramidal-II 〈c + a〉, which are distinct from the ground state hexagonal close packed α phase of Zr. Operation of these three modes can accommodate any deformation state. The relative preferences among the identified slip modes are examined using a mean-field crystal plasticity model and comparing the calculated deformation texture with the measurement. Knowledge of the basic crystallographic modes of slip is critical to understanding and analyzing the plastic deformation behavior of ω-Zr or mixed α-ω phase-Zr.

Atomistic fingerprint of hyaluronan-CD44 binding.

Hyaluronan is a polyanionic, megadalton-scale polysaccharide, which initiates cell signaling by interacting with several receptor proteins including CD44 involved in cell-cell interactions and cell adhesion. Previous studies of the CD44 hyaluronan binding domain have identified multiple widespread residues to be responsible for its recognition capacity. In contrast, the X-ray structural characterization of CD44 has revealed a single binding mode associated with interactions that involve just a fraction of these residues. In this study, we show through atomistic molecular dynamics simulations that hyaluronan can bind CD44 with three topographically different binding modes that in unison define an interaction fingerprint, thus providing a plausible explanation for the disagreement between the earlier studies. Our results confirm that the known crystallographic mode is the strongest of the three binding modes. The other two modes represent metastable configurations that are readily available in the initial stages of the binding, and they are also the most frequently observed modes in our unbiased simulations. We further discuss how CD44, fostered by the weaker binding modes, diffuses along HA when attached. This 1D diffusion combined with the constrained relative orientation of the diffusing proteins is likely to influence the aggregation kinetics of CD44. Importantly, CD44 aggregation has been suggested to be a possible mechanism in CD44-mediated signaling.

Special Features of Creep and Long-Term Strength of Single-Crystal Refractory Nickel-Base Alloys

Results of experimental studies of creep in a wide range of temperatures and stresses are presented for three modern single-crystal nickel-base alloys. A method for determining the creep characteristics by tests including step increase of the tension is suggested and tested. The effect of transition into plastic condition on the creep parameters, the influence of the chemical composition and loading conditions on the duration of creep stages I, II and III, and the conditions of appearance of crystallographic and not crystallographic modes of fracture are analyzed. Possible simple approximations of the creep curves are considered with allowance for the accumulation of damage and occurrence of unsteady creep stages.

High Cycle Fatigue Damage Mechanisms of MAR-M 247 Superalloy at High Temperatures

High cycle fatigue tests of the MAR-M 247 superalloy under constant load were conducted with the aim to determine S–N curves at temperatures 650, 800, 900 and 950 °C. It has been found that the slope of the S–N curves reflects changes of the cyclic deformation mechanism with temperature. At temperatures 650 and partly at 800 °C, fatigue crack propagation exhibits crystallographic character (propagation along the {111} system) in early stages of the fatigue life. The crystallographic mode of the fatigue crack propagation was found to disappear at temperatures 900 and 950 °C.

Effect of microstructure on appearance of near-threshold fatigue fracture in Cr–Mo–V steel

Near-threshold fatigue crack growth behavior in 25Cr2NiMo1V steel with different microstructures was investigated by utilizing the load-shedding technique at ambient temperature. Crack surface morphology was observed by SEM with special emphases on the incidence of intergranular fracture and the influence on crack growth rates. Results show that the maximum intergranularity occurs at the ΔK corresponding to the cyclic plastic zone size being equivalent to the prior austenitic grain size. Two types of crack growth mode were observed in the near-threshold regime, i.e., the crystallographic mode of crack growth and the striation mode of crack advance. The incidence of faceted fracture was mainly rationalized by comparing the cyclic plastic zone size with the grain size. It is concluded that, in the crystallographic mode, lower crack growth rates in samples with higher heat treatment temperatures are caused by a greater degree of roughness-induced crack closure (RICC), faceted fracture induced crack closure (FFICC), and oxide-induced crack closure (OICC). The faceted fracture shows negligible influence on crack growth rates when cracks grow in a striation controlled mode.

Four-point-bend fatigue of AA 2026 aluminum alloys

High-cycle fatigue tests were carried out on a newly developed high-strength AA 2026 Al alloy, which was in the form of extrusion bars with square and rectangular cross sections, using a self-aligning four-point-bend rig at room temperature, 15 Hz, and R = 0.1, in lab air. The fatigue strength of the square and rectangular bars was measured to be 85 and 90 pct of their yield strength, respectively, more than twice that of the predecessor to the 2026 alloy (the AA 2024 Al alloy). Fatigue cracks were found to be always initiated at large Θ′ (Al7Cu2(Fe,Mn)) particles and to propagate predominantly in a crystallographic mode in the AA 2026 alloy. The fatigue fractographies of the square and rectangular extrusion bars were found to be markedly different, due to their different grain structures (fibril and layered, respectively). Fracture steps on the crack face were found in both of these extrusion bars. Since the 2026 alloy was purer in terms of Fe and Si content, it contained much less coarse particles than in a 2024 alloy. This partially accounted for the superior fatigue strength of the 2026 alloy.

Evaluation of docking performance: comparative data on docking algorithms.

Docking molecules into their respective 3D macromolecular targets is a widely used method for lead optimization. However, the best known docking algorithms often fail to position the ligand in an orientation close to the experimental binding mode. It was reported recently that consensus scoring enhances the hit rates in a virtual screening experiment. This methodology focused on the top-ranked pose, with the underlying assumption that the orientation/conformation of the docked compound is the most accurate. In an effort to eliminate the scoring function bias, and assess the ability of the docking algorithms to provide solutions similar to the crystallographic modes, we investigated the most known docking programs and evaluated all of the resultant poses. We present the results of an extensive computational study in which five docking programs (FlexX, DOCK, GOLD, LigandFit, Glide) were investigated against 14 protein families (69 targets). Our findings show that some algorithms perform consistently better than others, and a correspondence between the nature of the active site and the best docking algorithm can be found.

A discrete dislocation model of Stage I fatigue crack growth and an analysis of Mode I to Mode II transition at low ΔK1

Simulations of dislocations nucleation and glide ahead of a crystallographic mode II crack are performed for push-pull and reversed torsion . An influence of the normal stress on the friction of crack flanks, as well as on the condition for dislocation emission is introduced.The crack growth rates are deduced from the dislocation fluxes at the crack tip. A comparison between the loading modes is made. Taking into account the presence of grain boundaries, the repeated decelerations and sometimes the arrest that characterise Stage I crack growth are described by the model. An analysis of the transition from mode I to mode II crack growth observed at low ΔK I in single crystals is proposed.

Matching chemistry and shape in molecular docking.

We have added a chemical filter to the ligand placement algorithm of the molecular docking program DOCK. DOCK places ligands in receptors using local shape features. Here we label these shape features by chemical type and insist on complementary matches. We find fewer physically unrealistic complexes without reducing the number of complexes resembling the known ligand-receptor configurations. Approximately 10-fold fewer complexes are calculated and the new algorithm is correspondingly 10-fold faster than the previous shape-only matching. We tested the new algorithm's ability to reproduce three known ligand-receptor complexes: methotrexate in dihydrofolate reductase, deoxyuridine monophosphate in thymidylate synthase and pancreatic trypsin inhibitor in trypsin. The program found configurations within 1 A of the crystallographic mode, with fewer non-native solutions compared with shape-only matching. We also tested the program's ability to retrieve known inhibitors of thymidylate synthase and dihydrofolate reductase by screening molecular databases against the enzyme structures. Both algorithms retrieved many known inhibitors preferentially to other compounds in the database. The chemical matching algorithm generally ranks known inhibitors better than does matching based on shape alone.

Effects of interfacial strength on fatigue crack growth in a fiber-reinforced titanium-alloy composite: Chan, K.S. and Davidson, D.L. Metall. Trans. A June 1990 21A, (6), 1603–1612

In recent years, there has been a renewed interest in understanding the mechanical properties and microstructural characteristics of aluminum alloys containing lithium, because of their potential for extensive use in aerospace applications. In the present paper the fatigue crack propagation behavior of aluminum alloy 2020 (AlCuLiMnCd) in the peak strength temper is examined over a wide range of growth rates (from 10−6 to 10−11 m cycle−1). It was found that alloy 2020 shows crack propagation rates that are up to an order of magnitude slower at intermediate ΔK levels and up to two orders of magnitude slower in the near-threshold region, when compared with aluminum alloy 7075 under conditions of similar yield strength, load ratio, frequency and environment. Extensive analyses of the crack path for alloy 2020 reveal that pronounced deflection and branching of the fatigue crack occur at all growth rate levels as a result of a highly crystallographic mode of crack advance. The micromechanisms of fatigue crack growth in alloy 2020 are examined in terms of crack profile, fracture surface features, microstructure and environment.

Fatigue crack growth behavior of aluminum alloy 2020 (AlCuLiMnCd)

In recent years, there has been a renewed interest in understanding the mechanical properties and microstructural characteristics of aluminum alloys containing lithium, because of their potential for extensive use in aerospace applications. In the present paper the fatigue crack propagation behavior of aluminum alloy 2020 (AlCuLiMnCd) in the peak strength temper is examined over a wide range of growth rates (from 10 −6 to 10 −11 m cycle −1). It was found that alloy 2020 shows crack propagation rates that are up to an order of magnitude slower at intermediate ΔK levels and up to two orders of magnitude slower in the near-threshold region, when compared with aluminum alloy 7075 under conditions of similar yield strength, load ratio, frequency and environment. Extensive analyses of the crack path for alloy 2020 reveal that pronounced deflection and branching of the fatigue crack occur at all growth rate levels as a result of a highly crystallographic mode of crack advance. The micromechanisms of fatigue crack growth in alloy 2020 are examined in terms of crack profile, fracture surface features, microstructure and environment.

FRACTOGRAPHIC ANALYSIS IN THE UNDERSTANDING OF CORROSION FATIGUE MECHANISMS

Abstract—Fractographic analyses have been used to explain the cyclic crack growth behaviour of A533‐B, Ducol W30, a C‐Mn steel and type 304 stainless steel in simulated light water reactor environments at ambient temperature. Fractographic observations have offered an explanation for anomalous crack growth behaviour and have also indicated where micro structural or environmental variables dominate in producing certain fracture modes and crack growth rates. An understanding of the operative corrosion fatigue mechanisms has been formulated through these fractographic analyses. Environmental crack growth in the ferritic steels has been described by a model involving both anodic dissolution and hydrogen embrittlement. Conditions where only one of these mechanisms would dominate have been identified and limits to their effect postulated. A crystallographic mode of failure observed in the austenitic type 304 stainless steel has also been explained by a selective dissolution process.