Pile-up Mechanism Research Articles

In situ scanning electron microscopy indentation studies on multilayer nitride films: Methodology and deformation mechanisms

Systematic studies of the deformation mechanisms of multilayer transition metal nitride coatings TiN/CrN, TiN/NbN, and NbN/CrN, and corresponding reference coatings of TiN, NbN, and CrN deposited by a direct current (dc) magnetron sputtering process onto silicon 〈100〉 have been performed. Mechanical characterization was conducted using a combination of microindentation and nanoindentation in the load range 30 to 150 mN and 0.5 to 3.5 mN, respectively. For both load ranges, scanning electron microscopy (SEM) in situ indentation was used to observe the indentation process including any pileup, sink-in, and fracture mechanisms specific to each coating. The coatings’ microstructure, both before and after indentation, was analyzed using transmission electron microscopy (TEM). It was possible to both correlate the indentation load–displacement response to surface roughness effects and fracture modes (substrate and film cracking) and observe deformation mechanisms within the coatings.

Micro-crack initiation at the tip of a semi-infinite rigid line inhomogeneity in piezoelectric solids

A dislocation emission mechanism for micro-crack initiation at the tip of a semi-infinite rigid line inhomogeneity in a piezoelectric solid is proposed in the present paper. For a rigid line inhomogeneity embedded in a piezoelectric matrix, dislocations of one sign are driven away from the tip due to high stress level, while the stationary dislocations of the opposite sign are left behind near the tip of the inhomogeneity. As a result, a micro-Zener–Stroh crack is initiated ahead of the line inhomogeneity. In the current study, a dislocation pileup mechanism for micro-crack initiation at the inhomogeneity tip is proposed. An interesting result is that the critical stress intensity factors for a line inhomogeneity perpendicular to the poling direction can be related to the fracture toughness of a conventional crack in the same material. Analytical solutions show that the critical plane shear stress intensity factor depends on the plane shear mechanical and displacement loadings, and the critical opening stress and electric displacement intensity factors depend on not only the mechanical and displacement loadings, but the electric field and displacement loadings as well.

A new look at driven magnetic reconnection at the terrestrial subsolar magnetopause

The physics of steady driven magnetic reconnection at Earth's subsolar magnetopause is addressed. Three‐dimensional, global magnetohydrodynamics (MHD) simulations of the magnetopause are compared with analytical solutions of the resistive MHD equations [Sonnerup and Priest, 1975] corresponding to magnetic field annihilation driven by an incompressible stagnation point flow. The simulations demonstrate that under steady southward interplanetary magnetic field conditions and when the plasma resistivity is spatially uniform, subsolar magnetopause reconnection occurs in long, thin Sweet‐Parker current sheets via a flux pileup mechanism [Sonnerup and Priest, 1975; Priest and Forbes, 1986] (rather than in Petschek slow shock configurations). Magnetic energy piles up upstream of the magnetopause current sheet to accommodate the sub‐Alfvénic solar wind inflow. The scaling of the pileup with Lundquist number, S, is consistent (approximately ∝ S1/4) with that predicted by the analytical, incompressible stagnation point flow solutions (though there are small corrections due to plasma compressibility in the simulations). Since there is a finite energy in the magnetosheath available to drive the magnetic pileup (and associated rapid magnetic reconnection), we expect the pileup to saturate and the reconnection rate to drop as the upstream plasma pressure drops to accommodate the pileup. Thus we expect the reconnection to stall, the rate vanishing in the limit S → ∞. We discuss the role of Hall electric fields in allowing the magnetic pileup to saturate before the reconnection begins to stall, permitting Alfvénic reconnection to occur in thin current sheets in the limit S → ∞.

In-situ observation of damage evolution and fracture in AlSi7Mg0.3 cast alloys

The mechanisms which occur during damage initiation, evolution and crack growth in AlSi7Mg0.3 cast alloys are studied by in-situ tensile testing in a scanning electron microscope. It is shown that microcracks in these alloys are predominantly formed in the Si particles. Shear bands are seen to precede the breaking of the Si particles and the dislocation pile-up mechanism can thus be confirmed as the dominant damage initiating process in the matrix. Both micro- and macrocrack coalescence have been observed in the course of the experiments. The effect of the microstructure of the AlSi7Mg cast alloys on damage nucleation, crack formation and compliance reduction is analysed.

Kinks and cracks in S1 ice under across-column compression

Fresh-water S1 ice has been slowly compressed uniaxially across the columns at- 10 C ( = 0.96). Orowan-type deformation kinks were observed and cracks were detected on the kink band boundaries. Each crack was associated with a step in the boundary. Nucleation of these cracks is explained in terms of a Stroh-type dislocation pile-up mechanism.

The cracking mechanism of silicon particles in an A357 aluminum alloy

The cracking of Si particles in an A357 Al alloy has been investigated over a spectrum of stress and strain by varying aging strength and applying different tensile strains. The variation of the fraction of broken Si particles with stress, strain, and cleavage plane orientation has been obtained. The features of cracking reveal that cracking of Si particles is a very localized event. A dislocation pileup mechanism is the most probable one among all crack-initiation theories for explaining the behavior. Based on this mechanism, further deduction has been made to obtain the relationship between the fraction of broken particles and metallurgical factors. The present data, along with Gurlandrss and that of Lowet al., have been found to verify this relationship for the effect of stress, strain, and cleavage plane orientation.

Effect of grain size on mechanical properties of nanocrystalline materials

The possibility of a dislocation mechanism in the deformation process of nanocrystalline materials is reviewed and analyzed. The present theoretical calculation, by taking the anisotropic characteristic of crystallographic symmetry and different choices of critical shear strength into account, results in a reasonable limit in grain size for applying dislocation pile-up theory to nanocrystalline materials. The deviation from the Hall—Petch relationship is rationalized in terms of a small number dislocation pile-up mechanism. A composite model is proposed to evaluate the strength of nanocrystalline materials. It is shown that this model can be used for interpreting the various cases observed in Hall—Petch studies. An analytical expression for assessing the creep rate of nanocrystalline materials by a diffusion mechanism, including triple line diffusion, is derived. It is predicted that the creep rate due to triple line diffusion will exhibit a stronger grain size dependence than that due to grain boundary diffusion.

Design of the microstructural scale for optimum toughening in metallic composites

The role of the microstructural scale in toughening of composites containing a metallic constituent is described by invoking a dislocation pile-up mechanism for debonding at metal ceramic (intermetallic) interfaces. Below a certain size of the metal phase, the pile-up length becomes subcritical precluding debonding. The toughening effect below and above this critical size separates into two kinds of behavior leading to a maximum toughness when plotted against the characteristic length of the metal phase. The position of this maximum defines the optimum microstructural scale. The concept is applied successfully to metal-ceramic and metal-intermetallic composites with a microstructural length scale of less than about 10 μm. Metal matrix composites where the metal forms a continuous network through the microstructure are shown to have much higher toughness than the case where the same volume of the metal phase is distributed as discrete particles in the composite.

Reexamination of a 1955 theory of climb-controlled steady-state creep

Observed steady-state creep rates of many metal class materials show stress dependence ϵ ̇ ∝ τ n where n ≈ 5. Many models of power-law creep predict n = 3 or 4, and require ad hoc assumption of the stress dependence of some geometric parameter to explain the observed n value. A 1955 climb-controlled model of steady-state creep assumes constant climb stress at the lead dislocation of an obstrcuted pileup to allow analysis. This analysis predicts n = 4. Invocation of a stress-dependent geometric parameter allows prediction of n ≈ 5. In the present work, numerical simulation of the model shows assuming constant climb stress to be incorrect and assuming stress-dependent geometry to be unnecessary to predict n ≈ 5. High and low stress limiting behaviors of the simulation agree with the results of earlier analytical models. Slip of pileup dislocations during climb of the lead dislocation is suggested as the physical explanation for the non-constant climb stress on the lead dislocation. Both this work and previous analytical models on pileup mechanisms are subjected to critism based on the lack of observations of dislocation pileups in the steady state creep.

Crack nucleation in polycrystalline ice

This paper examines in detail two likely mechanisms of microcrack formation in polycrystalline ice and pays special attention to the grain size dependencies of each mechanism. Under consideration are the Zener-Stroh dislocation pileup mechanism and an elastic mechanism based on the anisotropy of the ice lattice. Calculations for the pileup mechanism indicate that although the dislocation velocity is relatively low, a critical-sized pileup can form under plausible test conditions. Quantification of the elastic anisotropy mechanism indicates that it operates over approximately the same stress levels as the pileup mechanism and exhibits the same grain size dependency. The results of observations on the microcracking of laboratory-prepared freshwater ice having randomly oriented equiaxed grains are used to test the model predictions. The work gives detailed descriptions of the methods used to quantify each model.

Cleavage fracture of low carbon ferritic steels with fine grain size

AbstractMicromechanisms of transgranular cleavage fracture in ferritic steels have been investigated, particular attention being paid to the dependence of cleavage fracture stress σf on grain size over the range from 3·5 to 34·3 μm. It is found that σf increases less than linearly with d−1/2, where d is the grain diameter, whereas the Hall–Petch relation holds for the yield stress. Based on the experimental results, where σf was measured in the steels and their grain size and carbide size were changed independently, it is suggested that in the grain size range cited, the predominant mechanism was the propagation of a microcrack, which formed in the discrete carbide, into contiguous ferrite grains. The measured values of σf were discussed in a quantitative manner based on a model into which an equilibrium distribution of dislocations produced by the double pile-up mechanism was incorporated.

Mechanical behavior of rapidly solidified Al-Al2Cu and Al-Al3Ni composites

The eutectic alloys Al-Al2Cu and Al-Al3Ni have been unidirectionally solidified at rates from 1.05 to 6.80 in, per min by a semicontinuous casting technique, and then tested in tension at room temperature. In both alloys the flow stress and ultimate tensile strength increased with increasing solidification rate, except for the highest solidification rate. The increases in matrix work-hardening rate with solidification rate were too great to be accounted for by dislocation pileup mechanisms, but were found to correlate with elastic constraint effects of the matrix aluminum phase by the reinforcing phases. In the Al-Al2Cu eutectic the strength of the Al2Cu platelets increased as the platelet width decreased with increasing growth rate. Misalignment of the composite caused by either a cellular or a macroscopically concave solid-liquid interface resulted in a decrease in the ultimate strength, especially in the rod-like Al-Al3Ni alloy. This has been related to the fracture behavior of the composites. The very low fracture toughness of the lamellar Al-Al2Cu eutectic is consistent with models of composite materials, and seriously limits the alloy’s usefulness for engineering applications.

疲労したCu-Ge合金の内部微細構造

It is well-known that the fatigue strength of metals is much influenced by the frequency of cross-slip. So, dislocation arrangements, crack initiation and propagation mechanisms of fatigued metals are expected to be affected by stacking fault energy.Then, cold-rolled sheets of copper-germanium alloys which have a large variety of stacking fault energies, were fatigued in constant deflection with a bending fatigue machine, and after being thinned from one side of the surfaces, they were observed mainly by a transmission electron microscope.As a result, it is found that in copper-germanium alloys which have high stacking fault energies, striped high-dislocation-density walls (cell boundaries) parallel to [1\bar21] direction are formed by fatiguing. The lower is stacking fault energy, the more straight are cell boundaries. These straight cell boundaries are called “dark bands”. Dark bands are mainly composed of small dislocation loops. Fatigue cracks initiate and propagate along dark bands. These straight cell boundaries (dark bands) are assumed to correspond to persistent slip-bands on the surface.On the other hand, in copper-germanium alloys which have low stacking fault energies, many fatigue cracks initiate at the cross points of slip-line and twin boundary, grain boundary and/or other slip-lines, which suggests the importance of the dislocation pile-up mechanisms for fatigue crack initiation. While many cracks propagate through slip-lines. This fact indicates that fatigue cracks propagate along persistent slip-bands without regard to the order of magnitude of stacking fault energy. In the case of low stacking fault energy alloys, fatigue cracks sometimes propagate along grain boundaries or twin boundaries.

Microstrain characteristics of isostatically hot-pressed beryllium

Polycrystalline beryllium specimens were fabricated from powders by using significantly higher pressures and lower temperatures than are normally employed for commercially-compacted beryllium. The procedures involved encasement of a partially-compacted powder in a collapsible steel container; the container was externally isostatically pressurized by (a) hot gas (helium), or (b) a solid pressure-transmitting medium (steel or Al 2O 3, etc.) enclosed in a cavity and acted upon by the ram of a forging press. A finer initial powder-particle size and distribution were found to result in a smaller as-fabricated grain size. Microyield strength of as-fabricated, low-oxide beryllium decreased with decreasing grain size. However, upon heat treating at 924 °C for 30 min after pressing, microyield strength increased with decreasing grain size. Microyield strength also increased with decreased processing temperature, increased beryllium oxide content, and small alloying additions of chromium or silver. Flow stress of isostatically pressed beryllium was found to be a parabolic function of microstrain. The flow curves were characterized by three distinct ranges of microstrain, viz., 1. (1) heterogeneous basal glide at low microstrains, 2. (2) dispersion hardening, most probably by a dislocation pile-up mechanism at intermediate levels of microstrain, and 3. (3) prismatic glide and cross-slip at microstrains greater than 50 × 10 −6. Microflow behavior of isoforged beryllium was a sensitive function of the time held at pressure during final consolidation, probably due to the effect of pressure on the solubility of precipitates.

Hydrogen in steel—the stability of micro-cracks

The stability of a wedge shaped microcrack under constant external stress and internal pressure is examined. Some modifications are found necessary in the original analysis of the stability under external stress only; these reinforce the conclusion that a crack formed under tension by the pile-up mechanism will grow unless damped by plastic flow. However, because of the special orientation of the stress system, the analyses of the ductile-brittle criterion given by Cottrell and by Petch are unaffected. The theory presented by Garofalo, Chou and Ambegaokar, which includes also the influence of internal pressure, is analysed in the light of these modifications and of others which are shown to be necessary. The corrected analysis is compared quantitatively with that of these workers using formulae for the effect of hydrogen adsorption on the surface energy and for the pressure of gaseous hydrogen in equilibrium with lattice dissolved hydrogen. The most recent thermodynamic analysis of this latter equilibrium is also discussed. Finally the general interpretation of Garofalo, Chou and Ambegaokar is criticised and an alternative analysis, including a re-assessment of the effect of hydrogen adsorption, is given. In contrast to the conclusions of Garofalo, Chou and Ambegaokar, it is suggested that only a very small quantity of lattice dissolved hydrogen is required to exert an embrittling effect directly through pressure in wedge cracks. This view explains why under some conditions extreme sensitivity to hydrogen may result, and is in accord with evidence from data on the solubility, diffusion and internal friction, which as others have proposed, indicates that much of the hydrogen is bound in some kind of trap.