Slip Transition Research Articles

The mechanics of slip transition at intermediate temperatures in <001>-oriented NiAl single crystals I. Experimental observations of the decomposition of α<111> dislocations in Ni–44 at.% Al

Abstract Ni–44 at.% Al single crystals were studied after compressive deformation along the hard orientation between 700 and 825 K. In order to understand the mechanism of the slip transition occurring at intermediate temperatures, compression was performed at various strain rates and to various strain levels. Local decomposition of α<111> dislocations was observed between 750 and 800 K. At 800 K (the slip transition temperature), complete decomposition occurs into α<101> and α<010> dislocations. Several unique features are associated with this decomposition event, such as the preference for near-edge line directions (for the α<111> dislocations), significant strain softening between 800 and 900 K, and the effect of thermal activation in determining the extent of the actual decomposition event.

The mechanics of slip transition at intermediate temperatures in -oriented NiAl single crystals II. A metastable state for α{110} dislocations in NiAl and its role in their decomposition

Abstract We show in this paper that there exists a metastable arrangement of products in the reaction α<111> → α<110> + α<001> in B2 NiAl. Atomistic calculations show that the metastable state can be formed spontaneously by a local shuffle or climb decomposition. Once formed, the ‘piggyback’ configuration can glide as a unit, with a restoring force helping to keep it intact. However, if the product lines become separated locally beyond a critical distance, the result is a decomposition loop. Small dislocation loops (less than 100 Å) collapse, large loops (greater than 3000 Å) evolve into dislocation dipoles, and medium loops can fail to evolve further. These results are consistent with detailed observations of the microstructure. The implications for this metastable state for the mechanical properties are examined.

The mechanics of slip transition at intermediate temperatures in &lt;001&gt;-oriented NiAl single crystals II. A metastable state for a &lt; 111 &gt;{110} dislocations in NiAl and its role in their decomposition

Abstract We show in this paper that there exists a metastable arrangement of products in the reaction α → α + α in B2 NiAl. Atomistic calculations show that the metastable state can be formed spontaneously by a local shuffle or climb decomposition. Once formed, the ‘piggyback’ configuration can glide as a unit, with a restoring force helping to keep it intact. However, if the product lines become separated locally beyond a critical distance, the result is a decomposition loop. Small dislocation loops (less than 100 A) collapse, large loops (greater than 3000 A) evolve into dislocation dipoles, and medium loops can fail to evolve further. These results are consistent with detailed observations of the microstructure. The implications for this metastable state for the mechanical properties are examined.

A unified wall slip model

A unified slip model is developed, which predicts wall slip by either a disentanglement mechanism or by debonding mechanism, depending upon the adhesive energy of the wall-polymer pair. The model is based on the transient network theory, in which the activation processes of adsorption and desorption are considered to occur at the wall in parallel to the stretching of the adsorbed chains. It is shown that the stick–slip transition occurs due to the local non-monotonic flow behavior near the wall irrespective of the mechanism of slip. The model predictions of the critical wall shear stress are in good agreement with experimentally observed values of the critical stress for various adhesive energies of wall polymer pair. Another important prediction of the model is that the temperature dependence of the critical wall shear stress for debonding is different than that of disentanglement mechanism under certain experimental conditions. This may be useful for discerning the correct mechanism of slip. The unified model encompasses different systems (viz. entangled solutions and melts) and diverse mechanisms (viz. disentanglement and debonding) in a common mathematical framework.

A NEW ANALYTIC SOLUTION OF THE NAVIER-STOKES EQUATIONS FOR MICROCHANNEL FLOWS

In this article we present a new analytic solution of the Navier-Stokes equations for microchannel flows. The solution is based on the concept of the continuum approach using the Chapman-Enskog method, but built upon the proposal to introduce a hyperbolic tangent function of Kn number in the power series of the distribution function and slip boundary condition. The physics behind the mathematical modification are discussed. With the slip boundary condition accurate to O(tanh(Kn)), the solution of the Navier-Stokes equations is extended successfully to the transition flow regime. The analytic solutions are compared with results of DSMC in both slip flow and transition flow regimes. Satisfactory agreements on the velocity profiles and pressure distributions have been achieved. The extension of the upper Knudsen number limits of continuum approach is significant in molecular gas dynamics.

Interfacial Flow Behavior of Highly Entangled Polybutadiene Solutions

Strong flow behavior of entangled polymer solutions is investigated to illustrate a correlation between linear viscoelastic properties and interfacial slip flow behavior. It is found that entangled polybutadiene (PBD) solutions (made of 1,4-PBD and oligomeric BD of varying vinyl contents) exhibit an interfacial stick−slip transition at a critical stress about one-third of their plateau modulus. The magnitude of the transition, as quantified by the extrapolation length b, depends strongly on the level of chain entanglement which is determined by the concentration φ and reflected in the magnitude of the solution viscosity η. Analogous to the case of polymer melts, the experimental data reveal the same molecular weight dependence for b as that for η measured at a constant stress. The φ dependence of b is universal in the sense that its scaling behavior b ∝ φ 3 holds independently of temperature and the vinyl content of oligomeric BD solvent. The φ dependence of η is however more complicated because it depend...

Fatigue behavior of Fe-48at.%al polycrystals with B2 structure at high temperatures

In FeAl alloys with the B2 structure, slip transition from at low temperature to at high temperature occurs depending on crystal orientation, alloying composition and lattice defects such as excess thermal vacancies. The slip transition strongly influences the strength, ductility and fracture mode in these alloys. According to recent results using FeAl single crystals containing a low density of excess thermal vacancies, yield stress increased with increasing temperature showing an anomalous peak between 823 and 873K. The anomalous strengthening peak corresponded to the slip transition: superlattice dislocations were dominantly operative at temperatures below the peak, while dislocations appeared above the peak. In this article, the authors report on the fatigue behavior of an Fe-48at.%Al polycrystalline alloy deformed at high temperatures, focusing on the effect of anomalous strengthening and the transition in slip direction.

Slip in Entangled Polymer Melts. 1. General Features

Apparent violations of the no-slip boundary condition are studied using a series of narrow molecular-weight distribution polybutadiene melts (67300 ≤ M̄n ≤ 650000), subjected to plane-Couette shearing over clean silica glass surfaces. Simultaneous measurements of slip velocity and shear stress reveal several new molecular characteristics of slip in entangled polymers. log−log plots of slip velocity versus shear stress display three distinct power-law regimes: (i) A weak slip regime at low shear stresses that is characterized by extrapolation/slip lengths b of the order of a few micrometers; (ii) A stick-slip regime at intermediate shear stresses marked by periodic oscillations in slip velocity and shear stress; (iii) A strong slip regime beyond a defined critical shear stress σ*. Slip violations in this last regime are characterized by large slip velocities and massive extrapolation lengths (b∞ ∼ 100−1500 μm). For all polymers studied the critical stress σ* for the weak-to-strong slip transition is found to be proportional to the plateau modulus Ge of the bulk polymer, σ* ≈ (0.2 ± 0.02) Ge. This finding is consistent with a shear-induced polymer disentanglement explanation for apparent slip violations in entangled polymers. Our experimental observations are also found to be in good agreement with a recently proposed scaling theory for friction and slip in entangled polymers, which assumes noninteracting surface chains. We rationalize this last result in terms of a polymer adsorption model in which a single macromolecule spontaneously attaches to numerous surface sites, yet offers a sufficiently long tail to resist relative motion of a chemically identical bulk polymer that attempts to slide over it.

Slip band propagation and slip vector transition in B2 FeAl single crystals

At the peak temperature, 823K, of the yield strength anomaly of B2 FeAl, slip band propagation and slip vector transition were investigated using Fe–39mol% Al single crystals. The single crystal oriented along a compression axis close to the [123] direction showed serrated flow during work-hardening at small strains (<2%). Coarse slip bands propagate in the single crystal specimens, like Lüders band propagation in polycrystals, in the initial strain range. The slip vector was identified by TEM to be parallel to 〈111〉 in the early stage of strain corresponding to yielding. Beyond plastic strains of about 3%, serrations disappeared and significant work-softening occurred. The slip vector responsible for the later stage of deformation was observed to be 〈100〉. That is, the slip vector changes from 〈111〉 to 〈100〉 as the plastic strain increases. Because the density of 〈111〉 superdislocations is found to be very low after the slip transition, glide decomposition of 〈111〉 superdislocations is believed to the primary source mechanism for 〈100〉 dislocations. The slip band propagation in B2 single crystals is discussed in comparison with that of other intermetallic single-crystalline materials.

NON-LINEAR SPRING RESISTANCE AND FRICTION DAMPING OF FRICTIONAL CONSTRAINT HAVING TWO-DIMENSIONAL MOTION

Abstract A model is proposed to investigate the contact kinematics of a frictional constraint experiencing two-dimensional relative motion. In this model, a contact plane is defined and its orientation is invariant. In addition, the contact normal load is assumed constant. In this study, analytical criteria are developed to determine the transitions between stick and slip which characterize how the friction force relates to the two-dimensional relative motion. Using the stick–slip transition criteria, a stick–slip diagram of elliptical motion is developed. This stick–slip diagram illustrates the fundamental characteristics of the two-dimensional contact kinematics when the relative motion has an elliptical trajectory. Fourier series expansion is employed to divide the induced periodic friction force into two components: non-linear spring resistance and friction damping. In this study, a set of non-linear functions that relate the non-linear spring resistance and friction damping to the elliptical motion are developed. It is shown that these non-linear functions can be analytically derived for the two extreme cases: circular motion and one-dimensional motion. The single-term harmonic balance scheme along with the non-linear spring resistance and friction damping of the frictional constraint are then used to calculate the resonant response of a frictionally constrained two-degree-of-freedom oscillator. The accuracy of the method is demonstrated by comparing the results with those of the direct time integration method.

Extrudate swell behavior of polyethylenes: Capillary flow, wall slip, entry/exit effects and low-temperature anomalies

This paper describes a comprehensive experimental study of the extrudate swell behavior of high-density (HDPE) and linear low-density polyethylenes. The contributions from bulk melt elasticity, entry extensional flow, and exit boundary discontinuity are each explicitly investigated as a function of molecular weight and its distribution. The elusive apparent molecular weight dependence of the transient extrudate swell ratio (ESR) is elucidated to arise from the different molecular relaxation rates. A master curve is obtained at a fixed stress for the time-dependent ESR of different molecular weights when represented in terms of the rescaled time, t/τ, where t is the actual time allowed for growth of ESR and τ is related to the overall molecular relaxation time of each HDPE. The intricate influence of the exit boundary discontinuity on the extrudate swell is illustrated on the basis of the recently accumulated precise knowledge of wall slip behavior of linear polyethylenes (LPE). Wall slip induced by fluoropolymer coatings localized at the die exit wall is demonstrated to reduce the ESR of LPE under all stresses below an interfacial stick–slip transition. The entry effect is characterized in two ways: by comparing the ESR values below and above the stick–slip transition of the HDPE at the critical stress, and by comparing the ESR from a long die of aspect ratio L/D=15 with that from an orifice die of L/D=1/4. The HDPE samples are found to exhibit an anomalously larger ESR above a critical stress at low temperatures (from 150 to 160 °C).

Decomposition of a〈111〉 and a〈101〉 dislocations in hard-oriented NiAl single crystals

The B2 intermetallic compound NiAl and its alloys have been subjects of extensive study for potential use as high temperature structural materials, particularly in the aerospace industry. The factors that make them attractive candidates are their low density (5.85 g/cm{sup 3}), excellent oxidation and corrosion resistance and high melting points. But they suffer from lack of ductility at low temperatures and poor strength at higher temperatures. There are still several issues regarding deformation behavior in NiAl and other B2 alloys (e.g., FeAl) that merit further research. Study of the deformation behavior in NiAl should provide further insight into some of the outstanding issues in B2 alloys: chiefly, the nature of slip transition and the effect of various parameters such as temperature, strain rate and stoichiometry on the deformation behavior. The preferred slip system in NiAl involves a slip. But on stressing along the cube axes, also known as the hard orientation, single crystals of NiAl deform at much higher stresses through the motion of a dislocations at lower temperatures. Non-a slip dominates at higher temperatures, with an associated increase in ductility and dramatic decrease in strength. There is also some confusion regarding the mode of deformation beyond the slipmore » transition temperature. The exact process of this slip transition needs to be addressed in its entirety to completely understand the mechanical response of NiAl.« less

Effects of Deviation from Stoichiometry on Deformation Behavior of Hard-Oriented NiAl Single Crystals

ABSTRACTNi-44A1, Ni-50Al and NiAl-0.3 at.% Hf single crystals have been studied in compression to understand the effects that alloying additions and deviation from stoichiometry can have on the mechanical response of NiAl-based single crystals. While all three single crystals deform through a&lt;111&gt; slip at lower temperatures, the active slip systems differ at higher temperatures. Climb of a&lt;010&gt; dislocations contributes to deformation in Ni-50AI single crystals beyond the slip transition temperature, while Ni-44Al and NiAl-0.3Hf crystals deform through a&lt;101&gt; glide. But several microstructural differences have been observed in the mode of deformation between Ni-44Al and NiAl-0.3Hf crystals. In addition, significant strengthening is exhibited in the Hf-doped crystals at higher temperatures. The post-deformation microstructure is also observed to be sensitive to both strain and strain rate. A possible explanation is offered for some of the observed differences in deformation behavior between the three alloys.

Wall slip and absence of interfacial flow instabilities in capillary flow of various polymer melts

We conduct a comprehensive experimental study of melt/wall interfacial slip behavior of various polymer melts in capillary flow. Polymers under study include two polystyrenes of different molecular weights (MW=280 000 and MW=2×106, respectively), two low-density polyethylenes (LDPE), poly(ethylene vinyl acetate) (EVA), and polypropylene (PP). The experimental results reveal an important finding that has greatly extended our previous knowledge of the roles of melt/wall interfacial interactions and molecular entanglements in dictating capillary melt flow behavior. None of the six polymers exhibits an interfacial stick–slip transition or shows any sign of wall slip in bare aluminum dies. Yet, all are found to display a sizable wall slip when the strength of polymer/surface adsorption is reduced by lowering the die wall surface energy with a fluoropolymer coating. The degree of wall slip as characterized by the Navier–de Gennes extrapolation length is demonstrated to be explicitly proportional to the melt viscosity, such that the magnitude of wall slip in the coated die decreases with increasing shear thinning. Diminishing of a measurable wall slip at high stresses explains the well-known absence of spurt, flow oscillation, and sharkskin phenomena in capillary flow of commonly encountered polymers such as PS, LDPE, PP.

Exploring molecular origins of sharkskin, partial slip, and slope change in flow curves of linear low density polyethylene

This paper explores the molecular mechanism for sharkskin formation on extrudate of linear low density polyethylenes (LLDPE) and investigates the rheological origin of a characteristic curvature (i.e., a slope change) in the flow curve of LLDPE. Rheological measurements, performed at various temperatures from 160 to 240 °C with a controlled‐pressure capillary rheometer and a variety of dies, suggest that the slope change in the flow curve, interpreted by many as demonstrating wall slip in the die land, arises from a combination of interfacial slip and cohesive failure due to chain disentanglement, first initiated on the die wall in the exit region. Since the disentanglement state is unstable for the adsorbed chains within a certain stress range below the critical stress for the global stick–slip transition, a partial slip flow cannot sustain itself and occurs only periodically. This time‐dependent molecular entanglement–disentanglement fluctuation produces the sharkskin like extrudate in the regime where the slope change takes place. Sharkskin dynamics are found to precisely correlate with chain relaxation processes. Specifically, the characteristic time scale τ (i.e., the sharkskin periodicity) is found to be of the same magnitude and have the same WLF (Williams–Landel–Ferry) temperature dependence as that of the characteristic molecular relaxation time τ* as determined by oscillatory shear measurements in a parallel‐plate flow cell. The LLDPE resins are also observed to undergo interfacial stick–slip transitions as well as a rarely seen cohesive slip–slip transitions at various temperatures.

Stick−Slip Transition in Capillary Flow of Polyethylene. 2. Molecular Weight Dependence and Low-Temperature Anomaly

The present study focuses on molecular characteristics and temperature dependence of a superfluid-like stick−slip transition in a series of highly entangled high-density polyethylene (PE) resins and their rheological behavior in pressure-driven capillary flow. It is found that at sufficiently high stresses the capillary flow characteristics are largely dictated by the stick−slip transition occurring at the PE/die wall interfaces. The transition magnitude is found to have an explicit and strong molecular weight Mw dependence reminiscent of bulk chain entanglement, i.e., the extrapolation length bc ∝ Mw3.4. In addition to its normal temperature dependence for T ≥ 200 °C as described in paper 1, the critical stress for the stick−slip transition ceases to scale linearly with T from 200 down to 160 °C for all four molecular weights studied. The extrapolation length bc at the transition also becomes much larger at a lower T. The anomaly is clearly interfacial in nature and may involve a flow-induced ordered mes...

Slip Transition and Dislocation Structures in Off-Stoichiometric NiAl Single Crystals

ABSTRACTCompression tests have been performed on hard-oriented Ni-44A1 single crystals for several temperatures and strain levels. There is a slip transition from a&lt;111&gt; type slip to non-a&lt;111&gt; type slip which corresponds with the observation of yield points. Transmission electron microscopy studies provide evidence for decomposition of a&lt;111&gt; dislocations into a&lt;101&gt; and a&lt;010&gt; dislocations near the “knee” of the yield strength curve. The mechanism of this slip transition and the nature of the dislocation processes both below and above the “knee” are described.

Superfluid-Like Stick−Slip Transition in Capillary Flow of Linear Polyethylene Melts. 1. General Features

This paper explores rheological characteristics of and molecular mechanism for a superfluid-like stick−slip transition occurring under controlled pressure in capillary flow of a series of highly entangled linear polyethylene (PE) melts and establishes its connection with the spurt flow phenomenon. The transition is signified by a large discontinuity in the flow rate at a critical stress, resulting in a double value within the flow curve. The magnitude of the transition can be quantified in terms of an extrapolation length, b. In particular, the superfluid-like flow transition occurs throughout a range of temperatures from T = 180 to 260 °C as long as a critical stress, σc, is exceeded. It is found that σc increases linearly with T, and bc at the transition remains around 1.7 mm at all the temperatures for the PE (MH20) of weight-average molecular weight Mw = 316 600. Thus the observed remarkably large interfacial slip is believed to be due to complete disentanglement of the adsorbed chains from free chain...

Yield Stress Anomaly in B2 FeAl

ABSTRACTOur studies on yield stress anomaly of B2 FeAI single crystals are reviewed in this paper. A positive temperature dependence of yield stress, so-called “yield stress anomaly”, is observed in B2 FeAI in which excess vacancies are fully annealed out. Associated with the anomaly, characteristic asymmetry is found between tension and compression. While the strain-rate sensitivity is almost zero in the temperature range of the yield stress anomaly, the stress relaxation becomes significant with increasing temperature, indicating that a recovery process is thermally activated. It is ascertained by the two-surface trace analysis that slip transition from &lt;111&gt; direction at intermediate temperature to &lt;100&gt; at high temperature occurs around the peak temperature. Even at the peak temperature, in addition, operative slip vector for yielding is confirmed to be predominantly &lt;111&gt; by TEM. Also, it is observed that &lt;111&gt;-type superdislocations are frequently climb-dissociated in the temperature range of the anomaly. APB formation on {111} plane is energetically favorable, which is in agreement with the Flinn's calculation for the B2 superlattice that APB energy on {111} plane is lower than that on {110} plane. Such an anisotropy of APB energy would offer specific driving force for the climb dissociation on &lt;111&gt; superdislocations. On the basis of the observed results, the anomalous strengthening behavior of B2 FeAI single crystals is discussed.

Wall Slip of a Polymer Melt on a Polymer Brush

AbstractWe have investigated simple shear flows of poly-dimethyl-siloxane (PDMS) melts on PDMS brushes with well controlled grafting density Σ, using optical techniques based on near field laser anemometry to measure the local velocity of a polymer melt at a solid interface. We have observed a slip transition from a weak slip regime to a strong one as the shear rate crosses a critical value γ*. The influence of the molecular weight of the melt Mm, of the molecular weight of the brush chains Mb and of Σ on γ*, has been investigated to elucidate the molecular mechanisms. The results are compared with an existing molecular model.