Plastic deformation behavior of β-phase isotactic polypropylene in plane-strain compression at room temperature

Isotactic polypropylene (iPP) rich in β crystal modification (constituting 92% of crystalline phase) was deformed by the plane-strain compression with constant true strain rate, at room temperature. The evolution of phase structure, morphology and orientation was studied by DSC, X-ray and SEM.

The deformation sequence and the active deformation mechanisms were found out. The most important mechanisms were interlamellar slip operating in the amorphous layers, resulting in numerous fine deformation bands due to localization of deformation and the crystallographic slip systems, including the (110)[001] chain slip and (110)[10] transverse slip.

Shear within deformation bands leads to β → smectic and β →  solid state phase transformations. At room temperature the β → smectic transformation appeared to be the primary transformation, yielding the oriented smectic phase with high concentration of 19 wt.% at the true strain of e = 1.49. The β →  yields only about 4 wt.% of new -phase at the same strain. As a result of the deformation and phase transformation within numerous fine deformation bands β-lamellae are locally destroyed and fragmented into smaller crystals.

Another deformation mechanism is the cooperative kinking of lamellae, leading to their reorientation and formation of a chevron-like lamellar arrangement.

At high strains, above e = 1, an advanced crystallographic slip and high stretch of amorphous material due to interlamellar shear bring further heavy fragmentation of lamellar crystals, earlier fragmented partially by deformation bands. This fragmentation is followed by fast rotation of small unconstrained crystallites with chain axis towards the direction of flow, FD. This process leads to development of the final texture of the highly deformed β-iPP with molecular axis of both crystalline and smectic phases oriented along FD.     

Theoretical Model of Impact Damage in Structural Ceramics

This paper presents a mechanistically consistent model of impact damage based on elastic failures due to tensile and shear overloading. An elastic axisymmetric finite element model is used to determine the dynamic stresses generated by a single particle impact. Local failures in a finite element are assumed to occur when the primary/secondary principal stresses or the maximum shear stress reach critical tensile or shear stresses, respectively. The succession of failed elements thus models macrocrack growth. Sliding motions of cracks, which closed during unloading, are resisted by friction and the unrecovered deformation represents the "plastic deformation" reported in the literature. The predicted ring cracks on the contact surface, as well as the cone cracks, median cracks, radial cracks, lateral cracks, and damage-induced porous zones in the interior of hot-pressed silicon nitride plates, matched those observed experimentally. The finite element model also predicted the uplifting of the free surface surrounding the impact site.     

X-Ray Microbeam Studies of Fracture Surfaces in Alumina

Studies using a double-crystal X-ray spectrometer to measure crystal distortion at various depths below the fracture surfaces of polycrystalline alumina have shown two zones of distortion. Zone I, of high distortion, extends about 10μ; zone II, having much lower distortion, extends over 50μ (in excess of one grain diameter). Both zones are observed in specimens fractured at 20° and at 1700°C. The primary difference observed between specimens broken at the two temperatures is that zone I has a much higher misorientation in the 1700°C fractures than in 20°C fractures; zone I1 is quite similar for the two temperatures. Since zone I appears in all reflections and is about one half grain diameter in depth, it is proposed that this distortion arises from basal and nonbasal slip in those grains through which the fracture crack passes. Since zone II is observed only in the (0330) and (2240) reflections, it is proposed that this distortion is the result of basal slip only. From these data the plastic work of fracture is estimated as about 1500 ergs per cm². This estimate is probably high by 10 to 25%. The value for the work of fracture is relatively independent of temperature. The method described in this work permits study of the deformation in the thin layers near the fracture surface in any material.     

Mechanical Behavior of Samhire

The present state of the knowledge of the mechanical behavior of sapphire is reviewed. Sapphire deforms plastically at temperatures above 900°C, the most common mode of deformation being slip on the basal plane in the 〈11     0〉 direction. Under certain conditions, however, slip may occur on prism planes or twinning may occur. A yield point is often observed, which appears to be related to the multiplication of dislocations by a mechanism controlled by the motion of dislocations through the lattice, rather than by the tearing of dislocations from an impurity (or defect) atmosphere. The dynamics of yielding and flow is similar and can be expressed by either of two thermally activated equations. In one, the effect of stress is principally on the pre-exponential factor; in the other, its effect is principally on the activation energy. There are insufficient data to permit a positive decision between the two, or to positively identify the rate-controlling dislocation mechanism. For 60°-oriented crystals fracture generally occurs on a plane approximately normal to the tensile stress and the fracture surface is conchoidal. In the range 1100° to 1500°C, the tensile fracture stress decreases with increase in plastic strain, independent of temperature and strain rate. The mechanism of failure in this case seems to be the interaction of edge dislocations with pre-existing cracks.     

Mechanical Properties of Magnesia Single Crystals Corn pression

Stress-strain curves of single crystals of magnesia compressed in the [100] direction are reported at temperatures from –196° to 1200°C.; curves are also shown for different rates of loading at room temperature. The crystals show considerable ductility at all temperatures and at room temperature can be deformed plastically about 6% before fracture at stresses which are about one-quarter of reported polycrystalline fracture strengths. The macroscopic yield drops apparently exponentially from an extrapolated value of 50,000 lb. per sq. in. at absolute zero to about 4500 lb. per sq. in. at temperatures of 900°C. and higher. Heat-treatment has an appreciable effect on the yield stress. The resistance of the material to deformation increases with the number of slip systems and bands activated because of the barriers to dislocation movements which occur at slip band intersections. At about 2 to 3% strain, stress concentrations begin to be relieved by small internal cracks which are not easily propagated. This effect is extensive before final macroscopic failure of the crystal occurs. Preliminary creep tests above the macroscopic yield stress and in the temperature range 800° to 1000°C. show large instantaneous plastic deformations followed by slow constant-rate creep.     

Decoration of Dislocations by the Precipitation of Alumina in MgO · 2.9Al2O3 Spinel

A thermal etching technique is developed to reveal dislocations in MgO · nAl₂O₃ (n=2.9) single crystals decorated by Al₂O₃ precipitation. Large plastic deformation leads to a larger density of thermal etch pits. Also, dense arrays of pits, parallel to the traces of slip planes with a surface, are observed around a Vickers identation. The cracks developed by an indentation run selectively along the 〈100〉 directions and accompany dislocations arrayed in the possible slip planes.     

In-plane Shear Properties of 2-D Ceramic Matrix Composites

The in-plane shear properties of a range of 2-D ceramic matrix composites have been measured using the Iosipescu configuration. The nonlinear deformation is found to be associated primarily with matrix cracks. Consequently, the shear modulus decreases as strain proceeds. The flow strength in shear is found to be compatible with the stress at which multiple matrix cracking is expected to occur, without interface slip. Consequently, the shear strength scales with the shear modulus. The shear ductility is found to diminish as the matrix modulus increases. This effect is attributed to the influence of matrix modulus on the bending deformation of fibers between matrix cracks. The experimental observations are used to suggest a mechanism map that identifies (class III) materials which redistribute stress around notches by means of shear bands.     

Mechanical Behavior of Lithium Fluoride at High Temperatures

Single crystals of lithium fluoride were pulled in tension at high temperatures; the results of these tests are compared with data for magnesium oxide. Interpenetration of {110}〈110〉 slip systems does not occur in LiF as readily as in MgO. Because of this lack of interpenetrability, plastic instability and completely ductile fracture do not occur in LiF below 700°C (0.87 T _mp); the high-temperature tensile strength of LiF decreased very little from 300° to 700°C. This lack of interpenetrability of slip systems in LiF at high temperatures also had a profound effect on the deformation processes, the development of substructure, and the strain-hardening and fracture characteristics of the material. This work emphasizes the importance of interpenetration of slip not only in the high-temperature ductility and strain-hardening processes but also in the maximum tensile strength which ionic crystals develop before fracture.     

Effects of Crystalline Anisotropy and Indenter Size on Nanoindentation by Multiscale Simulation

Nanoindentation processes in single crystal Ag thin film under different crystallographic orientations and various indenter widths are simulated by the quasicontinuum method. The nanoindentation deformation processes under influences of crystalline anisotropy and indenter size are investigated about hardness, load distribution, critical load for first dislocation emission and strain energy under the indenter. The simulation results are compared with previous experimental results and Rice-Thomson (R-T) dislocation model solution. It is shown that entirely different dislocation activities are presented under the effect of crystalline anisotropy during nanoindentation. The sharp load drops in the load–displacement curves are caused by the different dislocation activities. Both crystalline anisotropy and indenter size are found to have distinct effect on hardness, contact stress distribution, critical load for first dislocation emission and strain energy under the indenter. The above quantities are decreased at the indenter into Ag thin film along the crystal orientation with more favorable slip directions that easy trigger slip systems; whereas those will increase at the indenter into Ag thin film along the crystal orientation with less or without favorable slip directions that hard trigger slip systems. The results are shown to be in good agreement with experimental results and R-T dislocation model solution.     

Plastic deformation in natural diamonds: Rose channels associated to mechanical twinning

Hollow channels in diamond are well acknowledged to be the result of dissolution processes. In this article we demonstrate that some hollow channels in natural diamonds are the consequence of intense plastic deformation by mechanical twinning. Two mixed-habit diamonds presenting numerous geometrical hollow tubes were studied. X-ray Laue analyses showed the presence of microtwins. At the intersection of microtwins, displacements and cracks are generated, creating the hollow channels observed. The presence of the cracks seems to have released the internal stress, as there was less to no signs of deformation at and around them. Further dissolutions are sometimes but not always seen within the cavities. Mechanical twinning, so far mostly identified in pink to purple diamonds, might be more widespread than originally thought in natural diamonds.     

Deformation mechanisms in polytetrafluoroethylene

The deformation of isotropic and oriented polytetrafluoroethylene has been examined by wide-angle X-ray diffraction and electron microscopy. The deformation mechanisms which have been found to operate are 101̄0〈0001〉 chain direction slip and 101̄0〈12̄10〉 transverse slip. At 20°C the critical resolved shear stress for chain direction slip has been found to be about 2.5 MN/m² and that for transverse slip to be about 15 MN/m².     

Contact Damage Accumulation in Tic3SiC2

The evolution of deformation—;microfracture damage below Hertzian contacts in a coarse-grain Ti₃SiC₂ is studied. The Hertzian indentation stress—;strain response deviates strongly from linearity beyond a well-defined maximum, with pronounced strain—;softening, indicating exceptional deformability in this otherwise (elastically) stiff ceramic. Surface and subsurface ceramographic observations reveal extensive quasi-plastic microdamage zones at the contact sites. These damage zones are made up of multiple intragrain slip and intergrain shear failures, with attendant microfracture at high strains. No ring cracks or other macroscopic cracks are observed on or below the indented surfaces. The results suggest that Ti₃SiC₂ may be ideally suited to contact applications where high strains and energy absorption prior to failure are required.     

Investigation of AlN ceramic anisotropic deformation behavior during scratching

To explore the anisotropic deformation behavior of aluminum nitride ceramic during processing, ramp and constant load scratch experiments were conducted with Vickers indenter. Characteristics of deformation anisotropy were observed in ramp-load scratch with differences found in the directions of pile-ups, slip lines and cracks produced by grains with various orientations. Constant-load scratches were conducted to explore the anisotropic deformation behavior in plastic stage. Obvious differences were found in scratch pile-up height and residual depth of grains with different orientations. As the grain orientation Euler angle changes from the basal to the two prism surfaces, the residual depth of the scratch gradually increases. Molecular dynamics simulation was performed to disclose the mechanisms of anisotropic dislocation and deformation during scratching. The activation of the slip system for different grain orientations was calculated by a scratch Schmid factor model which was verified by the observation of pile-ups and slip lines around the scratches.     

Plastic Deformation of Fine-Grained Alumina (Al2O3): I, Interface-Controlled Diffusional Creep

Plastic deformation in fine-grained alumina polycrystals (grain size 1 to 15 μm) was studied. At least three distinct deformation mechanisms are important: diffusional creep, basal slip, and unaccommodated grain-boundary sliding. The first and most important of these processes is addressed in this paper. Analysis of the deformation dynamics suggests that both lattice and grain-boundary diffusion are important in the diffusional creep. Aluminum, rather than oxygen, lattice and grain-boundary diffusion are rate-controlling because oxygen diffusion is very slow in the lattice but very rapid in grain boundaries. Significantly, the diffusional creep can become interfacecontrolled at low stresses, causing the often-reported non-Newtonian creep behavior of fine-grained alumina.     

Deformation Microstructure in (001) Single Crystal Strontium Titanate by Vickers Indentation

Recent interests on the plastic deformation of strontium titanate (SrTiO₃) are derived from its unusual ductile-to-brittle-to-ductile transition (DBDT). The transition is divided into three regimes (A, B, and C) corresponding to the temperature range of 113–1053 K (−160° to 780°C), 1053 to ∼1503 K (780° to ∼1230°C), and ∼1503–1873 K (∼1230° to 1600°C), discovered by Sigle and colleagues in the MPI-Stuttgart. We report the dislocation substructures in (001) single crystal SrTiO₃ deformed by Vickers indentation at room temperature, studied by scanning and transmission electron microscopy. Dislocation dipoles of screw and edge character are observed and confirmed by inside–outside contrast using ± g -vector by weak-beam dark field imaging. They are formed by edge trapping, jog dragging, and cross slip pinching-off. Similar to dipole breaking off in deformed sapphire (α-Al₂O₃) at 1200°C and γ-TiAl intermetallic at room temperature, the dipoles pinch off at one end, and emit a string of loops at trail. Two sets of slip systems {110}〈     〉 and {100}〈011〉 are activated under both 100 g and 1 kg load. The suggestion is that plastic deformation has reached the stage II work hardening, which is characterized by multiplication of dislocations through cross slip, interactions between dislocations, and operating of multiple slip systems.     

Contact Fatigue in Silicon Nitride

A study of contact fatigue in silicon nitride is reported. The contacts are made using WC spheres, principally in cyclic but also in static loading, and mainly in air but also in nitrogen and water. Damage patterns are examined in three silicon nitride microstructures: (i) fine ( F )-almost exclusively fully-developed cone cracks; (ii) medium ( M )-well developed but smaller cone cracks, plus modest subsurface quasi-plastic damage; (iii) coarse ( C )-intense quasi-plastic damage, with little or no cone cracking. The study focuses on the influence of these competing damage types on inert strength as a function of number of contacts. In the F and M microstructures strength degradation is attributable primarily to chemically assisted slow growth of cone cracks in the presence of moisture during contact, although the M material shows signs of enhanced failure from quasi-plastic zones at large number of cycles. The C microstructure, although relatively tolerant of single-cycle damage, shows strongly accelerated strength losses from mechanical degradation within the quasi-plastic damage zones in cyclic loading conditions, especially in water. Implications concerning the design of silicon nitride microstructures for long-lifetime applications, specifically in concentrated loading, are considered.     

Contact-Induced Transverse Fractures in Brittle Layers on Soft Substrates: A Study on Silicon Nitride Bilayers

An analysis of transverse cracks induced in brittle coatings on soft substrates by spherical indenters is developed. The transverse cracks are essentially axisymmetric and geometrically conelike, with variant forms dependent on the location of initiation: outer cracks that initiate at the top surface outside the contact and propagate downward; inner cracks that initiate at the coating/substrate interface beneath the contact and propagate upward; intermediate cracks that initiate within the coating and propagate in both directions. Bilayers consisting of hard silicon nitride (coating) on a composite underlayer of silicon nitride with boron nitride platelets (substrate), with strong interfacial bonding to minimize delamination, are used as a model test system for Hertzian testing. Test variables investigated are contact load, coating/substrate elastic-plastic mismatch (controlled by substrate boron nitride content), and coating thickness. Initiation of the transverse coating cracks occurs at lower critical loads, and shifts from the surface to the interface, with increasing elastic-plastic mismatch and decreasing coating thickness. This shift is accompanied by increasing quasi-plasticity in the substrate. Once initiated, the cracks pop in and arrest within the coating, becoming highly stabilized and insensitive to further increases in contact load, or even to coating toughness. A finite element analysis of the stress fields in the loaded layer systems enables a direct correlation between the damage patterns and the stress distributions: between the transverse cracks and the tensile (and compressive) stresses; and between the subsurface yield zones and the shear stresses. Implications of these conclusions concerning the design of coating systems for damage tolerance are discussed.     

Kinking and Cracking Caused by Slip in Single Crystals of Silicon Carbide

α-SiC single crystals were compressed parallel to the basal plane, (0001), at temperatures between 900° and 1500°C. Plastic deformation by slip on the basal planes which accompanied kinking occurred above 1000°C. At kink boundaries, two kinds of cracks were observed. One was the cracks elongated parallel to the basal plane. This kind of crack was initiated by the tensile stress produced by piled-up dislocations on the basal planes against a kink boundary. The other was on a kink boundary, and was induced by the stress of dislocations, heterogeneously distributed on the kink boundary. The initiation of cracks produced by dislocations was considered to be a possible cause of fracture in polycrystalline SiC at high temperatures.     

Fracture behavior of diamond-like carbon films on stainless steel under a micro-tensile test condition

We investigated the stability of the DLC film coated on 304 stainless steel substrate by r.f. PACVD method. Fracture and spallation behaviors of the coating were observed during micro-tensile test of the film/substrate composite. As the tensile deformation proceeded, the cracks of the film appeared in the perpendicular direction to the tensile axis. Further deformation resulting in the local necking with shear band of 55° inclined to the tensile axis, induced the spallation of the film, which was initiated at the cracks of the film, and was aligned along the slip directions. We found that both the cracking and the spallation behaviors are strongly dependent on the pretreatment condition, such as Ar plasma pretreatment or Si buffer layer deposition. The spallation of the film was significantly suppressed in an optimized condition of the substrate cleaning by Ar glow discharge. These results show that the spallation behavior during the tensile test can be used to estimate the interfacial strength of the coating with relatively poor adhesion.     

Slip Behavior and Hardness Indentations in MnSe and MnSe-MnS Solid Solutions

Single crystals in the solid solution series MnSe-MnS were prepared, and selected surfaces were indented with either a Vickers or a Knoop microindenter. Information on mechanical behavior was obtained by observation of slip traces around indentations and by study of Knoop hardness anisotropy. Manganous selenide exhibits {111}     and {110}     as primary and secondary slip mechanisms, respectively, in contrast to the {110}     mechanism preferred by MnS. In the system MnSe-MnS, the primary slip mechanism changes gradually with composition. The {110}     mechanism of MnS is much more sensitive to temperature than the {111}     mechanism of MnSe, and a large solid solution hardening effect accompanies the substitution of sul-fide for selenide ions.