Showing papers in "Materials Science and Engineering A-structural Materials Properties Microstructure and Processing in 2001"

Magnesium: Properties — applications — potential

Abstract: Magnesium is the lightest of all metals used as the basis for constructional alloys. It is this property which entices automobile manufacturers to replace denser materials, not only steels, cast irons and copper base alloys but even aluminium alloys by magnesium based alloys. The requirement to reduce the weight of car components as a result in part of the introduction of legislation limiting emission has triggered renewed interest in magnesium. The growth rate over the next 10 years has been forecast to be 7% per annum. A wider use of magnesium base alloys necessitates several parallel programs. These can be classified as alloy development, process development/improvement and design considerations. These will be discussed briefly and followed by some examples of the increasing uses of magnesium and future trends.

Improving the mechanical properties of magnesium and a magnesium alloy through severe plastic deformation

Abstract: Pure Mg and Mg alloys generally exhibit only limited ductilities at ambient temperatures. Experiments were conducted to evaluate the potential for improving the mechanical properties of pure Mg and an Mg–0.9% Al alloy at room temperature by subjecting these materials to severe plastic deformation through the procedure of equal-channel angular pressing (ECAP). It is shown that ECAP may be applied successfully to these materials at elevated temperatures and this leads to grain refinement due to the occurrence of recrystallization during the pressing process and to significant improvements in the strength and ductility of these materials. Since these improvements are apparent after only a single pass through the ECAP die, it is concluded that the introduction of ECAP provides a simple and effective procedure for improving the ambient temperature mechanical properties of materials, such as hcp metals, where the measured ductilities are generally limited.

Short fiber reinforced composites for fused deposition modeling

Abstract: Addressed in this paper are critical material property issues related to the short fiber reinforced composite used in rapid prototyping and manufacturing (RP&M). Acrylonitrile–butadiene–styrene (ABS) copolymer has been a popular choice of material used in fused deposition modeling (FDM), a commonly used RP&M process. However, conventional ABS polymers in the filamentary form for FDM are known to be of low strength and hardness. In order to overcome this deficiency, ABS was modified by incorporating several different property modifiers including the short glass fiber, plasticizer, and compatibilizer. Glass fibers were found to significantly improve the strength of an ABS filament at the expense of reduced flexibility and handleability. The latter two properties of glass fiber reinforced ABS filaments were improved by adding a small amount of plasticizer and compatibilizer. The resulting composite filament, prepared by extrusion, was found to work well with a FDM machine.

The science and technology of mechanical alloying

Abstract: Mechanical alloying (MA) is a powder metallurgy processing technique involving cold welding, fracturing, and rewelding of powder particles in a high-energy ball mill, and has now become an established commercial technique to produce oxide dispersion strengthened (ODS) nickel- and iron-based materials. MA is also capable of synthesizing a variety of metastable phases, and in this respect, the capabilities of MA are similar to those of another important non-equilibrium processing technique, viz., rapid solidification processing (RSP). However, the “science” of MA is being investigated only during the past 10 years or so. The technique of mechanochemistry, on the other hand, has had a long history and the materials produced in this way have found a number of technological applications, e.g., in areas such as hydrogen storage materials, heaters, gas absorbers, fertilizers, catalysts, cosmetics, and waste management. The present paper discusses the basic mechanisms of formation of metastable phases (specifically supersaturated solid solutions and amorphous phases) by the technique of MA and these aspects are compared with those of RSP. Additionally, the variety of technological applications of mechanically alloyed products are highlighted.

Modelling of TWIP effect on work-hardening

Abstract: Austenitic steels can exhibit both high strength and ductility due to a particularly high work hardening rate. Among all the possible deformation modes for austenitic steels, Twinning Induced Plasticity (TWIP) has the most beneficial effect on the work-hardening. It is believed that deformation twins increase the work-hardening rate by acting as obstacles for gliding dislocations. Many studies have investigated this point experimentally using microscopy. On a physical basis, the purpose of this study is to develop a work-hardening model taking into account the interaction between twinning and dislocation gliding. The results from the model are in good agreement with the tensile test results.

Influence of ECAP routes on the microstructure and properties of pure Ti

Abstract: Equal channel angular pressing (ECAP) is an innovative technique that can produce bulk ultrafine-grained (UFG) materials in product forms large enough for structural applications. It is well known that ECAP route, defined by the sequence of orientations of the billets relative to the die during the iterative ECAP passes, significantly affects the microstructural development of the work piece. Studies reported in the literature have so far focused on fcc metals such as Al and Cu. In this work, we have studied the influence of ECAP routes on the microstructures and properties of hcp commercially-pure Ti. Three ECAP routes, conventionally defined as BA, BC and C, were used to process the Ti billets. Surface quality, microstructures, microhardness, tensile properties, anisotropy, and thermal stability were studied. The route BC is shown to be the best route for processing hcp Ti.

Review: reaction synthesis processing of Ni–Al intermetallic materials

Abstract: Nickel aluminide intermetallic compounds possess attractive properties that make them good candidates for high temperature structural applications. The production of these materials in short processing times and low energy via reaction synthesis routes has been reviewed in this article including most recent novel reactive methods applied to Ni–Al intermetallics (e.g. microwave combustion synthesis and hot extrusion reaction synthesis). Future directions in this field have been suggested.

Magnesium strengthened by SiC nanoparticles

Abstract: Mg was reinforced with SiC nanoparticles by powder metallurgical technique. The SiC nanoparticles were generated by laser-induced gas phase reaction in a flow reactor and had a median particle diameter of 30 nm. In order to distribute the nanoparticles in the Mg matrix, Mg micropowder with a median particle diameter of 40 μm was mixed or ball milled with the nanoscaled ceramic powder followed by hot extrusion. The mechanical properties of the new material were investigated by tensile tests and creep measurements and the microstructure of the composites were examined by light and transmission electron microscopy (TEM).

Precipitation hardening in Al–Mg–Si alloys with and without excess Si

Abstract: The aluminum alloys of the 6xxx series contain an excess of Si above that required to form stoichiometric Mg 2 Si, which is added to improve the age hardening due mostly to precipitation of metastable β″ precipitates. The excess Si is not believed to alter the precipitation sequence, structure and lattice parameters of the different metastable precursors, but rather promotes formation of additional particles/phases which do not contribute to hardening significantly. The presence of excess Si changes the composition and density of metastable β″ particles, although a systematic study of the Mg/Si ratio in particles from alloys of different composition is lacking. In this paper, it is shown that the precipitation sequence in the balanced alloy is independent of the composition and the strength increases with Mg 2 Si level. This is due primarily to both a higher volume fraction and a refined distribution of the β″ particles. Excess Si increases the effective amount of the hardening phases above ∼0.9 wt.% Si. It modifies the Mg/Si ratio in the clusters/zones and β″ precipitates and improves strength by altering their size, number density and distribution. In addition, the extent and rate of strengthening increases until the overall Mg to Si ratio in the alloy is close to approximately 0.4. The hardening precipitates with reduced Mg to Si ratio become less stable with aging and cause a decrease in strength during over aging.

An investigation of the sliding wear behavior of Cu-matrix composite reinforced by carbon nanotubes

Abstract: The friction and wear behavior of Cu-matrix composite reinforced by carbon nanotubes (Nanotube/Cu composite) were investigated. By scanning electron microscopy (SEM) and X-ray diffraction (XRD), the worn surfaces and the worn chips were analyzed. The volume fraction of nanotubes is a main factor for the decrease of the wear rate of The Nanotube/Cu composite, which is associated with carbon nanotubes forming a protective oxide film on the sliding surface of the specimen. The optimum nanotubes content is 12–15%. Both the coefficients of friction and weight loss of the Nanotube/Cu composite are lower than those for Cu-matrix composite reinforced by carbon fiber (CF/Cu) owing to the much high intensity of nanotube.

Shear localization in dynamic deformation of materials: microstructural evolution and self-organization

Abstract: The plastic deformation of crystalline and non-crystalline solids incorporates microscopically localized deformation modes that can be precursors to shear localization. Shear localization has been found to be an important and sometimes dominant deformation and fracture mode in metals, fractured and granular ceramics, polymers, and metallic glasses at high strains and strain rates. Experiments involving the collapse of a thick walled cylinder enable controlled and reproducible application of plastic deformation at very high strain rates to specimens. These experiments were supplemented by hat-shaped specimens tested in a compression Hopkinson bar. The initiation and propagation of shear bands has been studied in metals (Ti, Ta, Ti–6Al–4V, and stainless steel), granular and prefractured ceramics (Al2O3 and SiC), a polymer (teflon) and a metallic glass (Co58Ni10Fe5Si11B16). The first aspect that was investigated is the microstructural evolution inside the shear bands. A fine recrystallized structure is observed in Ti, Cu, Al–Li, and Ta, and it is becoming clear that a recrystallization mechanism is operating. The fast deformation and short cooling times inhibit grain-boundary migration; it is shown, for the first time, that a rotational mechanism, presented in terms of dislocation energetics and grain-boundary reorientation, can operate within the time of the deformation process. In pre-fractured and granular ceramics, a process of comminution takes place when the particles are greater than a critical size ac. When they are smaller than ac, particle deformation takes place. For the granular SiC, a novel mechanism of shear-induced bonding was experimentally identified inside the shear bands. For all materials, shear bands exhibit a clear self-organization, with a characteristic spacing that is a function of a number of parameters. This self-organization is analyzed in terms of fundamental material parameters in the frame of Grady–Kipp (momentum diffusion), Wright–Ockendon, and Molinari (perturbation) models. © 2001 Elsevier Science B.V. All rights reserved.

Bulk amorphous and nanocrystalline alloys with high functional properties

Abstract: Recent progress of bulk amorphous, nanocrystalline and nanoquasicrystalline alloys in rod, sheet and ring forms produced by various casting processes has been reviewed by focusing on their formation, structure, mechanical strength, chemical properties, magnetic properties and applications. These bulk nonequilibrium alloys exhibit unique characteristics which cannot be obtained for conventional amorphous, crystalline and quasicrystalline alloys and have been commercialized in some application fields of electrodes and golf clubs, etc. The combination of new compositions, direct production to final material forms, novel structures and unique characteristics is promising for the future development of these nonequilibrium bulk alloys as basic science and engineering materials.

Experimental study of porosity and its relation to fatigue mechanisms of model Al–Si7–Mg0.3 cast Al alloys

Abstract: The microstructure and fatigue properties of three model AS7G03 cast aluminium alloys containing artificial pores have been studied. Synchrotron X-ray tomography has been used to characterise in three dimensions the pore population in the alloys. The development of fatigue cracks in relation with local crystallography has been studied by means of electron back scattered diffraction (EBSD). Both the average number of cycles to failure and the lifetime scatter depend on the pore content specially at high stress level. The mechanism leading to the initiation of a crack from a pore has been identified. The crack propagation at high stress level appears to be quite insensitive to microstructural barriers and can be reasonably well described by a Paris type law. At low stresses, however, short cracks are often observed to be stopped at grain boundaries and the fatigue life is no longer predicted by a simple propagation law.

Thermal stresses and deposition patterns in layered manufacturing

Abstract: In layered manufacturing, objects are constructed by sequential deposition of material layers. When the deposition process involves temperature gradients, thermal stresses develop. This paper examines the effect of deposition patterns on the resulting stresses and deflections in laser deposited metal parts. Finite element modeling of the deposition processes showed that the deposition pattern has a significant effect on the part stresses and deflections. Experiments performed using these same deposition patterns yielded sample deflections, which were in agreement with the finite element modeling predictions.

Microstructure and properties of pure Ti processed by ECAP and cold extrusion

Abstract: Equal channel angular pressing (ECAP) has been used to refine the grain size of commercially pure (CP) Ti as well as other metals and alloys. CP-Ti is usually processed at about 400°C because it lacks sufficient ductility at lower temperatures. The warm processing temperature limits the capability of the ECAP technique in improving the strength of CP-Ti. We have employed cold extrusion following warm ECAP to further refine the grains and improve the strength of CP-Ti. Ti billets were first processed for eight passes via ECAP route BC, with a clockwise rotation of 90° between adjacent passes. They were further processed by successive cold extrusions to an accumulative reduction in cross-section area by 47 or 75%. This paper reports the surface quality, microstructures, microhardness, tensile properties, and thermal stability of these Ti billets processed by a combination of ECAP and cold extrusion.

Localized heating during serrated plastic flow in bulk metallic glasses

Abstract: We have studied the serrated plastic flow observed in Zr40Ti14Ni10Cu12Be24 and Pd40Ni40P20 bulk metallic glass alloys tested in uniaxial compression. Quantitative measurements with sufficient temporal resolution to record the fine-scale structure of the data are reported. These data are used to predict temperature increases in single shear bands due to local adiabatic heating caused by the work done on the sample during plastic deformation. Since the predicted temperature increases are on the order of only a few degrees Kelvin, it seems unlikely that localized heating is the primary cause of flow localization. Instead, changes in viscosity associated with increased free volume in the shear band seem more consistent with experiment. Substantial shear band heating is, however, predicted for final failure, as corroborated by evidence of melting on the fracture surface.

Effects of solution treatment and continuous cooling on σ-phase precipitation in a 2205 duplex stainless steel

Abstract: A commercial 2205 duplex stainless steel with three different solution treatments (at 1020, 1080 and 1200°C for 3 min) followed by continuous cooling at four respective cooling rates (1, 0.5, 0.25 and 0.1°C s −1 ) has been investigated by means of optical metallography, transmission electron microscopy, electron probe microanalysis, X-ray diffraction and differential thermal analysis. It is found that the lower solution treatment temperature with the lower cooling rate significantly enhanced the σ phase transformation. The σ precipitate could be detected when the specimens were solution-treated at 1020 and 1080°C and cooled at the rate of 0.25°C s −1 ; substantial amounts of σ phase formed when the specimens were cooled at 0.1°C s −1 . The precipitation of σ phase was considerably retarded as the solution temperature increased from 1080 to 1200°C. The precipitation behaviors of σ phase and M 23 C 6 carbide were characterized by transmission electron microscopy. The results indicated that the Cr- and Mo-rich σ phase preferentially nucleated at the pre-formed M 23 C 6 carbide particles, which were located at δ / γ interface or within δ ferrite grain. The selected area diffraction patterns displayed the complicated crystallography of σ phase, and revealed the orientation relationships between the interfacial σ precipitate and adjacent matrix phases.

Solution heat treatment response of a third generation single crystal Ni-base superalloy

Abstract: The standard solution heat treatment of the third generation, single crystal Ni-base superalloy, CMSX-10, requires temperatures up to 1365°C and lasts a total of approximately 45 h. These high temperatures and long times result in a heat treatment that is costly. To determine if the heat treatment could be simplified and/or shortened to reduce the cost, a detailed study was completed on the standard heat treatment. The solution anneal heat treatment dissolves the eutectic γ/γ′ regions early in the heat treatment cycle at temperatures up to about 1340°C. However, the chemical segregation from the partitioning of elements during solidification, was not eliminated until much higher temperatures were reached. In particular, the segregation of W and Re to the dendrite core was not significantly reduced until temperatures in excess of 1360°C were reached in the heat treatment cycle. Reducing the heat treatment temperature and/or shortening the time of the heat treatment would be expected, therefore, to result in residual segregation of W and Re to the dendrite cores, a locally unstable microstructure, and, possibly, the formation of TCP phases.

A comparative study of elastic constants of Ti-Ni-based alloys prior to martensitic transformation

Abstract: Single crystal elastic constants of Ti–Ni alloys without (quenched) and with (aged) Ti 3 Ni 4 precipitates were measured systematically by rectangular parallelepiped resonance method as a function of composition and temperature, and compared with Ti–Ni–Cu and Ti–Ni–Fe alloys, in an attempt to answer some long-standing questions as to the origin of the unique monoclinic B19′ martensite, and why composition and thermomechanical treatment greatly changes the path of martensitic transformation. The results showed that softening in c 44 , in additional to c ′, is a common feature for all Ti–Ni binary (both quenched or aged) and ternary alloys. This general feature just corresponds to the fact that all these alloys ultimately transform into B19′, suggesting that softening in c 44 is responsible for the unique B19′ martensite, which found no analogy in other β phase alloys. We also found an interesting correspondence between the temperature dependence of anisotropy factor and transformation path. Prior to B2–B19′ transformation anisotropy shows a decrease with lowering temperature; prior to B2–B19 an anisotropy increase, while prior to B2–R transformation a constant anisotropy. We further showed that three possible martensite candidates (R, B19, B19′) are rooted in anomalies in specific phonon modes and elastic softening. We showed that the multi-stage transformations are restricted by a general rule: multi-stage transformation occurs in the sequence of increasing transformation strain. With this rule we can explain all known transformation paths by considering the effect of alloying addition and fine precipitates/dislocation network on relative stability of different martensites. We further predict that there may exist a new transformation path in Ti–Ni-based alloys: B2–R–B19–B19′.

Quantitative evaluation of critical cooling rate for metallic glasses

Abstract: Evaluation of critical cooling rate R c for glass formation was carried out quantitatively for metallic glasses. A fundamental equation proposed for oxide glasses was modified for metallic glasses. In the calculation both Gibbs free energy of the molten alloys and the effect of the differences in atomic size were taken into account. The calculation was carried out for three groups: pure metals, typical glass-forming systems, and the latest metallic glasses with a large glass-forming ability found after 1990. The calculated results were in agreement with previous data. As examples, the R c for Ni metal, Co- and Pd–Cu-based metallic glasses in the present work were 9.1×10 8 , 1.2×10 5 and 1.6×10 −2 K/s , respectively, instead of 3×10 10 ,3.5×10 5 and 1×10 −1 K/s in the previous work. The following points are clarified: (1) T m 2 / η characterizes R c as is typical for the Pd–Cu-based metallic glass with a low melting point as well as high viscosity at the melting point; (2) negative heats of mixing and atomic size mismatch above about 12% affect the reduction of R c from 10 −2 to 10 −7 and from 10 −1 to 10 −2 ; and (3) the reduction of R c is understood as a stabilization of liquid state.

Nanostructure formation on the surface of railway tracks

Abstract: The microstructure of the surface layer of railway tracks is investigated. It is shown that the surface layer of the rail transforms during exploitation into a nanocrystalline Fe–C alloy. The mechanism of the nanostructure formation is discussed. It is shown that the transformation of pearlite to the nanostructured Fe–C alloy layer is caused by the heavy plastic deformation at the wheel–rail contact zone. The transformation of the microstructure of the surface may take place at rail–wheel contact temperatures less than 230°C and its mechanism is similar to that taking place during mechanical alloying.

The potential for scaling ECAP: effect of sample size on grain refinement and mechanical properties

Abstract: The potential for scaling equal-channel angular pressing (ECAP) for use with large samples was investigated by conducting tests on an aluminum alloy using cylinders having diameters from 6–40 mm. The results show the refinement of the microstructure and the subsequent mechanical properties after pressing are independent of the initial size of the sample and, for the largest sample with a diameter of 40 mm, independent of the location within the sample at least to a distance of ∼5 mm from the sample edge. By making direct measurements of the imposed load during ECAP, it is shown that the applied load is determined by the sample strength rather than frictional effects between the sample and the die walls. The results demonstrate the feasibility of scaling ECAP to large sizes for use in industrial applications.

Fabrication and evaluation of plasma sprayed nanostructured alumina-titania coatings with superior properties

Abstract: Reconstituted nanostructured powders were plasma sprayed using various processing conditions to produce nanostructured alumina‐titania coatings. Properties of the nanostructured coatings were related to processing conditions through a critical plasma spray parameter (CPSP) that in turn, can be related to the amount of unmelted powder incorporated into the final coating. Those coatings that retain a significant amount of unmelted powder show optimum microstructure and properties. Selected physical and mechanical properties were evaluated by X-ray diffraction (XRD), optical and electron microscopy, quantitative image analysis and mechanical testing. Constituent phases and the microstructure of the reconstituted particles and plasma sprayed coatings were examined with the aid of quantitative image analysis as a function of processing conditions. Mechanical properties including hardness, indentation crack growth resistance, adhesion strength, spallation resistance during bend- and cup-tests, abrasive wear resistance and sliding wear resistance were also evaluated. These properties were compared with a commercial plasma sprayed alumina‐titania coating with similar composition. Superior properties were demonstrated for nanostructured alumina‐titania coatings plasma sprayed at optimum processing conditions. © 2001 Elsevier Science B.V. All rights reserved.

Effects of pitting corrosion on the fatigue behavior of aluminum alloy 7075-T6: modeling and experimental studies

Abstract: The effects of pitting corrosion on the fatigue behavior of bare 7075-T6 aluminum alloy were investigated. Pitting corrosion decreased the fatigue lives by a factor of about 6 to 8. The fatigue lives were also calculated assuming an equivalent initial flaw corresponding to pits of average and maximum dimensions. The measured fatigue lives generally agreed with the predictions using the average rather than the maximum pit size as the initial crack size. This result could be explained by the pit size distributions offering a significantly larger population of pits near the average size. This work has demonstrated the promise of standardized spray tests for obtaining quantitative measures of corrosion that can be used as inputs in analytical models for fatigue life prediction for evaluating integrity of aircraft structures.

Dislocation densities, arrangements and character from X-ray diffraction experiments

Abstract: X-ray diffraction peak profile analysis has become a powerful tool during the last two decades for the characterisation of microstructure either in the bulk or in loose powder materials. The evaluation and modelling procedures have developed together with the experimental techniques. It will be shown that the different features of diffraction peak profiles such as (i) broadening, (ii) asymmetric peak shape, (iii) peak shifts and (iv) anisotropic broadening provide a variety of microstructural parameters by modelling crystallite size and strain. Modelling strain by assuming dislocations will be more extensive. Two different procedures will be considered: (1) evaluation by using characteristic parameters of individual peak profiles, especially the FWHM, the integral breadths and the Fourier coefficients and (2) multiple whole profile fitting (MWPF) procedure using ab initio size and strain functions scaled by the contrast factors of dislocations. The two procedures will be discussed and illustrated by different case studies.

Mechanisms of grain growth in nanocrystalline fcc metals by molecular-dynamics simulation.

Abstract: To elucidate the mechanisms of grain growth in nanocrystalline fcc metals, we have performed fully three-dimensional molecular-dynamics simulations with a columnar grain structure and an average grain diameter of 15 nm. Based on the study of coarse-grained materials, the conventional picture is that grain growth is governed by curvature-driven grain-boundary migration. However, our simulations reveal that in a nanocrystalline material grain rotations play an equally important role, at least during the early stages of grain growth. By eliminating the grain boundary between neighboring grains, such rotations lead to grain coalescence and the consequent formation of highly elongated grains. A detailed analysis exposes an intricate coupling between this mechanism and the conventional grain-boundary-migration dominated mechanism. Incorporation of these insights into mesoscopic models should enable more realistic mesoscopic simulations of grain growth in nanocrystalline materials. (A short movie showing the overall evolution of the grain microstructure can be viewed at http://www.msd.anl.gov/im/movies/graingrowth.html.)

Development of a micromechanical life prediction model for plasma sprayed thermal barrier coatings

Abstract: A widely used method to produce thermal barrier coating (TBC) systems is the vacuum plasma spraying of a highly dense bondcoat layer with a defined surface roughness and the atmospheric plasma spraying (APS) of a porous (10–15%) Y 2 O 3 -stabilized zirconia top coat. In thermal cycling operation these systems often fail by crack initiation and propagation close to the bondcoat–top coat interface. This failure is attributed to stresses arising from the formation of a thermally grown oxide (TGO) layer on the rough bondcoat surface. The actual stress situation is rather complex due to TGO formation, creep effects in both bondcoat and top coat and due to the roughness of the bondcoat. All these factors have been take into account in the present work by using a finite element method (FEM) to calculate stress development during thermal loading. These results can then be introduced into a crack propagation model to estimate crack development during the thermal cycling operation. The predictions of this approach are compared to experimental results on the influence of bondcoat roughness on coating life. In these experiments TBC systems with bondcoat layers having three different levels of roughness were cycled in a gas burner rig until failure.

Magnetic and X-ray diffraction measurements for the determination of retained austenite in TRIP steels

Abstract: The accurate determination of the volume fraction of retained austenite is of great importance for the optimization of transformation induced plasticity (TRIP) steels. In this work, two aluminium-containing TRIP steels are studied by means of magnetization and X-ray diffraction (XRD) measurements. By fitting the field dependence of the approach to saturation in the magnetization curves, the saturation magnetization is determined, which is linearly related to the volume fraction of retained austenite. Moreover, information with respect to the microstructure can be obtained from the fitting parameters and the demagnetizing factor for the magnetization curve. The volume fractions obtained from the magnetization measurements are compared with data from XRD measurements. A discussion of the data suggests that magnetization measurements lead to more reliable results and a more sensitive detection of the retained austenite than XRD measurements.

Non-basal slip systems in HCP metals and alloys: source mechanisms

Abstract: A possible source mechanism for non-basal 〈c+a〉 slip dislocations is proposed based on the formation of an attractive junction between glissile 〈a〉 and sessile c dislocations from the prism plane into a pyramidal plane. The driving force for the junction formation, which comes from the long-range elastic interaction between c and 〈a〉 dislocations, is relatively large in most hexagonal close-packed (hcp) metals. Beryllium, which has an unusually low Poisson's ratio, is an exception to this rule. The cross-slip process is energetically unfavorable in Mg and Ti, from a viewpoint of the change in anisotropic elastic line tension, but it becomes favorable in Ti at elevated temperatures above 300°C. Discussion is given on intrinsic stacking fault energies and kinetic aspects of the cross slip.

Tailoring of interfaces in glass fiber reinforced polymer composites: a review

Abstract: An overview of research on the characterization of interfaces in glass fiber and particulate silica reinforced organic polymers is presented. Descriptions of the chemical and mechanical nature of the siloxane interphases that bond the polymer matrix to the reinforcing phase and their effects on the mechanical properties of fiber reinforced composites are described. While protection of the reinforcement from environmental damage and improvement of adhesion between phases have been well documented, neither the characterization of the properties of the interphase nor its consequences on the durability of a composite material have been definitively resolved. It is generally agreed, however, that synthetically created interphases are often mechanical weak links and a potential source for the initiation of defects in a structure. Recent research has focused on the attainment of monomolecular, reactive surface layers that form strong chemical bonds with both constituents. In this case, the polymeric ‘interphases’ are reduced in dimension to as close to a true ‘interface’ as molecular dimensions allow. Recent work in this area and speculation on the properties required are presented.