Ultra-fine Grain Materials by Severe Plastic Deformation: Application to Steels

Severe plastic deformation techniques are known to produce grain sizes up to submicron level. This leads to conventional Hall-Petch strengthening of the as-processed materials. In addition, the microstructures of severe plastic deformation processed materials are characterized by relatively lower dislocation density compared to the conventionally processed materials subjected to the same amount of strain. These two aspects taken together lead to many important attributes. Some examples are ultra-high yield and fracture strengths, superplastic formability at lower temperatures and higher strain rates, superior wear resistance, improved high cycle fatigue life. Since these processes are associated with large amount of strain, depending on the strain path, characteristic crystallographic textures develop. In the present paper, a detailed account of underlying mechanisms during SPD has been discussed and processing-microstructure-texture-property relationship has been presented with reference to a few varieties of steels that have been investigated till date.

High-Temperature (1023 K to 1273 K 750 degrees C to 1000 degrees C]) Plastic Deformation Behavior of B-Modified Ti-6Al-4V Alloys: Temperature and Strain Rate Effects

A minor addition of B to the Ti-6Al-4V alloy, by similar to 0.1 wt pct, reduces its as-cast prior beta grain size by an order of magnitude, whereas higher B content leads to the presence of in situ formed TiB needles in significant amounts. An experimental investigation into the role played by these microstructural modifications on the high-temperature deformation behavior of Ti-6Al-4V-xB alloys, with x varying between 0 wt pct and 0.55 wt pct, was conducted. Uniaxial compression tests were performed in the temperature range of 1023 K to 1273 K (750 degrees C to 1000 degrees C) and in the strain rate range of 10(-3) to 10(+1) s(-1). True stress-true strain responses of all alloys exhibit flow softening at lower strain rates and oscillations at higher strain rates. The flow softening is aided by the occurrence of dynamic recrystallization through lath globularization in high temperature (1173 K to 1273 K 900 degrees C to 1000 degrees C]) and a lower strain rate (10(-2) to 10(-3) s(-1)) regime. The grain size refinement with the B addition to Ti64, despite being marked, had no significant effect on this. Oscillations in the flow curve at a higher strain rate (10(0) to 10(+1) s(-1)), however, are associated with microstructural instabilities such as bending of laths, breaking of lath boundaries, generation of cavities, and breakage of TiB needles. The presence of TiB needles affected the instability regime. Microstructural evidence suggests that the matrix cavitation is aided by the easy fracture of TiB needles.

Curing reactions in plastic prepolymers and propellants. I. Polystyrene

Curing reactions of the viscous PS prepolymer and PS/AP propellant slurry have been studied. The molecular weight of the binder (separated from the propellant) and the prepolymer was found to increase to a maximum value, remain constant for some time, and then fall off between 50–125°C. The molecular weight of the binder was found to be less than corresponding prepolymer between 100–150°C but at lower temperatures (50–75°C) the reverse was found to be true. The increase in the molecular weight during curing at lower temperatures has been explained on the basis of Trommsdorff effect which gets support from the estimated activation energy (9 kcal mole−1) for the curing process. Curing was recognized as chain extension where the rate of polymerization becomes diffusion controlled below 75° C.

A method for estimation of the radius of elastic-plastic boundary around cold-worked holes

The radius of an elastic-plastic boundary was measured by the strain gage method around the cold-worked region in L72-aluminum alloy. The relative radial expansion was varied from 2.5 to 6.5 percent during the cold-working process using mandrel and split sleeve. The existing theoretical studies in this area are reviewed. The experimental results are compared with existing experimental data of various investigators and with various theoretical formulations. A model is developed to predict the radius of elastic-plastic boundary, and the model is assessed by comparing with the present experiments.

Continuity of plastic strain rate in stress relaxation tests

Stress relaxation testing is often utilised for determining whether athermal straining contributes to plastic flow; if plastic strain rate is continuous across the transition from tension to relaxation then plastic strain is fully thermally activated. This method was applied to an aged type 316 stainless steel tested in the temperature range 973–1123 K and to a high purity Al in the recrystallised annealed condition tested in the temperature range 274–417 K. The results indicated that plastic strain is thermally activated in these materials at these corresponding test temperatures. For Al, because of its high strain rate sensitivity, it was necessary to adopt a back extrapolation procedure to correct for the finite period that the crosshead requires to decelerate from the constant speed during tension to a dead stop for stress relaxation.

Examination of Stress-Fields In Elastic Plastic Indentation

A block of high-purity copper was indented by a 120-degrees diamond-tipped cone. Strain gauges were placed on the surface to measure the radial strains at different surface locations, during loading as well as unloading. The competence of three stress fields proposed for elastic-plastic indentation is assessed by comparing the predicted surface radial strains with those experimentally observed.

The scratch hardness and friction of a soft rigid-plastic solid

The paper describes an experimental and analytical study of the normal and scratch hardnesses of a model soft rigid-plastic solid. The material known as ‘Plasticine’, a mixture of dry particles and a mineral oil, has been deformed with a range of rigid conical indentors with included angles of between 30° and 170°. The sliding velocity dependence of the computed scratch hardness and friction has been examined in the velocity range 0.19 mm/s to 7.3 m/s. Data are also described for the time dependence of the normal hardness and also the estimated rate dependence of the intrinsic flow stress. The latter values were estimated from data obtained during the upsetting of right cylinders. Three major conclusions are drawn from these data and the associated analysis. (1) A first-order account of the scratching force may be provided by adopting a model which sums the computed plastic deformation and interfacial sliding contributions to the total sliding work. This is tantamount to the adoption of the two-term non-interacting model of friction. (2) For this system during sliding, at high sliding velocities at least, the interface shear stress which defines the boundary condition is not directly related to the bulk shear stress. The interface rheological characteristics indicate an appreciable dependence on the imposed strain or strain rate. In particular, the relative contributions of the slip and stick boundary conditions appear to be a function of the imposed sliding velocity. (3) The computed normal and scratch hardness values are not simply interrelated primarily because of the evolving boundary conditions which appear to exist in the scratching experiments.

Influence of water and thermal history on ion transport in lithium salt-succinonitrile plastic crystalline electrolytes

Important issues of water and thermal history affecting ion transport in a representative plastic crystalline lithium salt electrolyte: succinonitrile (SN)-lithium perchlorate (LiClO4) are discussed here. Ionic conductivity of electrolytes with high lithium salt amounts (similar to 1 M) in SN at a particular temperature is known to be influenced both by the trans-gauche isomerism and ion association (solvation), the two most important intrinsic parameters of the plastic solvent. In the present study both water and thermal history influence SN and result in enhancement of ionic conductivity of 1 M LiClO4-SN electrolyte. Systematic observations reveal that the presence of water in varying amounts promote ion-pair dissociation in the electrolyte. While trace amounts (approximate to 1-15 ppm) do not affect the trans-gauche isomerism of SN, the presence of water in large amounts (approximate to 5500 ppm) submerges the plasticity of SN. Subjugating the electrolyte to different thermal protocol resulted in enhancement of trans concentration only. This is an interesting observation as it demonstrates a simple and effective procedure involving utilization of an optimized set of external parameters to decouple solvation from trans-gauche isomerism. Observations from the ionic conductivity of various samples were accounted by changes in signature isomer and ion-association bands in the mid-IR regime and also from plastic to normal crystal transition temperature peak obtained from thermal studies. (C) 2010 Elsevier B.V. All rights reserved.

A crosslinked ``polymer-gel'' rechargeable lithium-ion battery electrolyte from free radical polymerization using nonionic plastic crystalline electrolyte medium

A cross-linked polymer-gel soft matter electrolyte with superior electrochemical, thermal and mechanical properties obtained from free radical polymerization of vinyl monomers in a semi-solid organic nonionic plastic crystalline electrolyte for application in rechargeable lithium-ion batteries is discussed here.

Stress-strain response of plastic waste mixed soil

Recycling plastic waste from water bottles has become one of the major challenges worldwide. The present study provides an approach for the use plastic waste as reinforcement material in soil. The experimental results in the form of stress-strain-pore water pressure response are presented. Based on experimental test results, it is observed that the strength of soil is improved and compressibility reduced significantly with addition of a small percentage of plastic waste to the soil. The use of the improvement in strength and compressibility response due to inclusion of plastic waste can be advantageously used in bearing capacity improvement and settlement reduction in the design of shallow foundations. (C) 2010 Elsevier Ltd. All rights reserved.

Multiscale Modeling Approach to Acoustic Emission during Plastic Deformation

We address the long-standing problem of the origin of acoustic emission commonly observed during plastic deformation. We propose a framework to deal with the widely separated time scales of collective dislocation dynamics and elastic degrees of freedom to explain the nature of acoustic emission observed during the Portevin-Le Chatelier effect. The Ananthakrishna model is used as it explains most generic features of the phenomenon. Our results show that while acoustic emission bursts correlated with stress drops are well separated for the type C serrations, these bursts merge to form nearly continuous acoustic signals with overriding bursts for the propagating type A bands.

Dielectric Relaxation Spectroscopy for Evaluation of the Influence of Solvent Dynamics on Ion Transport in Succinonitrile-Salt Plastic Crystalline Electrolytes

Influence of succinonitrile (SN) dynamics on ion transport in SN-lithium perchlorate (LiClO4) electrolytes is discussed here via dielectric relaxation spectroscopy. Dielectric relaxation spectroscopy (similar to 2 x 10(-3) Hz to 3 MHz) of SN and SN-LiClO4 was studied as a function of salt content (up to 7 mol % or 1 M) and temperature (-20 to +60 degrees C). Analyses of real and imaginary parts of permittivity convincingly reveal the influence Of trans gauche isomerism and solvent-salt association (solvation) effects on ion transport. The relaxation processes are highly dependent on the salt concentration and temperature. While pristine SN display only intrinsic dynamics (i.e., trans-gauche isomerism) which enhances with an increase in temperature, SN-LiClO4 electrolytes especially at high salt concentrations (similar to 0.04-1 M) show salt-induced relaxation processes. In the concentrated electrolytes, the intrinsic dynamics was observed to be a function of salt content, becoming faster with an increase in salt concentration. Deconvolution of the imaginary part of the permittivity spectra using Havriliak-Negami (HN) function show a relaxation process corresponding to the above phenomena. The permittivity data analyzed using HN and Kohlrausch-Williams-Watta (KWW) functions show non-Debye relaxation processes and enhancement in the trans phase (enhanced solvent dynamics) as a function of salt concentration and temperature.