Influence of Martensite Content and Morphology on Tensile and Impact Properties of High-Martensite Dual-Phase Steels

A series of dual-phase (DP) steels containing finely dispersed martensite with different volume fractions of martensite (V-m) were produced by intermediate quenching of a boron- and vanadium-containing microalloyed steel. The volume fraction of martensite was varied from 0.3 to 0.8 by changing the intercritical annealing temperature. The tensile and impact properties of these steels were studied and compared to those of step-quenched steels, which showed banded microstructures. The experimental results show that DP steels with finely dispersed microstructures have excellent mechanical properties, including high impact toughness values, with an optimum in properties obtained at similar to 0.55 V-m. A further increase in V-m was found to decrease the yield and tensile strengths as well as the impact properties. It was shown that models developed on the basis of a rule of mixtures are inadequate in capturing the tensile properties of DP steels with V-m > 0.55. Jaoul-Crussard analyses of the work-hardening behavior of the high-martensite volume fraction DP steels show three distinct stages of plastic deformation.

Effects of particle size distribution in the response of model granular materials in multi-ring shear

Rail track undergoes complex loading patterns under moving traffic conditions compared to roads due to its continued and discontinued multi-layered structure, including rail, sleepers, ballast layer, sub-ballast layer, and subgrade. Particle size distributions (PSDs) of ballast, subballast, and subgrade layers can be critical in cyclic plastic deformation of rail track under moving traffic on frequent track degradation of rail tracks, especially at bridge transition zones. Conventional test approaches: static shear and cyclic single-point load tests are however unable to replicate actual loading patterns of moving train. Multi-ring shear apparatus; a new type of torsional simple shear apparatus, which can reproduce moving traffic conditions, was used in this study to investigate influence of particle size distribution of rail track layers on cyclic plastic deformation. Three particle size distributions, using glass beads were examined under different loading patterns: cyclic sin-gle-point load, and cyclic moving wheel load to evaluate cyclic plastic deformation of rail track under different loading methods. The results of these tests suggest that particle size distributions of rail track structural layers have significant impacts on cyclic plastic deformation under moving train load. Further, the limitations in con-ventional test methods used in laboratories to estimate the plastic deformation of rail track materials lead to underestimate the plastic deformation of rail tracks.

Dry sliding wear behaviour of magnesium alloy based hybrid composites in the longitudinal direction

In the present investigation, the wear behaviour of a creep-resistant AE42 magnesium alloy and its composites reinforced with Saffil short fibres and SiC particles in various combinations is examined in the longitudinal direction i.e., the plane containing random fibre orientation is perpendicular to the steel counter-face. Wear tests are conducted on a pin-on-disc set-up under dry sliding condition having a constant sliding velocity of 0.837 m/s for a constant sliding distance of 2.5 km in the load range of 10-40 N. It is observed that the wear rate increases with increase in load for the alloy and the composites, as expected. Wear rate of the composites is lower than the alloy and the hybrid composites exhibit a lower wear rate than the Saffil short fibres reinforced composite at all the loads. Therefore, the partial replacement of Saffil short fibres by an equal volume fraction of SiC particles not only reduces the cost but also improves the wear resistance of the composite. Microstructural investigation of the surface and subsurface of the worn pin and wear debris is carried out to explain the observed results and to understand the wear mechanisms. It is concluded that the presence of SiC particles in the hybrid composites improves the wear resistance because these particles remain intact and retain their load bearing capacity even at the highest load employed, they promote the formation of iron-rich transfer layer and they also delay the fracture of Saffil short fibres to higher loads. Under the experimental conditions used in the present investigation, the dominant wear mechanism is found to be abrasion for the AE42 alloy and its composites. It is accompanied by severe plastic deformation of surface layers in case of alloy and by the fracture of Saffil short fibres as well as the formation of iron-rich transfer layer in case of composites.

A simulation study of the mixing, atomic flow and velocity profiles of crystalline materials during sliding

Self-contained Non-Equilibrium Molecular Dynamics (NEMD) simulations using Lennard-Jones potentials were performed to identify the origin and mechanisms of atomic scale interfacial behavior between sliding metals. The mixing sequence and velocity profiles were compared via MD simulations for three cases, viz.: sell-mated, similar and hard-softvcrystal pairs. The results showed shear instability, atomic scale mixing, and generation of eddies at the sliding interface. Vorticity at the interface suggests that atomic flow during sliding is similar to fluid flow under Kelvin-Helmholtz instability and this is supported by velocity profiles from the simulations. The initial step-function velocity profile spreads during sliding. However the velocity profile does not change much at later stages of the simulation and it eventually stops spreading. The steady state friction coefficient during simulation was monitored as a function of sliding velocity. Frictional behavior can be explained on the basis of plastic deformation and adiabatic effects. The mixing layer growth kinetics was also investigated.

Molecular dynamics simulations of sliding in an Fe-Cu tribopair system

Experimental evidence suggests that high strain rates, stresses, strains and temperatures are experienced near sliding interfaces. The associated microstructural changes are due to several dynamic an interacting phenomena. 3D non-equilibrium molecular dynamics (MD) simulations of sliding were conducted with the aim of understanding the dynamic processes taking place in crystalline tribopairs, with a focus on plastic deformation and microstructural evolution. Embedded atom potentials were employed for simulating sliding of an Fe-Cu tribopair. Sliding velocity, crystal orientation and presence of lattice defects were some of the variables in these simulations. Extensive plastic deformation involving dislocation and twin activity, dynamic recrystallization, amorphization and/or nanocrystallization, mechanical mixing and material transfer were observed. Mechanical mixing in the vicinity of the sliding interface was observed even in the Fe-Cu system, which would cluster under equilibrium conditions, hinting at the ballistic nature of the process. Flow localization was observed at high velocities implying the possible role of adiabatic heating. The presence of preexisting defects (such as dislocations and interfaces) played a pivotal role in determining friction and microstructural evolution. The study also shed light on the relationship between adhesion and plastic deformation, and friction. Comparisons with experiments suggest that such simulations can indeed provide valuable insights that are difficult to obtain from experiments.

Experimental study of the damping coefficient variations in zinc crystal

Much work has been done on obtaining empirical stress-velocity relations and evaluating the temperature dependence and activation energy of plastic deformation /1, 2/. Another prevalent concept is that of the drag coefficient and its variation with degree of crystal imperfection /3/. Significant differences and discrepancies exist in the reported values /2, 4/. Although it is recognised that the yield point is caused by point interstitials and aggregates, little has been done on the evaluation of specific crystal-solute combinations and interaction parameters. Some of the first efforts, in this direction were performed by Wain and Cottrell /5/.

Wear lip formation during dry sliding

A pin-on-disc test configuration has been used to examine the formation of the strain-hardened projection, or wear lips, especially at the trailing edge of the pin during dry sliding of aluminium alloys against steel discs. The mechanism of formation of such wear lips is studied with the aid of optical and electron microscopes. The plastic deformation of the pin, growth and eventual removal of the wear lip as wear debris are elucidated. The size and shape of the wear lips in pins of different shapes, i.e. square, rectangular, triangular and circular cross-sections, are described.

On the hardness of shear bands in amorphous alloys

The nanoindentation hardness of individual shear bands in a Zr-based metallic glass was investigated in order to obtain a better understanding of how shear band plasticity is influenced by non-crystalline defects. The results clearly showed that the shear band hardness in both as-cast and structurally relaxed samples is much lower than the respective hardness of undeformed region. Interestingly, inter-band matrix also exhibited lower hardness than undeformed region. The results are discussed in terms of the influence of structural state and the prevailing mechanism of plastic deformation.

Free-volume dependent pressure sensitivity of Zr-based bulk metallic glass

Instrumented indentation experiments on a Zr-based bulk metallic glass (BMG) in as-cast, shot-peened and structurally relaxed conditions were conducted to examine the dependence of plastic deformation on its structural state. Results show significant differences in hardness, H, with structural relaxation increasing it and shot peening markedly reducing it, and slightly changed morphology of shear bands around the indents. This is in contrast to uniaxial compressive yield strength, sigma(y), which remains invariant with the change in the structural state of the alloys investigated. The plastic constraint factor, C = H/sigma(y), of the relaxed BMG increases compared with that of the as-cast glass, indicating enhanced pressure sensitivity upon annealing. In contrast, C of the shot-peened layer was found to be similar to that observed in crystalline metals, indicating that severe plastic deformation could eliminate pressure sensitivity. Microscopic origins for this result, in terms of shear transformation zones and free volume, are discussed.

High-strength bulk Al-based bimodal ultrafine eutectic composite with enhanced plasticity

An in situ bulk ultrafine bimodal eutectic Al-Cu-Si composite was synthesized by solidification. This heterostructured composite with microstructural length scale hierarchy in the eutectic microstructure, which combines an ultrafine-scale binary cellular eutectic (alpha-Al + Al2Cu) and a nanometer-sized anomalous ternary eutectic (alpha-Al + Al2Cu + Si), exhibits high fracture strength (1.1 +/- 0.1 GPa) and large compressive plastic strain (11 +/- 2%) at room temperature. The improved compressive plasticity of the bimodal-nanoeutectic composite originates from homogeneous and uniform distribution of inhomogeneous plastic deformation (localized shear bands), together with strong interaction between shear bands in the spatially heterogeneous structure.

Constitutive equation for elevated temperature flow behavior of plain carbon steels using dimensional analysis

Dimensional analysis using π-theorem is applied to the variables associated with plastic deformation. The dimensionless groups thus obtained are then related and rewritten to obtain the constitutive equation. The constants in the constitutive equation are obtained using published flow stress data for carbon steels. The validity of the constitutive equation is tested for steels with up to 1.54 wt%C at temperatures: 850–1200 °C and strain rates: 6 × 10−6–2 × 10−2 s−1. The calculated flow stress agrees favorably with experimental data.

Microstructural stability and creep in nanocrystals

Microstructural stability is an important consideration during high temperature deformation and processing of nanomaterials. We will address issues relating to triple junctions in limiting grain growth during creep as well as densification. Although early studies on deformation have considered diffusion creep as a possible rate controlling deformation mechanism in nanocrystals, a critical inspection of available data indicates that there is no strong evidence for conventional diffusion creep in such materials. The possibility of diffusion creep by rapid diffusion along triple junctions will be analyzed, and interface controlled diffusion creep will also be discussed critically. It is shown that the critical grain size for dislocation activity is similar to that for occurrence of conventional diffusion creep.

Room-temperature equal channel angular extrusion of pure magnesium

In this paper, we demonstrate a way to impart severe plastic deformation to magnesium at room temperature to produce ultrafine grain size of similar to 250 nm through equal channel angular extrusion (ECAE). The strategy to deform magnesium at lower temperature or to achieve such grain sizes has been proposed as: (i) to obtain a suitable initial orientation with high Schmid factor for basal slip and low Schmid factor for pyramidal/prismatic slip; (ii) to take advantage of low stacking fault energy of basal and high stacking fault energies of prismatic/pyramidal planes in order to relatively work-harden the basal plane with respect to the pyramidal/prismatic plane; and (iii) to lower the temperature of deformation in steps, leading to continual refinement of grains, resulting in finer grain size. The experimental as well as simulated texture of ECAE-processed samples indicate that the deformation mechanism leading to ultrafine grain size is slip-dominated. The recrystallization mechanism during ECAE has been found to be orientation-dependent. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The Evolution of Tribomaterial During Sliding: A Brief Introduction

This brief introductory article summarizes key findings from experiments and from computer simulations concerning the dramatic changes that commonly occur adjacent to sliding interfaces. We conclude that a wide range of observed features depends on a few basic processes (plastic deformation, interactions with the environment (including the counterface) and mechanical mixing) and that sliding leads to flow patterns similar to those expected in fluid flow.

An elastic-viscoplastic constitutive model for the hot-forming of aluminum alloys

Under hot-forming conditions characterized by high homologous temperatures and strain-rates, metals usually exhibit rate-dependent inelastic behavior. An elastic-viscoplastic constitutive model is presented here to describe metal behavior during hot-forming. The model uses an isotropic internal variable to represent the resistance offered to plastic deformation by the microstructure. Evolution equations are developed for the inelastic strain and the deformation resistance based on experimental results. A methodology is presented for extracting model parameters from constant true strain-rate compression tests performed at different temperatures. Model parameters are determined for an Al-1Mn alloy and an Al-Mg-Si alloy, and the predictions of the model are shown to be in good agreement with the experimental data. (C) 2000 Kluwer Academic Publishers.