Analysis of acoustic emission during the melting of embedded indium particles in an aluminum matrix: a study of plastic strain accommodation during phase transformation

Acoustic emission is used here to study melting and solidification of embedded indium particles in the size range of 0.2 to 3 um in diameter and to show that dislocation generation occurs in the aluminum matrix to accommodate a 2.5% volume change. The volume averaged acoustic energy produced by indium particle melting is similar to that reported for bainite formation upon continuous cooling. A mechanism of prismatic loop generation is proposed to accommodate the volume change and an upper limit to the geometrically necessary increase in dislocation density is calculated as 4.1 x 10^9 cm^-2 for the Al-17In alloy. Thermomechanical processing is also used to change the size and distribution of the indium particles within the aluminum matrix. Dislocation generation with accompanied acoustic emission occurs when the melting indium particles are associated with grain boundaries or upon solidification where the solid-liquid interfaces act as free surfaces to facilitate dislocation generation. Acoustic emission is not observed for indium particles that require super heating and exhibit elevated melting temperatures. The acoustic emission work corroborates previously proposed relaxation mechanisms from prior internal friction studies and that the superheat observed for melting of these micron-sized particles is a result of matrix constraint.Comment: Presented at "Atomistic Effects in Migrating Interphase Interfaces - Recent Progress and Future Study" TMS 201

An acoustic emission study of martensitic and bainitic transformations in carbon steel

Steel is one of the most commonly used materials today, especially in industrial sectors such as ship building and the automotive industry. In order to meet the requirements for steel applications, new multi-phase steels are being developed. The microstructure of these steels consists of a variety of different phases, which leads to superior material properties - a combination of high strength with good formability. For the development of such steels research is required to gain more insight into the underlying microstructure and the mechanisms by which it is formed. This thesis describes unique acoustic emission experiments during martensitic and bainitic transformations in steel. The main objective of this work is to obtain a better understanding of the growth mechanism and kinetics of these solid-state phase transformations that can occur in carbon steel. In view of fact that acoustic emission is an unexplored technique in this kind of steel research, this study also aims to give a good overview of the possibilities and limitations of acoustic emission as a real-time monitoring technique for the evolution of bainite and martensite formation.Applied Science

Modeling Start Curves of Bainite Formation

It is demonstrated that calculations with a physically based model give an accurate description of the start curve of bainite formation in a wide range of steels. The temperature dependence of the overall kinetics, which determines the characteristic C shape of the start curve, is controlled by both the undercooling below the start temperature (T h ? T) and an effective activation energy Q b . A systematic analysis of the model parameters extracted from the best fits of published time-temperature-transformation (TTT) data reveals a material-independent relationship, which means that the activation energy is in accordance with known details of the dislocation-based nucleation model of bainite. It is shown that the C shape of the start curve can be determined for a given alloying content using an empirical relationship derived for Q b and, in combination with the material-independent relationship, the kinetics of bainite can be predicted at all temperature levels.Materials Science and EngineeringMechanical, Maritime and Materials Engineerin

Acoustic emission monitoring of bainite formation during continuous coating

This paper concerns acoustic emission (AE) measurements during continuous cooling of steel C45 using a Gleeble 1500 thermo-mechanical simulator. After austenizing at a certain temperature, the studied specimen was cooled down and the root mean square (rms) voltage ($V_{\rm rms}$) of the AE signals was measured. During cooling two distinct peaks in the $V_{\rm rms}$ data were observed at temperatures of 500–600 °C and 200–300 °C, which are attributed to bainite and martensite formation respectively. The observed bainite peak unambiguously implies that the mechanism of growth of bainite is displacive, i.e. is martensitic in nature. From the ratio of the martensite peak to the bainite peak information can be obtained about the amounts of martensite and bainite formed. The AE monitoring of bainite and martensite formation is supported by dilatation measurements, which were performed simultaneously. The effect of the austenite grain size on the evolution of the bainite and martensite formation was studied by varying the austenizing temperature

Experimental observations elucidating the mechanisms of structural bcc-hcp transformations in ?-Ti alloys

The formation mechanisms of two hcp ? phase morphologies in Ti-4.5Fe-6.8Mo-1.5Al have been investigated by optical microscopy (OM), atomic force microscopy (AFM), electron probe microanalysis (EPMA) and dilatometry. At relatively high temperatures primary ? forms predominantly on prior bcc ? grain boundaries, whereas at lower temperatures so-called bainitic ? plates nucleate both at grain boundaries and intragranularly. This morphological transition with decreasing temperature is associated with a change in transformation mechanism. The combined results of EPMA, OM and dilatometry show that the growth of these bainitic ? plates is partitionless, and not accompanied by a volume change. Subsequently, a posttransformation redistribution of Fe takes place, which causes a dilatation that can be modelled based on the diffusion of Fe and the interface-area density. This mechanism as well as the formed microstructure are similar to bainite in steel, and therefore we chose to denote this transformation product as bainitic ?. In addition, the AFM observations on bainitic ? plates show an invariant plane strain surface relief with tilt angles that are consistent with the Burgers’ transformation model based on shear. In contrast, the AFM results show that the formation of primary ? is accompanied by an irregular dip on a free surface, which is in agreement with the volume decrease measured using dilatometry. Furthermore, the EPMA results show that primary ? is formed by a partitioning transformation. The change in transformation mechanism with decreasing temperature is supported by the observed trend in both the dilatation and the volume fraction ? as a function of temperature.Materials Science and EngineeringMechanical, Maritime and Materials Engineerin

Charge transport in doped polythiophene

The conductive properties of the conjugated polymer polythiophene doped with FeCl3 are studied as a function of frequency co/2n, electric field E, temperature T and dopant concentration c. The data show a transition from quasi-one-dimensional (for low doping levels) to three dimensional (for high doping) variable range hopping. At all doping levels the macroscopic carrier mobility µ is determined by inter-chain hopping

Martensite Formation in Partially and Fully Austenitic Plain Carbon Steels

The progress of martensite formation in plain carbon steels Fe-0.46C, Fe-0.66C, and Fe-0.80C has been investigated by dilatometry. It is demonstrated that carbon enrichment of the remaining austenite due to intercritical annealing of Fe-0.46C and Fe-0.66C does not only depress the start temperature for martensite, but also slows the progress of the transformation with temperature compared to full austenitization. In contrast, such a change of kinetics is not observed when the remaining austenite of lean-Si steel Fe-0.80C is stabilized due to a partial transformation to bainite, which suggests that the stabilization is not of a chemical but of a mechanical nature. The growth of bainite and martensite is accompanied by a shape change at the microstructural scale, which leads to plastic deformation and thus strengthening of the surrounding austenite. Based on this stabilizing mechanism, the athermal transformation kinetics is rationalized by balancing the increase in driving force corresponding to a temperature decrease with the increase in strain energy required for the formation of martensite in the strengthened remaining austenite.Materials Science and EngineeringMechanical, Maritime and Materials Engineerin