The role of phase interface energy in martensitic transformations: a lattice Monte-Carlo simulation

To study martensitic phase transformation we use a micromechanical model based on statistical mechanics. Employing lattice Monte-Carlo simulations and realistic material properties for shape-memory alloys (SMA), we investigate the combined influence of the external stress, temperature, and interface energy between the austenitic and martensitic phase on the transformation kinetics and the effective material compliance. The one-dimensional model predicts well many features of the martensitic transformation that are observed experimentally. Particularly, we study the influence of the interface energy on the transformation width and the effective compliance. In perspective, the obtained results might be helpful for the design of new SMAs for more sensitive smart structures and more efficient damping systems.Comment: 10 pages, 3 figures, 22 reference

Damping behaviour of vibrating shape memory alloy rods investigated by a novel constitutive model

International audienceHysteretic behaviour of shape memory alloys (SMAs) is highly important for design and applicability of these materials in active structural elements like rods. Especially the damping performance of SMAs depend strongly on their hysteretic characteristics. Experimental investigations show the influences of stress on the hysteretic cycle. The current study shows some computational results of a constitutive model which is capable to investigate the effect of an external applied stress field on the hysteretic cycle according to a recently developed method on the basis of statistical mechanics metho

Embedded NiTi wires for improved dynamic thermomechanical performance of silicone elastomers

The extraordinary properties of shape memory NiTi alloy are combined with the inherent viscoelastic behavior of a silicon elastomer. NiTi wires are incorporated in a silicon elastomer matrix. Benefits include features as electrical/thermal conductivity, reinforcement along with enhanced damping performance and flexibility. To gain more insight of this composite, a comprehensive dynamic thermomechanical analysis is performed and the temperature- as well as frequency-dependent storage modulus and the mechanical loss factor are obtained. The analyses are realized for the composite and single components. Moreover, the models to express the examined properties and their temperature along with the frequency dependencies are also presented

Ultrasonic Degradation of Polystyrene for Tailoring Molecular Weight and Polydispersity of Polystyrene Fragments

Ultrasonic degradation of polymers attracts more and more attention in the field of chemical recycling of polymers due to the promising opportunity to tailor molecular weight and polydispersity of the gained polymer fragments. In this work, the influence of solvent, gas atmosphere, and ultrasound amplitude on the ultrasonic degradation process of polystyrene is investigated. Therefore, an experimental procedure to perform ultrasonic degradation of polystyrene under homogeneous temperature conditions in the solvents cyclohexane and toluene under the gas atmospheres CO$_{2}$ and N$_{2}$ for different ultrasonic amplitudes was designed. It could be shown that a significant effect on the molecular weight and polydispersity of the polymer could only be revealed for N$_{2}$ and not for CO$_{2}$ atmosphere

Molecular weight as a key for electroactive phase formation in poly(vinylidene fluoride)

Outstanding electroactive properties of certain crystallographic phases of poly(vinylidene fluoride)(PVDF) bring much attention to its melting and crystallisation behaviour for tailoring crystallographic nature. In the past, the focus was put on operating conditions in terms of melting and crystallisation kinetics, whereas a deeper understanding of the molecular structure–property relationship of PVDF is sparsely addressed. This study is the first survey to investigate systematically the structure–property relationship by clarifying the question, how molecular weight distribution affects thermal–caloric properties and hence polymorphous phase behaviour. It is shown that molecular weight strongly influences electroactive phase formation and plays a key role in phase design

Li-Distribution in compounds of the Li2O-MgO-Al2O3-SiO2-CaO system: a first survey

The recovery of critical elements in recycling processes of complex high-tech products is often limited when applying only mechanical separation methods. A possible route is the pyrometallurgical processing that allows transferring of important critical elements into an alloy melt. Chemical rather ignoble elements will report in slag or dust. Valuable ignoble elements such as lithium should be recovered out of that material stream. A novel approach to accomplish this is enrichment in engineered artificial minerals (EnAM). An application with a high potential for resource efficient solutions is the pyrometallurgical processing of Li ion batteries. Starting from comparatively simple slag compositions such as the Li-Al-Si-Ca-O system, the next level of complexity is reached when adding Mg, derived from slag builders or other sources. Every additional component will change the distribution of Li between the compounds generated in the slag. Investigations with powder X-Ray diffraction (PXRD) and electron probe microanalysis (EPMA) of solidified melt of the five-compound system Li2O-MgO-Al2O3-SiO2-CaO reveal that Li can occur in various compounds from beginning to the end of the crystallization. Among these compounds are Li1−x(Al1−xSix)O2, Li1−xMgy(Al)(Al3/2y+xSi2−x−3/2y)O6, solid solutions of Mg1−(3/2y)Al2+yO4/LiAl5O8 and Ca-alumosilicate (melilite). There are indications of segregation processes of Al-rich and Si(Ca)-rich melts. The experimental results were compared with solidification curves via thermodynamic calculations of the systems MgO-Al2O3 and Li2O-SiO2-Al2O3

A statistical mechanics approach describing martensitic phase transformation

International audienceA novel theoretical approach modeling martensitic phase transformation is demonstrated in the present study. The generally formulated model is based on the block-spin-approach and on renormalization in statistical mechanics and is applied to a representative volume element which is assumed to be in a local thermodynamic equilibrium. Using fundamental physical properties of a shape memory alloy (SMA) single crystal as input data the model predicts the order parameter "total strain", the martensitic phase fraction and the stress assisted transformation accompanied by pseudoelasticity without the requirement of evolution equations for internal variables and assumptions on the mathematical structure of the classical free energy. In order to demonstrate the novel approach the first computations are carried out for a simple one-dimensional case. Results for total strain and martensitic phase fraction are in good qualitative agreement with well known experimental data according to their macroscopic strain rearrangement when phase transformation occurs. Further a material softening effect during phase transformation in SMAs is predicted by the statistical physics approach

Kinetics and rates of martensitic phase transformation based on statistical physics

International audienceModeling martensitic phase transformation in SMAs has become a key to understand the transformation behavior in terms of their temperature and stress response. In the present work a new method based on a statistical physics concept has been developed for the description of transformation kinetics, total strain and their rates in polycrystalline solids undergoing displacive temperature induced phase transformations. As a result the model explains the kinetics and strain evolution for a polycrystal without the requirement of assumptions on internal variables and their constraints. Instead it is based on fundamental properties of the SMA single crystal, only

Modelling Binder Degradation in the Thermal Treatment of Spent Lithium-Ion Batteries by Coupling Discrete Element Method and Isoconversional Kinetics

Developing efficient recycling processes with high recycling quotas for the recovery of graphite and other critical raw materials contained in LIBs is essential and prudent. This action holds the potential to substantially diminish the supply risk of raw materials for LIBs and enhance the sustainability of their production. An essential processing step in LIB recycling involves the thermal treatment of black mass to degrade the binder. This step is crucial as it enhances the recycling efficiency in subsequent processes, such as flotation and leaching-based processing. Therefore, this paper introduces a Representative Black Mass Model (RBMM) and develops a computational framework for the simulation of the thermal degradation of polymer-based binders in black mass (BM). The models utilize the discrete element method (DEM) with a coarse-graining (CG) scheme and the isoconversional method to predict binder degradation and the required heat. Thermogravimetric analysis (TGA) of the binder polyvinylidene fluoride (PVDF) is utilized to determine the model parameters. The model simulates a specific thermal treatment case on a laboratory scale and investigates the relationship between the scale factor and heating rate. The findings reveal that, for a particular BM system, a scaling factor of 100 regarding the particle diameter is applicable within a heating rate range of 2 to 22 K/min

A mean-field model for transformation induced plasticity including backstress effects for non-proportional loadings

International audienceA viable model for the phenomenon of transformation induced plasticity must be able to predict the strain response to arbitrary, also non-proportional loading paths. The constitutive model presented in this paper is interfacing between the macro- and the mesoscale by stress partitioning in the parent and product phase, using a nonlinear scale transition rule. As one of the key features a mean field tensor representing the orientation strain contribution is introduced taking into account backstress effects. A good agreement of the model results with experiments on a maraging steel is achieved