I. Guillot

Design and tensile properties of a bcc Ti-rich high-entropy alloy with transformation-induced plasticity

ABSTRACT A new bcc Ti-rich high-entropy alloy (HEA) of composition Ti35Zr27.5Hf27.5Nb5Ta5 was designed using the ‘d-electron alloy design’ approach. The tensile behavior displays a marked transformation-induced plasticity effect resulting in a high normalized work-hardening rate of 0.103 without loss of ductility when compared to the reference composition Ti20Zr20Hf20Nb20Ta20. In this paper, a detailed microstructural analysis was performed to understand the deformation process, revealing architectural-type microstructures and a high volume fraction (65%) of internally twinned stress-induced martensite α″ after mechanical testing. This study opens the way to mechanical properties optimization and enhancement of titanium-based HEAs by combining multiple alloying designs. IMPACT STATEMENT For the first time, proof is given that transformation-induced plasticity was triggered in a bcc refractory high-entropy alloy, leading to a twofold increase in the normalized work-hardening rate. GRAPHICAL ABSTRACT

Corrosion of iron from heritage buildings: proposal for degradation indexes based on rust layer composition and electrochemical reactivity

Abstract In the present work, the authors tried to establish degradation indices for heritage ferrous artefacts, especially those used in ancient buildings and submitted to indoor atmospheric corrosion. The authors focused on the site of the Amiens Cathedral in the north of France. Samples coming from this reference site were carefully characterised in order to identify the different phases constituting the corrosion scale. The scale consists in a matrix of iron oxyhydroxide goethite embedded with several ferrihydrite marblings. Other phases such as lepidocrocite and akaganeite are scarcely present in the external part of the corrosion scale. Moreover, electrochemical measurements on both references and ancient samples enable to define the reduction reactive phases. From the nature of these phases and their localisation, two degradation indices were defined to evaluate rust reactivity. Finally, a curve that links these two factors is proposed as a first step towards a corrosion diagnosis.

Simulation of the concomitant process of nucleation-growth-coarsening of Al2Cu particles in a 319 foundry aluminum alloy

The precipitation of Al2Cu particles in a 319 T7 aluminum alloy has been modeled. A theoretical approach enables the concomitant computation of nucleation, growth and coarsening. The framework is based on an implicit scheme using the finite differences. The equation of continuity is discretized in time and space in order to obtain a matricial form. The inversion of a tridiagonal matrix gives way to determining the evolution of the size distribution of Al2Cu particles at t +Δt. The fluxes of in-between the boundaries are computed in order to respect the conservation of the mass of the system, as well as the fluxes at the boundaries. The essential results of the model are compared to TEM measurements. Simulations provide quantitative features on the impact of the cooling rate on the size distribution of particles. They also provide results in agreement with the TEM measurements. This kind of multiscale approach allows new perspectives to be examined in the process of designing highly loaded components such as cylinder heads. It enables a more precise prediction of the microstructure and its evolution as a function of continuous cooling rates.

Reactivity studies of atmospheric corrosion of heritage iron artefacts

Abstract: The authors consider the case of iron atmospheric corrosion to illustrate the possibility of developing a conservation diagnosis for a given material in a given environment. In the particular case of iron atmospheric corrosion, samples from the site of the Amiens Cathedral in the North of France have been characterised in order to identify the different phases constituting the corrosion product layers. The layers consist of a matrix of iron oxy-hydroxide goethite embedded with several ferrihydrite marblings. Other phases such as as lepidocrocite, maghemite and akaganeite are present in minor quantities. A degradation index is first defined from the phase proportions and from the intrinsic electrochemical properties of those phases. Further, the electrochemical reactivity of scratched rust powders has been studied to define a second degradation index. From these two degradation indices a first step towards a corrosion diagnosis method is proposed.

Influence of high-pressure torsion on the microstructure and the hardness of a Ti-rich high-entropy alloy

High-Pressure Torsion (HPT) is one of the most effective severe plastic deformation techniques in grain refinement. The goal of this study was to investigate the influence of HPT on the microstructure and hardness of a Ti-rich High-Entropy Alloy (HEA). The evolution of the grain size due to 1 turn of HPT was studied by transmission electron microscopy. Besides the refinement of the microstructure, a phase transition also occurred during HPT, as revealed by X-ray diffraction. The initial bcc structure transformed into a martensitic phase throughout the material. The features of this phase transformation were studied on a sample compressed to low strain values. The hardness as a function of the distance from the center in the HPT-processed disk was measured and correlated to the microstructure.