Strong particles in Metal | EPFL Graph Search

We all know that strong fibers make most of today¹s composite materials what they are : exceptionally stiff, strong, light and tough materials that serve in a number of demanding structural applications. This presentation will try to make the case for strong particles, proposing that they hold equally high promise as a reinforcing phase in metal ¬ if they can be made to be as strong as today¹s engineering fibres. The talk will be articulated in two parts. In the first, experimental data from the author¹s laboratory, combined with simple micromechanical calculations, will be used to show how strong ceramic particles combined with metal do indeed have the potential to produce highly attractive materials. The second part of the presentation will address the question of measuring particle strength : unlike fibres, which are easily tested in uniaxial tension, miroscopic second phases or particles used for the reinforcement of metals are far more difficult to characterize for strength. A few methods devised to this end in recent work within the author¹s laboratory will be presented, together with experimental results gathered on strong brittle particles that are, or can be, used, for the reinforcement of metals. The presentation will conclude with a few practical implications of this work.

Probing the local strength and toughness of microscopic hard second phases in metals and composites

Marco Cantoni, Raphaël Charvet, Cyril Dénéréaz, Willy Dufour, Marta Fornabaio, Lionel Michelet, Andreas Mortensen, Martin Guillermo Mueller, Václav Pejchal, Goran Zagar

Many engineering alloys and composites combine a ductile matrix with discrete microscopic reinforcing phases, e.g. silicon particles in aluminium alloys, carbides in steels or ceramic particles in metal matrix composites. It is well understood that the mechanical properties of such alloys and composites are strongly influenced by those of their reinforcing particles; yet not much is known of the nature and size of strength-limiting flaws within such microscopic second phases. To investigate the strength and fracture toughness of these microscopic particles directly, we have developed test methods that combine focused ion beam (FIB) milling with microtesting techniques and bespoke finite element simulation. Local strength is probed by micromachining individual reinforcing particles in such a way that tensile stresses are produced in a volume of material free of FIB and polishing artifacts upon the application of a load via nanoindenter. Fracture toughness is measured by the chevron-notch fracture test technique adapted for microscopic samples. Testing methods are established using nanocrystalline alumina fibers as the testbench material, so as to compare data thus obtained with known fiber properties from tensile tests on long sections of the fibers. These test methods are currently being transposed towards the microscopic testing of more irregularly shaped and anisotropic brittle second phases, with the objective to measure their local mechanical properties and identify microstructural flaws that govern the strength of such particles, using in-depth electron microscopy characterization combined with microtesting.

2015

Exotic phases of quantum frustrated magnets

Julien Dorier

In this work, we study two examples of frustrated magnetic systems whose degrees of freedom are spins (or pseudo-spins) on two-dimensional lattices. The first part presents the results obtained for the quantum compass model on a square lattice. This model is a minimal model for the orbital degrees of freedom of transition metal compounds. We use exact diagonalizations, quantum Monte Carlo simulations as well as a perturbative approach in order to study the ground state degeneracy in the thermodynamic limit and the type of order in the ground state. The second part of this work is devoted to the study of the magnetic properties of SrCu2(BO3)2, which is one of the few quasi two-dimensional quantum magnets whose magnetization curve as a function of the magnetic field shows plateaus. The magnetic degrees of freedom are described by a spin 1/2 Heisenberg model on the Shastry-Sutherland lattice in the presence of an external magnetic field. In order to study the magnetic properties at zero-temperature, we use perturbative continuous unitary transformations to derive an effective Hamiltonian in which the triplets Sz = 1 are particles moving on a background of singlets. This Hamiltonian is characterized by a kinetic energy dominated by correlated hopping, which let a particle hop only if there is another particle nearby. In order to better understand the effect of the correlated hopping, we start by studying a minimal model. By using exact diagonalizations, quantum Monte Carlo and a semi-classical approximation we show that the correlated hopping can stabilize a phase characterized by a condensation of pairs of particles and strongly favor supersolid phases. Next, we determine the zero-temperature phase diagram of the effective model by using a classical approximation. A comparison to previous theoretical works and experimental measurements shows that the Shastry-Sutherland model cannot reproduce the series of magnetization plateaus measured experimentally. This suggest that the residual interactions, which were neglected, could be crucial for SrCu2(BO3)2.

EPFL2008

Perturbative Techniques in Hadron Collider Physics

Paolo Torrielli

High-energy particle physics is going through a crucial moment of its history, one in which it can finally aspire to give a precise answer to some of the fundamental questions it has been conceived for. On the one side, the theoretical picture describing the elementary strong and electroweak interactions below the TeV scale, the Standard Model, has been well consolidated over the decades by the observation and the precise characterization of its constituents. On the other hand, the enormous technological potentialities nowadays available, and the skills accumulated in decades of collider experiments with increasingly high complexity, render for the first time plausible the possibility of addressing complicated and conceptually deep questions like the ones at hand. The best incarnation of this high level of sophistication is the CERN Large Hadron Collider (LHC), the most powerful experimental apparatus ever built, which is designed to shed light on the true nature of fundamental interactions at energies never attained before, and which has already started to open a new era in physics with the recent discovery of the longed-for Higgs boson, a true milestone for the human knowledge as well as one of the most important discoveries in the modern epoch. The knowledge that has been and is going to be reached in these crucial years would of course not be conceivable without a deep interplay between the theoretical and the experimental efforts. In particular, on the theoretical side, not only there are wide groups of researchers devoted to building possible extensions to the Standard Model, which draws the guidelines of current and future experiments, but also there is a vast community whose research is rather aimed at the precise predictions of all the physical observables that could be measured at colliders, and at the systematic improvement of the approximations that currently constrain such predictions. On top of representing the state-of-the-art of the human understanding of the properties that regulate elementary-particle interactions and of the formalisms that describe them, the developments of this line of research have an immediate and significant impact on experiments. Firstly, these detailed calculations are the very theoretical predictions against which experimental data are compared, so they are crucial in establishing the validity or not of the theories according to which they are performed. Secondly, the signals one wants to extract from data at modern colliders are so tiny and difficult to single out that the experimental searches themselves need be supplemented by a detailed work of theoretical modelling and simulation. In this respect, high-precision computations play an essential role in all analysis strategies devised by experimental collaborations, and in many aspects of the detector calibration. It is clear that, for theoretical computations to be useful in experimental analyses and simulations, the predictions they yield should be reliable for all possible configurations of the particles to be detected. Thus the key feature for the present theoretical collider physics is not particularly the computation of observables with high precision only in a limited region of the phase space, but the capability of combining (‘matching’) in a consistent way different approaches, each of which is reliable in a particular kinematic regime. With this perspective, matching techniques represent one of the most promising and successful theoretical frameworks currently available, and are considered as eminently valuable tools both on the theoretical and on the experimental sides. Matched computations are based on a perturbation-theory approach for the description of configurations in which the scattering products are well separated and/or highly energetic: in particular the precision currently attained for all but a few of the relevant processes within the Standard Model is the next-to-leading order (NLO) in powers of the strong quantum-chromodynamics (QCD) coupling constant αS; for the description of configurations in which the particles outgoing the collisions are close to each other and/or have low energy, it can be shown that the perturbation-theory expansion breaks down, and then a complementary method, like the parton shower Monte Carlo (PSMC), has instead to be employed. The task of matching is precisely that of giving a prediction that interpolates between the two approaches in a smooth and theoretically-consistent way. This thesis is focused on MC@NLO, a high-energy physics formalism capable of matching computations performed at the NLO in QCD to PSMC generators, in such a way as to retain the virtues of both approaches while discarding their mutual deficiencies. In particular, the thesis reports on the work successfully achieved in extending MC@NLO from its original numerical implementation, tailored on the HERWIG PSMC, to the other main PSMC programs currently employed by experimental collaborations, PYTHIA and Herwig++, confirming the advocated universality of the method. Differences in the various realizations are explained in detail both at the formal level and through the simulation of various Standard-Model reactions. Moreover we describe how the MC@NLO framework has been developed so as to render its implementation automatic with respect to the physics process one is about to simulate: beyond yielding an enormous increase in its potential for present and future collider phenomenology, and upgrading the standard of precision for high-energy computations to the NLO+PSMC level, this development allows for the first time the application of the MC@NLO formalism to a huge number of relevant and highly complicated reactions, through an implementation which is also easily usable by people well-outside the community of experts in QCD calculations. As example of this new version, called aMC@NLO, recent results are presented for complex scattering processes, involving four or five final-state particles. Finally, possible extensions of the framework to theories beyond the Standard Model, like the supersymmetric version of QCD, are briefly introduced.