Lionel Michelet
Carbides constitute a class of materials used for many applications, where excellent mechanical properties, such as stiffness, hardness and fracture toughness, are required. The machining industry is a typical example where carbide are crucial materials. Whether it be in tool steels or in cermets, the most performant machining alloys are actually composed of ternary carbides, i.e. contain two different metallic elements in addition to carbon, present as a reinforcement in a metallic matrix. Ternary carbides constitute better reinforcements providing enhanced mechanical properties in comparison with binary carbides; however, our knowledge of ternary carbides per se is insufficient, and only the properties of the alloys have been characterized. The main reason explaining such a lack of data is the relative small size of these particles, in the micro-range, which complicate the measurement of their mechanical properties. This thesis aims to contribute towards filling this gap by means of micromechanical methods able to measure locally the main properties of individual microscopic carbide particles. This is now possible thanks to the recent developments of experimental techniques such as nanoindentation and FIB. The challenges of this thesis are then to be able to 1) produce carbides particles embedded in iron with tailored compositions, 2) test mechanically individual particles and 3) explain the evolution of the properties with changes in composition, in order to find an optimal composition characterized by enhanced mechanical properties. The main challenges come from the small dimension of the particles (< 40 µm) and the fact that such particles are embedded in a steel matrix, more compliant than the particles, which can then bias the measurement of elastic properties. Standard nanoindentation techniques cannot measure accurately the mechanical properties of such matrix/particle combinations and new methods have been developed in this thesis in order to overcome this problem. Carbide particles have been grown in situ by arc-melting cast iron with transition metal high purity chips, in order to properly tune the composition. Three mechanical properties have been measured: elastic modulus, hardness and fracture toughness, by indenting a polished surface, or by bending up to fracture notched micro-cantilever beams. This trio of properties is essential in order to characterize properly a material in view of an application in machining industry. By using binary carbide compositions as references, we have characterized five ternary carbide systems, namely (Ti,W)C, (Ti,V)C, (Ti,Ta)C, (Ti,Nb)C and (Ta,V)C, as well as the quaternary (Ti,Ta,V)C system. The ratio of the metallic elements has been varied in order to cover the entire range of compositions. For the ternary systems, we found compositions that exhibited both a higher hardness and modulus than the two corresponding binary carbides, enhancing the properties by ~15-20%. By investigating the filling of atomic bonds involving d-orbitals, we can provide a partial explanation to the elastic modulus and hardness evolution with composition, the maxima in hardness corresponding to a valence electron concentration between 8.4 and 8.6. The quaternary system aims to further improve mechanical response of the ternary carbides: it allows to highlight a very promising quaternary composition, characterized by the combination of both a very high modulus and hardness of respectively 636 and 41 GPa.
EPFL2018
