Size effects in LiF plasticity: new insights into the lattice resistance contribution

• Autores: Rafael Alfonso Soler Arnedo
• Directores de la Tesis: Javier Segurado Escudero (dir. tes.), Jon Mikel Molina Aldareguia (dir. tes.)
• Lectura: En la Universidad Carlos III de Madrid ( España ) en 2014
• Idioma: inglés
• Tribunal Calificador de la Tesis: Javier Gil Sevillano (presid.), Elena Gordo Odériz (secret.), Bill W.J. Clegg (voc.)
• Materias:
  • Física
    • Mecánica
      • Mecánica de medios continuos
      • Medida de propiedades mecánicas
• Enlaces
  • Tesis en acceso abierto en: e-Archivo
• Resumen

  • Previous studies on the mechanical response under compression of single-crystal micropillars suggest that the effect of sample size on the flow stress is material dependent. This investigation addresses the role of the intrinsic lattice resistance of the material on this dependency. In particular, the objective of this study is to ascertain whether different slip systems can be characterized individually using micro-compression and to see how size effects differ as a function of the bulk critical resolved shear stress of the operative slip system. For this, LiF was chosen as the model material as it presents a marked plastic anisotropy as a result of the large difference in the critical resolved shear stress between the “soft" {110}{110} and the “hard" {100}{110} active slip systems, and because their operative slip systems depend strongly on the micropillar crystallographic orientation. Plasticity in LiF was evaluated in terms of crystal orientation and slip system activation by means of crystal plasticity finite element simulations, focusing on the distinctive response of two micropillar crystallographic orientations, the [100]- and the [111]-orientation, where only the “soft" and the “hard" slip systems activate, respectively. Furthermore, the influence of three potential sources of experimental uncertainties associated with the alignment of the micropillars were assessed: geometrical tilts, lattice rotations, and misalignments between the surfaces of the at punch and the head of the pillar, concluding that micropillars oriented in the [111]-direction are extremely sensitive to experimental uncertainties, thus to have artifacts present in the data. The compressive response of LiF single-crystal micropillars oriented in the [111]-direction was also studied experimentally. Micropillars of different diameter (in the range 1-5 ?m) were obtained by etching the matrix away in directionally-solidified NaCl-LiF and KCl-LiF eutectic compounds. Initial micro-compression tests carried out at room temperature did not show any significant effect of the micropillar diameter on the flow stress. These results were discussed to the light of previous results in LiF in the [100]-orientation, that showed a strong size effects on the flow stress (Nadgorny et al., 2008), confirming previous observations that suggest that the extent of the size effect on the flow stress scales with the intrinsic lattice resistance of the material. To evaluate the effect of the ion-irradiation induced damage associated with focused ion beam (FIB) fabrication of the micropillars (the conventional method to fabricate micropillars), selected [111]-oriented pillars were exposed to high-energy Ga+ ions to ascertain the effect of ion irradiation on the mechanical response. Ion irradiation led to an increase of approximately 30% in the yield strength and the maximum compressive strength but no effect of the micropillar diameter on ow stress was found either. The role of the lattice resistance was further analyzed by performing elevated temperature micro-compression tests at temperatures up to 250 °C. Results showed that size e_ects on LiF [111]-oriented micropillars are strongly dependent on temperature. It was demonstrated that the size effect observed during micropillar compression comes about as a result of the relative weights of the size-independent (lattice resistance plus forest hardening) and size-dependent (operation of single arm dislocation sources) contributions to strength. The former dominated at room temperature (and no size effect was found) while both were of the same order at 250 °C for the micropillar diameters studied, leading to a strong size effect. Thus, the role of the lattice resistance on the size effect of micrometer-size single-crystals was demonstrated unambiguously for this first time. This result rationalizes the different values of power-law exponent for the size effect found in the literature for FCC and BCC metals as well as covalent and ionic solids. ------------------------