Princípios estruturais de formação e evolução da austenita expandida sob condições extremas de pressão e temperatura

Abstract

The present study comprises basic aspects of formation and evolution of the nitrogen (N) expanded austenite (𝛾𝑁 or the S-phase), especially those related with stresses and strains caused in the crystal lattice by the interstitial solute. Through a theoretical and experimental approach, crystal structure analyses of formation, anisotropy, evolution and decay of the S-phase were investigated, based on mass transport and conservation, thermodynamics and solid mechanics. The three main experiments were as follows. (𝑖) The first analysis evaluated the nature of the S-phase produced in austenitic stainless steels with different stoichiometries, with regard to the plasma immersion ion implantation (PIII) parameters, in the light of the mass conservation in the crystal lattice. (𝑖𝑖) The N diffusion in the S-phase was studied by in situ structure characterizations during thermal treatments in the XRD-2 beamline at the Brazilian Synchrotron Light Laboratory (LNLS); also observed was the correlation between initial solute distribution and thermal evolution of the system, which involve anisotropic and thermodynamic aspects of the 𝛾𝑁 phase transition. (𝑖𝑖𝑖) The third experiment also consisted in in situ structure analysis of the modified surfaces, now subjected to very high pressures (in the order of GPa) at the XDS beamline from LNLS, which allowed to extend the discussion to the formation and intrinsic anisotropy of the S-phase. The N uptake rate, diffusion, anisotropy and phase transition phenomena occur simultaneously during nitriding. The major contributions of this work is to stablish connections among them, since such aspects are usually treated separately in the literature. The continuity equation applied to the N flux through the modified layer was found to describe the mass transport during the S-phase formation. It considers the N inlet rate (dependent on the nitriding parameters) and the N outlet rate (controlled by diffusion), as well as the solute distribution and redistribution in the material. The retained N amount (𝐶𝑁) has dependence on the lattice stresses and strains caused the own interstitial solute; hence, strains and anisotropy of the S-phase were characterized by varying PIII conditions and chemical composition of substrates, which directly affected the mass transport. The directional bulk modulus 𝐾 [𝜙] for both S-phase (𝛾𝑁) and austenite (𝛾) were measured from the high pressures experiments. After the N entrance in the material, it transited from 𝐾𝛾 〈100〉>𝐾𝛾 〈111〉 to 𝐾𝛾𝑁 〈100〉<𝐾𝛾𝑁 〈111〉, whereas elastic modulus changed from 𝐸𝛾 〈100〉<𝐸𝛾 〈111〉 to 𝐸𝛾𝑁 〈100〉>𝐸𝛾𝑁 〈111〉. This is a result of the inversion of the anisotropic factor, from 𝐴𝛾>1 to 𝐴𝛾𝑁<1. Elastic moduli measured for the 𝛾 substrate were 𝐸𝛾[200]=110 𝐺𝑃𝑎 and 𝐸𝛾[111]=312 𝐺𝑃𝑎, and for the S-phase they were 𝐸𝛾𝑁[200]=190 𝐺𝑃𝑎 and 𝐸𝛾𝑁[200]=74 𝐺𝑃𝑎, in good agreement with theoretical predictions. Studies of the thermal evolution disclosed a symmetry with the anisotropic evolution, properly simulated here. Besides, nitriding parameters and materials compositions are determining factors for the N distribution, with consequences for the solute mobility and phase transitions in the modified layer.