Papers by Author: Pedro E.J. Rivera-Díaz-del-Castillo

Abstract: The design of novel ultra high strength steels for aerospace applications is subjected to stringent requirements to ensure their performance. Such requirements include the ability to withstand high loads in corrosive environments subjected to temperature variations and cyclic loading. Achieving the desired performance demands microstructural control at various scales; e.g. fine lath martensite is desired in combination with nanoprecipitate networks at specified volume fractions, and controlled concentrations of alloying elements to prevent alloy embrittlement. The design for a specified microstructure cannot be separated from the processing route required for its fabrication. Alloys displaying exceptional properties are subjected to complex interactions between microstructure and processing requirements, which can be described in terms of evolutionary principles. The present work shows how genetic alloy design principles have been utilised for designing stainless steels displaying strength exceeding that of commercial counterparts. Such designed alloys become feasible for fabrication by tailoring their microstructure employing thermodynamic and kinetic principles, while fracture toughness properties can be controlled via performing quantum mechanical cohesion energy computations.

Abstract: Due to Plasticity induced Transformation in metastable β-Ti-10V-2Fe-3Al (wt.%) alloy (PiTTi) upon deformation, a noticeable improvement in mechanical properties is observed. Among the main factors controlling such effect are the β grain size and its composition. Such phase transforms into martensite upon quenching. Its martensite start temperature (Ms) varies in accordance with its composition. Following Ghosh and Olson’s theory, a thermodynamics based model to predict the compositional dependence of the Ms temperature is developed, and successfully validated for Ti-X (X = Fe, Cr, Mo, V, Nb, Zr and Al) binary alloys. The model has been used to design new alloys displaying a tailored PiTTi effect.

Abstract: A non-equilibrium thermodynamics-based approach is proposed to predict the dislocation density and flow stress at the steady state of high temperature deformation. For a material undergoing dynamic recovery and recrystallization, it is found that the total dislocation density can be expressed as ( )2 ρ = λε& b , where ε& is the strain rate, b is the magnitude of the Burgers vector and λ is a dynamic recovery and recrystallization related parameter.

Abstract: Non-equilibrium thermodynamics theory is applied to the description of plastic deformation in pure FCC metals at the steady state. The saturation flow stress is predicted as a function of temperature and strain rate for Al, Cu, Ni and Ag. The implications on the cell/subgrain size and dislocation density are explored.

Abstract: High molybdenum high strength stainless steels can contain the so-called Chi phase (Fe36Cr12Mo10). The presence of this phase, which normally occurs at grain boundaries, depletes the chromium content leading to intergranular corrosion. This may cause alloy embrittlement during long term use. The presence of such phase has proven to be highly sensitive to alloy processing parameters such as the cooling rate after a final heat treatment. The present work provides a model to quantify the effects of processing parameters aimed at controlling the Chi phase. The model is based on nucleation and growth classical theories involving capillarity effects for the early stages; it is applied to a range of heat treatment conditions and compared to experimental results.

Abstract: A novel statistical mechanics approach to quantify the effects of hot rolling and deformation on the formation of dislocations in a single grain scenario is presented. The dislocations are dealt as equilibrium defects in the crystal structure, which is assumed to be deformed via the formation of dislocations or single atom displacements at the grain boundary, which involve breaking their bonds and are thus termed “bond breaking atoms”. The deformation process is applied to steels of a variety of grain size and dislocations densities. The model has the capacity to describe the grain energy increase as a function of crystallography, grain sizes, temperature and degree of deformation, providing thus an aid in predicting the conditions for dynamic recovery and recrystallization.