P.E.J. Rivera-Díaz-del-Castillo

Modelling strength and ductility of ultrafine grained BCC and FCC alloys using irreversible thermodynamics

Abstract A novel grain size dependent strain hardening model is derived from the theory of irreversible thermodynamics. The model yields the evolution of the dislocation densities in the grain interior and at the grain boundary, as well as their contributions to the flow stress. It is found that submicron grain sizes have a lower dislocation density in the grain interior, causing ductility to decrease greatly. The predicted stress–strain curve shapes, uniform elongation and ultimate tensile strength values for interstitial free steels (body centred cubic) and aluminium alloys (AA1100, face centred cubic) show good agreement with experimental observations.

Rolling contact fatigue in bearings: multiscale overview

Abstract For over a century, rolling contact fatigue in bearings has been recognised as a key feature limiting bearing life. The phenomenon is manifested through dark etching regions, 30 and 80° white etching bands as well as white etching areas, the latter often forming intricate defects commonly referred to as butterflies. Their presence depends on testing conditions, such as contact pressure, temperature, number of revolutions and steel cleanliness. Microstructural inspection demonstrates that precipitate shearing, dissolution, cell and nanocrystal formation as well as matrix/inclusion debonding may take place throughout bearing life. Such microstructural features have a negative effect on bearing hardness, strength, ductility and toughness, usually preceding failure. The present review shows how such phenomena are interconnected, highlighting the need for integral characterisation and modelling across the scales. This will aid in the conception of new heat treatments, steel grades and microstructures for enhanced rolling contact fatigue, leading to increased bearing life.

Hydrogen-Trapping Mechanisms in Nanostructured Steels

Nanoprecipitation-hardened martensitic bearing steels (100Cr6) and carbide-free nanobainitic steels (superbainite) are examined. The nature of the hydrogen traps present in both is determined via the melt extraction and thermal desorption analysis techniques. It is demonstrated that 100Cr6 can admit large amounts of hydrogen, which is loosely bound to dislocations around room temperature; however, with the precipitation of fine coherent vanadium carbide traps, hydrogen can be immobilized. In the case of carbide-free nanostructured bainite, retained austenite/bainite interfaces act as hydrogen traps, while concomitantly retained austenite limits hydrogen absorption. In nanostructured steels where active hydrogen traps are present, it is shown that the total hydrogen absorbed is proportional to the trapped hydrogen, indicating that melt extraction may be employed to quantify trapping capacity.

Genetic alloy design based on thermodynamics and kinetics

A theory-guided computational approach for alloy design is presented. Aimed at optimising the desired properties, the microstructure is designed and an alloy composition optimised accordingly, combining criteria based on thermodynamic, thermokinetic and mechanical principles. A genetic algorithm is employed as the optimisation scheme. The approach is applied to the design of ultra-high strength stainless steels. Three composition scenarios, utilising different strengthening precipitates (carbides, Cu and NiAl/Ni3Ti), are followed. The results are compared to a variety of existing commercial high-end engineering steels, showing that the design strategy presented here may lead to significant improvements in strength beyond current levels.

Plasticity induced transformation in a metastable β Ti-1023 alloy by controlled heat treatments

Abstract Investigations into the possibility of improving the strength–ductility relation in a metastable β-titanium alloy (Ti–10V–2Fe–3Al) through plasticity induced transformation (PiTTi) have been carried out. Various heat treatments in the β and/or α+β condition were performed to study their influence on both the microstructure and solute partitioning, which eventually control the PiTTi effect. Stress induced martensite formation promoting such effect has been observed upon compression testing for β and β+(α+β) microstructures. The stress–strain curves exhibiting stress induced martensite show a ∼20% increase in strength, while still retaining a reasonable ductility level. Microstructural parameters such as grain size and solute concentration (especially V) in β have been related to the alloys ability to exhibit PiTTi.

Computational design of UHS maraging stainless steels incorporating composition as well as austenitisation and ageing temperatures as optimisation parameters

An extended integral alloy design approach for the development of new ultra high strength maraging steels is presented, which incorporates not only chemical composition effects but also criteria accounting for the influence of the entire heat treatment. The approach considers the desired strengthening precipitates formed during the final ageing treatment as well as undesirable equilibrium phases present during the preceding high temperature homogenisation treatment. The results are compared with the predictions of a previous model, which considered the combination of composition and final precipitation tempering stage only.

Computational design of nanostructured steels employing irreversible thermodynamics

Abstract Recent theory demonstrates that the Kocks–Mecking formulation of plasticity has a foundation in multiscale irreversible thermodynamics. The key terms in the formulation can be obtained form experiments and from fundamental calculations. This offers two advantages to materials scientists and alloy designers: the Kocks–Mecking approach goes beyond being a phenomenological approach, gaining a solid physical foundation in multiscale computational physics; the new formulation can be employed to conceive new alloys displaying complex synergistic interactions at several scales and among several phases. This approach is ideal for designing and modelling nanostructured steels. This work incorporates four concomitant strengthening effects: solid solution, Hall–Petch, dislocation forest and precipitation. The new formulation is applied to nanostructured martensitic, dual phase and twinning induced plasticity steels, describing with excellent accuracy of their stress–strain behaviour.

Precipitate coarsening in multicomponent systems

Abstract Expressions to obtain composition shifts due to capillarity are presented when the chemical potential shifts of the precipitate phase are taken into account. These are applied to calculate the precipitate coarsening kinetics in multicomponent systems, showing dependence on the solution thermodynamics followed by the continuous phase.

Heat Treatment and Composition Optimization of Nanoprecipitation Hardened Alloys

A modeling strategy for designing nanoprecipitation strengthened alloys is presented here. This work summarises the application of a new thermokinetics approach wherein multiple design criteria are enforced: corrosion resistance and high strength combined with affordable thermomechanical processing schedules. The methodology presented here iteratively performs thermodynamic and kinetic calculations, which are aimed at determining the best precipitate nanostructures following multiple design objectives. A genetic algorithm is employed to more rapidly find optimal alloy compositions and processing parameters consistent with the design objectives. The strength was maximized, while conditions on the microstructure were imposed: corrosion resistance, fine martensite formation, and the prevention of primary and undesirable precipitate particles. It is possible to computationally design new alloys strengthened by Ni-based nanoprecipitates and carbides with yield strengths exceeding 1.6 GPa and good corrosion resistance. A major limitation in the methodology is the determination of optimum processing times, which require the computation of the formation energies of non-equilibrium precipitates employing other techniques. A method to circumvent this limitation is discussed.

A thermostatistical theory for solid solution effects in the hot deformation of alloys: an application to low-alloy steels

The hot deformation of low-alloy steels is described by a thermostatistical theory of plastic deformation. This is based on defining a statistical entropy term that accounts for the energy dissipation due to possible dislocation displacements. In this case, dilute substitutional and interstitial atom effects alter such paths. The dislocation population is described by a single parameter equation, with the parameter being the average dislocation density. Solute effects incorporate additional dislocation generation sources. They alter the energy barriers corresponding to the activation energies for dislocation recovery, grain nucleation and growth. The model is employed to describe work hardening and dynamic recrystallization softening in fifteen steels for a wide range of compositions, temperatures and strain rates. Maps for dynamic recrystallization occurrence are defined in terms of processing conditions and composition.