Papers by Author: Sybrand van der Zwaag

Abstract: The degradation of creep resistance in Nickel-based single crystal superalloys is essentially ascribed to their microstructure evolution. Yet there is a lack of work that manages to simulate the effect of alloying element concentrations on microstructure degradation. In this research, a computational model is developed to connect the rafting kinetics of Ni superalloys with their chemical composition, by combining thermodynamics calculation and an energy-based microstructure model. The isotropic coarsening rate and γ/γ misfit stresses have been selected as composition related parameter, and the effect of service temperature, time and applied stress are also taken into consideration to simulate the evolutions of microstructure parameters during creep process. The different generations of commercial Ni superalloys are selected and their chemical compositions are calculated based on this model. The simulated microstructure parameters are validated by the results from experimental results and the existing analytical model. The capability of the model in predicting the microstructure characteristics may provide instructional thought in developing a novel computational guided design approach in Ni superalloys.

Abstract: This work presents an artificial intelligence based design of a series of novel advanced high performance steels for ambient and high temperature applications, following the principle of the materials genome initiative, using an integrated thermodynamics/kinetics based model in combination with a genetic algorithm optimization routine. Novel steel compositions and associated key heat treatment parameters are designed both for applications at the room temperature (ultra-high strength maraging stainless steel) and at high temperatures (ferritic, martensitic and austenitic creep resistant steels). The strength of existing high end alloys of aforementioned four types are calculated according to the corresponding design criteria. The model validation studies suggest that the newly designed alloys have great potential in outperforming existing grades.

Abstract: Recently a novel strategy to improve the fatigue resistance of precipitation hardened aluminium alloys has been proposed, which is based on dynamic precipitation in partially aged material. In this paper, the effect of mean stress and alternative temperature treatments that can enhance the high cycle fatigue resistance through the mentioned mechanism were investigated. The material used is an under-aged 2024 aluminium alloy, which showed superior fatigue life compared to the peak aged condition. The recorded behaviour was observed to be more effective at lower stress amplitudes and lower R values. As the dynamic precipitation may not be able to keep up with the damage evolution, dedicated experiments were conducted to insert periods of controlled healing in a stress free state.

Abstract: This work concentrates on assessment of the TEP change of AA6061 during isothermal aging at 177 °C and the following interrupted aging at 65 °C. The results show that the TEP is sensitive to follow the microstructural changes undergone during all aging stages. Multiple sub-ambient temperature dependences TEP of binary Al-X alloys as well as the AA6061 subjected to the above mentioned heat treatments were undertaken. The solute level of individual element of the alloy, particularly those contributing to the clustering/precipitation, can be extracted and evaluated.

Abstract: The anelastic behavior of AA2024 alloy is studied in the temperature range between room temperature and 325 °C. The internal friction technique is shown to be very sensitive to the microstructural changes that take place at these temperatures. Interrupted aging performed at low temperature induces increase in the peak height at ~230 °C indicating the slow release of vacancies aiding the aggregation of Mg and Cu which further transforms into semicoherent precipitates. Stretched specimens indicate increase in background which is attributed to anelastic or viscoelastic of dislocations. TDIF of T6I4 samples is strongly affected to the point of deformation, whilst TDIF of T6I6 samples is affected by the deformation but irrespective to the point of deformation.

Abstract: A cyclic phase transformation concept has been proposed to investigate the growthkinetics of the austenite (γ) to ferrite (α ) and vice versa in Fe-Mn-C and Fe-C alloys. In the caseof cyclic partial transformations in Fe-Mn-C alloys, two new and special stages are observed:a stagnant stage in which the degree of transformation does not vary while the temperaturechanges and an inverse phase transformation stage, during which the phase transformationproceeds in a direction contradictory to the temperature change. The local equilibrium (LE)and paraequilibrium (PE) are both applied to analyzing the new observations. The stagnantstage was found to be caused by the Mn partitioning, while the inverse phase transformationstage was due to equilibrium conditions not being reached at the transition temperatures.A mixed-mode model is applied to simulating the cyclic phase transformation in Fe-C alloy,and it is found that the cyclic phase transformation concept is a very promising method forinvestigating the interface mobility.

Abstract: Aluminium alloys display complex phase transitions to achieve their desired properties.Many of these involve elaborated precipitation sequences where the main role is not played by ther-modynamically stable species, but by metastable precipitates instead. An interplay between phasestability, crystal symmetry, diffusion, volume and particle/matrix interfaces sets the pace for the ki-netics. Thermodynamic modelling, which focuses on stable precipitates, is not an aid in describingsuch processes, as it is usually transitional phases that achieve the desired properties. The model pre-sented here combines first--principles to obtain the transition precipitate energetics (both at the bulkand at the interface with the matrix) with thermochemical databases to describe the overall kineticsof stable precipitates. Precipitate size and number density are captured via the Kampmann--Wagnernumerical approach, which is embedded in a genetic algorithm to obtain optimal compositional andheat treatment scenarios for the optimisation of the mechanical properties in aluminium alloys of the 7000 series.

Abstract: We have performed in-situ magnetization and high-energy X-ray diffraction measurements on two aluminum-based TRIP steels from room temperature down to 100 K in order to evaluate amount and stability of the retained austenite for different heat treatment conditions. We have found that the bainitic holding temperature affects the initial fraction of retained austenite at room temperature but does not to influence significantly the rate of transformation upon cooling.

Abstract: The original mixed-mode model is reformulated by considering the soft impingement effect and applying a general polynomial method of dealing with the concentration gradient in front of the interface. Comparison with the numerical solution shows that the reformulated mixed-mode model is more precise than the original model. The effect of soft impingement on the kinetics of partitioning phase transformation depends on both the growth mode and the degree of super-saturation.

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.