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Design of Phase Transition Kinetics in Non-Equilibrium Metals

Periodic Reporting for period 2 - TRANSDESIGN (Design of Phase Transition Kinetics in Non-Equilibrium Metals)

Reporting period: 2019-08-01 to 2021-01-31

The first technological use of non-equilibrium phase transitions in metals for designing properties of materials is documented as ~800 BC, but the time has come to make a leap forward. Nearly all classes of materials show non-equilibrium phase transitions. Understanding how fast these transitions occur is a key question in materials science. In metals, kinetics is connected to diffusion via atomic lattice vacancies. However, there is no universal sound and predictive physical understanding of the kinetics under non-equilibrium situations so far, because theory cannot be verified experimentally. The in situ measuring of non-equilibrium kinetics and the corresponding vacancy evolution is not possible at industrially relevant and controlled high thermal rates of nowadays. Moreover, direct atomistic observation of vacancies in bulk metals has not been achieved in the past.
The development of unique strategies for in situ measuring of non-equilibrium phase transition kinetics and the microscopic observation of the underlying processes are the main objectives of TRANSDESIGN. Unveiling kinetic evolution of non-equilibrium vacancies and their atomic lattice motion in metals are seen as breakthroughs. Hence, we observe non-equilibrium vacancy annihilation via ultrafast chip calorimetry, which offers unique advantages to understand non-equilibrium diffusion. Moreover, we aim to establish ultrafast in situ measurements of non-equilibrium phase transitions via chip calorimetry as a standard in thermal analysis for metallic systems of technological/economic relevance or with high potential. Within TRANSDESIGN we further utilize high image contrast solutes, which trap vacancies, as markers for an identification of the role of vacancies via scanning transmission electron microscopy. This unique strategy should enable the observation of “vacancies-at-work” in the bulk of metals.
The project aims to close longstanding experimental-theoretical gaps with significant impact on the optimization and design of new kinetically driven processes and products in the field of metallurgy. Bridging chip calorimetry and atomic characterisation experiments with thermo-kinetic and atomistic simulations are used to validate existing theoretical models and to create new universal guidelines to understand and to design phase transition kinetics in non-equilibrium metallic systems. However, the fundamentals gained within the TRANSDESIGN project are intended to be universal to significantly contribute to the advancement of the European competence in materials science.
We have already accomplished the goal of work package 1 – “Utilizing chip calorimetry for non-equilibrium phase transitions in relevant metals” – to apply chip calorimetry for non-equilibrium phase transitions in relevant metals. Our chip-calorimetry system is now already set-up for its use above 500 °C under well-controlled conditions together with a measurement strategy for increased sensitivity. The gained methodical results in the field of advanced chip calorimetry have already been published and satisfy our expectation on the indented high-performance high rate calorimetry. Currently our methodology is applied on in situ measurements of non-equilibrium phase transitions in different alloys (e.g. Al alloys for additive manufacturing).
In work package 2 – “Measuring the non-equilibrium vacancy evolution” – we currently develop methods for in situ observation of the non-equilibrium vacancy evolution at rapid rate thermal histories. First promising results have been gained, but further work is necessary to increase significance of data and to enhance the stability of the measurement conditions for such ultrasensitive measurements, which is fully in the time line of the description of the action.
Within work package 3 – “Microscopic imaging of atomic vacancies in metals” – we elaborate the microscopic observation of atomic vacancies in metals. Single vacancy-marker atom detection is already possible in our own laboratory. Moreover, we could already report on this track that the sample dimensions play a crucial role for microscopic in situ observations of vacancy-based non-equilibrium phase transitions. Following this, we currently test various observation conditions such as sample type, temperature, acceleration voltage and the type of solute to study the dynamics in various metals.
We have definitely already made progress in the proposed methodological areas covered by the TRANSDESIGN project.
For chip calorimetry of non-equilibrium phase transitions we showed that a measurement of specific heat capacity with high sensitivity is possible in metals for high rate processes. This was beyond the state-of-the-art and now serves as an important achievement in establishing the method as a standard in thermal analysis for metallic systems. Moreover, the gained knowledge sets a base for the targeted ultimate goal of in-situ measuring the non-equilibrium vacancy kinetics, since a method to significantly improve sensitive was also presented in this study. This let us expect to finally reach the goal of an in situ study of non-equilibrium vacancy evolution to be made possible via chip calorimetry.
In the field of non-equilibrium phase transition kinetics and their microscopic observation we could report that the sample dimensions play a crucial role for vacancy-based non-equilibrium phase transitions. This is of special importance when solid state solute clustering reactions in alloys are studied as it means that the conventional generation of a significant number of thermal vacancies by quenching is not possible at the nanoscale. This is far beyond the state of the art and pushes forward not only our one ultimate idea on the solute marker methodology to study solute vacancy interactions, but also the microscopy community working in this field. We have already good indication of solute motion driven by non-equilibrium vacancies in scanning transmission electron microscopy (STEM), but this happens at extremely high kinetics. The large accessible potential parametric space of such observations let us expect to find conditions where it will be possible to study this motion in detail.
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