Spinodal decomposition in advanced stainless steels and cemented carbides
Spinodal decomposition may occur in materials with a miscibility gap in the phase diagram and leads to demixing on the atomic scale. After prolonged heat treatment or service at elevated temperatures this leads to an increased hardness and strength. However, it may also cause embrittlement at low temperatures. In fact, the upper service temperature of duplex stainless steels is limited by the embrittlement caused by spinodal decomposition whereas the microstructure development in carbide phases in cemented carbides is a potential strengthening mechanism that could increase performance and tool life.
The scientific goal is to develop a method for prediction of the effect of individual alloying elements, stress state and temperature on phase equilibria, reaction kinetics, morphology and finally the effect of the morphology on the resulting properties. The industrial goal is to improve the performance of the materials, either utilizing the effect to increase tool performance of cemented carbides or decreasing kinetics via an alloying strategy to increase the upper service temperature of the important duplex stainless steels.
So far the work has concentrated on the binary Fe-Cr system, the base system for all stainless steels, and the carbide system TiC-ZrC. Since phase separation in the Fe-Cr system takes place at relatively low temperatures the thermodynamic description below 500 °C needed revision. Consequently a new thermodynamic description of the binary Fe-Cr has been developed and shows significant differences compared to the currently accepted description at low temperatures. Extensive ab-initio computations have provided new insights to the importance of a proper handling of the magnetic contribution to the total energy and its effect on various physical properties. The microstructure evolution during phase separation has been modeled using the phase field approach based on the non-linear Cahn-Hilliard equation in 3D and the new thermodynamic description. The simulations show that the atomic mobilities at low temperatures need revision and thermal fluctuations need to be included.
On the experimental side we have studied phase separation in binary Fe-Cr alloys and commercial duplex stainless steel grades (base metal, simulated HAZ and weld metal) for different ageing times, temperatures and for the commercial materials also the effect of load with micro-hardness measurements, transmission electron microscopy and Atom-Probe- Tomography.
Figure: Phase separation in Fe-Cr (Xiong et al., Calphad 2011)
Figure: Decomposition in TiZrC by STEM-HAADF