Thermodynamic Modelling and Microstructural Study of Z-Phase Formation in a Ta-Alloyed Martensitic Steel

Area of Science:

• Materials Science
• Computational Materials Science
• Metallurgy

Background:

• Martensitic Z-steels are crucial advanced materials.
• Understanding precipitate transformation is key to optimizing their properties.
• Tantalum (Ta) and Chromium (Cr) play significant roles in Z-steel microstructures.

Purpose of the Study:

• To develop a thermokinetic computational framework for simulating precipitate transformations in Tantalum-containing martensitic Z-steels.
• To accurately model the transformation of MX phase to Z-phase, driven by Chromium diffusion.

Main Methods:

• Developed a thermokinetic framework integrating CALPHAD thermodynamics and diffusion mobility data.
• Assessed thermodynamics using solubility data, enthalpies, and experimental dissolution temperatures (differential scanning calorimetry).
• Performed simulations using MatCalc, incorporating precipitation sites, interfacial energies, and dislocation density evolution, validated by transmission electron microscopy.

Main Results:

• Simulations showed excellent agreement with experimental data for precipitate size, number density, and chemical composition.
• The framework successfully modeled the MX to Z-phase transformation driven by Chromium diffusion.
• Validated the usability and accuracy of the developed thermokinetic simulation approach.

Conclusions:

• The developed thermokinetic framework provides a reliable tool for predicting precipitate behavior in Z-steels.
• This computational approach aids in the design and optimization of advanced martensitic steels.
• Accurate simulation of Z-phase precipitation is achievable with detailed thermodynamic and kinetic parameters.