Modeling of Z-phase formation from disordered precursor nitride-phases MX and M2X during heat-treatment and creep exposure of martensitic Z-steel

Description

1.) Introduction/ Purpose:
Z-steels are a new promising generation of creep resistant martensitic Cr-steels which contain Tantalum for the formation of long-term stable precipitates CrTaN of type Z-phase. As the demand for simulating the microstructural stability and evolution of this new material rose in the past years, there was a strong need to create a new CALPHAD-type thermodynamic database including the relevant precipitate phases containing Tantalum.
2.) Methods:
A thermodynamic Z-steel database for the precipitation kinetic software MatCalc was developed by implementing Tantalum. Precipitate sizes, chemical compositions, binary/ ternary diagrams of investigated systems (such as Ta-C, Ta-N, Ta-C-N), diffusion data, atom solubilities and first principle calculations from literature were collected and used in the database creation. Additionally, DSC measurements were conducted to determine the dissolution temperature of Z-phase. In addition to thermodynamic equilibrium calculations of the system, it is now possible to model nucleation and transformation of fcc MX precipitates and of bcc M2X precipitates into Z-phase in MatCalc version 6 by applying the concept of equivalent interface energy and minimum nucleation radius.
3.) Results:
The precipitation evolution during manufacturing and high-temperature aging of two sample alloys was modelled and compared to TEM, SEM and EBSD measurements. The results show good agreement between simulation and experiment.
4.) Conclusion:
The new thermodynamic database is able to model MX and M2X nucleation, their transformation into Ta-containing Z-phase and the coarsening phenomena during high-temperature aging.

Period	4 Sep 2019
Event title	European Congress and Exhibition on Advanced Materials and Processes: EUROMAT 2019
Event type	Conference
Location	Stockholm, Sweden
Degree of Recognition	International