Microstructural pattern formation during eutectoid transformation in multicomponent steels

• Applicant: Professorin Dr. Britta Nestler, since 3/2020
• Subject Area: Metallurgical, Thermal and Thermomechanical Treatment of Materials ; Mechanical Properties of Metallic Materials and their Microstructural Origins ; Thermodynamics and Kinetics as well as Properties of Phases and Microstructure of Materials
• Term: from 2016 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 314490367

• Final Report Year: 2021

Final Report Abstract

The research and outcomes of the project, which largely coincides with the working package of the proposal, can be summarised as follows: ˆ For the accurate handling of multicomponent steels in phase-field framework, existing grand-potential model is revisited. Two distinct approaches have been followed to incorporate interstitial diffusion in grand-potential formalism. These two approaches vary in treatment of concentration. While conventionally mole fraction is adopted to handle concentration, in the revisited grand-potential models the composition of the alloying are treated either by molar number-density or site fraction. Such treatment of concentration facilitates in distinguishing substitutional diffusion from the interstitial migration of chemical species. ˆ Both steady-state and non steady-state growth of pearlite in multicomponent steel, particularly, manganese steel, is modelled under phase-field framework. Appropriate quantitative driving-forces are incorporated through polynomial approximation of free-energy densities rendered by CALPHAD database. Moreover, an approach to introduce surface diffusion is discussed to ensure that the different modes of mass transfer as sufficiently introduced in the formulation. The steady-state evolution of pearlite yields a regular lamellar arrangement of ferrite and cementite. This microstructure is achieved by the time-independent equilibrium condition which is established at the transformation front. Whereas, owing to the varying equilibrium condition in the non steady-state transformation, divergent pearlite with increasing interlamellar spacing is formed. Corroborating with the experimental observations, it is realised that the decrease in the carbon content in the austenite matrix is the primary reason for the change in equilibrium condition at the interface. Exhaustive study to identify the factors influencing the evolution of divergent pearlite is pursued to aide the Jackson-Hunt treatment of this unique microstructural evolution. ˆCurrent understanding of divorced eutectoid transformation is deepened in this project by investigating morphological transformation of three-dimensional cementite structures. Preliminary studies involving twodimensional setups have identified that the curvature-driven transformation play a vital role in the growth of cementite during non-cooperative divorced eutectoid transformation. Therefore, morphological of analytically ill-defined three-dimensional cementite plates have been extensively investigated. These analysis, besides unravelling the hitherto unknown mechanism of spheroidization, have rendered crucial insights on the kinetics of the evolution which can be employed to perfect the heat treatment cycles associated with sub-critical annealing. Moreover, in-keeping with the proposal, the interaction of the evolving cementite structure and grain boundaries in polycrystalline system has been analysed. In order to perfect the multicomponent phase-field formulation, which is employ to model solid-state transformation, elastic driving-force has been augmented. Under this chemo-elastic framework, solid-state phase transformations wherein mechanical components like eigenstrain play a key role can be coherently modelled. The introduction excess energy, which is unavoidable in most elastic formulations, is circumvented in the present technique by choosing appropriate dynamic-variables based on the jump conditions. By employing this chemo-elastic approach, microstructural evolution pertaining to Widmanstatten ferrite and bainite have been modelled, which are consistent with experimental observation.

Publications

• Phase-field analysis of quenching and partitioning in a polycrystalline Fe-C system under constrained-carbon equilibrium condition, Computational Material Science, 159, 281-296 (2019)
  ˆ P G Kubendran Amos, E Schoof, N Streichan, D Schneider and B Nestler
  
• Mechanisms of pearlite spheroidization: Insights from 3D phase-field simulations, Acta Materialia, 161, 400-411 (2018)
  P G Kubendran Amos, A Bhattacharya, B Nestler and K Ankit
  
• Phase-field simulations of pearlitic divergence in Fe-C-Mn steels, Acta Materialia, 93m 78-87 (2018)
  L T Mushongera, P G Kubendran Amos, B Nestler and K Ankit
  
• Distinguishing interstitial and substitutional diffusion in grand-potential based phase-field model, Materialia, 12, 100820 (2020)
  ˆ P G Kubendran Amos and B Nestler
  
• Influence of stress-free transformation strain on the autocatalytic growth of bainite: A multiphase-field analysis, Materialia, 9, 100620 (2020)
  E Schoof, P G Kubendran Amos, D Schneider and B Nestler
  
• Limitations of preserving volume in Allen-Cahn framework for microstructural analysis, Computational Materials Science, 173, 109338 (2020)
  P G Kubendran Amos, E Schoof, J Santoki, D Schneider and B Nestler
  
• Multiphase-field modelling of concurrent grain growth and coarsening in complex multicomponent systems, Journal of Material Science and Technology, 45, 215 - 229 (2020)
  ˆ P G Kubendran Amos, R Perumal, M Selzer and B Nestler
  
• Quadrijunctions-stunted grain growth in duplex microstructure: A multiphase-field analysis, Scripta Materialia, 182, 16-20 (2020)
  R Perumal, P G Kubendran Amos, M Selzer and B Nestler
  
• The non-steady-state growth of divergent pearlite in Fe–C–Mn steels: a phase-field investigation., Journal of Material Science, 55, 5280-5295 (2020)
  L T Mushongera, P G Kubendran Amos, E Schoof, P Kumar and B Nestler