[Paper Review] Harnessing metastability for grain size control in multiprincipal element alloys during additive manufacturing

Akane Wakai, Jenniffer Bustillos+5 more|arXiv (Cornell University)|May 6, 2024

Additive Manufacturing Materials and Processes4 citations

TL;DR

This study introduces a novel alloy design strategy in FeMnCoCr multiprincipal element alloys (MPEAs) for additive manufacturing by leveraging metastability induced by increasing Mn content to suppress columnar grain growth and achieve significant grain refinement. By tuning phase stability through composition, the authors enable precise microstructural control via solidification engineering, demonstrating a pathway to tailorable grain sizes in AM-fabricated MPEAs.

ABSTRACT

Controlling microstructure in fusion-based metal additive manufacturing (AM) remains a challenge due to numerous parameters directly impacting solidification conditions. Multiprincipal element alloys (MPEAs) offer a vast compositional design space for microstructural engineering due to their chemical complexity and exceptional properties. Here, we establish a novel alloy design paradigm in MPEAs for AM using the FeMnCoCr system. By exploiting the decreasing phase stability with increasing Mn content, we achieve notable grain refinement and breakdown of columnar grain growth. We combine thermodynamic modeling, operando synchrotron X-ray diffraction, multiscale microstructural characterization, and mechanical testing to gain insight into the solidification physics and its ramifications on the resulting microstructure. This work paves way for tailoring grain sizes through targeted manipulation of phase stability, thereby advancing microstructure control in AM.

Motivation & Objective

• To address the persistent challenge of microstructural control in fusion-based additive manufacturing of complex alloys.
• To explore the role of compositional complexity in modulating solidification behavior and grain structure in multiprincipal element alloys (MPEAs).
• To establish a new design paradigm for grain size engineering in MPEAs by exploiting metastability through controlled Mn content variation.
• To link thermodynamic stability, solidification dynamics, and resulting microstructure in AM-processed MPEAs using in situ and multiscale characterization.

Proposed method

• Employing thermodynamic modeling to predict phase stability trends across varying Mn concentrations in FeMnCoCr MPEAs.
• Conducting operando synchrotron X-ray diffraction during laser melting to capture real-time solidification dynamics and phase evolution.
• Performing multiscale microstructural characterization (e.g., electron backscatter diffraction, electron microscopy) to analyze grain morphology and texture.
• Integrating mechanical testing to correlate microstructure with mechanical response and validate performance outcomes.
• Systematically varying Mn content to tune phase stability and assess its impact on nucleation and grain growth kinetics.
• Correlating experimental observations with thermodynamic predictions to establish a design rule for grain size control in AM-MPEAs.

Experimental results

Research questions

• RQ1 How does increasing Mn content influence phase stability and solidification behavior in FeMnCoCr MPEAs during additive manufacturing?
• RQ2 Can metastable phase formation be harnessed to suppress columnar grain growth and promote equiaxed grain nucleation?
• RQ3 What is the relationship between phase stability trends and microstructural evolution under AM processing conditions?
• RQ4 To what extent can grain size be tailored through compositional tuning of Mn in MPEAs?
• RQ5 How do solidification dynamics, as observed in operando, correlate with final microstructure and grain morphology?

Key findings

• Increasing Mn content in FeMnCoCr MPEAs reduces phase stability, promoting metastable phase formation during solidification.
• The transition to metastable phases suppresses columnar grain growth, leading to a significant refinement of grain size.
• Operando synchrotron X-ray diffraction revealed that metastable phase nucleation occurs rapidly during solidification, coinciding with grain refinement.
• Microstructural analysis confirmed a shift from columnar to equiaxed grain structures with higher Mn content, indicating effective nucleation promotion.
• Mechanical testing demonstrated improved strength and ductility in the refined-grain microstructures, validating the performance benefits of the approach.
• The study establishes a direct link between compositional tuning of Mn and controllable grain size via metastability engineering in AM-fabricated MPEAs.