Atomic-scale grain boundary engineering to overcome hot-cracking in additively-manufactured superalloys

There are still debates regarding the mechanisms that lead to hot cracking in parts build by additive manufacturing (AM) of non-weldable Ni-based superalloys. This lack of in-depth understanding of the root causes of hot cracking is an impediment to designing engineering parts for safety-critical applications. Here, we deploy a near-atomic-scale approach to investigate the details of the compositional decoration of grain boundaries in the coarse-grained, columnar microstructure in parts built from a non-weldable Ni-based superalloy by selective electron-beam melting. The progressive enrichment in Cr, Mo and B at grain boundaries over the course of the AM-typical successive solidification and remelting events, accompanied by solid-state diffusion, causes grain boundary segregation induced liquation. This observation is consistent with thermodynamic calculations. We demonstrate that by adjusting build parameters to obtain a fine-grained equiaxed or a columnar microstructure with grain width smaller than 100 $\mu$m enables to avoid cracking, despite strong grain boundary segregation. We find that the spread of critical solutes to a higher total interfacial area, combined with lower thermal stresses, helps to suppress interfacial liquation.Comment: Accepted version at Acta Materiali

Current Challenges and Opportunities in Microstructure-Related Properties of Advanced High-Strength Steels

This is a viewpoint paper on recent progress in the understanding of the microstructure–property relations of advanced high-strength steels (AHSS). These alloys constitute a class of high-strength, formable steels that are designed mainly as sheet products for the transportation sector. AHSS have often very complex and hierarchical microstructures consisting of ferrite, austenite, bainite, or martensite matrix or of duplex or even multiphase mixtures of these constituents, sometimes enriched with precipitates. This complexity makes it challenging to establish reliable and mechanism-based microstructure–property relationships. A number of excellent studies already exist about the different types of AHSS (such as dual-phase steels, complex phase steels, transformation-induced plasticity steels, twinning-induced plasticity steels, bainitic steels, quenching and partitioning steels, press hardening steels, etc.) and several overviews appeared in which their engineering features related to mechanical properties and forming were discussed. This article reviews recent progress in the understanding of microstructures and alloy design in this field, placing particular attention on the deformation and strain hardening mechanisms of Mn-containing steels that utilize complex dislocation substructures, nanoscale precipitation patterns, deformation-driven transformation, and twinning effects. Recent developments on microalloyed nanoprecipitation hardened and press hardening steels are also reviewed. Besides providing a critical discussion of their microstructures and properties, vital features such as their resistance to hydrogen embrittlement and damage formation are also evaluated. We also present latest progress in advanced characterization and modeling techniques applied to AHSS. Finally, emerging topics such as machine learning, through-process simulation, and additive manufacturing of AHSS are discussed. The aim of this viewpoint is to identify similarities in the deformation and damage mechanisms among these various types of advanced steels and to use these observations for their further development and maturation

Phase nucleation through confined spinodal fluctuations at crystal defects in Fe-Mn alloys

Segregation or adsorption to crystalline defects like grain boundaries and dislocations has been widely associated with a wide variety of relevant phenomena for the design of new materials like medium manganese steels. Mobility and cohesion are two important properties of grain boundaries that are influenced by the local chemistry of the defect. Segregation plays a key role as well during the initial stages of diffusional nucleation and growth of a new phase from a supersaturated solid solution. Notwithstanding, these phenomena are often treated in a disconnected manner. This work aims to clarify the role of Mn segregation to crystalline defects during the early stages of austenite reversion in medium manganese steels. Three binary Fe-Mn alloys, identified as Fe3Mn, Fe4Mn and Fe9Mn (in wt.%), were annealed at 450°C up to 2 months in order to follow the segregation of Mn to crystalline defects, the early stages of nucleation of austenite and finally the growth of this phase. Additionally, a Fe-7Mn-0.1C-0.5Si (in wt.%) medium manganese steel was annealed at 450°C up to 2 weeks in order to investigate the effect of carbon on the manganese segregation, carbide nucleation and austenite reversion. The samples from different annealing times were characterized at different length scales by using various techniques, including atom probe tomography (APT), electron back scattered diffraction (EBSD), Kikuchi transmission diffraction (TKD), x-ray diffraction (XRD) and transmission electron microscopy (TEM). Additionally, thermodynamic-kinetic calculations were performed in order to describe the observed sequence of segregation, nucleation and growth during phase transformation.The results reveal that Mn segregation or adsorption to the crystalline defects proceeds in a spinodal-fashioned way with well-defined compositional fluctuations. These low-dimensional spinodal fluctuations act as a precursor to the nucleation of the austenite phase when they become strong enough in composition and wavelength. The co-segregation of carbon with Mn leads to much stronger fluctuations in the segregated regions allowing the formation of the nucleus of M23C6 carbide before the nucleation of austenite. A non-classical multi-step mechanism through confined spinodal fluctuations mechanism is proposed for the nucleation of austenite and transition carbides in medium manganese steels together with a model to estimate the amount of Mn segregated to the grain boundaries before the nucleation event

Segregation assisted grain boundary precipitation in a model Al-Zn-Mg-Cu alloy

International audienceUnderstanding the composition evolution of grain boundaries and grain boundary precipitation at near-atomic scale in aluminum alloys is crucial to tailor mechanical properties and to increase resistance to corrosion and stress corrosion cracking. Here, we elucidate the sequence of precipitation on grain boundaries in comparison to the bulk in a model Al-Zn-Mg-Cu alloy. We investigate the material from the solution heat treated state (475°C), through the very early stages of aging to the peak aged state at 120°C and further into the overaged regime at 180°C. The process starts with solute enrichment on grain boundaries due to equilibrium segregation accompanied by solute depletion in their vicinity, the formation of Guinier–Preston (GP) zones in the solute-enriched grain boundary regions, and GP zones growth and transformation. The equilibrium segregation of solutes to grain boundaries during aging accelerates this sequence compared to the bulk. Analysis of the ~10 nm wide precipitate-free zones (PFZs) adjacent to the solute-enriched grain boundaries 2 shows that the depletion zones are determined by (i) interface equilibrium segregation; (ii) formation and coarsening of the grain boundary precipitates and (iii) the diffusion range of solutes in the matrix. In addition, we quantify the difference in kinetics between grain boundary and bulk precipitation. The precipitation kinetics, as observed in terms of volume fraction, average radius, and number density, is almost identical next to the depletion zone in the bulk and far inside the bulk grain remote from any grain boundary influence. This observation shows that the region influenced by the grain boundaries does not extend beyond the PFZs

An Automated Computational Approach for Complete In-Plane Compositional Interface Analysis by Atom Probe Tomography

International audienc

An Automated Computational Approach for Complete In-Plane Compositional Interface Analysis by Atom Probe Tomography

International audienc

Microstructural Characterisation of High-and Medium Mn Steels

4th HMnS / IEHK, RWTH Aachen University Proceedings (2019) 246 - 249RWTH-2019-0375