High Performance Steels: Metastability in Steels
Sponsored by: TMS Structural Materials Division, TMS: Steels Committee
Program Organizers: Ana Luiza Araujo, CBMM North America Inc.; C. Tasan, Massachusetts Institute of Technology; Jonah Kleem-Toole, Colorado School of Mines; Louis Hector, General Motors Global Technical Center; Tilmann Hickel, Max-Planck-Institut Fuer Eisenforschung; Benjamin Adam, Oregon State University

Tuesday 2:30 PM
March 1, 2022
Room: 252C
Location: Anaheim Convention Center

Session Chair: Ana Araujo, CBMM NA; Melissa Thrun, Colorado School of Mines

2:30 PM  
A Novel Approach to Improve Strength-ductility Combinations in Medium Manganese Steels: Dawn Van Iderstine1; Matthew Cagle1; YubRaj Paudel1; Shiraz Mujahid1; Hongjoo Rhee1; Haitham El Kadiri2; 1Center for Advanced Vehicular Systems, Mississippi State University; 2Department of Mechanical Engineering, Mississippi State University
    Stabilizing austenite with manganese is a potential path to new third-generation advanced high-strength steels. Large retained austenite fractions (>30%) have been obtained in intercritically-annealed medium manganese steels. During deformation, this austenite gradually transforms to martensite, prolonging work hardening to give excellent strength-ductility combinations. However, intercritical annealing in its present form has multiple scalability limitations. Resulting mechanical properties are highly sensitive to annealing temperature, and impractical annealing times are often required to sufficiently stabilize austenite. To address such issues, a modified intercritical annealing method is proposed that is expected to promote austenite nucleation with limited growth, enhancing austenite distribution and stabilizing it more efficiently than previously reported for similar compositions. The ductile austenite grains accommodate strain, delaying failure from interactions between the harder matrix ferrite grains. Tensile testing, EBSD, and neutron diffraction are used to study mechanical properties and corresponding austenite transformation behavior, which are compared with literature on similar steels.

2:50 PM  
NOW ON-DEMAND ONLY – The Influence of Temperature on the Strain-hardening Behavior of Fe-22/25/28Mn-3Al-3Si TRIP/TWIP Steels : Dean Pierce1; Jake Benzing2; Jose Jiménez3; Tilmann Hickel4; Ivan Bleskov4; Jong Keum1; Dierk Raabe4; Jim Wittig5; 1Oak Ridge National Laboratory; 2National Institute of Standards and Technology; 3Centro Nacional de Investigaciones Metalurgicas (CSIC); 4Max-Planck-Institut für Eisenforschung; 5Vanderbilt University
    The influence of temperature and stacking fault energy (SFE) on the strain-hardening behavior, microstructural evolution, and critically resolved shear stress for mechanical twinning was investigated for three Fe–22/25/28Mn–3Al–3Si wt.% transformation- and twinning-induced plasticity (TRIP/TWIP) steels. The intrinsic SFEs were calculated by two different methods, density functional theory and statistical thermodynamic modeling. The steels were systematically mechanically tested at elevated temperatures and their dislocation and defect structures characterized by optical and transmission electron microscopy. New insight into factors controlling controlling mechanical twinning and the critical resolved shear stress for twinning at elevated temperatures were discovered.

3:10 PM  
Effects of Austenite Stability on Sheet Forming of Advanced High Strength Steels: Christopher Finfrock1; Benjamin Ellyson1; John Copley1; Brady McBride1; C. Becker1; Diptak Bhattacharya1; Doug Smith1; Chloe Johnson1; Connor Rietema1; Raj Banerjee2; Kamel Fezzaa3; Cody Kirk4; Nesredin Kedir4; Jinling Gao4; Weinong Chen4; Tao Sun5; Niranjan Parab3; Jonah Klemm-Toole1; Amy Clarke1; Kester Clarke1; 1Colorado School of Mines; 2University of North Texas; 3Argonne National Laboratory; 4Purdue University; 5University of Virginia
    Third-generation advanced high strength steels use the deformation-induced phase transformation of austenite to martensite to enhance sheet formability. However, stamping conditions, such as strain rate, strain state, and temperature, influence the balance of strain accommodation via transformation-induced plasticity and dislocation slip. This presentation will explore the processing-property relationships that dictate sheet formability and formed part properties. First, synchrotron diffraction experiments will elucidate the effects of strain rate and temperature on the martensitic transformation. Second, sheet formability experiments, coupled with in-situ measurements of surface strain and temperature evolution, will be used to provide a basis for optimizing microstructures for enhanced formability.

3:30 PM  
Stacking Fault Energy Dependent Deformation Mechanisms in Medium-Mn Steels: Krista Limmer1; Daniel Field1; Daniel Magagnosc1; Timothy Walter1; Christopher Meredith1; Jeffrey Lloyd1; 1DEVCOM Army Research Laboratory
    Medium-Mn steels with a range of stacking fault energy (SFE) values were mechanically tested at a range of temperatures and strain rates and characterized to determine the dominant deformation mechanisms: twinning, phase transformation, or dislocation glide. The temperature and rate dependence of twinning and phase transformation is a function of the alloy dependent intrinsic stacking fault energy as well as the unstable stacking fault energy (USFE). Empirical models were used to predict the composition dependent SFE and first principles calculations were used to determine USFE in representative medium-Mn alloys. Temperature and rate effects on the effective stacking fault energy and resulting deformation mechanisms are discussed in terms of ex-situ characterization, in-situ neutron diffraction, and Taylor impact tests.

3:50 PM  
Reverted Austenite in Precipitation Hardened Stainless Steels: Design Considerations and Mechanical Effects : Hyunseok Oh1; Jiyun Kang1; Menglei Jiang1; Cemal Cem Tasan1; 1Massachusetts Institute of Technology
    Engineering the reverted nanoscale austenite dispersed in a martensitic matrix has been known as a promising way to increase ductility and toughness without sacrificing strength in various advanced high strength steels (AHSS). The reverted austenite has also been introduced in precipitation-hardened martensitic stainless steels to increase the toughness. However, the precise role of reverted austenite on the mechanical properties of the stainless steels is not fully understood and requires a systematic investigation. Here, we study the mechanical effects of the reverted austenite with different stacking fault energies in a PH 17-4 stainless steel via integrated in-situ scanning electron microscope and in-situ synchrotron deformation experiments. In particular, we will discuss the effects of austenite on 1) strength and 2) strain localization with their fundamental mechanisms. This study will pave the way toward a well-targeted design of stainless steels for simultaneously increasing ductility and toughness without sacrificing strength.

4:10 PM Break

4:25 PM  
Tailoring Transformation Plasticity to Resist Microvoid Shear Localization: Brandon Snow1; G. Olson1; D. Parks1; 1Massachusetts Institute of Technology
    Ductile fracture toughness in high strength steels is limited by nucleation, growth, and coalescense of voids. Shear localization of void sheets originating from submicron grain-refining carbides limits local ductility and macroscopic toughness. In recent years, attention has focused on developing steels exhibiting transformation-induced plasticity (TRIP) in order to increase fracture toughness while maintaining high strength levels. Here finite element models of particle-containing representative volume elements (RVEs) are used to investigate the effects of key material properties, including austenite stability and interface adhesion, on the local voiding processes. The studies probe the mechanisms by which TRIP steels achieve high toughness and identify directions to further optimize toughness.

4:45 PM  
Tempering & Austempering of Double Soaked Medium Manganese Steels: Alexandra Glover1; Cheng Liu1; Emmanuel DeMoor2; John Speer2; 1Los Alamos National Laboratory; 2Colorado School of Mines
    The double soaking heat treatment, when applied to a 0.14C 7.17Mn (wt%) medium manganese steel, has been shown to generate a strength ductility combination desired for the third generation of advanced high strength steels. This work builds upon the double soaking heat treatment through the addition of a tempering or austempering step. Microstructures following double soaking and (aus)tempering were shown to contain a combination of retained austenite, athermal or tempered martensite, and blocky or bainitic ferrite. Microstructural characterization utilized X-ray diffraction, dilatometry and transmission Kikuchi diffraction. The resulting mechanical properties and localized deformation behavior were measured using uniaxial tensile tests in conjunction with digital image correlation. The double soaking plus tempering heat treatment was shown to produce an ultimate tensile strength of 1,340 MPa and 28% total elongation, while the double soaking plus austempering heat treatment resulted in an ultimate tensile strength of 1,675 MPa and total elongation of 22%.

5:05 PM  
Deformable Plastic Strain-induced Epsilon-martensite in FeMnCo Alloys: A Pathway Towards Overcoming the Limits of Metastability : Shaolou Wei1; Cem Tasan1; 1Massachusetts Institute of Technology
    In contrast to pseudo-elastic martensitic phases, typical BCT-martensite formed through plastic straining often exhibits brittle-like features, largely ascribed to the extensive defect density and the pronounced plastic accommodation in the vicinal parent phase. A somewhat intermediate situation occurs when epsilon-martensite is nucleated during plastic deformation: the similar atomic stacking sequence between HCP and FCC lattices enables more moderate interfacial mismatch, alleviating plastic accommodation. In light of this, we will show that through appropriate compositional design, highly deformable strain-induced epsilon-martensite can be achieved in ternary FeMnCo alloys. We will detail in this presentation: (1) the exploration of a sequential FCC-HCP-FCC martensitic transformation chain by plastic straining; (2) the understanding of an atomic shuffle-involved “twinning” in strain-induced epsilon-martensite; and (3) the atomistic procedures that accomplish “twinned” epsilon-martensite to final FCC transformation. Finally, a discussion towards the feasibility of multi-stage TRIP/TWIP metastability engineering will also be included.

5:25 PM  
Manganese-diffusion Controlled Kinetics of Austenite Growth and Cementite Dissolution during Intercritical Annealing of Medium-Mn Steels : Josh Mueller1; John Speer2; David Matlock2; Emmanuel De Moor2; 1Los Alamos National Laboratory; 2Colorado School of Mines
    Simulations for microstructural evolution during intercritical annealing (IA) of medium-Manganese steels predict slow dissolution of cementite while austenite growth occurs via a Mn-diffusion controlled transformation. These simulations have been shown to be in good agreement with in situ austenite fractions during intercritical annealing measured with high energy X-ray diffraction. The present work aims to further elucidate austenite transformation and cementite dissolution mechanisms during IA. Consistent with simulation predictions, micrographs via scanning electron microscopy, and energy dispersive X-ray spectroscopy via scanning transmission electron microscopy, of a Fe 0.2C 4.5Mn steel indicate that cementite was enriched in Mn prior to IA, and was slow to dissolve during IA. Substantial growth of austenite with Mn enrichment also occurred during IA. Thermodynamic assessments were made which reveal Mn-diffusion controlled kinetics associated with the dissolution of cementite, and the inhibition of rapid initial austenite growth via so-called negligible-partitioning local-equilibrium.