First-principles investigations of iron-based alloys and their properties

Fundamental understanding of the complex interactions governing structure-property relationships in iron-based alloys is necessary to advance ferrous metallurgy. Two key components of alloy design are carbide formation and stabilization and controlling the active deformation mechanism. Following a first-principles methodology, understanding on the electronic level of these components has been gained for predictive modeling of alloys. Transition metal carbides have long played an important role in alloy design, though the complexity of their interactions with the ferrous matrix is not well understood. Bulk, surface, and interface properties of vanadium carbide, VCx, were calculated to provide insight for the carbide formation and stability. Carbon vacancy defects are shown to stabilize the bulk carbide due to increased V-V bonding in addition to localized increased V-C bond strength. The VCx (100) surface energy is minimized when carbon vacancies are at least two layers from the surface. Further, the Fe/VC interface is stabilized through maintaining stoichiometry at the Fe/VC interface. Intrinsic and unstable stacking fault energy, γisf and γusf respectively, were explicitly calculated in nonmagnetic fcc Fe-X systems for X = Al, Si, P, S, and the 3d and 4d transition elements. A parabolic relationship is observed in γisf across the transition metals with minimums observed for Mn and Tc in the 3d and 4d periods, respectively. Mn is the only alloying addition that was shown to decrease γisf in fcc Fe at the given concentration. The effect of alloying on γusf also has a parabolic relationship, with all additions decreasing γisf yielding maximums for Fe and Rh --Abstract, page i

On the Grain Growth Kinetics of a Low Density Steel

The grain growth kinetics of an age-hardenable Fe-Mn-Al-C steel were investigated. Kinetics of grain growth were determined between 1173 and 1348 K (900–1075 °C) to obtain a range of grain sizes from 30 to 475 μm. It was found that grain growth was negligible at 1173 K (900 °C) for times up to 15 h. The activation energy for grain growth was found to be 467 kJ/mol. The hardness and mean linear intercept (L3) were correlated to follow a traditional Hall-Petch relationship. Tensile properties of the alloy were determined after various solution treatments performed for 2 h followed by water quenching. Tensile strength increased from 810 to 960 MPa and ductility was reduced from 80 to 60% as the grain size decreased from 200 μm to 30 μm as grain coarsening was mitigated by lowering the solution treatment temperature

Effect of Nickel, Copper and Chromium on Stacking Fault Energy in FCC Iron

In this study, ab-initio density functional methods are used to examine the effects of nickel, copper, and chromium substitutions on unstable and intrinsic stacking fault energies in FCC iron. The aim of this study was to determine if these alloy additions favor the formation and stability of e-martensite. Nickel and copper additions are shown to increase intrinsic stacking fault energy whereas chromium is shown to have a parabolic relationship. Effects on the unstable stacking fault energy are also examined indicating chromium decreases the unstable stacking fault energy whereas Ni and Cu have a complex effect and are dependent upon proximity to the stacking fault

Microstructural Influence on Mechanical Properties of a Lightweight Ultrahigh Strength Fe-18Mn-10Al-0.9C-5Ni (wt%) Steel

This study evaluates the role of thermomechanical processing and heat treatment on the microstructure and mechanical properties of a hot rolled, annealed, and aged Fe-18Mn-10Al-0.9C-5Ni (wt%) steel. The steel exhibited rapid age hardening kinetics when aged in the temperature range of 500-600 ⁰C for up to 50 h, which has been shown in other work to be the result of B2 ordering in the ferrite and K-carbide precipitation within the austenite matrix. The ultimate tensile strength increased from 1120 MPa in the annealed condition to 1230 MPa after 2 h of aging at 570 ⁰C. Charpy V-notch toughness was evaluated at -40 ⁰C in sub-sized specimens with a maximum in the annealed and quenched condition of 28.5 J in the L-T orientation

Developing a Third-Generation Advanced High-Strength Steel with Two-Stage Trip Behavior

Previous success in achieving exceptional tensile properties of \u3e1100 MPa ultimate tensile strength and \u3e30% elongation to failure in alloys that exhibit a two-stage transformation induced plasticity mechanism (γ-→ε→α) has prompted the continued development of this alloy system. First principles investigations of stacking fault energy revealed Si has the same effect as Al in decreasing the barrier to nucleate e-martensite from parent austenite while decreasing the relative stability of ε-martensite compared to austenite. Insight from ab-initio calculations has been combined with thermodynamic driving force and Ms temperature calculations to develop two alloys of composition Fe-15.1Mn-1.95Si-1.4Al-0.08C- 0.017N (Fe-15-2-1.4-0.08) and Fe-14.3Mn-3.0Si-0.9Al-0.16C-0.022N (Fe-14-3-1-0.16). The Fe-15-2-1.4-0.08 alloy achieved a triplex hot band microstructure of austenite, e-martensite, and a-martensite, which exhibited two-stage TRIP character and a UTS of 1058 MPa at 29.1% elongation to failure. The work hardening rate in this alloy has been related to a grain refinement mechanism characterized by the γ→ε→α\u27 transformation. The Fe-14-3-1-0.16 alloy achieved a hot band microstructure consisting predominately of ε- and α- martensite. The limited fraction of austenite resulted in the absence of Stage I (γ→ε) TRIP and the material work hardened directly after yielding via Stage II (ε→α) TRIP. Over-stabilization of e-martensite led to incomplete transformation to a-martensite that resulted in premature failure at 726 MPa UTS and 11.0% elongation. It is concluded that the ideal hot band microstructure to achieve exceptional tensile properties via two-stage TRIP behavior is composed primarily of austenite and e-martensite which can be controlled via C, Si, and Al alloying

A Modified Embedded Atom Method (MEAM) Interatomic Potential for the Fe-C-H System

We develop an Fe-C-H interatomic potential based on the modified embedded-atom method (MEAM) formalism based on density functional theory to enable large-scale modular dynamics simulations of carbon steel and hydrogen

Alloy Partitioning Effect on Strength and Toughness of κ-Carbide Strengthened Steels

Alloy partitioning during heat treatment in a lightweight precipitation hardened steel was investigated using transmission electron microscopy and atom probe tomography. The mechanical properties are discussed as a function of the effect of solution treatment temperature and aging time, giving rise to variations in chemical modulation. A wrought lightweight steel alloy with a nominal composition of Fe-30Mn-9Al-1Si-1C-0.5Mo (wt. %) was solution-treated between 1173–1273 K and aged at 773 K. Lower solution treatment temperatures retained a finer grain size and accelerated age hardening response that also produced an improved work hardening behavior with a tensile strength of −1460 MPa at 0.4 true strain. Atom probe tomography indicated these conditions also had reduced modulation in the Si and Al content due to the reduced aging time preventing silicon from diffusing out of the κ-carbide into the austenite. This work provides the framework for heat-treating lightweight, age hardenable steels with high strength and improved energy absorption