Thermodynamic Investigation of New High-Strength Low-Alloy Steels with Heusler Phase Strengthening for Welding and Additive Manufacturing: High-Throughput CALPHAD Calculations and Key Experiments for Database Verification

Abstract

High-strength low-alloy (HSLA) steels often comprise of Cu clusters and M 2 C (M: Mo, Cr) carbides as strengthening particles. In this work, three new HSLA steels with alternative strengthening phases, Fe 2 SiTi and Ni 3 Ti, are investigated by using HSLA-115 steel as the reference. To evaluate the weldability for potential fabrication using casting, welding, and additive manufacturing, freezing ranges are studied using differential thermal analysis (DTA), and CALPHAD (Calculation of Phase Diagrams) approach under equilibrium and nonequilibrium conditions. While the cooling signals in the DTA analysis are not pronounced enough for thermal analysis, the trend of freezing range change based on the nonequilibrium and equilibrium calculations are consistent with the heating signals. High-throughput calculations are performed to deduce the effect of variation of each alloying element on the freezing range. Moreover, the experimental and calculated phase fractions and compositions of the as-cast and heat-treated alloys were compared. Though CALPHAD model-prediction can provide valuable insights into the phase stability of these new alloys, there is a remarkable difference regarding phase fraction and composition of individual phases. Therefore, this study indicates that the application of the CALPHAD approach in new alloy discovery requires a careful model-validation and database calibration.Keywords CALPHAD Á Heusler phase Á HSLA steels Á thermal analysis Á thermodynamics This article is an invited paper selected from presentations at ''PSDK XIV: Phase Stability and Diffusion Kinetics-Gibbs: Phase Equilibria, Diffusion and Materials Design'' held during MS&T'19, September 29-October 3, 2019, in Portland, Oregon. The special sessions were dedicated to honor Dr. Patrice Turchi, recipient of the ASM International 2019 J. Willard Gibbs Phase Equilibria Award ''for outstanding and pioneering contributions in the application of first-principles, quantum-mechanical calculations to the modeling of phase equilibria and thermodynamic behavior of alloys.'' It has been expanded from its original presentation.

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