Optimization of L12 precipitation strengthening in ferrous multi-component alloys

Abstract

Abstract

Optimization of L12 precipitation strengthening in ferrous multi-component alloys

Guanghui Yang Department of Materials Science and Engineering The Graduate School Hanyang University

Multicomponent alloys (MCAs) with face-centered-cubic (FCC) structures, often referred to as high- or medium-entropy alloys, have garnered significant attention over the past two decades due to their expansive alloy-design capabilities and remarkable properties. However, their industrial application has been constrained by low performance-to-cost ratio and limited strength. This necessitates the development of low-cost MCAs with exceptional strength.

Traditional methods for strengthening metallic materials often suffer from a trade-off between strength and ductility. However, L12 precipitation strengthening is a promising avenue to significantly enhance strength while maintaining ductility in FCC MCAs. Nevertheless, the formation of undesired brittle phases, such as Heusler and σ phase alongside L12 particles, has resulted in early failure. Furthermore, previously reported L12-strengthened MCAs have exhibited high production costs, primarily due to the utilization of expensive elements such as Co. Consequently, there is an urgent need to develop low-cost, high-performance MCAs with L12 precipitation strengthening. In this thesis, we address the following key objectives: 1. Design of a novel, low-cost, and high-strength ferrous MCA with L12 precipitation. 2. Optimization of mechanical performance of ferrous MCAs containing L12 particles with high density and heterogeneous distribution. 3. Enhancement of L12 precipitation strengthening of ferrous MCAs via activation of incoherent L12 particles.

In the first part, we propose an innovative approach to design low-cost ferrous MCAs containing L12 particles by alloying Ti and Al with the base composition of Fe50Ni30Cr20 (atomic percent, at.%), which closely resembles conventional stainless steels. Through experimental screening, we identified an optimal composition of Fe46.5Ni27.3Cr19.9Al3.4Ti2.8 MCA, which precipitated dense L12 particles without unwanted brittle phases.

Next, we present a method to optimize the mechanical properties of a novel Co-free ferrous Fe46.0Ni28.6Cr18.5Al3.9Ti3.0 MCA by inducing heterogeneous precipitation of L12 particles via cold-rolling and annealing. During annealing at 600 °C for 3 hours, various types of L12 particles such as continuous L12 particles, discontinuous L12 particles, and incoherent L12 particles were triggered, resulting in an exceptional combination of strength and ductility (ultimate tensile strength: 1465 MPa, yield strength: 1265 MPa, total elongation: 17%). This outstanding performance is attributed to the synergistic effects of enhanced precipitation strengthening and activation of multiple strengthening mechanisms such as dislocation plasticity, deformation-induced 9R, multi-variant stacking faults (SFs), and deformation-induced D024 phase.

Finally, we optimize the L12 precipitation strengthening through the activation of incoherent L12 particles via a two-step annealing process in the novel Fe45.2Ni30.1Cr17.9Al3.5Ti3.2 MCA. We discover that moving grain boundaries upon second annealing at 700 °C can bypass the pre-precipitated coherent L12 particles formed during first annealing at 900 °C, thereby inducing a coherent-to-incoherent transition. These incoherent L12 particles significantly increase the resistance to the dislocation gliding, consequently yielding enhanced strength (yield strength of 714 MPa and ultimate tensile strength of 1114 MPa) but also preserving exceptional ductility (total elongation of 28%). This remarkable combination of mechanical properties is attributed to the enhanced precipitation hardening and synergistic activation of diverse deformation mechanisms, including dislocation plasticity, microband-induced plasticity, and formation of dense SFs in both FCC-matrix and L12 particles.

This thesis furnishes crucial insights for the design of cost-effective ferrous MCAs strengthened by L12 precipitates, offering an escape from the dilemma associated with the high production cost of conventional MCAs precipitated with L12 particles. Additionally, it introduces two pivotal strategies to enhance the mechanical properties of the newly devised ferrous MCAs, encompassing heterogeneous precipitation and co-precipitation of both coherent and incoherent L12 particles. We believe that this thesis serves as a critical guide for the development of high-performance, cost-effective MCAs precipitated with L12 particles, thereby facilitating their practical applications in the industry.

Keywords: Multi-component alloys, alloy design, precipitation strengthening, L12 precipitation, continuous precipitate, discontinuous precipitate, 9R phase, D024 phase, stacking fault, incoherent L12 particle.

Student Number: 2017208579