Influence of precipitation strengthening on high-manganese steels and multi-principal element alloys

Precipitation strengthening, or precipitation hardening, is a widely utilized technique in metallurgy which enhances the mechanical properties of metal alloys by impeding dislocation movement. Initially discovered in aluminum alloys in 1911, this method relies on mechanisms such as dislocation bowing (Ashby–Orowan mechanism) and shearing (Kelly–Fine mechanism) to overcome obstacles posed by precipitates. This technique does not only increase yield strength and ultimate tensile strength, but can also contribute to improved fatigue life, high temperature resistance, wear resistance, and numerous other benefits while being easily applicable at the same time. For these reasons, precipitation strengthening has been used extensively in various alloy systems. However, novel alloy systems with increasingly complex microstructural relationships also increase the requirements and the design opportunities for precipitation strengthening. Therefore, the objective for this thesis is to demonstrate how precipitation strengthening can be used effectively in modern alloy systems like multi-principal element alloys (Fe30Mn10Co10Cr2.5V1.5C) and high-manganese steels (X110MnAl30-8). By employing CALPHAD calculations and finite-element methods microstructures and suitable heat treatment routes were analyzed before manufacturing began. Multiple processing options were chosen, including conventional thermomechanical treatment in addition to additive manufacturing, including in situ heat treatment. With the use of various microscopy methods at different length scales from light optical microscopy to atom probe tomography, precipitation itself and its effects on the microstructure were determined and quantified. In conjunction with mechanical testing—including tensile testing, fatigue testing, and hardness measurements—the resulting mechanical properties and their correlations with the microstructural features were established. Ultimately, the study demonstrates how precipitation behavior in these alloys is heavily reliant on underlying kinetics and diffusivity, which creates a notable difference in the required aging times. The necessary aging times were drastically shortened through adjusting precipitate–matrix combinations and introducing in situ heat treatments into additive manufacturing. It was also found that the formation of undesirable, secondary precipitation at grain boundaries exerted a negative influence on the mechanical properties. The study further concludes that precipitates can strongly affect the deformation behavior, either through changes in the stacking fault energy and the chemical composition of the matrix or through their specific interaction with the dislocation movement. Moreover, such changes to the deformation mechanism in conjunction with the overall strength increase lead to increased fatigue life, based on the precipitation and the subsequent changes to the chemical composition. In summary, this thesis outlines the potential of precipitation strengthening to improve various properties of modern alloy systems outside of the thoroughly defined increases in strength, as well as what challenges can be expected and how those challenges could be addressed.