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Abstract

In recent decades, considerable efforts have been made in the production of steel and the modification of its microstructure in order to improve the mechanical properties through the incorporation of nano-particles. Nanotechnologies offer potential in the manufacture of nano-engineered steels. While typical production methods for Metal Matrix nano-Composites (MMnCs) are difficult and expensive, the main drawback of the casting method is the agglomeration of the nano-particles and a poor interface between the nano-particles and the metal matrix. Therefore, the aim of this doctoral thesis was to investigate the potential of different nano-particle additions as reinforcement elements in the conventional liquid-metal casting process, which cannot be obtained or at least not easily through precipitation and to study their effect on the microstructure, mechanical properties and wear behavior. The investigation was focused on the various approaches to the production, modification and addition of nano-particles, as well as the influence of the type, concentration, size and surface modification of nano-particles added to a steel matrix on the homogeneity and distribution. The right combination of mentioned factors can provide superior mechanical strength, while maintaining or even improving other properties, such as toughness, damping capacity, wear resistance, creep behavior as well as electrical and thermal properties. The results show that also in the case of the conventional casting process, it is possible to produce reinforced steel-matrix nano-composites with a homogeneous distribution of the nano-particles in the matrix, resulting in improved properties. The identification and interface between the nano-particles and the steel matrix is highlighted and the properties evaluated on the example of austenitic stainless steel (AISI 316L type), used due to its simple two-phase microstructure. The properties were found to depend on the type and distribution of the reinforcing nano-particles, with the best results shown by Al2O3, Y2O3 and TiB2 nano-particles when mixed with a dispersive medium. When the nano-particles were used with a dispersive medium, a more uniform distribution of the nano-particles was achieved, leading to improved hardness, fatigue and wear resistance, while maintaining the yield and ultimate tensile strength of the reference non-modified steel. On the other hand, nano-particle reinforcement in general resulted in a reduced high-temperature creep resistance, evaluated at 650 °C. However, the main reason lies in the precipitation of the σ phase, starting above 600 °C, with the incorporated nano-particles acting as nucleation sites. A higher fraction of the σ phase is therefore present in the nano-particle-reinforced material, thus having a detrimental effect on the high-temperature properties of the investigated steel.