Analysis of physical phenomena occurring in selected metallic alloys subjected to various metallurgical processes.

S. Dymek

AGH University of Science and Technology, Faculty of Metals Engineering

and Industrial Computer Science, Krakow, Poland

Metallic alloys constitute a class of engineering materials that are widely applied as structural materials in numerous branches of industry. However, apart from their high application potential, they provide an interesting source for theoretical studies of versatile physical phenomena occurring during their processing. The theoretical studies are base on experimental observations that are performed on different scales beginning from the macro down to the atomic scale. This lecture shows examples of materials engineering investigations that originate from the industrial practice and evolve toward the materials science.

         The first example deals with joining of metallic materials by a friction stir welding (FSW). The FSW is a solid-state joining process, i.e. the metal is not melted during the process as it occurs in classical welding processes. The FSW of the material is facilitated by severe plastic deformation in the solid state involving dynamic recovery and recrystallization of the base material. Also, numerous phase transformation may occur that change materials properties. The flow of material during FSW is a complex process that is still not fully understood despite numerous investigations and proposed models. This lecture will discuss friction stir welding of selected aluminum alloys  – wrought and heat-treatable in different configurations, i.e. as similar or dissimilar joints. The characterization of FSW welds was performed by light and electron microscopy (scanning and transmission), diffraction analysis (X-ray as well as electron and neutron), Positron Annihilation Lifetime Spectroscopy (PALS) as well as standard mechanical tests (hardness and tensile). The investigation was supported by developing of a numerical model of material flow and temperature distribution in the mixing zone that allowed for the comprehension of the relationship between forming microstructure and temperature distribution across the weld.

     The other example pertains to our investigation of an unique Ni–25Mo–8Cr (wt.%) superalloy, known under its commercial name Haynes 242 alloy. This is an age-hardenable alloy that utilizes long range order as a primary mechanism for providing room and elevated temperature strength. The standard heat treatment (aging at 650°C for 72 hours) produces coherent, extremely small, metastable long-range ordered domains with the Ni2(Mo,Cr) stoichiometry. This aging nearly doubled the alloy's unaged yield stress, while preserving notable ductility of more than 30% in room temperature. The alloy in an unaged condition deforms predominantly by crystallographic slip while in the aged specimens, microtwinning was found to be a predominant deformation mechanism. The strengthening phase is metastable, however, it is not clear in what manner the microstructure of this alloy will evolve to its stable condition. This is our ongoing study that is realized together with the Joint Institute for Nuclear Research in Dubna.