Papers by Author: M. Topic

Abstract: We investigated two coatings, Pt:Al and Pt3:Al, deposited onto pure Al substrates. After annealing at 500°C for 2 hours in vacuum, the Pt:Al coating became “matt silver” while the Pt3:Al colour was “old gold”. Although the deposited coatings were relatively smooth, rope-and ridge-like morphologies were observed in the annealed Pt:Al and Pt3:Al coatings, respectively. X-ray diffraction analyses revealed the presence of Al₂₁Pt₆ and Al₂Pt intermetallics in annealed Pt:Al while only the Al₂Pt intermetallic phase was observed in the annealed Pt3:Al coatings. RBS spectra showed an increase of coating thickness by four times in the Pt:Al whilst the coating thickness was doubled in the Pt3:Al coatings. The changes to color, surface morphology, and thickness caused by thermal annealing were attributed to the structural characteristics of the most dominant phases.

Abstract: Platinum and platinum alloys, due to their good electrical and mechanical properties, are commonly used in fields such as jewellery, catalysis and electronics. In this work, a two-layer system of Pt and V, deposited on Si substrates by electron beam deposition technique, were investigated amongst others for residual stress development in the coating. The investigation was carried out using diffraction techniques employing laboratory X-rays on the BRUKER D8 Discover instrument equipped with a High Star detector and analyzed with LEPTOS v6 software. The results showed the stress state to be tensile, relaxing significantly on annealing. In addition, complementary results of phase composition, and coating morphology will also be presented.

Abstract: Bending of metal plates with high-energy laser beams presents a flexible materials forming technique where bending results from the establishment of a steep temperature gradient through the material thickness. This inevitably leads to non-uniform thermal expansion/contraction and subsequently residual stresses. Non destructive residual strain mapping with neutron diffraction through the 8mm thickness of a series WA 300 grade structural steel plate samples, focused on the region straddling the centerline of the heating bead location, shows the presence of large residual stress fields. Directly below the laser track the longitudinal strains are tensile and dominant, normal strains compressive and transverse strains slightly tensile. The magnitudes of the strains decrease outside the width of the laser beam footprint. The first laser pass induces throughthickness strains close to yield, whereafter their magnitudes decrease with increased number of laser beam passes. A comprehensive mapping of the longitudinal stresses as function of the number of laser passes is given.

Abstract: The kinetics of plastic deformation and microstructural evolution, and the residual stress in particular, were investigated on the steel plates (SABS 1431) bent by laser beam. The steel plates were bent by different number of laser scans and therefore, each was bent to a different extent. The stress results obtained by x-ray diffraction (sin2ψ-method) show a higher compressive stress along the laser path than in the transverse direction. It was also found that stress relaxation occurs during multi-scan laser forming process and most importantly, that the stress is not significantly different in comparison to the stress, which initially existed in rolled steel plates. The metallographic analyses show that phase transformation, dynamic recovery and recrystallization processes occur during laser forming.