Issue 22

V. Di Cocco, Frattura ed Integrità Strutturale, 22 (2012) 31-38 ; DOI: 10.3221/IGF-ESIS.22.05 37 LOM observations of Zn-Sn and Zn-Ti coatings sections are shown in Fig. 5 and 6 respectively . Both Zn-Sn and Zn-Ti coatings don’t show any damage at low dipping time values in the compression sides, but at high dipping time values it is possible to have a delamination at  interface (in the Zn-Sn coatings,) or longitudinal cracks in  phase in Zn-Ti coatings. In tension sides of Zn-Ti coatings double longitudinal cracks are present in iron-  and  - tri-phases interfaces, as shown in Fig. 6 at 900s. Furthermore other cracks are present in outer layer, but restricted to one of the three phases constituting outer tri-phases layer. However the main damage in tension sides, for all investigate coatings, is due to the presence of radial cracks. In Zn-Sn coatings radial cracks generate at substrate-  interface and propagate in  , arresting at  interface, or sometimes in  phase or at  interface while in Zn-Ti coatings radial cracks generate in the substrate-  interface and propagate towards outer layer, arresting at interface  - tri-phases layer. Radial damage quantification is shown in Fig. 7 in terms of number of radial cracks per millimeter of deformed arc. In Zn- Sn coatings  phase is characterized by high values of damage due to its brittle behavior. Moreover, as said above, radial cracks don’t start in  phase but propagate from  . 0 5 10 15 20 25 0 200 400 600 800 1000 Damage [Cracks/mm] Dipping time [°] delta zeta ( a) 0 5 10 15 20 25 0 200 400 600 800 1000 Damage [Cracks/mm] Dipping time [°] delta ( b) Figure 7 : Intermetallic phases radial damage: a) in Zn-Sn coatings, b) in Zn-Ti coatings. For those reasons damage level in  phase is lower than the damage level in  for all the investigated conditions. In Zn-Ti coatings, radial cracks are present only in  phase. In this case damage decreases with dipping time because of the growth of outer layer at 360s, where radial cracks becomes less relevant than the longitudinal ones, which are not given in the chosen damage parameter. C ONCLUSIONS n this work bending cracks in two Zn-based coatings are investigated in order to evaluate mechanical properties, cracks paths and damage of intermetallic phases. The presence of Sn doesn’t change intermetallic phases characteristic in traditional Zn coatings, but the presence of Ti leads to the presence of an outer zone, formed by a double phases ductile matrix and a brittle dispersed phase. Zn-Sn coatings are characterized by high bending strength and good elastic recovery properties unlike Zn-Ti coatings, probably due to high values of damage. In Zn-Sn, radial cracks initiate at substrate-  interface and propagate in  phase, arresting at  interface or in  phase or at  interface. In Zn-Ti coatings radial cracks are present in tension side at all investigated dipping time, but beyond 360s longitudinal cracks are the main damage. R EFERENCES [1] A.R. Marder: Progress in Materials Science, 45 (2000) 191. I