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Aim: Testing and Evaluation of SCC in presence of solid copper Experiment Procedure: • U-bend (d≈13.6mm) constant stress corrosion test as per SAE ARP 1795 A as in fig.1 • Materials: Titanium sheet (mill annealed) : Ti-8Al-1Mo-1V (Ti-811), Ti-6Al-4V(Ti-64) • Embrittling Agents: 3wt.% Nacl+water, copper paste, copper coating, gold coating • Heat treatment: •Method A: 480±10°C, 8±0.2 h •Method B: 260±10°C,168±4h • Metallographic examination: 20x (preliminary), 500x (final) Hypothesis for SCC in contact with solid metal [3] : • Diffusion of embrittling species through fresh TiO2 film until crack tip • Crack propagation along intergranular, penetration of oxide film along grain boundary as shown in fig.2 Conclusion: • Ti-811 is susceptible to SCC at higher temperatures ≈480°C for incubation period of ≈8hrs when it is in contact with solid Cu Future Research: • To study SCC mechanism in detail by SEM-EDS analysis and fractography • To find thresholds where SCC starts such as Cu content, temperature, time, composition Acknowledgments: Authors acknowledge NGSST, NFFP and VAC for supporting the work. @ Corresponding e-mail: [email protected] Effect of small trace elements on mechanical properties of titanium alloys Testing and Evaluation of Stress- Corrosion Cracking with copper Clément Philippot * , Raghuveer Gaddam *” , Pia Åkerfeldt * , Robert Pederson *# Background Titanium alloys are frequently used in the aerospace industry because of their superior strength to weight ratio. During manufacturing of these alloys it is of prime importance to produce alloys with appropriate composition and at the same time minimizing/avoiding getting detrimental elements in the finished material, especially when used for fracture critical applications in aerospace industry. To achieve this, the understanding of the effect of interstitials (O, H) and alloying elements (B, Cu) on mechanical properties and processing routes is crucial. At Volvo Aero Corporation (VAC) a new standard is being developed which includes issues regarding handling of titanium components during the manufacturing processes. One part of this standard deals with what matters that are allowed and not allowed to be in direct contact with a titanium component surface due to the risk of Stress Corrosion Cracking (SCC) [1]. From literature it has been reported that titanium alloys undergo SCC in contact with solid Cadmium, Silver and Gold [2] . However, from ‘in-house’ experience indications were found that Copper in contact with titanium could also lead to SCC. In order to clarify whether Copper in contact with titanium will lead to SCC or not, this project work was initiated. * Division of Engineering Materials , #VAC Fig.2 Tarnish rupture model References: 1.Volvo Aero Corporation Standard, ‘Foreign Matter in Titanium’,VOLS: 10073879 2. R.E.Stoltz et. al., Solid Metal Embrittlement of Ti-6Al-6V-2Sn by Cadmium, Silver and Gold, Corrosion,Vol. 35, No.4, 1979 3. D.G.Kolma, Solid Metal Induced Embrittlement, ASM Handbook,Vol.13A, p.393-397,2003 4. H. Jones Russel, Stress - Corrosion Cracking: Materials Performance and Evaluations, ASM International, p265-295, 1992 5. SAE Standard on, ‘Stress Corrosion of Titanium Alloys’ ARP 1795A Results and Discussion: • Tests are validated as per SAE ARP 1795 A [5] • Ti-811 showed susceptibility to SCC in contact with Cu at 480°C which is probably due to presence of more Al content (α-stabilizer) [4] • Ti-64 exhibited resistance to SCC at 480°C which is probably due to less Al and more V (β-stabilizer) [4] • Observed crack length was smaller for samples with copper paste than coating which could be due to less amount of Cu in paste TiO2 Fig.1 Schematic of U-bend testing (a) (b) (c) (d) Fig.3 SCC cracks examined optically at 500x as per SAE ARP 1795 A (a) No solution, (b) 3 wt.% Nacl, (c) copper paste, (d) sputtered copper coating d
