PSI - Issue 14

M S Nandana et al. / Procedia Structural Integrity 14 (2019) 314–321 M S Nandana/ Structural Integrity Procedia 00 (2018) 000–000

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2.2. Heat treatment

The received alloy was subjected to peak ageing condition by performing solution treatment at 490˚ C for 6 hrs followed by ageing at 120˚ C for 24 hrs. Whereas the RRA treatment involves three stages of heat treatment following the solution treatment and cold water quenching. Retrogression or reversion treatment of the peak aged (T6) alloy was performed at 200 °C for 20 min. The selection of temperature and time of retrogression was based on the optimized parameters to obtain equivalent tensile strength as that of the T6 treated condition. The alloy after retrogression was re-aged to peak ageing condition to complete the heat treatment process. The complete process of heat treatment cycle is schematically represented in Fig. 2.

Solution treatment at 490 °C for 6 hrs

Water quench

Retrogression at 200 °C 20 min

Temperature

Re-ageing at 120 °C, 24 hrs

Ageing at 120 °C for 24 hrs

Time

Fig. 2. Schematic diagram representing the RRA heat treatment process

2.3. Microstructural studies

The microstructural characterization of the heat treated sample was performed by using transmission electron microscope (TEM, JEM-2100, JEOL make) operated at 200 kV. TEM samples were prepared by mechanical polishing and thinning down the sample to 80 μm thick. Discs of 3 mm diameter were punched from the thin section, which was followed by lapping to make the surface flat. The samples were dimple ground to 15 μm using dimpling unit (Gatan make). Ion milling was carried out on the dimpled region by using Gatan PIPS instrument with beam energy of 5 keV, at beam angle 6 ° top and 6 ° bottom followed by fine milling at 2 keV. Round tensile test samples were fabricated as per ASTM E8/E8M-16a standard with its tensile axis parallel to the rolling direction. The tests were performed by using universal testing machine operated at a cross head speed of 1 mm/min. The dimensions of the tensile test sample are shown in Fig. 3a. FCGR tests were carried out as per ASTM standard E647-15e1 using a computer controlled servo-hydraulic test machine, at room temperature and laboratory air environment. A sinusoidal loading pattern with stress ratio (R = σ min /σ max ) of 0.5 and frequency of 10 Hz was applied on the test samples. Standard compact tension (CT) samples of width W = 50.5 mm, thickness B = 12.5 mm was used. A sketch of the sample is shown in Fig. 3b. The crack length was measured based on the compliance technique using CMOD gauge fitted to the test sample. 2.4. Mechanical testing