PSI - Issue 14

Amit Singh et al. / Procedia Structural Integrity 14 (2019) 78–88

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Amit Singh et al./ Structural Integrity Procedia 00 (2018) 000 – 000

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R. Hill (1973), used a Crussard- Jaoul (C-J) model to describe the strain hardening behavior of dual phase materials. Since, IMI 834 alloy is a near α titanium alloy which is consist of two phases α and β. Hence, to understand strain hardening behavior better of this alloy a modified Crussard- Jaoul (C-J) model used. This modified C-J model based on Equation (3). The modified C-J model can be expressed as per following ln ( ) = (1 − 1 ) + ln( ∗ 1 ) , or ln ( ) = (1 − ) + ln (9) where,  ൌ ͳȀ and ൌ ∗ 1 . This logarithmic form of strain hardening equation is able to differentiate the contribution of two phases in strain hardening. 3. Experimental IMI 834 alloy with a nominal composition of Ti -5.8Al-4.0Sn-3.5Zr-0.7Nb-0.5Mo-0.35Si-0.06C (wt %) was procured from M/s. Timet Ltd. (Birmingham, UK) in the form of forged and machined bars of 172 mm diameter. Samples were extracted from the as-received bars, coated with deltaglaze 347, heated in a resistance heating furnace to a temperature of 1213K. A standard soaking time of 1 hrs per inch of ruling section thickness was provided to attain the thermal equilibrium. The soaked billets were transferred to a 2000 MT forge press and isothermally forged to 70% of their height with a strain rate of 0.003/s followed by air cooling. Isothermally forged billets were then cut into two halves and a total of 20 such halves were generated. Out of the 20 samples, 5 samples were used for each solution treatment (ST) temperature; 1288 K, 1303 K, 1318 K and 1333 K for the study. After solution treatment at the identified temperatures the samples were air cooled and aged at 973 K for 2 hrs followed by air cooling does not have any impact on strain hardening. While, the fully plastic region is believed to have impact on strain hardening by a change in microstructural feature such as grain size and shape, solid-solution, precipitates, prior history of materials (e.g. initial dislocation density), sub-grains and crystallographic texture etc. 3.1. Material processing

Fig. 1. Schematic of ASTM E8 standard used for tensile testing of IMI 834 alloy.

3.2. Material characterization and testing

From the heat treated (solution treated and aged) material, samples were extracted and mechanically polished for metallographic examination. The polished samples were etched with conventional Kroll’s reagent (92 ml D.I. water + 6 ml Nitric acid + 2 ml Hydrofluoric acid) at room temperature by swabbing for 60 seconds. The microstructures of samples were then examined and recorded using OLYMPUS © BX53M metallurgical microscope. The phase fractions present were then estimated from the optical microstructures using ImageJ image analysis software. From