PSI - Issue 71

S.R. Reddy et al. / Procedia Structural Integrity 71 (2025) 172–179

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temperature workability is one of the important aspects to be considered for efficient industrial applicability of these materials (Ahmed et al., 2020)(Zhang et al., 2023; Zhang et al., 2018) particularly in aerospace applications. Superplastic forming (Gifkins T., 1978) (Chokshi A.K. and Langdon T.G., 1993) has been used extensively in aerospace applications due to their unique combination requirement of materials with superior mechanical properties with increased safety requirement. High strain rate superplastic forming with strain rates >10 -2 s -1 (Higashi M. and Langdon T.G., 1996) have been observed as one of the alternatives to conventional metal forming industry owing to its high output and economic viability. The major requirement high strain rate superplasticity is stable fine grain sizes with high strain rate sensitivity required for flow stability and extended elongations. Recently HEAs have shown superplastic behavior above 0.5T m (Kuznetsov et al., 2013; Nguyen P. and et al., 2018) owing to their stable fine grain sizes. The strain rate (( ̇ )) is given by ̇ ∝ d p . σ 1/m where the grain size exponent (p ≥ 2) and strain rate sensitivity (m ≥ 0.3) should be optimum to exhibit superplastic behavior. Several reports show that the dominant mechanism for super plastic behavior is grain boundary sliding(Chokshi A.K. and Langdon T.G., 1993; Mohamed, 2020). AlCoCrFeNi 2.1 EHEA has shown high temperature superplastic behavior (Reddy et al., 2024a) above 800 ° C in duplex microstructure with extremely fine grain sizes of 0.6 µm (Wani et al., 2016). However, a detailed study of the high temperature behavior at different strain rates and varied temperatures has not been performed which is the main objective of the present research work. This study further delves into the activation energy required for deformation which can enhance the understanding the behavior in these materials. 2. Experimental Procedure The AlCoCrFeNi 2.1 EHEA material used in the present research work is arc melted from elements with purity >99.9%. A cast block with length x width x thickness of 90 mm x 15 mm x 3 mm is prepared by suction casting in a water-cooled copper mold. Small samples of 30 mm length are extracted for further cold rolling up to 90% reduction in laboratory two high roll mill of 140 mm roll diameter by giving 5% reduction per pass to achieve a final thickness of 0.3 mm. Tensile samples with gauge length of 3 mm and width of 1 mm using wire Electrical Discharge Machining (EDM) and are heat treated at 800 ° C for 1 hr to achieve a fully recrystallized grain size of 0.6 µm. The tensile samples are finely polished to remove any surface defects and used for high temperature tensile testing using a table top mechanical testing machine (Model: Instron 5567). The tensile samples were carefully heated to the test temperatures of 700 ° C, 800 ° C, 900 ° C at a heating rate of 10 ° C/min and soaked for 20 minutes to attain equilibrium. Tests were performed at different strain rates varying between 3x10 -4 s -1 to 10 -1 s -1 to understand the materials behavior. Further strain rate sensitivity measurements are performed 10 -4 s -1 to 10 -1 s -1 to measure the strain rate sensitivity at different temperatures. Microstructural characterization was performed on the fractured materials to evaluate the microstructural changes using Scanning Electron Microscope- Electron Back scatter Diffraction (SEM-EBSD) technique (Model: SUPRA 40, Carl Zeiss). Multiple areas were scanned very near to the fracture tip with fine step size varying from 20 - 50 nm from each sample to evaluate the grain size and phase analysis. All the scans were analyzed using TSL-OIM ™ (EDAX Inc., USA) software for determining the grain size and phase fraction data. 3. Results and discussion Figure 1 shows duplex microstructure of the starting material after recrystallization. The average grain size measured from EBSD maps is ~0.6 um and phase fraction is 55:45 of FCC/L1 2 :BCC/B2 phases respectively. Both phases are ordered where L1 2 phase is Ni 3 Al type and B2 phase is NiAl type with elements distributed accordingly (Wani et al., 2016). Fig. 2 shows true stress vs. elongation (%) curves acquired at different temperatures varying between 700 °C -900 °C. The specimens tested at 700 °C yielded an UTS (ultimate tensile strength) of ~770 MPa where as it has dropped significantly at higher temperatures to ~380 MPa at 800 °C and ~180 MPa at 900 °C at a strain rate of 10 -1 s -1 . In contrast the ductility has significant improvement from elongation to failure (ef) ~90% at 700 °C to an exceptional ductility of ~960% at 900 °C. Lower strain rates and high temperatures lead to improved superplastic behaviour with ductility >300%. Specimens tested at 700 °C show increased ductility from 10 -1 s -1 to