Feasibility of in situ AlCrFeCoNi high entropy alloying by reactive friction stir processing

The feasibility of in-situ high entropy alloying of the AlCrFeCoNi stoichiometry through reactive friction stir processing (FSP) was evaluated. Three methods of elemental mixing were developed to investigate the progress of in-situ high entropy alloy (HEA) formation during the solid-state friction stirring treatment. In the first scenario (F1), the elements Cr, Fe, Co, and Ni were simply mixed and subsequently pre-placed into a commercial pure aluminum sheet, and successful FSP was carried out to consolidate the HEA composition. In the second method (F2), the powders of the four elements were mechanically alloyed for one-hour before processing, then inserted into the aluminum sheet, followed by the FSP modification. For the last method (F3), the elemental powders were mixed through ball milling prior to being subjected to FSP and mechanically alloyed longer up to 18-hours and then incorporated into the aluminum substrate for in-situ HEA formation during multi-pass processing. Subsequently, microstructural details and potential phase formations were characterized using advanced analysis techniques, along with their stability across all three processing scenarios. Thermo-mechanical modeling of FSP was also conducted to predict temperature profiles under the employed processing conditions, allowing for discussions about the stability and formation of possible phases based on thermodynamic calculations. Elemental maps of energy dispersive X-ray spectroscopy (EDS), accompanied by field emission scanning electron microscopy (FE-SEM) imaging, indicated that the best in-situ alloying occurred in the F3 processing scenario. In conjunction with microstructural developments, mechanical properties for the produced HEAs through friction stirring consolidation were examined using micro-hardness indentation testing and longitudinal tensile testing from the stirred regions. The maximum hardness reached approximately 280 HV, with the uppermost tensile strengths of yield and ultimate value recorded at around 110 MPa and 140 MPa, respectively, representing significant achievements in relation to the as-received aluminum alloy substrate.