Phase Evolution and Characteristics of Mechanically Alloyed and Spark Plasma Sintered High Entropy AlCoCrFeTi Alloy

Formation of simple solid solution apart from complex inter-metallic phase made High Entropy Alloys (HEAs) unique among other alloy systems. The current study illustrates phase evolution and mechanical properties of high entropy AlCoCrFeTi HEA alloy developed using mechanical alloying (MA) and spark plasma sintering (SPS) at 45 MPa pressure and 1000 ̊C temperature. The alloy possess high value of configurational entropy (13.38 J/Kmol), negative enthalpy of mixing (-16.48 kJ/mol), and lower geometrical parameter (atomic size difference of 7.173%) which are highly desirable for the formation of single phase HEA solid solution. Mechanical alloying resulted in the evolution of a mono-phase nanocrystalline AlCoCrFeTi HEA BCC solid solution with an average crystallite size of 9.12 nm within a shorter milling time of 15 h. This was confirmed by the XRD analysis and SAED Pattern of the Powder TEM analysis. During the thermal analysis, the alloy powder showed higher thermal stability without indicating melting over the entire measurement range. However, a minor FCC phase was observed during sintering. This was confirmed by the XRD analysis and BSE SEM analysis of the sintered alloy. The alloy retains its nanocrystalline nature (72 nm), even after sintering due to restricted grain growth and shorter sintering times. The synthesized alloy possesses superior density (99.38% relative density) with significantly lower sintering time. AlCoCrFeTi HEA alloy exhibits superior hardness (1091.5HV) and high compressive strength (880.02MPa) due to its inherent BCC nature and induced lattice distortion during mechanical alloying and solid solution strengthening.