Microstructure and mechanical properties of AlCoCrNiTi–C High Entropy Alloy processed through Spark Plasma Sintering

Highlights: • Composite microstructure resulting from in situ reactions. • Enhanced strength due to grain boundary strengthening and precipitation hardening. • Limited grain growth at temperatures of 800 °C, 900 °C and 1000 °C. • Heat treatment studies reveals increase in volume fraction of Cr rich phase. AlCoCrNiTi–C High Entropy Alloy (HEA) fabricated through mechanical alloying (10 h) and Spark Plasma Sintering (SPS) at 1000 °C develops a mixture of NiAl B2 phase, Cr rich phase and fine TiCs. Transmission Electron Microscope (TEM) images before and after milling confirms the presence of BCC phases in the alloy. Carbon Hydrogen and Nitrogen (CHN) analysis reveal the presence of carbon content in milled powders after 10 h. Back Scattered Electron (BSE) images and elemental mapping indicates the presence of TiCs along grain boundaries of NiAl B2 phase. Microstructural characterization before and after heat treatments reveals the fact that there is an increase in the volume fraction of Cr rich phase. Moreover the alloy develops a good room temperature compressive strength of 2750 ± 4.3 MPa, high temperature strength of 871 ± 3.1 MPa at 800 °C along with micro hardness of 860 ± 7.8 HV_0.5. Nano indentation results shows that the major contribution to hardness and strength comes from NiAl phase which has a value of 10.2 ± 2.1 GPa. Enhanced properties of the alloy can be attributed to the presence of NiAl B2, disordered Cr rich BCC phase along with fine TiCs.