Investigation of the effects of Al and Ti on degradation, ion release, and bioactivity in CoCrFeNi high-entropy alloys

This study aims to investigate the effect of Al and Ti addition on the degradation, ion release and bioactivity properties of CoCrFeNi alloys for biomaterial applications. Three different high entropy alloys (CoCrFeNi, CoCrFeNiAl and CoCrFeNiTi) were produced by mechanical alloying and arc plasma sintering methods. Mechanical alloying was used to obtain the alloy powders and it was observed that a homogeneous element distribution was achieved in all samples with 80 hours of alloying time. Arc plasma sintering was used to sinter the pressed powders at high temperature and it was found that Al and Ti addition delayed the amorphization. Characterization analyses were performed using SEM, EDS and XRD analysis methods to determine the microstructure, element distribution and phase structure of the alloys. Degradation analyses were carried out by measuring the weight changes of the alloys before and after immersion in simulated body fluid (SBF) to determine the dissolution amount of the alloys. It was observed that Al and Ti addition reduced the degradation severity of CoCrFeNi alloys. Ion release analyses were performed by measuring the metal ion concentrations in the SBF samples taken after 4 weeks of immersion to determine the metal ion release of the alloys. It was observed that the metal ion release was at ppb levels in all sample groups and the lowest ion release was obtained in the CoCrFeNiTi sample group. Bioactivity analyses were performed by determining the Ca-P nucleations and the Ca-P ratios on the sample surfaces after 4 weeks of immersion in SBF using EDS analyses to examine the Ca-P nucleation behavior of the alloys. It was determined that the bioactivity was very low in CoCrFeNi alloy, Al addition increased the bioactivity and the highest bioactivity was achieved in CoCrFeNiTi alloys. This study reveals that CoCrFeNi-based high entropy alloys have a strong potential as an alternative biomaterial.