Microstructure and mechanical properties of mechanically-alloyed CoCrFeNi high-entropy alloys using low ball-to-powder ratio

The main issue of this work was to analyse the microstructural evolution and mechanical properties of FCC high entropy alloy (HEA) when BPR (ball-to-powder ratio) was limited to 5:1. The motivation of our work is to increase the amount of milled fraction without losing efficiency of the milling process. Nowadays many papers describe HEAs by using powder metallurgy processes, but higher BPR is used. In consequence less amount of powder is milled in one period and the process is not effective enough from the industrial point of view.

In this work four equiatomic CoCrFeNi samples were made by Mechanical Alloying plus Spark Plasma Sintering using different milling times: 10, 20, 30, 40 hours. We used 200 Φ5 mm WC balls and milled with intervals 15:15 minutes. Milling speed was 250 rpm. After the mechanical alloying has been finished samples were sintered by using Spark Plasma Sintering technique. We chose 950 °C as a process temperature with heating rate 100 °C/min. Sintering pressure was 50 MPa. Samples were then homogenise in 1050 °C for 12 hours. Then samples were water quenched.

The densification of samples during sintering was in satisfied level, what was confirmed by relative densities of samples (>90 %) The microstructure observation of sintered samples revealed Cr-rich particles evenly distributed in samples volume. The number of particles decreases with increasing the milling time. Elements are randomly distributed in the matrix phase except a small Cr-depletion. XRD technique shows multiple FCC structure. As the milling time exceeds, the main FCC structure is promoted. Microhardness increased as a function of milling time. After annealing microstructures were almost out of Cr-rich phase. Only the biggest particles remained. EBSD revealed the grain size decrement as a function of milling time. Also X-ray diffractograms presented significant homogenisation of manufactured samples. Despite the microhardness decrease after heat treatment, the longest milled sample still possess very promising properties. Moreover hardness of samples is not indent’s size dependent (micro- and nanohardness).

During milling time the particles are joining and fracturing many times. As a consequence elements are mixing and promoting the new phase(s) growing. We deduced that Hall-Petch effect is the most important factor determining better mechanical properties in longer milled samples. However the milling process need to be improvement. Cr-rich phase observed in sintered samples is the effect of low efficiency of the process, which might be improved by either smaller fraction of Cr at the beginning (premilling process) or increase the other process parameters (milling speed, sintering time).