An Investigation of Elastic Modulus in Zr Doped CoCrFeMoNi HEA by Three-Point Bending

In recent years there has been a great interest in high entropy alloys (HEAs) since they have good mechanical properties, high corrosion resistance and good thermal conductivity among others, which makes them interesting for various fields of research. For this reason, it is important to know their behavior in different conditions that allows us to define the possible applications of these HEA's [1–3]. The literature has been confirming the effect of doping with different elements in the improvement of the mechanical properties and corrosion resistance of HEA's [4]. One of the aspects to be considered in the mechanical properties of a HEA is the strength and fracture toughness of the alloy. These mechanical characteristics are related to the value of the modulus of elasticity (E) of the alloy, which provides information on the cohesion of the material as it is related to the binding energy of the atoms. To calculate the modulus of elasticity of the prepared alloys, a three-point bending test is performed on fibers of the material. A non-equiatomic high entropy alloy is made with the elements: Co, Cr, Fe, Mo and Ni of chemical composition in percent by weight, wt %, of 20.67, 19.99, 19.88, 20.20 and 19.27 respectively, which we call Sample 1. From this alloy a second one, Sample 2, is obtained by doping with 0.71%wt. Zr. Two ingots are manufactured with high purity elements, 99.00%, in an arc melting furnace model MRF ABJ 900 VAR in a protective argon atmosphere. Six remelting operations are carried out on each ingot to achieve a good homogenization. To obtain an average value of the modulus of elasticity, 10 fibers obtained with a Buehler IsoMet® 4000 precision linear saw machine using 0.5 mm thick tungsten disks are made from the specimens. The fibers have a rectangular section with an average thickness of 0.45 mm and variable length. A three-point bending test is performed in an Electroforce 3100 universal testing machine providing a maximum force (F) of 22N. The fibers are placed in the bending test fixture, see Fig. 1a, and a vertical force is applied at its midpoint until the strand breaks, as in Fig. 1b, or the maximum force is reached. Finally, an analysis of the breakage zones is performed by electron fractography using a Sigma 300 VP field emission scanning electron microscope (FESEM). The observation is performed with the secondary electron detector (SE) at 15 kV at different magnifications. With the data obtained in the three-point bending test, the force (F) displacement (w) graphs are generated (see Fig. 2). From the straight line obtained in the elastic zone, we take its slope and calculate the modulus of elasticity as dF/dw = 48EI/L3, where E is the modulus of elasticity, I is the moment of inertia of the rectangular section and L is the length between supports. a) Three-point bending test of one of the specimens and b) detail of the fracture of one of the specimens. Three-point flexural test diagrams for: a) Sample 1 and b) Sample 2. For Sample 1 a value of E of 173.99 ± 4.25 GPa is obtained and for Sample 2 the calculated value is 144.33 ± 5.38 GPa. The value obtained for the sample with Zr is 20.5% lower than for the sample without the addition of Zr which causes a decrease in the stiffness of the alloy. FESEM image of the a) fracture surface of Sample1 and b) detail of the marked zone. In the FESEM micrographs of the samples, Fig. 3 and Fig. 4, it can be seen how on the surface of Sample 1 there are fracture planes that indicate a brittle fracture of the material and, on contrary, on the surface of Sample 2 there are small dimples characteristic of a material that presents certain ductility. FESEM image of the a) fracture surface of Sample1 and b) detail of the marked zone. In this work a high entropy alloy from CoCrFeMoNi system and then doped by 0.71 wt% Zr were fabricated by vacuum arc melting. FESEM and three-points bending tests were performed to analyze the fracture surface and to evaluate the elastic modulus. It was determined that the doping with only 0.71 wt% Zr, the elastic modulus decreased with more than 20% and this is associated with great strength, high elasticity and high wear resistance [5].