Figure 2. Mechanical properties and failure characteristics of the CrCoNi medium-entropy alloy.

(a) Tensile tests show a significant increase in yield strength, σ_y, ultimate tensile strength, σ_UTS and strain to failure, ɛ_f, with decreasing temperature from room temperature, 293 K, to cryogenic temperatures, 198 and 77 K. In the same temperature range, the work of fracture increases from 3.5 MJ m⁻² to 6.4 MJ m⁻². (b) Fracture toughness tests on compact-tension, C(T), specimens show an increasing fracture resistance with crack extension and crack initiation, K_JIc, values of 208, 265 and 273 MPa m^1/2 at 293, 198 and 77 K, respectively. (c) Stereo microscopy and scanning electron microscopy images show a clear transition from the notch to the pre-crack and a pronounced stretch-zone between the pre-crack and the fully ductile fracture region of a sample that was tested at 198 K. (d) The fracture surface shows ductile dimpled fracture and Cr-rich particles that act as void initiation sides. (Data points shown are mean±s.d.; see Supplementary Table 1 for exact values.) The scale bars in c and d, and the insets of d are 75, 5 and 2 μm, respectively.