Enhanced hardness and fracture toughness of the laser-solidified FeCoNiCrCuTiMoAlSiB 0.5 high-entropy alloy by martensite strengthening Advisor : Tzu-Yao Tai Advisee ﹕ Chung-Lin Huang Department of Mechanical Engineering Southern Taiwan University Date ﹕ 2015/12/22 Scripta Materialia 69 (2013) 342–345 School of Materials Science and Engineering, Anhui University of Technology, Ma’anshan , Anhui, People’s Republic of China Jiangsu Key Lab of Advanced Metallic Materials, School of Materials Science and Engineering, Southeast University, Nanjing , Jiangsu, People’s Republic of China Hui Zhang, Yizhu He and Ye Pan

2  In this work, a efficient laser solidification strategy was proposed to prepare FeCoNiCrAlSiCuTiMoB0.5 HEA (denoted B 0.5 -HEA), where the boron content is 5.25 at.% and the others are 10.5 at.%.  The laser technique employed here gives a rapid solidification rate of ( ℃ s –1 ), which greatly favors the formation of solid-solution phase.  Si and B were added as ferrosilicon (77 wt.% Si) and ferroboron (18 wt.% B) powders, respectively.  The laser parameters were: 2.0 kW laser power, 4.5 mm beam diameter and 400 mm min -1 scanning speed velocity. a thickness of about 1.5 mm was obtained  The boron content is reduced to 3.2 at.% by burn-off after laser cladding.

3 XRD SEM TEM Nano Indenter-2N

 Two arrows are centered on the (110) reflection, which suggest the nucleation of martensite phase in B 0.5 -HEA.  The reflections are in good agreement with previous reports on martensite formation in BMGs and shape memory alloys. 4  After annealing treatment, the intensity of the martensitic diffraction peaks greatly deceases and the (100) ordered peak slightly increases.

5 B0.5-HEA is mainly composed of equiaxed grains The substructure in the equiaxed grains exhibits typical lath-like martensitic morphology in the TEM observation. Some other phases are inevitably precipitated at the grain boundaries, as indicated by the squares The reason is due to the highly complex composition of the coating.  Figure 2c demonstrates the magnified lath martensitic microstructure containing a high density of dislocations.  After annealing at 900 ℃  Abundant precipitated nanocrystals are evident in the B 0.5 -HEA sample.

6 Olson and Cohen derived a classic model for strain- induced free energy change (ΔG) of martensitic transformation  The boron atoms might occupy octahedral interstitial sites and inevitably result in elongation and simultaneous softening along the shear modulus directions of c 44 ({001} ) and c’ ({110} ) in the cubic crystal, leading to high non-chemical energy change during martensitic transformation.

7  This flow pattern is explained by repeated solute locking and unlocking of dislocations during plastic deformation in crystalline alloys

8  The central cross-section of the alloys and are the average of ten arbitrary points. Much higher than those of most previously reported HEAs(hardnesses of 500–900 HV) and about 1.5 times higher than those of Al 3 -HEA. The fracture toughness of B 0.5 -HEA is 50.9 MPa m 0.5, which is six times higher than that of Al 3 -HEA.

9 Thanks for your attention!