Effects Of Ni, Mo And Zr Additions On Microstructure And Properties Of Laves Phase NbCr₂ Compound

Laves phase NbCr₂ has potential application as new type high temperature because of its excellent high-temperature properties, such as high melting point, high strength, high creep resistance, and good oxidation resistance at high-temperatures. However, this potential is retarded by its brittleness at room temperature. How to improve the brittleness of Laves phase NbCr₂ at room temperature is one of the hot points in material science.Samples were prepared by mechanical alloying (milling time is 20h and ball-to-powder is 13:1) and hot-pressing (hot-pressing temperature is 1250℃and hot-pressing time is 0.5h). The effects of Ni, Mo and Zr single or multi-additions on microstructure and properties of NbCr₂-based Laves phase compounds are investigated systematically for the first time with optical microscope, scanning electron microscope, X-ray diffraction and energy disperse X-ray.The results show that the average grain sizes of Nb-Cr-Ni powders milled for 20h are nano-scale. The electron concentration is an important factor for Laves-phase of NbCr₂-Ni compounds. The electron concentration of C15 to C14 is higher than that (i.e., 5.88) reported in the literature, which is more than 6.103. The optimum content is 4.5 wt.% for Laves phase NbCr₂ in this research. For the first time the results show that relatively density of NbCr₂-4.5wt.%Ni is 97.79%, Vicker hardness is 8.67GPa, fracture toughness is 6.87 MPa.m1/2, yield strength is 2016MPa, compressive strength is 2161MPa and plastic strain is 8.21%. The toughening mechanism is fine-grain, Ni replacing Cr, solid solution harder, and crack deflection or microbridging.For the first time the average grain sizes of Nb-Cr-Mo powders milled for 20h are 50-131 nm and microstrain is not higher than 1.0587%. The optimum content is 10.8 wt.% for Laves phase NbCr₂ in this research. The relatively density of NbCr₂-10.8wt.%Mo is 93.467%, Vicker hardness is 10.9GPa, fracture toughness is 7.10 MPa.m1/2, yield strength is 2988MPa, compressive strength is 3140MPa and plastic strain is 8.9%. The toughening mechanism is fine-grain, Mo replacing Nb and crack deflection or microbridging.For the first time the average grain sizes of Nb-Cr-Zr powders milled for 20h are 48-75 nm and microstrain is not higher than 1.2100%. The microhardness of powders milled 20h is remarkable higher than that of powders without milling. The optimum content is 23.4 wt.% for Laves phase NbCr₂ in this research. The relatively density of NbCr₂-23.4wt.%Zr is 94.187%, Vicker hardness is 9.23GPa, fracture toughness is 6.50 MPa.m1/2, yield strength is 1933MPa, compressive strength is 1998MPa and plastic strain is 6.59%. The toughening mechanism is fine-grain, Zr replacing Nb and crack deflection or microbridging.The toughening effects of Ni, Mo and Zr single additions on NbCr₂-based Laves phase compounds with Cr rich or Nb rich are more enhanced than stoichiometry NbCr₂ because of alloying and soft-second phase.For the first time the average grain sizes of the powders with Ni, Mo and Zr multi-alloying milled for 20h are 100-110 nm and microstrain is in the range of 0.4000% to 0.5500%. Adding Ni and Mo is the best for toughening among multi-alloying. The optimum content is 10.8 wt.% Mo and 4.5wt.%Ni for Laves phase NbCr₂ in this research. The relatively density of NbCr₂-10.8wt.%Mo-4.5wt.%Ni is 96.8%, Vicker hardness is 14.82GPa, fracture toughness is 7.51MPa.m1/2, yield strength is 3109MPa, compressive strength is 3508MPa and plastic strain is 10.12%. The toughening mechanism is fine-grain, Mo replacing Nb and Ni replacing Cr, and crack deflection or microbridging.