Bulk Amorphous Ni75−xNb5MxP20−yBy (M=Cr, Mo) Alloys with Large Supercooling and High Strength

Abstract

Bulk amorphous Ni-based alloys with high strength were formed in the diameter range up to 1.0 mm for Ni₆₅Nb₅Cr₅Mo₅P_14-16B_4-6 alloys by copper mold casting. The T_g and ΔT_x(=T_x−T_g) of the bulk amorphous alloys are 703 and 50 K, respectively, for the 4%B alloy and 701 and 61 K, respectively, for the 6%B alloy. There is no difference in thermal stability between the bulk and ribbon alloys. The crystallization takes place through a single exothermic reaction in which crystalline phases of Ni, Ni₃P, Ni₃B, Ni₆Mo₂P₃, Cr₃P and Cr_0.75Ni_0.25P precipitate almost simultaneously. The necessity of the long-range atomic rearrangement leading to the simultaneous precipitation of the six crystalline phases seems to be the reason for the high glass-forming ability of the Ni-based alloys. The Ni-based bulk amorphous alloys exhibit high tensile fracture strength (σ_f) of 2690 to 2760 MPa, Young’s modulus (E) of 95 to 103 GPa, fracture elongation (ε_f) of 2.8% and Vickers hardness (H_v) of 840 to 880. The ratios of 9.8H_v⁄3E and σ_f⁄E are nearly the same as those for the corresponding melt-spun amorphous ribbons. The synthesis of the Ni-based bulk amorphous alloys with the same thermal and mechanical properties as those for the alloy ribbons is encouraging for the future development of bulk amorphous nonferrous alloys with high strength.