We studied the glass forming ability of Ni-Nb binary alloys and found that some of the alloys can be prepared into bulk metallic glasses by a conventional Cu-mold casting. The best glass former within the compositional range studied is off-eutectic Ni 62 Nb 38 alloy, which is markedly different from those predicted by the multicomponent and deep eutectic rules. The glass formation mechanism for binary Ni-Nb alloys was studied from the thermodynamic point of view and a parameter ␥ * was proposed to approach the ability of glass formation against crystallization. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2158130͔ Bulk metallic glasses ͑BMGs͒ as a prominent class of functional and structural materials with unique properties have attracted intensive interests due to their considerable significance in science and technology. [1][2][3][4][5][6][7][8][9][10][11] In order to make the best use of these noncrystalline materials, the key problem is to develop BMGs with improved properties and excellent glass forming ability ͑GFA͒. It is commonly regarded that the formation of metallic glasses is controlled by two factors, i.e., the cooling rate and the composition of the alloys. The critical cooling rate, which is the most effective gauge for GFA of the alloys, is hard to be measured experimentally. Hence, a great deal of efforts have devoted to the investigation on the composition of glass forming alloys. Inoue et al. 11 and Johnson 1 framed the empirical rules to predict the element selection and compositional range of glass forming alloys. These rules have played an important role as a guideline for synthesis of BMGs for the last decade.Binary alloys are usually considered to have a lower GFA due to their lack of complicated structure with atomic configuration according to the "confuse principal." 2,3 However, recent experimental results have shown that binary Zr-Cu alloys can also be vitrified into BMGs. [12][13][14] In contrast with the deep eutectic rule, the better glass formers in Zr-Cu binary alloy system such as Zr 35.5 Cu 64.5 and Zr 50 Cu 50 are off eutectic. Therefore, the empirical rules for glass formation, that is, multicomponent alloys with composition near deep eutectic, could be no longer the major concern for designing BMGs.In this work, we reported that some of the Ni-Nb binary alloys can be prepared into fully glassy rods up to 2 mm in diameter by a conventional Cu-mold casting method. The best glass former within the compositional range studied is off-eutectic Ni 62 Nb 38 alloy, which is different from those predicted by the multicomponent and deep eutectic rules. The glass formation mechanism of the binary alloys was studied from the thermodynamic point of view based on Miedema's calculation model.Ingots of Ni-Nb binary alloys with different compositions were prepared separately by arc-melting of 99.9% ͑at. %͒ pure Ni and Nb in titanium-gettered argon atmosphere. The rods of 2 mm in diameter were prepared by suction casting under argon atmosphere. The structure of the samples was charact...