Role of Nb in glass formation of Fe–Cr–Mo–C–B–Nb BMGs

Abstract

A new Fe-based bulk metallic glass with superior glass-forming ability (GFA), Fe46Cr15Mo14C15B6Nb4, was developed based on the Fe–Cr–Mo–C–B alloy system by minor addition of Nb. The effects of Nb addition on glass formation of the Fe50−xCr15Mo14C15B6Nbx (x=0, 2, 4 and 6at.%) alloys were investigated. The optimum addition content of Nb was determined as 4at.% by X-ray diffraction and differential scanning calorimeter analysis. A fully amorphous rod sample with 3mm in diameter was produced by using commercial-grade raw materials and a copper mold casting technique. This alloy shows an ultimate compressive strength of 1920MPa and Vicker’s hardness 1360HV, which is two to three times that of conventional high strength steel and suggests a promising potential for applications combining outstanding corrosion and wear resistance properties. The crystallization kinetics studies found that the activation energies for glass transition, onset of crystallization and crystallization peak were higher than those of other reported Fe-based bulk metallic glasses. The value of the fragility parameter m for the Fe46Cr15Mo14C15B6Nb4 alloy was calculated to be 34, indicating that the Fe–Cr–Mo–C–B–Nb alloy system is a strong glass former according to the Angell’s classification scheme. It is inferred that the more sequential change in the atomic size, the generation of new atomic pairs with large negative heats of mixing and the amount of oxygen in the molten liquid neutralized into Nb oxides provide a synergetic effect for the remarkably improved GFA and thermal stability.