Fe- and Co-based bulk glassy alloys with ultrahigh strength of over 4000 MPa

Since the first synthesis of Fe-based bulk glassy alloys in Fe–(Al,Ga)–(P,C,B,Si) system in 1995, a number of Fe- and Co-based bulk glassy alloys have been developed up to date because their alloys are expected to exhibit high mechanical strength and good soft magnetic properties. The maximum diameter of Fe- and Co-based bulk glassy alloys exhibiting high fracture strength of over 4000 MPa is 5 mm for Fe–Co–B–Si–Nb system and 3 mm for Co–Fe–Ta–B–Mo–Si system. The addition of a small amount of Nb or Ta is essential for the increase in their glass-forming ability through the formation of network-like atomic configurations. The primary crystalline phase from supercooled liquid is a metastable complex FCC Fe23B6 or Co23B6 phase and hence the change to the local atomic configurations leading to the precipitation of the metastable Fe23B6 or Co23B6-type phase is thought to play an important role in the achievement of high glass-forming ability. The highest fracture strength reached as high as 4250 MPa for Fe–Co–B–Si–Nb alloy and 5545 MPa for Co–Fe–Ta–B–Mo alloy. The fracture strength has a good linear relation with Young's modulus, glass transition temperature or liquidus temperature. It is, therefore, concluded that the origin for the ultrahigh strength is attributed to the strong bonding nature among the constituent elements. Considering that Fe–Si–B amorphous alloy wires developed for several years between 1979 and 1983 have been used as high strength materials for the last two decades, the newly developed high-strength Fe- and Co-based bulk glassy alloys are expected to be used as a new type of ultrahigh strength material by utilizing the advantage points of much higher strength and three-dimensional material form.