Ductile Bulk Amorphous Steel Alloys
Iron-based bulk metallic glasses (BMGs) are being studied as a potentially new types of structural materials because of their high strength, good corrosion and wear resistance coupled with relatively low materials cost. Among these Fe-based BMGs are the previously reported amorphous steels based on Fe-Mn-Cr-Mo-C-B and Fe-(Mn,Cr)-(Ln,Y)-Mo-C-B (Ln=Lanthanides), known as DARVA-Glass 1 and DARVA-Glass 101, respectively. The fracture and yield strengths of amorphous steels are found to be three times those of high-strength stainless steel alloys, and their elastic moduli are near 200 GPa, comparable to those of super-austenitic steels. However, these amorphous steels are mostly brittle, which limits their promise as structural materials. Specifically, the alloys exhibit nearly zero plastic strain in compression test and very low fracture toughness ~3-4 MPa-m1/2. Many current studies are now directed toward improving the ductility or damage tolerance of bulk amorphous steels.
Recently, it was reported that the plasticity of BMGs was closely related to the shear modulus (G) to bulk modulus (K) ratio (G/K), or equivalently, the Poisson’s ratio. The G/K ratios of ductile metallic glasses were found to be smaller than 0.41-0.43, or the Poisson’s ratios larger than 0.31-0.32. The correlation between mechanical properties and elastic moduli indicates that the brittleness problem of metallic glasses can be alleviated by tuning the elastic properties of individual alloy systems via changes in alloy content. A well-defined transition from plasticity to brittleness was indeed found in amorphous steels as the critical Poisson’s ratio fell below ~0.32. The embrittlement (or ductilization) of Fe-BMGs is attributed to the interplay of local structure and bonding configuration in the Voronoi network. It is the interplay of the intra- and inter-cluster bonds that determine the thermal stability and mechanical properties of Fe-based BMGs. In BMG samples that exhibit good ductility under compression loading, shear bands are easily initiated and multiplied. That is, the tendency towards shear stress concentration that tends to nucleate cracks is alleviated. Accordingly, BMGs that exhibit lower shear moduli are likely to undergo shear deformation instead of brittle fracture. By chemically tuning the strengths and local arrangements of the interatomic interactions in the amorphous network, one can lower the shear modulus and therefore increase the Poisson’s ratio to enhance the ductility of amorphous steels. This can be achieved by selecting appropriate combinations of metalloids and metals in the alloys.
Poon, Shiflet, Gu
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