Computation Mechanics of Microstructures group from The State Key Lab. of Nonlinear Mechanics at the Institute of Mechanics, CAS has achieved progress on the failure and the deformability of bulk metallic glasses (MGs). The respective results have been published in Scientific Reports from the Nature Publishing Group and in Acta Materialia.
Metallic glasses owe a series of excellent mechanical, physical and chemical properties such as ultra-high modulus, high strength and hardness, and superb corrosion resistance, etc. Vitreloy 1 BMG, for example, has a strength of about 2GPa, an elastic limit of 2% and a fracture toughness of 55MPa.m-1/2. On the other side, since BMGs lack long-range order, they cannot deform plastically through dislocations commonly seen in crystalline metals. Hence BMGs have poor tensile ductility at room temperature, and this is the Achilles heel of metallic glasses. The major novel observations regarding the failure and the deformation behavior of BMGs include:
Tensile strength of the net section in circumferentially notched BMG cylinders increases with the constraint quantified by the ratio of notch depth over notch root radius; in contrast, the ceramic exhibit notch weakening. The strengthening in the former is due to resultant deformation transition: Shear failure occurs in intact samples while samples with deep notches break in normal mode fracture, see Figure A. Scientific Reports 5, 10537 (2015); doi: 10.1038/srep10537.
We observed spiral fracture in the Vitreloy 1 BMG subjected to both shear and normal stresses, and the Mohr–Coulomb type of failure is essential for the unique spiral fracture. The use of spiral angles resulted from torsion–tension experiments provide another novel experimental strategy to examine critically the applicability of different strength/failure criteria to metallic glass and other materials, see Figure B. Acta Materialia. 99, 206–212 (2015).
The reformation capability of the medium range connection in clusters regulates the deformation ability of the metallic glasses. Researches have shown that the metallic glasses have a short-range order (SRO) cluster structure or medium range connection structure beside its long range disorder, and the clusters are mainly made up of icosahedrons. From comparison and analysis of two typical metallic glasses (a Zr-based glass with high ductility and a Fe-based one with low ductility) we find that, the degree of short-range order cluster structure for brittle Fe-based glass decreases dramatically during the stretch, while mild change occurs in ductile Zr-based glass. We further discovered that Zr-based glass has a significant higher reformation capability (the fracture, restructuring and reformation of the SRO cluster structure or medium range connection structure) over the Fe-based glass, see Figure C. Our study here provides important insights from atom scale about the mechanisms accounting for ductility or brittleness of bulk metallic glasses. Sci. Rep. 5, 12177 (2015); doi: 10.1038/srep12177.
Figure A | Strengthening mechanism analysis of the circumferentially notched metallic samples: (1) tensile set up and definition the notch depth-a and notch width-2ρ; (2) tensile stress displacement curves; (3) strengthening phenomenon; (4)-(6) Failure mode transition process; Sci. Rep. 5, 12177 (2015). B | Spiral fracture of metallic glass subjected to axial and shear stress state: (1) fractography under uniaxial tensile; (2)–(8) spiral fractography under different combined normal shear stress states; (9) fractography under uniaxial compression; (10) illustration of the spiral angle θ+β definition; (11) Stress analysis; Acta Mater. 99 (2015) 206–212.
Figure C | (a) Illustration of the atom cluster icosahedron and cluster groups; (b) degree variation of the Zr-based and Fe-based clusters; (c) comparison of the cluster reformation capability; (d) illustration of the reformation process during loading; (e) illustration of the fracture of the Fe-based cluster groups.