Indentation fracture toughness of amorphous steel
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S. Joseph Poon Department of Physics, University of Virginia, Charlottesville, Virginia 22904-4714
G.J. Shiflet Department of Materials Science and Engineering, University of Virginia, Charlottesville, Virginia 22904-4745
Reinhold H. Dauskardta) Department of Materials Science and Engineering, Stanford University, Stanford, California 94305-2205 (Received 22 November 2004; accepted 4 January 2005)
Indentation fracture toughness experiments were performed on amorphous steel. Measured toughness values were 3.2 ± 0.3 MPa √m for 3-mm-thick specimens and 3.8 ± 0.3 MPa √m for 10-mm-thick specimens. Crack geometry was determined to be of the “Palmqvist” or radial configuration. High indentation loads >40 N were necessary for crack formation, and indentation cracks did not form adjacent to every indent, or in a uniform pattern. Possible reasons for crack formation thresholds and geometry are discussed.
An emerging new class of bulk Fe-based metallic glasses is known as amorphous steel. Amorphous steel alloys offer higher strengths than other bulk metallic glasses, with high stiffness and yield strengths three times higher than high-strength crystalline steels.1,2 They have been produced in rods as thick as 12 mm.3,4 Compared to Zr-based bulk metallic glass, amorphous steel has higher strength and stiffness, lower material cost, and higher glass transition temperature. However, amorphous steel alloys suffer from lower glass-forming ability than the more established Zr-based and Pd-based systems. Bulk metallic glasses are not generally brittle, with nominal toughness values typically around 15–25 MPa √m for Zr-based alloys, which can reach as high as 70 MPa √m with extensive crack branching.5–8 As a result, conventional fracture mechanics-based specimens satisfying small-scale yielding and plane strain conditions were required to measure valid fracture toughness values.9 Indentation fracture mechanics techniques used to characterize fracture behavior in brittle materials could not be used since cracking could not be initiated. In brittle materials, indenting with a diamond pyramidal Vickers indenter produces a pattern of cracks, typically a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0104 J. Mater. Res., Vol. 20, No. 4, Apr 2005
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emanating from the four corners of the indent. Provided the materials exhibit brittle fracture with minimal crack tip plasticity, this crack pattern can be used to establish fracture toughness values. Other requirements for valid indentation fracture measurements include materials that do not densify like certain anomalous glasses and porous materials.10 Indentation fracture has been used to characterize the fracture toughness of several classes of materials, including glasses, ceramics, intermetallics, and some polymers. In the present study, we demonstrate that Fe48Cr15Mo14Er2C15B6 amorphous steel, known as DARPA-University of Virginia-Glass 101 (DARVAGlass 101), exhibits indentation c
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