Embrittlement of amorphous Fe 40 Ni 38 Mo 4 B 18 alloy by electrolytic hydrogen
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I.
INTRODUCTION
IT has long been recognized that hydrogen may embrittle many crystalline metals, including Fe- and Ni-base alloys. The embrittlement is manifested by the nonductile fracture mode, reduced ductility, and reduced tensile strength in a tensile test. Recently, some Fe- and Ni-base amorphous alloys also have been found to exhibit similar hydrogen embrittlement (HE) in a tensile or bending test by cathodic charging in acidic solutions, f~-81 Amorphous alloys are distinguished from the crystalline alloys, particularly in their deformation behavior. M9-~31 Macroscopically, they behave in a brittle manner. In a tensile test, the linear relationship of stress strain extends to the fracture point. Plastic deformation occurs only shortly before or simultaneously with fracture. Plastic strain is localized in sharp shear bands, c~~ Their fracture is preceded by large local plastic shear, which produces a featureless smooth zone, followed by a catastrophic shear, which produces a "vein" or "ridge" pattern. 1~~ The lack of ductility has been ascribed to the inhomogeneous deformation. Therefore, the ways in which hydrogen affects the deformation and fracture processes in amorphous alloys might be different from those in crystalline alloys. The changes in mechanical properties and fracture mode due to HE in amorphous alloys have been well documented, l~-sj but Very few quantitative studies on the embrittlement have been performed. In some of our previous work, it was observed that an Fe-Ni base amorphous alloy Fe40Ni38Mo4Bt8 could be embrittled in ambient hydrogen gas 1141and cracking could be induced by static charging, tjsj In this article, we studied further
the effects of hydrogen charging on the mechanical properties of this alloy. The effects were quantitatively correlated with hydrogen diffusivity and concentration within the specimen.
II.
EXPERIMENTAL
The material studied was the commercial metallic glass Fea0Ni3sMo4B18 (Metglas 2826MB). The asreceived ribbon was approximately 25.4-mm wide and 25-~m thick. Reduced-section specimens, with the width and length of the gage section being 6.3 and 25 mm, respectively, were prepared. Prior to the test, they were polished with 600-grit emery paper to give a fresh surface, and the thickness was reduced to about 20/zm. The area outside the gage section was insulated with lacquer. Hydrogen charging at current densities of 0.2, 0.5, and 2.0 m A / c m 2 was carried out prior to and during deformation at room temperature (20 ~ using a platinum anode in a solution of 0.1 N H2SO 4 with an addition of 5 m g / L NaAsO2 to promote the ingress of hydrogen. A tensile test was performed at room temperature at strain rates in the range 7.58 • 10 6 to 2.67 • 10 -4 s -1. Aluminum shims were glued onto both ends of the specimen for gripping. Three to five specimens in each condition were tested and the arithmetic mean was calculated. The fracture surfaces were examined by scanning electron microscopy (SEM).
III.
RESULTS
A . Tests in Ai r J.-J. LIN, formerly Graduate Student,
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