Environmental Cracking of Amorphous Alloys

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ENVIRONMENTAL CRACKING OF AMORPHOUS ALLOYS S. ASHOK, T. P. SLAVIN, N. S. STOLOFF AND M. E. GLICKSMAN Rensselaer Polytechnic Institute, Troy, New York 12181 USA ABSTRACT Six amorphous metals have been tested in liquid metal environments or while charging with hydrogen. Differences in sensitivity to each environment are correlated with fractographic features. Evidence in support of enhanced shear as the mechanism of environmental cracking is presented. INTRODUCTION The mechanical performance of stressed engineering materials often is influenced by the nature of the surrounding environment. Although amorphous metals are known to display excellent resistance to general corrosion, several cases of hydrogen embrittlement of Fe-base alloys have been reported [1]. More recently, liquid metal embrittlement also has been induced in a variety of amorphous alloys [2]. In the case of crystalline metals, it has been suggested that there may be an underlying link between several types of environmentally induced cracking - whether due to hydrogen, liquid metals or aqueous environments capable of inducing stress corrosion cracking [3]. Such a link may be easier to establish in amorphous metals, since the often complicating effects of grain boundaries and low energy cleavage planes present in crystalline alloys are not encountered in the amorphous metals. Therefore, critical tests of proposed mechanisms of embrittlement, e.g., decohesion [4,5] and enhanced shear [3] might be more readily devised. EXPERIMENTAL PROCEDURE Six amorphous alloys were tested: Fe 8 1 . 5 B1 4 . 5 Si 4 , Fe 8 1 5B13.5Si2.5C2.5' Fe 4 ONi40P14B 6 , (Metglas 2826), Fe4 0 Ni 3 8 B1 8 Mo4 (2826MB)) Ni8 1 P1 9 (BNi-6) and Ni 7 8Si 8 Bl 4 (BNi-3). Tensile and bend experiments were performed in air or in liquid Hg-In at room temperature. At 200 0 C the tests were performed in silicone oil or in liquid lithium. Specimens also were tested either after precharging or while simultaneously cathodically charging with hydrogen from various solutions. Tension tests were performed at a constant strain rate of 0.002 S-1. Bend tests were carried out between parallel glass plates as described previously [2]. RESULTS AND DISCUSSION Table 1 summarizes results from tensile and bend tests conducted in air and in Hg-In. The Fe-Si alloys are more embrittled than the Fe-Ni Metglas alloys. At 200 C Li severely embrittles Metglas 2826 and the nickel-base alloys, see Table 2 and Fig. 1. Alloy 2826MB, which is similar in composition to 2826 except for 4% Mo in the former, showed no embrittlement. The Fe-base alloys were not embrittled, even though carbon is present in FeCBSi (embrittlement of crystalline steels in the presence of lithium reportedly arises from rapid attack of grain boundary carbides) [6]. Tensile tests performed while charging with hydrogen show that the degree of embrittlement depends on the type of solution and varies from alloy to alloy,

310 see Table 3. Under all charging conditions Metglas 2826 was not embrittled while FeCBSi displayed maximum sensitivity to h