The effect of ordering on the hydrogen embrittlement susceptibility of Ni 2 Cr
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IT has recently been found that highly cold worked H A S T E L L O Y C-276 is susceptible to hydrogen embrittlement after aging at temperatures in the range of 200 to 500 ~ It was further shown that the grain boundary segregation of phosphorus is a key factor for the increased susceptibility to hydrogen induced intergranular failure. 2 In contrast, it has been suggested by Asphahani 3 that an ordering reaction, as opposed to segregation, is responsible for the increased susceptibility. It was hypothesized that A2B type order in the form of Ni 2 (Cr, Mo) nucleated during aging of H A S T E L L O Y C-276 and that it was the growth of this ordered phase which caused an increase in the hydrogen embrittlement susceptibility? In order to study this phenomenon as a possible mechanism for increased embrittlement, we investigated the effect of ordering on the hydrogen embrittlement susceptibility of Ni2Cr, a model ordering alloy. Ni2Cr undergoes a first order, order-disorder transformation at the critical temperature 863 K. 4 However, a high degree of long range order is obtained only when the alloy is heated for long periods at temperatures somewhat below the critical temperature. The ordering reaction appears to occur most rapidly at 773 K. Taunt and Ralph 5 have found that samples initially heat treated at 1173 K and quenched ordered more rapidly at 773 K than samples which are initially highly cold worked. They suggested that at 1173 K, short range order fluctuations exist and are frozen in when the alloy is rapidly quenched. At 773 K these fluctuations grow in amplitude and wavelength producing the fine, ordered domains found in ordered Ni2Cr. The need for nucleation at grain boundaries or other defect sites is eliminated and the result is homogeneous growth of the ordered state within the disordered matrix. The ordered structure in Ni2Cr was determined by B. J. BERKOWITZ is Staff Metallurgist, Exxon Research and Engineering Company, Corporate Research Science Laboratories, Linden, NJ 07036. C. MILLER is a Graduate Student at Columbia University, Department of Metallurgy, New York, NY 10027. Manuscript submitted October 18, 1979.
X-ray, 6 neutron, 7 and electron 8 diffraction (and found to be isomorphous with Pt2Mo). In the disordered state, the lattice is face centered cubic (fcc) with a lattice parameter of 3.566~, (after quenching from 1273 K). The ordered structure is an orthorhombic superlattice containing six atoms. The slightly smaller lattice parameter in terms of the original cubic coordinate is 3.562~ (after aging at 460 ~ for 3200 h). 8 Figure 1 illustrates the crystallographic orientations and the relationship between the disordered fcc and the ordered orthorhombic superlattice. The (100)orth and (010)or,h planes are parallel to the (110)fcc and (T10)fcc planes respectively and the (001)o,h plane is parallel to the (001)fc~ plane. The superlattice parameters are described by: a = ao/x/-2, b = 3ao/X/2 and c = a0 .6 The degree of long range order has been determined as a function of aging time by neutron dif
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