Effect of ordering on hydrogen embrittlement of Ni-Cr alloy

In order to clarify the role of deformation in hydrogen cracking, effect of ordering (disordered fcc --> Ni2Cr-type ordered phase) on tensile properties in air and with hydrogen charging has been investigated by using a single crystalline Ni-Cr alloy (70%Ni-30%Cr), grown by an optical floating-zone furnace and aged for from 0h to 1000h at 773K. The hydrogen cracks have been found to initiate at the intersection of slip lines and to propagate along {001}-plane when deformed in tension along <001> with hydrogen charging in the disordered condition. The mode of fracture, however, has been changed by ordering and the hydrogen cracks have initiated and propagated preferentially along [111]-plane. TEM observation has shown that, in the ordered specimens, there are two types of dislocations, superdislocation triplet and ordinary single dislocation which is enhanced by hydrogen charging. The interaction of these dislocations has led to anisotropic deformation identified with twinning deformation and pile-ups of dislocation on {111}-plane. It is concluded that the anisotropic deformation plays a major role in the fracture along {111}-plane in Ni2Cr ordered alloys.