Effect of high pressure hydrogen gas on the mechanical properties of three Ni based eutectic composites

Smooth and notched tensile bars of directionally solidified Ni-45.5W, Ni-19.7Nb-6.0Cr-2.5Al, and Ni-21.75Nb-2.55Al were tested to failure in 34.5 MN/m2 hydrogen gas and 34.5 MN/m2 helium gas. Decreases in the ultimate tensile strength, uniform elongation, and reduction in area values are found for the samples tested in hydrogen as compared to those tested in helium. The most marked and consistent changes in properties are found for the notched tensile bars. Since the presence of the hydrogen appears to have little effect on the shape of the stress strain curve at small strains, the notched bar test is the better test of hydrogen-induced damage in these materials. Hydrogen damage in the three materials is evidenced by cracking not usually found in samples tested in helium. The damage in the Ni-W eutectic is the most dramatic with extensive transverse and longitudinal cracking prior to failure observed. Additionally, the presence of hydrogen greatly weakens the reinforcement/matrix interface in each case, so that the interface becomes a preferred path for longitudinal cracking. In the other two alloys, grain boundary longitudinal cracking is also found. While no mechanisms have been conclusively established, the most likely source of the degradation is hydrogen trapping at the interfaces related to the presence of segregated trace element S or to the disordered nature of the interfacial region