The effect of second-phase Ni solid solution on environmental embrittlement of L12-type Ni3(Si,Ti) ordered alloys

The environmental embrittlement of the L12-type Ni3(Si,Ti) alloys was studied in terms of the effect of morphology, volume fraction, size and interfacial structure of a second-phase Ni solid solution by a tensile test, fractographic and microstructural observation. For the Ni80.5Si10.5Ti9 (expressed by at.%) alloy that contained fine Ni solid solution precipitates in the L12 matrix and a high volume fraction of the eutectoid-like microstructure consisting of Ni solid solution and L12 phase, environmental embrittlement rarely occurred under all aging conditions. For the Ni80Si10.7Ti9.3 alloy that contained fine Ni solid solution precipitates in the L12 matrix and a very small volume fraction of the eutectoid-like microstructure, environmental embrittlement was reduced with increasing aging time, i.e., with the coarsening of the Ni solid solution precipitates, and was eventually fully suppressed in a prolonged aging time. For the Ni79.7Si10.9Ti9.4 alloy that contained only fine Ni solid solution precipitates in the L12 matrix, environmental embrittlement was only slightly reduced with increasing aging time. These results were discussed in association with the interaction of hydrogen with the second-phase Ni solid solution in a eutectoid-like microstructure as well as in L12 matrix grains.