LONG-RANGE ORDERING BEHAVIOR AND MECHANICAL-PROPERTIES OF NI-MO-BASEDALLOYS

Thermal ageing for up to 1000 h at 600-800 degrees C was used to study the long-range ordering behaviour to Ni4Mo and related ordered phases in selected Ni-Mo based alloys with varying Fe and Or contents, and t he corresponding effects on mechanical properties. Analytical electron microscopy and X-ray diffraction were used to characterize the micros tructure. Mechanical properties were determined from microhardness and tensile tests. During the early stages of ordering, the crystallograp hically-related Pt2Mo-type, DO22 and D-1a superlattices coexisted in a ll alloys studied. However, depending upon the exact chemical composit ion, particularly the Mo, Cr and Fe contents, some of these superlatti ces became unstable as the ordering reaction progressed. Chromium was found to act as a stabilizer of either a Pt2Mo-type superlattice or Ni 3Mo depending upon the Mo content; however Fe acted as a stabilizer on ly of Ni3Mo. For both Cr and Fe, the tendency to stabilize Ni3Mo was r ealized at relatively higher Mo contents. Ordering behaviour of commer cial alloys containing minor concentrations of Or and Fe was found to significantly vary from one heat to another depending upon the exact M o content. Although ordering to Ni4Mo and Ni3Mo could lead to severe e mbrittlement, ordering to a Pt2Mo-type superlattice was found to have beneficial effects on mechanical strength.