EFFECT OF NONEQUILIBRIUM GRAIN-BOUNDARY SEGREGATION OF ALLOYING ELEMENTS ON MECHANICAL-PROPERTIES IN FE-5MN-9NI-5MO ALLOY

Nonequilibrium grain boundary segregation of alloying elements in an F e-5Mn-9Ni-5Mo alloy and its effect on the mechanical properties have b een investigated through AES and tensile test. The alloy showed a duct ile-brittle-ductile transition during isothermal aging at 430 or 450 d egrees C, and intergranular fracture was shown in the brittle region. This transition was due to manganese segregation to the grain boundari es and its desegregation into the matrix, and was deeply related to th e precipitation reaction of fct MnNi. The segregation level of mangane se showed a peak value with aging time which corresponded to the lowes t ultimate tensile strength, but the segregation level of nickel at th e grain boundaries steadily increased with no peak level. This was due to the interaction coefficient between nickel and molybdenum in longe r aging times, which augmented the free energy of segregation of nicke l. In longer aging times, molybdenum participating in the precipitatio n reaction of Fe,Mo at the grain boundaries or in the bulk acted as a potent strengthener of the grain boundaries. The segregation level of molybdenum increased to a saturation value due to the interaction betw een nickel and molybdenum.