Abstract
Nonequilibrium grain boundary segregation of alloying elements in an Fe-5Mn-9Ni-5Mo alloy and its effect on the mechanical properties have been investigated through AES and tensile test. The alloy showed a ductile-brittle-ductile transition during isothermal aging at 430 or 450°C, and intergranular fracture was shown in the brittle region. This transition was due to manganese segregation to the grain boundaries and its desegregation into the matrix, and was deeply related to the precipitation reaction of fct MnNi. The segregation level of manganese showed a peak value with aging time which corresponded to the lowest ultimate tensile strength, but the segregation level of nickel at the grain boundaries steadily increased with no peak level. This was due to the interaction coefficient between nickel and molybdenum in longer aging times, which augmented the free energy of segregation of nickel. In longer aging times, molybdenum participating in the precipitation reaction of Fe 2 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 between nickel and molybdenum.
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