The ageing characteristics of an Fe-19 wt% Ni-9 wt% Co-5 wt% W alloy

Abstract

In the present paper, we compare the behaviour during ageing in the temperature range 380 to 530° C of the two martensitic alloys Fe-18.65 Ni-8.99 Co-4.87 Mo and Fe-18.65 Ni-8.99 Co-4.87 W (wt %). The kinetics of precipitation were followed by measurements of hardness, by resistometric measurements and electron microscopy. In the case of the alloy with Mo, Mo at first concentrates in local clusters, then an ω phase is formed, and above 450° C, finally a Ni3Mo or Laves phase (Fe, Ni, Co)2 Mo is formed. In the case of the alloy with W, ω phase formation was not observed but an intermetallic precipitate isomorphous with Ni3Mo, (Fe, Ni, Co)3 W = A3W was found. The precipitates are rod-shaped, of several nm in length. The preferred direction of growth of these precipitates is 〈1 1 1〉 of the martensitic matrix and the orientation relationship with the martensitic matrix is (0 1 1)M ∥ (0 1 0)A3W and [1 1 1]M ∥ [1 0 0]A3W, identical to that found for the Ni3Mo precipitate in the (Fe, Ni, Co, Mo, Ti) industrial maraging steel.