Investigation into the effect of an increase in tungsten content on the behaviour during ageing of maraging type Fe-Ni-Co-W alloys

Abstract

In the present paper, the behaviour during ageing is compared in the temperature range 400 to 500° C of the three martensitic alloys Fe-18.5 wt % Ni-8.99 wt % Co-4.87 wt % W, Fe-17.5 wt % Ni-8 wt % Co-8 wt % W and Fe-17 wt % Ni-8 wt % Co-10 wt % W. The kinetics of precipitation were followed by hardness measurements and small-angle X-ray scattering and electron microscopy studies. The ageing effect can be observed in three successive stages. In the second stage,with which a marked increase of the hardness is associated, there is formation and growth of intermetallic precipitates of (Fe, Ni, Co)₃W≡A ₃W. These precipitates are elongated ellipsoids, of several nanometres in length. The preferred direction of growth of these precipitates is 〈111〉 of the martensitic matrix and the orientation relationship with the martensitic matrix is$\{ 011\} _M //(010)_{A_3 W}$ and$\left\langle {1\bar 11} \right\rangle _M //[100]_{A_3 W}$, identical to that found for the Ni₃Mo precipitate in the quaternary alloys of the same composition but with the W-content substituted for by Mo; but none of the three above-mentioned stages corresponds to the formation ofω-phase.