A study has been made of the precipitation processes occurring at 550°C and 620°C in ferritic Fe-Ni alloys, with Al contents from 1% to 3% and Ti contents from 0% to 3.4%. Hardness tests, resistivity, density and lattice parameter measurements, and electron microscopy techniques have been employed. In addition, detailed tensile testing and electron microscopy have been used to determine the strengthening mechanisms and the rate-controlling deformation mechanisms in these alloys.
Ageing at both temperatures results in substantial hardness increments which increase with increasing Al and Ti contents. The predominant intermetallic compound responsible for the increase in strength, is the ordered body-centered cubic Ni (Al, Ti) in alloys with a Ti/Al ratio less than 2, and is the ordered face-centered cubic Ni2 Al Ti in the alloy with a Ti/Al ratio of 3.4. Precipitation of the intermetallic compounds occurs by a normal nucleation and growth process, and subsequent particle coarsening at constant volume fraction closely follows the kinetics predicted by the LSW theory. At long ageing times a significant deviation from the particle distribution predicted by the LSW theory occurs as a result of particle coalescence and in alloys which have high particle-matrix mismatch the particles become aligned at large particle sizes. The evidence suggests that in addition to precipitation of the intermetallic phases the matrix contains ordered zones at all ageing treatments, which do not correspond to the thermodynamically defined supersaturations and which have limited, if any, growth with ageing time.
The origin of the strengthening is the extra shear stress increment required to deform the intermetallic particles at all ageing treatments investigated. The matrix ordering, present at all ageing times, does not contribute significantly to the observed increments. The observed shear stress increment of 180 N/mm 2, in an alloy of 0.5% mismatch, consists approximately of 40% order hardening, 40% shear modulus hardening and 20% coherency hardening. The greater shear stress increment of 340 N/mm 2 obtained in an alloy of 1.5% mismatch is attributed more to an increased contribution from coherency hardening than to the effect of the greater volume fraction of particles present in this alloy. At all strains, the work-hardening rate during tensile deformation attains a maximum value in the alloy condition corresponding to near peak strength. The high work-hardening rates, characteristic of Orowan looping, are not evident even in the grossly overaged condition.
Ageing at both temperatures results in a progressive increase in the thermal component of the flow stress from the solution-treated value to peak and post-peak aged conditions. The rate-controlling deformation mechanism in all conditions until gross overageing occurs, is the shearing of the coherent intermetallic particles. At long ageing times the effective stress drops rapidly to a value lower than that in the solution-treated condition. This is associated with the lattice obstacles of the body-centred cubic matrix becoming the rate-controlling obstacles as a critical value of the inter-particle spacing is exceeded.