Microstructural evolution of austenite in a microalloyed Fe30% Ni alloy.

The study of the physical metallurgy of microalloyed steels has been an important field
of research for nearly forty years. During this time the hot working characteristics have
been comprehensively investigated, simulated and modelled. Unfortunately, the actual
microstructural behaviour during hot working cannot be followed completely due to the
unavoidable phase transformation of these steels upon cooling. This transformation
prohibits direct study of the deformed austenite, by disordering the dislocation
structures developed during hot working.
In order to avoid the problem of transformation, a model alloy has been developed. This
allows the retention of the austenitic structure to room temperature, while retaining
similar thermodynamic and deformational properties to conventional microalloyed
steels. The alloy was based on a matrix of iron with 30wt. % nickel, and niobium and
carbon additions to the level of 0.1% and 0.09% respectively. The use of such an alloy
to simulate the hot working behaviour of traditional microalloyed steels means that the
study of softening and precipitation events in the austenite matrix is possible as the
phase transformation is avoided.
The Fe-30%Ni-Nb alloy has undergone thermomechanical processing. Hot plane strain
compression tests have been carried out in order to study the precipitation kinetics.
Before testing, the material was solution treated at 1250°C to allow supersaturation of
the niobium at lower temperatures. Double-deformation plane strain compression
testing has been carried out over a range of temperatures (900-1050°C) at a strain rate of
10s 1 and with delay times between deformations varying from is to 1000s. This testing
has allowed the study of both the static precipitation and recrystallisation kinetics from
the resulting flow stress behaviour. Precipitation has been evident from the stress-strain
curves.
Transmission electron microscopy of thin foils of the hot-worked material has been
completed to investigate the dislocation structures produced. This shows the presence of
a microbanded substructure. The particle populations have been studied using
conventional transmission electron microscopy. Direct observation of particles
precipitated in the austenite matrix has been achieved by electron spectroscopic imaging
studies of the as-deformed material. This is important, as it shows preferential
precipitation upon the dislocation structure, and not within the matrix.
The overall study shows that the iron-nickel alloy is a good model austenite in many
respects. It has similar hot-working characteristics: deformation behaviour, work
hardening response, and recrystallisation behaviour. The present work also reports
similar NbC precipitation behaviour to that found in conventional C-Mn based
microalloyed steels. It appears that the alloy is an excellent model for microalloyed
austenite under hot-working conditions, and should prove to be a valuable material for
future investigations.