The application of friction stir processing (FSP) to a cast NiAl bronze (NAB) material is presented
as a means for selective modification of the near-surface layers by converting as-cast
microstructures to a wrought condition in the absence of macroscopic shape change. This may enable
selective surface hardening of cast components. The complex physical metallurgy of the NAB is
reviewed, and microstructure changes associated with FSP for a selected set of processing
parameters are examined by optical microscopy (OM) and transmission electron microscopy (TEM)
methods. Direct temperature measurement in the stir zone is infeasible and, so, these
microstructure changes are used to estimate peak temperatures in the stir zone. The persistence of
a Fe3Al phase (Kii) indicates that peak temperatures are below the solvus for this phase, while the
presence of transformation products of the f3 phase, including fine Widmanstätten a., bainite, and
martensite, indicates that peak temperatures exceed the eutectoid temperature for the reaction
β →ᵅ + Kiii throughout the stir zone.Defense Advanced Research Projects Agency (DARPA