The effect of concurrent straining on phase transformations in NiAl bronze during the friction stir processing thermomechanical cycle

Abstract

Equivalent strains up to a value of ≈2.7 were determined by evaluation of the shape changes of the phases in a duplex α(fcc)/β(bcc) microstructure formed ahead of the pin tool extraction site during the friction stir processing (FSP) thermomechanical cycle in a cast NiAl bronze alloy. Correlation of the local strains with volume fractions of the various microstructure constituents in this alloy shows that the concurrent straining of FSP results in acceleration of the α + β → β reaction in the thermomechanically affected zone (TMAZ) ahead of the pin extraction site. The resulting volume fraction of β (as determined by the volume fraction of its transformation products formed during post-FSP cooling) corresponds closely to the volume fraction expected for the peak stir zone temperature measured separately by means of thermocouples embedded within the tool pin profile along the tool path. The stir zone (SZ) in this material exhibits near-equilibrium microstructures despite brief dwells near the peak temperature (T peak ≈ 0.95T melt), reflecting large local strains and strain rates associated with this process. © 2011 The Author(s).