  The mechanisms that govern a previously unexplained hardening effect of a
single phase Cu-30wt%Zn {\alpha}-brass after heating have been investigated.
After cold-work, the alloy possesses an increased yield strength and hardening
rate only when heat treated to temperatures close to 220{^\circ}C, and is
otherwise softer. Crystallographic texture and microstructure, explored using
electron backscatter diffraction (EBSD), describe the deformation heterogeneity
including twin development, as a function of heat treatment. When heated, an
increased area fraction of deformation twins is observed, with dimensions
reaching a critical size that maximises the resistance to dislocation slip in
the parent grains. The effect is shown to dominate over other alloy
characteristics including short range order, giving serrated yielding during
tensile testing which is mostly eliminated after heating. In-situ X-ray
diffraction during tensile testing corroborates these findings;
dislocation-related line broadening and lattice strain development between as
worked and heated {\alpha}-brass is directly related to the interaction between
the dislocations and the population of deformation twins. The experiments
unambiguously disprove that other thermally-induced microstructure features
contribute to thermal hardening. Specifically, the presence of recrystallised
grains or second phases do not play a role. As these heat treatments match
annealing conditions subjected to {\alpha}-brass during deformation-related
manufacturing processes, the results here are considered critical to
understand, predict and exploit, where appropriate, any beneficial
process-induced structural behaviour.
