These values of Φ and Ψ lead to an imposed
strain of ∼1 on each passage of the sample through the
die [17]. An earlier investigation, also conducted using pure
Al, showed that the use of a solid die with an arc of curvature
of Ψ =20◦ has no significant influence on measurements of
homogeneity in the as-pressed samples by comparison with a
conventional split die where the channel is bent through 90◦
and there is no arc of curvature so that Ψ =0◦ [18]. Samples
were processed by ECAP through selected numbers of passes
using route BC in which the samples are rotated by 90◦ in the
same sense between each pass [19]. This processing route
was selected because earlier experiments on high-purity Al
showed it leads most expeditiously to an array of equiaxed
grains separated by boundaries having high angles of misorientation
[12].