during heating and cooling agree very well with each other and confirm the upper boundary of the u03b4 h u03b1 phase field in fig1 a the corresponding composition range between 20 0 and 26 0 at cu nearly coincides with the composition range over which annealing at 850 u00b0 c and quenching produces homogeneous u03b1 solid circles only between 18 5 and 20 0 at cu supercooling retards the transformation u03b4 h r u2192 u03b1 as we have observed by dta analysis carried out at a cooling rate of 5 k min the phase diagram in fig1 b actually constitutes a u201c realization diagram u201d u2014 the phase diagram drawn in dotted lines refers to the particular heat treatment and quenching procedure described above for comparison solid lines indicate the equilibrium phase diagram the dotted lines commence in the maximum of the u03b1 phase field and extend over the entire section of the phase diagram shown in this figure above 24 8 at cu supercooling shifts the borderlines of the phase fields towards higher cu contents the critical tie line t k7 moves to a lower temperature t k7 u2032 and the eutectoid point follows a line that extrapolates the lower boundary of the u03b4 h phase field on the right of the a maximum between 18 0 and 19 0 at cu the critical tie line t k9 shifts to a lower temperature t k9 u2032 comparison between the dotted and the solid lines in fig1 b indicates that supercooling can reduce the width of the two phase field u03b4 r u03b1 to 0 6 at field u03b4 rm u03b1 m according to the convention introduced in section 5 1 non equilibrium phases are denoted by the subscript u201c m u201d the field u03b4.