Spinodal ordering in the equiatomic AuCu alloy

L10 nanotwinned gold-rich Au–Cu–Pt

“…with the c axis of the L1 0 structure locally oriented along any one of [1 0 0], [0 1 0] or [0 0 1]), which produces a typical "tweed" contrast in transmission electron microscopy (TEM) [4][5][6]. This orientation-modulated tweed structure later coarsens and evolves, by gradual elimination of one variant and rearrangement of its intervariant boundaries, into a structure of AuCuI containing only two variants arranged in parallel band-shaped twins typically a few tens of nanometres wide, each containing only one orientation variant and a fine network of anti-phase boundaries (APBs) [4,6,7]. These fine-scale twins, typically called "microtwins" (despite the fact that these twins can be nanoscopic in size) [8,9], can initially be arranged along both perpendicular {1 1 0} twinning planes [4], but later evolve to be all oriented along only one set of parallel {1 1 0} planes, producing a characteristic arrangement often designated as the "polytwin" structure [5,[10][11][12][13].…”

“…Upon rapid quenching to temperatures below 360°C, the disordered A1 structure, which initially displays a certain measure of short-range order, instead evolves homogeneously in "spinodal" fashion, to produce a coherent nanoscale structure displaying all three orientation variants (i.e. with the c axis of the L1 0 structure locally oriented along any one of [1 0 0], [0 1 0] or [0 0 1]), which produces a typical "tweed" contrast in transmission electron microscopy (TEM) [4][5][6]. This orientation-modulated tweed structure later coarsens and evolves, by gradual elimination of one variant and rearrangement of its intervariant boundaries, into a structure of AuCuI containing only two variants arranged in parallel band-shaped twins typically a few tens of nanometres wide, each containing only one orientation variant and a fine network of anti-phase boundaries (APBs) [4,6,7].…”

“…This orientation-modulated tweed structure later coarsens and evolves, by gradual elimination of one variant and rearrangement of its intervariant boundaries, into a structure of AuCuI containing only two variants arranged in parallel band-shaped twins typically a few tens of nanometres wide, each containing only one orientation variant and a fine network of anti-phase boundaries (APBs) [4,6,7]. These fine-scale twins, typically called "microtwins" (despite the fact that these twins can be nanoscopic in size) [8,9], can initially be arranged along both perpendicular {1 1 0} twinning planes [4], but later evolve to be all oriented along only one set of parallel {1 1 0} planes, producing a characteristic arrangement often designated as the "polytwin" structure [5,[10][11][12][13]. This arrangement is energetically favoured because it lowers internal stresses caused by the unit cell distortion that accompanies the transition from cubic A1 crystals to the tetragonal L1 0 ordered structure while maintaining a coherent (heavily twinned) crystal structure [9][10][11][12][13][14].…”

“…Ordering in this alloy is a first-order transition [2] documented to take place above 360°C by classical nucleation and growth [3][4][5]. Upon rapid quenching to temperatures below 360°C, the disordered A1 structure, which initially displays a certain measure of short-range order, instead evolves homogeneously in "spinodal" fashion, to produce a coherent nanoscale structure displaying all three orientation variants (i.e.…”

“…This arrangement is energetically favoured because it lowers internal stresses caused by the unit cell distortion that accompanies the transition from cubic A1 crystals to the tetragonal L1 0 ordered structure while maintaining a coherent (heavily twinned) crystal structure [9][10][11][12][13][14]. At higher transformation temperatures, the same ordered structure develops, albeit by nucleation and growth [3][4][5]. The polytwin structure also often contains a second series of much coarser (micrometre-scale) twins that preserve the two-variant nanometre-scale parallel microtwin structure, arranging the microtwins in a symmetric fishbone arrangement about each side of the coarser twin boundaries [9,15].…”