Formation, modulation and adaptive twinning of martensite in the Au7Cu5Al4 shape memory system

Abstract

The Au 7 Cu 5 Al 4  electron phase transforms displacively from an L2 1 parent to a nominally bodycentred tetragonal martensite with c/a<1. The compound is of interest because it has the potential to serve as an 18 carat shape memory alloy in jewellery. Analysis of its X-ray diffraction spectra indicates that the martensite is modulated by a [110]/[ 10 1 ] transverse shear wave, showing that it belongs, strictly speaking, to the generic B19 structure type. The martensite is also twinned, and the probable twinning structure is explored. A ) 6 , 9 ( 15R stacking sequence is deduced, which for reasons of the L2 1 ordering inherited from the parent phase, must be doubled to produce a notional ) 6 , 9 , 6 , 9 ( 30R martensite that properly repeats. However, although the measured X-ray diffraction spectra can be substantially explained by the structures derived, the martensite probably also has additional, higher-order lattice modulations.

Thin Films of AuCuAl Shape Memory Alloy for Use in Plasmonic Nano-actuators

“…At higher magnification, fringes spaced at about 1.54 nm become visible in this film. If a nominal spacing of the close-packed planes of 0.222 nm is accepted [10,18], then a martensite phase of the 7R type is indicated by the presence of these fringes [10]. The lack of crystallinity when the (Au 0.58 Cu 0.42 ) 1-x Al x films were deposited at room temperature is similar to the situation found for very thin, nanoscale films of TiN [14].…”

“…It has been shown to be relatively resistant to aging compared to its copper-based counterparts and has been previously studied in its bulk form [2][3][4][5][6][7][8][9][10][11][12]. However no work has been done on any thin film or nanostructured properties.…”

Determination of the 76 wt.% Au section of the Al–Au–Cu phase diagram

“…The structures of some of the less familiar phases were subsequently determined, and are reported elsewhere [9][10][11]. The degree of ordering of the AuCu-I phase was primarily determined by computation of the ratio a/c, determined for a four-atom fct (a/c>1) or bct unit cell (a/c<1).…”

“…In contrast, laths were barely evident or absent in the as-cast, arc-melted and unmounted state. X-ray diffraction studies conducted at ambient and elevated temperatures revealed that the lathy phase at room temperature has the structure of a nominally tetragonal martensite [10], the atomic positions of which are further modulated [11]. The parent phase for this martensite is a little-known ternary β electron phase, with the nominal composition Au 7 Cu 5 Al 4 and an e/a ratio of 1.48 [9].…”

A2 → B2 → L21 ordering transitions in Au–Cu–Al alloys

“…The martensite phase transformed from the ordered L2 1 phase has a modulated body-centered tetragonal structure with the martensitic transformation starting temperature (Ms) of about 296 K [3,4]. The disordered A2 phase quenched into liquid nitrogen can also transform to martensite with the Ms below 273 K. However, the ordered B2 phase cannot transform to martensite even quenched into liquid nitrogen [5].…”