Displacive transformations in Au-18 wt pct Cu-6 wt pct Al

Abstract

A gold alloy with 18 wt pct Cu and 6 wt pct Al undergoes a reversible displacive phase transformation between an incompletely ordered L2 1 parent phase and a tetragonal product. The characteristics of these transformations were studied using acoustic emission, dilatometry, X-ray diffraction, and metallography. The morphology of the transformation products, the structure of the parent phase, and the generation of significant acoustic emission during the transformations indicate that they are at least quasi-martensitic, if not martensitic, and that this system is an example of a ␤-phase shape-memory alloy (SMA). The onset temperatures of the transformations depend on the prior thermal history of the sample. The martensite start (M s ) temperature is between 30 ЊC and 20 ЊC. The system exhibits hysteresis and will revert to the parent phase when reheated, with an austenite start (A s ) temperature between 55 ЊC and 80 ЊC. However, freshly cast or solution-annealed and quenched samples of the alloy do not transform to the tetragonal phase. Aging of such material at temperatures between 30 ЊC and 200 ЊC is required before they will manifest the displacive transformation. The "martensite" phase is considerably more resistant to aging-induced stabilization than that of most other SMAs.

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

“…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]. The reversible transformation between the two structures has been studied, and is reported elsewhere [18]. It is sufficient to say here that the M s temperature is in the range 10 to 30°C, while the A s temperature varies between about 55°C and 77°C ( Figure 3).…”

Ternary β and γ phases in the Al–Au–Cu system at 750°C

“…For example, Sample 10 (Au 6.9 Cu 5.7 Al 3.4 ), which had the lowest transformation temperatures, A s was at ~88˚C and M s at ~26˚C. The M s is within the known range for Spangold but the A s is a little higher than the ~78-85˚C usually reported for that alloy [14,29]. The small difference may have been caused by the slightly lower e/a ratio of Sample 10 relative to standard Spangold (1.42 vs. 1.48).…”

Phase diagram of Au-Al-Cu at 500 °C

“…A β (Au 7 Al 4 Cu 5 ) phase exhibiting martensite-like properties was developed [3][4][5]. The embossed and fine-layered surface of this material, which originates from a martensitic transformation, exhibits a characteristic shining color and is used in Spangold jewelry.…”

“…Al-Au binary systems exhibit five intermediate phases at 500°C, namely, AuAl 2 , AuAl, Au 2 Al, Au 8 Al 3 (Au 5 Al 2 ), and Au 4 Al [6]. Al-Cu binary systems exhibit five intermediate phases at 500°C, namely, θ (Al 2 Cu), η 2 (AlCu), ζ 2 (Al 9 Cu 11 ), δ (Al 2 Cu 3 ), and γ 1 phases [7].…”