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Gold-based bulk metallic glass alloys based on Au-Cu-Si are introduced. The alloys exhibit a gold content comparable to 18-karat gold. They show very low liquidus temperature, large supercooled liquid region, and good processibility. The maximum casting thickness exceeds 5 mm in the best glassformer. Au 49 Ag 5.5 Pd 2.3 Cu 26.9 Si 16.3 has a liquidus temperature of 644 K, a glass transition temperature of 401 K, and a supercooled liquid region of 58 K. The Vickers hardness of the alloys in this system is ϳ350 Hv, twice that of conventional 18-karat crystalline gold alloys. This combination of properties makes the alloys attractive for many applications including electronic, medical, dental, surface coating, and jewelry. Gold became known to mankind thousands years ago, and has been of inestimable value to civilization ever since. It is the noblest of the noble metals, and has a good combination of high thermal conductivity, high electrical conductivity, and high corrosion resistance. These properties have resulted in extensive use of gold and its alloys in electronic, aerospace, astronomy, medical, and industrial applications. For instance, gold can be found in many of today's sophisticated electronic devices. However, gold has always been most valued as a jewelry material. Gold alloys are easy to fashion, are nonallergenic, have a bright pleasing color, and remain tarnish free indefinitely. Pure gold and higher karat gold alloys, however, are rather soft and therefore vulnerable to wear and scratching. This results in diminished aesthetic appearance, and is a drawback of conventional crystalline gold alloys.In the last two decades, several alloys based on Pd, 1-3 La, 4 Zr, 5,6 Fe, 7-9 and Pt 10 were found to form bulk amorphous phases. Fully amorphous samples are obtained when the alloys are cast into copper molds of diameter up to centimeters which indicates critical cooling rates for glass formation of 100 K / s or less. These bulk metallic glasses ͑BMGs͒ exhibit properties such as high strength, large elastic strain limit, high hardness, and, in some cases, substantial ductility. 11 The compositions of these BMGs are typically close to a deep eutectic composition. Consequently, their melting temperatures are much lower than estimated from interpolation of the alloy constituents' melting temperatures. The resulting low liquidus temperature is an attractive property for casting alloys. The extraordinary stability of BMG forming alloys against crystallization also results in a large supercooled liquid region, ⌬T, ͑⌬T = T x -T g , T x : crystallization temperature, T g : glass transition temperature͒, the temperature region in which the amorphous phase first relaxes into a highly viscous liquid before eventually crystallizing.In this temperature region, BMG's are amenable to superplastic processing using netshape processing methods similar to those employed for thermoplastics. 12 The binary gold silicon eutectic composition was the first alloy found to exhibit metallic glass formation by Duwez and co-workers in ...

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We report on the solidification of Au 49 Ag 5:5 Pd 2:3 Cu 26:9 Si 16:3 bulk metallic glass under various strain rates. Using a copper mold casting technique with a low strain rate during solidification, this alloy is capable of forming glassy rods of at least 5 mm in diameter. Surprisingly, when the liquid alloy is splat cooled at much higher cooling rates and large strain rates, the solidified alloy is no longer fully amorphous. Our finding suggests that the large strain rate during splat cooling induces crystallization. The pronounced difference in crystallization behavior cannot be explained by the previously observed strain rate effect on viscosity alone. A strain rate induced phase separation process is suggested as one of the explanations for this crystallization behavior. The strain-rate-dependent critical cooling rate must be considered in order to assess the intrinsic glass forming ability of metallic liquid. DOI: 10.1103/PhysRevLett.96.075503 PACS numbers: 61.43.Dq, 64.70.Dv, 64.75.+g, 82.60.Lf In the 1950s, Turnbull predicted that glass formation in metals is possible if heterogeneous nucleation could be suppressed [1]. Shortly after, Duwez and co-workers reported the synthesis of the first metallic glass [2] by rapidly cooling an Au-Si alloy at a rate of approximately 10 6 K=s. In the 1970s, ribbons, splats, powder, droplets, wires, and thin films were the typical forms of early metallic glasses obtained by rapid quenching [3,4]. Pd-Cu-Si alloy system was the first exception -Chen's discovery of this alloy system provided the first evidence that metallic glass could be cast in bulk form [5].During the past few decades, bulk metallic glasses have been discovered in a wide range of alloys [6 -11]. In some alloys, a critical cooling rate to avoid crystallization as low as 0:005 K=s [12] and critical casting thickness of up to 7 cm were reported [13] when heterogeneous nucleation was reduced.With the exception of some exotic bulk metallic glass (BMG) synthesis methods [14 -17], most amorphous alloys have been created by cooling the liquid from above its liquidus temperature. In this case, the intrinsic glassforming ability (GFA) and critical casting thickness, d max , were understood to be completely characterized by the critical cooling rate, R c (see, e.g., [18].)This Letter presents solidification studies on gold bulk metallic glass-forming alloy Au 49 Ag 5:5 Pd 2:3 Cu 26:9 Si 16:3 , which could be cast into fully amorphous rods of at least 5 mm in diameter using a conventional copper mold injection method. When the liquid alloy was splat cooled, where it was exposed to both high cooling rate and high strain rate, it crystallized during solidification. This suggests that the high strain rate induces crystallization. This effect becomes less pronounced with increasing processing temperature.Ingots of Au 49 Ag 5:5 Pd 2:3 Cu 26:9 Si 16:3 alloy were prepared by arc-melting the elements (purity: Au, 99.95%; Cu, 99.9%; Ag, 99.5%; Pd, 99.95%; Si, 99.95%) in a titaniumgettered, argon-filled atmosphere. Fu...

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