This invention relates to amorphous metallic alloys, commonly referred to as metallic glasses, which are mostly formed by solidification of alloy melts by cooling the alloy to a temperature below its glass transition temperature before appreciable crystallization or nucleation of crystals can occur.
Metallic alloys having an amorphous or glassy phase are useful for several industrial applications. Normally, metals and intermetallic alloys crystallize during solidification from the liquid phase. Some metals and intermetallic alloys may be undercooled and remain as a viscous liquid phase or amorphous phase or glass at ambient temperatures when cooled rapidly. Typical cooling rates are about 1,000 to 1,000,000° K/sec.
To achieve rapid cooling rates of 10,000° K/sec or greater, a very thin layer (e.g., less than 100 micrometers) or small droplets of molten metal are brought into contact with a conductive substrate maintained at near ambient temperature. The small dimension of the amorphous material is a consequence of the need to extract heat at a sufficient rate to suppress crystallization. Thus, previously developed amorphous alloys have only been available as thin ribbons or sheets or as powders. Such ribbons, sheets or powders may be made by melt-spinning onto a cooled substrate, such as a spinning copper wheel, or by thin layer casting on a cooled substrate moving past a narrow nozzle.
Many efforts have been directed to searching for amorphous alloys with greater resistance to crystallization for achieving lower cooling rates and hence thicker metallic glasses, often also called bulk metallic glasses. The further crystallization may be suppressed at lower cooling rates, and thicker bodies of amorphous alloys may be obtained.
During formation of amorphous metallic alloys, undercooled alloy melt may crystallize. Crystallization occurs by a process of nucleation and growth of crystals driven by the energetically optimum structure and thereby setting the crystallization energy free. To form an amorphous solid intermetallic alloy, the melt has to be cooled from or above the melting temperature (Tm) to below the glass transition temperature (Tg), without the occurrence or with only minor occurrence of crystallization. Tx is the temperature at which crystallization occurs upon heating the amorphous alloy above the glass transition temperature. Crystallization of the metallic glass occurs at temperatures below crystallization temperature Tx but at a lower rate. The crystallization temperature Tx is not a sharply defined first order phase transition.
The metallic glasses are brought into the desired form by heating the metallic glass to a temperature above the glass transition temperature Tg and then forming the metallic glass. For forming the metallic glass, it is therefore desirable to find a system where the difference DT between the glass transition temperature Tg and the crystallization temperature Tx is substantial. A substantial difference in temperature DT allows the metallic glass to be formed without crystallization or, more precisely, without creating high amounts of unwanted crystalline phase in the metallic glass.
For bulk metallic glasses, it is therefore desirable to use an alloy having a substantial temperature difference (DT) between the crystallization temperature (Tx) and the glass transition temperature (Tg).
Intermetallic alloys that form bulk metallic glasses include zirconium-based alloys. One group of such Zr-based alloys is the Zr—Ti/Nb—Cu—Ni—Al alloys, which are known for example from X. H. Lin et al., “Effect of Oxygen Impurity on Crystallization of an Undercooled Bulk Glass Forming Zr—Ti—Cu—Ni—Al Alloy,” Materials Transactions, Vol. 38, No. 5 (1997), pages 473 to 477; U.S. Pat. No. 5,735,975; U.S. Patent Application Publication 2004/238,077; European Patent Application Publication EP 2 597 166 A1; X. Zeng et al., “Influence of melt temperature on the compressive plasticity of a Zr—Cu—Ni—Al—Nb bulk metallic glass,” Journal of Materials Science 46 (2011), pages 951-956; Z. Evenson et al., “High temperature melt viscosity and fragile to strong transition in Zr—Cu—Ni—Al—Nb(Ti) and Cu47Ti34Zr11Ni8 bulk metallic glasses,” Acta Materialia 60 (2012), pages 4712 to 4719; Y. F. Sun et al., “Effect of Nb content on the microstructure and mechanical properties of Zr—Cu—Ni—Al—Nb glass forming alloys,” Journal of Alloys and Compounds 403 (2005), pages 239-244.
Another group of Zr-base alloys forming bulk metallic glasses is the Zr—Ti—Nb—Cu—Ni—Be alloy known for example from C. Hays et al., “Improved mechanical behavior of bulk metallic glasses containing in situ formed ductile phase dendrite dispersions,” Materials Science and Engineering: A, Volumes 304-306, (2001), pages 650-655; or F. Szuecs et al., “Mechanical properties of Zr56.2Ti13.8Nb5.0Cu6.9Ni5.6Be12.5 ductile phase reinforced bulk metallic glass composite,” Acta Materialia, Volume 49, Issue 9, (2001), pages 1507-1513. A further group of Zr-based alloys forming bulk metallic glasses and bearing beryllium is Zr—Ti—Cu—Ni—Be, known from U.S. Pat. No. 5,288,344 and U.S. Pat. No. 5,368,659.
In some of the above mentioned systems, the temperature difference DT between the crystallization temperature Tx and the glass transition temperature Tg is less than 70° K, causing difficulties when forming these metallic glasses. A further drawback of some metallic glasses may be found in the difficulties to obtain the metallic glass from the melt. When the melting temperature Tm of the alloy is high compared to the glass transition temperature Tg, a higher amount of energy has to be extracted from the alloy to create the metallic glass. If the activation energy to form crystal nuclei in the alloy is low, seed crystals will form during the cooling of the alloy. Both problems may be encountered with a higher cooling rate. As thermal energy has to be conducted from the cooling metal alloy melt, a higher cooling rate results in unfavorably thinner metallic glass samples. The obtainable critical thickness of about 5 mm is still not sufficient for many technical applications, e.g., parts of clocks, springs, elastic contacts for electronic devices, etc.