Glass-forming metallic alloys

Johnson, W. L. (1999) Bulk glass-forming metallic alloys science and technology, Mater. Res. Bull., 24, 42-56. 

It is very difficult to cool pure metals and other pure elements fast enough to form glasses. However, metallic alloys can often be converted into glasses, particularly if they contain a mixture of small and large atoms such as iron and boron, or they are multi-component mixtures of metals that crystallize into more than one intermetallic compound (i.e., eutectic compositions). Thus, covalent chemical interactions of the atoms are important because they stabilize liquids and thereby inhibit crystallization. 

The numerous glass-forming binary alloys may be divided into two main categories metal-metal alloys and metal-metalloid alloys. The former category may again be subdivided into three subgroups comprising alloys of transition metals (3d, 4d, 5d), alloys of simple metals and alloys of transition metals with either rare earths... 

Metallic Glasses. Under highly speciali2ed conditions, the crystalline stmcture of some metals and alloys can be suppressed and they form glasses. These amorphous metals can be made from transition-metal alloys, eg, nickel—2irconium, or transition or noble metals ia combination with metalloid elements, eg, alloys of palladium and siUcon or alloys of iron, phosphoms, and carbon. 

Thallium phthalocyanine (PcT12) can be obtained by heating phthalonitrile with an inter-metallic alloy of indium and thallium in an evacuated glass ampule.145 The structure of this compound is unique among metal phthalocyanines as the two thallium cations occupy two opposite corners of an octahedron, which is formed by the thallium cations and the four iso-indolinc nitrogen atoms facing the center of the macrocycle.147 Another unusual type of phthalocyanine can be prepared by heating phthalonitrile with thallium metal.148 It was identified as a bicyclic thallium(III) phthalocyanine (Pc3/2T1).14S... 

The formation of barium chromate often leads to the physical separation of the sealing glass and the metal alloy due to barium chromate s high thermal expansion. Along interfacial regions where oxygen or air access is blocked, chromium or chromia can react with barium-calcium-aluminosilicate (BCAS) glass-ceramic to form a chromium-rich solid solution and a series of pores. 

The precursor alloy is quenched to form small grains readily attacked by the caustic solution [31], Quenching can also enable specific intermetallic phases to be obtained, although this is less common. Yamauchi et al. [32-34] have employed a very fast quench to obtain a supersaturation of promoter species in the alloy. It is even possible to obtain an amorphous metal glass of an alloy, and Deng et al. [35] provide a review of this area, particularly with Ni, Ni-P, Ni-B, Ni-Co, and Ni-Co-B systems. The increased catalytic activity observed with these leached amorphous alloy systems can be attributed to either chemical promotion of the catalyzed reaction or an increased surface area of the leached catalyst, depending on the components present in the original alloy. Promotion with additives is considered in more detail later. 

The Ni-Pd-Cu-P family of alloys shows among the metallic systems the highest glass-forming ability known to date (Inoue et al. 1997, Loftier 2003). 

The ternary alloys of a lanthanide with Al and a late transition metal have very high glass-forming ability and low critical cooling rate bulk samples may be cast. 

A simple method for predicting limits for glass-forming ability (GFA) in metallic alloys was proposed by Saunders and Miodownik (1983). lliey utilised the To criterion to predict the limit to the glass-forming range (GFR) of a number of binary and ternary systems. The To criterion follows the premise diat, if cooling conditions... 

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