The preparation, characterization, and surface reactivity of crystalline and amorphous strongly interacting alloys : TiAl and AgMn

Abstract

The formation of the amorphous phase of the Ti-Al alloy system has been carried out by co-condensation, the physical vapor deposition of the two metals, onto substrates at both ambient and liquid nitrogen temperatures. The composition range over which thermally stable amorphous phases of Ti-Al can be formed by co-deposition of the pure elements has been found to extend from at least 15 to 60 atomic percent titanium for both deposition temperatures. This is the widest composition range found to date for the Ti-Al alloy system, and it may be possible to extend it to even higher concentrations of aluminum. The crystallization temperatures have been estimated for the amorphous phases and are: for Ti[50]Al[50] 500°C < T[cryst] [less than or equal to] 600°C, Ti[25]Al[75] 300°C < T[cryst] [less than or equal to] 350°C, and for Ti[15]Al[85] T[cryst] [approximately] 325°C. The reactivity of the Ti-Al co-deposited alloys has been studied over a wide range of compositions, Ti[x]Al[100-x] (13 [less than or equal to] x [less than or equal to] 85). The reactivity was probed by examining the oxidation behavior of the alloys in high vacuum and in 5x10^-3 Torr O2 from temperature to 600°C. Additionally, the reactivity of three Ag-Mn alloys (5, 9, and 14 atomic % Mn) was investigated from room temperature to 650°C in high vacuum, 5x10^-3 Torr O2, and 5x10^-1 Torr O2, and from room temperature to 450°C in 5 Torr O2. The main controlling factors involved in the formation and stability of the oxide overlayer have been shown to be: the free energy of formation for the oxides of the alloy components, the stepwise oxidation of titanium and manganese, bulk alloy composition, oxygen activity, temperature, oxide decomposition, and oxygen solubility in the alloy. In the Ag-Mn alloy system, the sublimation of metallic manganese from the surface at high temperatures in high vacuum was found to be a critical factor. The chemical state of the species involved in the formation of the oxide overlayer depend much more strongly on temperature and oxygen activity for the Ag-Mn alloy system than for the Ti-Al alloy system discussed above. The two alloy systems have been used to extend the applicability of the Cabrera-Mott model in thin oxide overlayer formation.